Dry pyroclouds promote longer-lasting extreme wildfire events intensification.

Dry pyroclouds promote longer-lasting extreme wildfire events intensification. | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Dry pyroclouds promote longer-lasting extreme wildfire events intensification. Marc Castellnou Ribau, Mercedes Bachfischer, Marta Miralles Bover, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4053550/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Global occurrence of pyroclouds leads to an accelerated wildfire rate-of-spread (ROS), creating extreme wildfire events (EWE). Pyroclouds form during wildfires under unstable atmospheres. Recent EWEs unexpectedly created deep moist pyrocloud and accelerated ROS through the night, beyond the unstable atmosphere’s daily cycle. Here, we analyzed the dependence of the ROS acceleration on pyroclouds and atmospheric instability. We used 190 EWEs observed worldwide, supporting their analysis with a coupled fire‒atmosphere model. We find that accelerated ROS depends on dry pyroclouds processes driven by atmospheric instability, particularly that of the free troposphere (FT). Dry pyroclouds form when plume deepening reaches the transition layer between the atmospheric boundary layer (ABL) top and the lifting condensation level (LCL). The depth of dry pyrocloud turbulence, modulated by the FT stability, plays a leading role in further enhancing the downward entrainment of warm and dry air. This entrainment process mixes ABL to form a deeper fireABL, optimizing conditions for moist pyrocloud events. A novel finding is the role of the dry pyrocloud entrainment process in maintaining a decoupled fireABL from surface conditions during the day-to-night transition. This dynamical transition explains the recently observed nocturnal extreme fire events that lasted up to 17 hours, a phenomenon present in 40.2% of observed EWEs worldwide. We argue that during wildfires, the dry pyrocloud persistence shifts the turbulent driving conditions from surface to atmosphere, disrupts the daily cycle of the lower atmosphere, and increases the number of longer-lasting pyroconvective EWEs. Earth and environmental sciences/Natural hazards Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 One-sentence summary Dry pyrocloud shifts the driving turbulent conditions from the surface to the atmosphere, deepening a fire-modified atmospheric boundary layer and facilitating its maintenance when decoupled during the day-to-night transition, resulting in intensified and longer wildfires under pyroconvective conditions. 1. Introduction Pyrocumulonimbus (pyroCb) 1 driven wildfires cause extreme wildfire events (EWE) to spread day and night regardless of the surface weather and fuel conditions, triggering unprecedented fire storms 2 . Such wildfires are increasing in occurrence 3 and have been reported worldwide, including in Canada, California, Chile, Amazonia, Australia, Portugal, Spain, and Greece (Table S1). The spread of extreme fire at night raises questions about the dependence of fire‒atmosphere coupling on high fireline intensity (FLI) during optimal burning conditions 4 . Their unpredictability and sustained FLI well over 1·10 4 kW·m -1 compromise the suppression capacity 5 , and firefighter safety 6 . Therefore, unstoppable EWEs driven by pyroCu/Cb 2 are shockingly impacting lives, property, and biodiversity 7 (Table S1). Pyroconvection intensifies wildfire’s behaviour 8 . First, the coupling of FLI to the above atmosphere enhances the boundary layer turbulence and atmospheric thermodynamic instability 9 . Second, under unstable ABL conditions, convective plume dynamics form a moist pyrocloud 10,11 , characterized by accelerating plume updrafts 12 and downdrafts once it reaches the stage of a mature pyroCb. Finally, the pyroconvective turbulent atmosphere creates events dominated by coherent vortical structures such as tornadoes 13 . The physical explanation of this concatenation of processes is still poorly understood 14 . The ROS bias (ROS b ) is a metric to quantify the impact of those pyroconvective phenomena on a fire 15,16 . The metric compares, in the form of a ratio, the observed wildfire ROS with the modeled one only accounting for weather conditions at the surface. ROS b > 3 indicates our incapacity to explain fire-weather interaction with the current theory. Pyroconvective fires have previously been associated with optimal burning conditions (OBC) linked with diurnal dry and warm weather, facilitating fuel load availability 17 . The OBC facilitating extreme fires correlates more strongly with Tª and RH than wind 18 . Even when mesoscale intense wind events spread unusually large extreme fires day and night, such large extreme fires require those winds to be warm and dry 19 , thus confirming fuel availability as the core of OBC. When fuel availability at central daytime hours coincides with an unstable atmosphere 8 , it creates optimal conditions for fire‒atmosphere coupling 10,11,14 , resulting in pyroconvection and ROS b . These more optimal burning conditions are increasingly frequent under climate change conditions 20 , facilitating a trend to EWE intensification 21 . Global warming significantly increases fuel load through enhanced aridity 22 and fire weather episodes intensification that extend its duration with warmer and drier nights 23 . Strikingly, the observed increase in EWEs goes beyond the scope of intensified midday OBC or warmer and drier nights. More specifically, sustained pyroconvection and extreme ROS b last longer and emerge during day-to-night transitions (DtN) 24 . Nocturnal EWEs started to be recorded within the fire community after 2017’s Las Maquinas fire in Chile and the Serta-Arganil fire in Portugal. ROS of 8-13 km/h and ROSb of 14-16 were observed between 20:00-06:00 LT (Table S1). These phenomena contradict the evening decaying of unstable atmospheric thermodynamic conditions 25 . Understanding the process underlying the extreme fire spread is of pivotal importance 26 . Efforts to advance our comprehension of pyroconvection intensification of fire spread have focused on fire physics 11,27 . Nevertheless, during DtN pyrocloud-dominated wildfire events, prediction uncertainties on ROSb persists 28 . Here, we propose a new physical explanation for the intensification of the longer-lived wildfire event by integrating a global dataset of 182 EWEs with detailed 849 hours of extreme spread rates and a conceptual fire-atmosphere model. We analyze the interrelationships between day, day-to-night transition, and night ROS b events with lower and free troposphere (FT) stability and pyrocloud type occurrence 15 . We hypothesize that during extreme wildfires, fire-atmosphere interaction shifts from turbulence driven by surface-fire conditions to turbulence governed by the ABL entrainment-pyrocloud conditions. 2. Global links relating abnormal fire spread to atmospheric thermodynamics The Observed EWE worldwide (Figure 1a) exhibits regional differences when analyzed in terms of daily spread (ha·day -1 ) and time of maximum spread. In short, local fuel, landscape morphology, and weather shape the spread of wildfires. However, we have identified a counterintuitive new worldwide pattern of extreme ROS events outside optimal burning conditions. Timeframes serve here as a surrogate for the burning conditions. Maximum fuel availability is expected during daylight (0600 and 1800 local time, LT), aligning with daily cycles of solar radiation, temperature and humidity 30 , and satellite-reported peak fire activity 31 . Reduced fuel load availability is assumed at day-to-night transition (DtN, 1800 to 2400 LT), and poor fuel availability at night (2400 to 0600 LT) due to cooler, moister, and darker conditions. Notably, our data set shows that up to 42% of the maximum ROS events are outside of daytime (DtN, night), thus outside optimal burning conditions. Such a new extreme fire spread pattern during DtN and night also contradicts the need for ABL instability as a necessary condition to create fire-atmosphere coupled extreme fires and moist pyroconvection. Using the vertical profiles of the state variables in ERA5, with a spatial resolution of 30 x 30 km 2 , we calculate a combined dual gradient classification of hourly ROS b events according to the ABL and FT thermodynamic stability 32 (Figure 1b). As a metric within the ABL, we use the potential temperature gradient δθ/δz (K·m -1 ). Our criteria are the following: δθ/δz 0.001 K·m -1 as a stable boundary layer (SBL). To describe the FT, we use the potential temperature lapse rate ꙋ θ (K·m -1 ), considering ꙋ θ > 0.004 K·m -1 an indication of thermal stable stability. Our analysis, therefore, links the global ROS b distribution to moist pyroclouds, both governed by thermodynamic instability in the atmospheric vertical profile (Figure 1b). Most ROS b fire spreads in our dataset concentrate under ABL δθ/δz < 0.003 K·m-1 and FT ꙋθ 2000 m AGL). It physically describes the unstable ABL conditions that facilitate the intensification of plume-dominated 16 wildfires producing moist pyroclouds 10 . It is here worth stressing that this pyroconvection regim e goes beyond the prototypical CBL conditions 33 (δθ/δz < 0.001 K·m -1 ) and includes fire-extended CBL up to δθ/δz < 0.003 K·m -1 . From now on, we will refer to it as eCBL. Our interpretation is that fire-driven turbulence at the surface triggers the mixing from the eCBL into CBL conditions, driving the fire-atmosphere coupling. The eCBL also facilitates pyroclouds and ROS b under strong stable FT (Figure 1b, square II). Wildfires observed under these conditions are convective wind-driven 16 under a ‘ shallow pyroconvection regime.’ They are strongly influenced by FT ꙋθ, abruptly limiting plume's deepening. To integrate eCBL conditions into CBL, strong wind mechanical turbulence 34 is crucial in enhancing the plume's ability to entrain geostrophic winds. The resulting acceleration of the ABL wind leads to an extreme spreading fire that can persist throughout the day and night (Figure S1). Remarkably, the explanation of extreme fire events 10,11,14,35 driven by moist pyroclouds, based on ABL thermodynamic instability, falters when confronted with ROS b under strong ABL thermodynamic stable conditions (Figure 1b, square III). Strikingly, PyroCb-driven spreads occur in those highly stable ABL. These events occur in shallow ABL (<1000 m AGL) during the evening and night when fuel availability is reduced and turbulence decays in the lower atmosphere 25 . Vertical pyrocloud deepening in this deep pyroconvective regime is governed by FT instability. Our observations confirm that fire turbulence from an ongoing fire transitioning from day to night can penetrate the shallow SBL formed on the surface. After that process, the fire plume reaches the weaker stability on top and deepens suddenly. This process leads to extreme fire behaviour in otherwise decaying surface burning conditions. Two representative examples of this phenomenon are The Ateca fire of 2022 (Spain) and the Santa Ana fire of 2023 (Chile) (Figure S1). Our observations reveal new insights into the fire-atmosphere coupling processes. Enhanced ABL instability facilitates pyroconvective conditions, but the FT stability determines the type of pyrocloud (Figure 1c), as it controls its vertical growth capacity. In shallow pyroconvection, a stable FT constraint overshooting and wind-driven fires produce shallow oPyroCu. A weak stable FT modulates the deepening of shallow and deep pyroCu, while a weaker stable FT promotes extreme pyroCb in a deep pyroconvective regime. In the deep pyroconvection regime , we notice a plume-deepening process primarily driven by the FT stability conditions rather than the fire itself. We refer to such events as decoupled pyroconvection. The findings suggest a shift in the fire-atmosphere coupling driving turbulence towards the top of the ABL when the plume begins to deepen in the FT. Based on these findings, we propose a unified double process that currently drives extreme wildfire events worldwide: a concatenation of fire-atmosphere coupling-decoupling processes governed by FT stability. In Figure 2, we provide visual evidence of the coupling process during diurnal conditions and the decoupling process during the evening transition conditions. First, during the day (Figure 2a), we show the different coupling stages based on a combination of real cases that occurred in California (USA) and Catalonia (Spain). The plume penetration 35 through the ABL leads to a dry pyrocloud formation (Figure 2.a1) in the transition layer between the ABL top and the LCL (video S3). Such a layer separates the regimes of moist cloud convection above from those of dry convection and mechanical mixing below 36 . The dry pyrocloud turbulence in this layer enhances a combination of processes: first, the entrainment of the FT dry and warm air into the ABL, second, the subsiding shells from cloud evaporative cooling 37 , and third, the plume mass flux transport 38 . This combined mechanism gradually deepens and mixes the ABL (Figure 2.a1 to 2.a4) in a relatively large area and, therefore, forms a fire-induced ABL or fireABL (video S4). We estimate an increase of up to 2000 m fireABL from our observation's statistics. Key in our study is these processes' impact on the ROS metric, with an increase of up to 20 times. Although the dry pyrocloud can evolve to a moist pyrocloud thousands of meters deep through the latent heat release 10,11 , we consider the fireABL as the layer below the plume injection height, visually identified by the plume umbrella. We have observed the dry pyrocloud turbulence in a wide range of convective plumes by in-plume radiosondes, confirming that it is a well-established process in extreme wildfires (Figure S5). During the evening transition (Figure 2b), the fireABL decoupling sequence is key in the fire spread intensification. The fireABL, once formed under diurnal conditions, is maintained downwind below the plume umbrella (Figure 2.b2). This occurs since the fireABL mixing is produced by combining the fire turbulence at the surface and the dry pyrocloud entraining turbulence on top of the fireABL. During the DtN transition, as the surface cools down, an SBL is formed, leading to an inhibition of the vertical mixing 39 and a decoupling between the SBL and the above fireABL (Figure2.b3). Under those conditions, the fire plume deepens and forms pyroclouds through the night after easily overcoming the shallow SBL, as discussed in Figure 1. As such, this process prolongs the daytime fireABL conditions and extreme fire behaviour conditions towards the DtN and night hours. The best quantification of the process by which the dry pyrocloud turbulent dynamics exacerbates fire behaviour and facilitates moist convection is obtained by analyzing the change from ABL to fireABL height with the LCL during ROS b events (Figure 3a1-a2). By combining a conceptual energy balance coupled plume model 40 with the kinematic fireline intensity (K·m 2 ·s -1 ) retrieved from the observed fire spread, we estimate fireABL by the resulting plume injection height. Our criterion is that moist convection occurs when ABL height > LCL height, being dry convection otherwise. While fires around the black dashed line in Figure 3a1 correspond to the moist convection ROS b cases, it is essential to notice the set of fires that fall within dry convective ABL with a height below 2000 m AGL (dashed square in Figure 3a1). This same set situates above 2000 m AGL when we correlate their fireABL with the LCL (Figure 3a2). They cluster with the original moist convection cases around the LCL level and its overshooting buffer zone (violet shadow). Due to the fireABL deepening in dry convection situations, the events with moist convection favorable conditions (violet shadow) increases from 42,1 % of ABLs (Figure 3a) to 76,4 % of fireABLs (Figure 3b). This finding reveals that dry pyroclouds are a key phenomenon in intensifying extreme fire behaviour by increasing the probability of moist pyroclouds occurrence. The group of fire events in Figure 3 that change fireABL position and get closer to LCL depth contain the extreme fire events occurring at DtN and night (red and blue dots), supporting the hypothesis of fireABL relevance on expanding the CBL conditions during the DtN timeframe as suggested in our hypothesis in Figure 2B. (Figures S6). The local expansion of fireABL convective conditions beyond the prototypical CBL cycle (Figure 4) becomes clear when we compare the ABL height against the fireABL height daily cycle. To highlight the dry pyrocloud turbulence effect on fireABL formation we base our quantification using both, the ‘observed fireABL’ based on the real observed fire behavior and the ‘expected fireABL’ based on the simulated fire behavior using surface burning conditions. Our statistics show an ‘observed fireABL’ 17 h long cycle of 2000 m deep (black dashed line) CBL conditions (11:00 to 03:00) in contrast of the classic 33 8 hours long (11:00 to 19:00 LT) CBL depth 8 h cycle for ABL and ‘expected fireABL’. The observed fireABL staying significantly deeper (p=0.005, Table S2) than the ABL and expected fireABL after its afternoon maximum contradicts the ABL depth decrease below 2000 m during the evening transition 41 . Filtering the extreme events by ROSb (grey lines in Figure 4), the process of fireABL maintenance is even pronounced. The expected fireABL and ABL failure to capture the vertical development due to plume injection and its maintenance in the DtN and night hours reinforces the dry pyrocloud importance on complementing the fire fire-atmosphere coupling. The fireABL expansion coincides with 16 continuous hours of ROS b > 3 (boxplot in Figure 4), confirming the correlation between the strength of the plume updraft and fire ROS 15 . The prolonged and coinciding fireABL and ROS b cycles support the hypothesis of extended extreme fire behaviour events by coupling-decoupling processes by fire plume and dry pyrocloud hypothesized in Figure 2. Data from a radiosone launched at 23:52 LT below the ‘umbrella’ smoke-cloud during the Santa Ana fire in Chile 2023 (Figure S7), directly observed a 2800 m AGL nocturnal residual fireABL layer (0,003 k·m -1 ) on top of a 700 m AGL SBL maintaining the height of the diurnal smoke injection height layer. This observation confirms the nocturnal residual fireABL dependence on the daily fireABL formation. 3. Towards new dynamics of the fire-atmosphere system Our results confirm that wildfires need a pyroconvective regime based on weak stability in the lower and upper atmosphere to produce pyroCu/Cb 10,11 and intensify ROS b events. However, the need for weak stability in the ABL is challenged when we observe extreme events outside the pyroconvection regime (Figure 1b) in the stable ABL of both the wind-driven regime and the shallow nocturnal SBL of the pyroCb regime . The ABL stability 4,35 importance in promoting extreme fire behaviour is diminished by the capacity of fire or wind turbulence to mix eCBL δθ/δz values up to 0.006 K·m-1 into CBL conditions. It is the FT stability that modulates the pyrocloud depth, facilitating pyrocloud events independently of the CBL daily cycle. As a result, pyrocloud events concatenate during the day and night, allowing abnormal long-lasting firestorms. This phenomenon has been observed in Australia 2020, Portugal 2017 and Canada 2023 (Figure S8). Such FT-driven firestorms are paradoxically facilitated by two dry convection processes in the ABL: an initiation process driven by fire-buoyant turbulence, mixing the ABL from the bottom, and an intensification process by dry pyrocloud entrainment turbulence, mixing the ABL from the top. These two simultaneous mixing turbulence processes, the fire plume, and dry pyrocloud modify the ABL’s depth into a fire-induced ABL or fireABL. They resemble the direct heating and entrainment heating cycles in the land-atmosphere system 42 . The fire plume turbulence mixing capability is fueled by FLI from the surface. It drives fireABL depth increase, creating fire-atmosphere coupling in slightly stable ABL (eCBL conditions). The ABL mixing is driven by the fire thermals’ buoyancy, as with CBL in the classical mixing layer theory 33 , and the plume detrainment of its core temperature 11 . The process is described as the penetration and deepening stages 8 through the atmospheric vertical profile. Those are the fundamental processes in the extreme pyroCb theory, governing the plume’s capacity to reach the free convection level and subsequently trigger (very) rapid growth 10,11,28 . The dry pyrocloud turbulence enhances the entrainment of dry and warm air from the ABL top, mixing it into the already existing fireABL. It accelerates ABL depth increase 43 , intensifying fire-atmosphere coupling. The dry pyrocloud turbulence is observed to expand the cloud driven mixing process for the fireABL also horizontally (figure S9). It extends downwind from the fire , with the evolution of the pyrocloud into a thick cloud resembling a stratocumulus-like smoke cloud 44 (Figure 2). We name such cloud pyro-stratocumulus (pyroStrCu). It continuously contributes to the entrainment of air through radiative and evaporative cooling turbulence 45,46 , typical of such cloud type. The process introduces a new fire‒atmosphere coupling effect in a much broader area than the plume itself. As a process it expands further into the night than the observed time lag (2 to 5 hours) between the maximum surface heat flux and the CBL’s deepening peak 41 . The PyroStrCu plays a crucial role in preserving fireABL depth and facilitating its decoupling from ABL daily cycle (Figure4), thus facilitating extreme fire behaviour. The process is even more important when fireABL will become a residual layer by the formation of nocturnal surface stability. The residual fireABL persistence relies on the radiative and evaporative cooling turbulence from pyroStrCu. Is this residual layer that facilitates the plume deepening into FT, triggering moist convection when the LCL depth declines at night 47 (Figure S6). Our explanation is related to the identified role played by residual layers in enhancing the penetration of plumes 48 . Overall, the residual fireABL resembles the deep residual layer cycle created by the deep hot and dry CBL during persistent mega-heatwaves 49 The pyroStrCu enhancement of fire spread is opposed to the reported fuel availability reduction by persistent smoke cover 50 . However, our observations indicate pyroStrCu turbulence downwind from the head fire within the immediate distance (< 2 to 5 km), eventually evolving into a thin smoke layer in the fire region. The observed cloud-driven mixing processes redefine our understanding of fire-atmosphere coupling, shifting the focus from surface to top ABL mixing processes. The surface and pyrocloud turbulence combination reveals diverse, unaccounted fireABL mixing processes (Figure 5). This introduces a new framework explaining various observed phenomena. It supports the generalization of moist pyrocloud events, as the increased plume injection height 51 facilitates LCL reach. It explains the observed longer-lasting extreme fire behaviour, as the deep fireABL extends from day to night involving diurnal coupling and nocturnal decoupling processes. The observed stable plume injection and pyrocloud depth despite strong surface fire changes 10 can be attributed to pyrocloud-governed mixing processes. The framework also highlights extreme fire events (Table S1) driven by FT conditions entrainment. Together, the plume and pyrocloud mixing turbulence drive a concatenation of coupled and decoupled processes (Figure 4) that supports a higher importance of the ‘weather hypothesis’ in explaining the behaviour of extreme fires, to the detriment of the fuel hypothesis 52 . Our research connects both explanations. First, the bottom-up-driven formation of fireABL explains the need for fuel availability and OBC for fire-atmosphere coupling and fireABL formation. Once fireABL is formed, dry pyrocloud modulated by the influence of the FT stability facilitates the fireABL deepening during the day and its maintenance, regardless of fuel conditions, when decoupled at night. Our findings provide us with observational evidence of how events will continue to unfold. As the climate change increases aridity 22 and ABL height 53 , we will experience deeper dry pyroclouds. This will result in the enhancement of moist pyroconvection and ROS b late in the day when the decoupled fireABL is closer to LCL. The more frequent heat waves 54 already promote a positive feedback on creating and preserving the residual fireABL. Moreover, warmer nights 23 during the heat waves make it easier for the fires to erode nocturnal SBL and access the residual fireABL. Overall, we can expect an increase in abnormal deep and long-lasting pyroconvection events at night. The findings critically compromise suppression capacity, calling for revising fundamental concepts, enhancing observation techniques, and developing new simulation strategies for wildfire management. Declarations Authors contribution : Marc Castellnou conceived the research and hypothesis, wrote the article with support from Marta Miralles and Mercedes Bachfischer. Marc Castellnou and Mercedes Bachfischer designed the data gathering. Jordi Vila-Guerau de Arellano supervised the research and deeply involved in the writing task. Acknowledgments: This paper and its research would not have been possible without the support of fire Analysts from different Fire and Rescue Services in complementing fire spread data: The Bombers GRAF Generalitat de Catalunya, Jaume Cendra from Diputación General Aragon, Paco Senra from INFOCA, Miguel Angel Botella from GVA, Fernando Chico from INFOCAM, Gerard Grau from Aude Sapeurs-Pompiers, Fabio Silva from GAUF in Portugal Civil Protection, Jorge Saavedra from CONAF in Chile, Luca Tonarelli from antincendi boschivi della Regione Toscana in Italia, Juaquin Ramirez from Technosylva in California and Sisoula Tanidou from Civil Protection Greece. We thank Andrea Duane, Rut Domenech, Laia Estivill, and Brian Verhoeven for their expertise and assistance throughout our study. Open data: All data used in this work is published at: https://doi.org/10.5281/zenodo.10799438 Reanalysis data used in the research can be found at: https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5 The general fire spread information data can be obtained from: https://www.earthdata.nasa.gov/learn/find-data/near-real-time/firms Competing Interest statement : The authors declare no competing interests. References AMS. Pyrocumulonimbus - Glossary of Meteorology. https://glossary.ametsoc.org/wiki/Pyrocumulonimbus (2023). Peterson, D. A. et al. Australia’s Black Summer pyrocumulonimbus super outbreak reveals potential for increasingly extreme stratospheric smoke events. npj Clim Atmos Sci 4 , 1–16 (2021). Di Virgilio, G. et al. Climate Change Increases the Potential for Extreme Wildfires. Geophysical Research Letters 46 , 8517–8526 (2019). Werth, P. A. et al. 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Interactions between dry-air entrainment, surface evaporation and convective boundary-layer development. Quarterly Journal of the Royal Meteorological Society 135 , 1277–1291 (2009). Pino, D., Vilà-Guerau De Arellano, J. & Duynkerke, P. G. The Contribution of Shear to the Evolution of a Convective Boundary Layer. J. Atmos. Sci. 60 , 1913–1926 (2003). Reid, D. G. & Vines, R. G. A Radar Study of the Smoke Plume from a Forest Fire . (Melbourne, CSIRO, 1972). doi:10.25919/vpk1-he24. Bretherton, C. S. et al. An intercomparison of radiatively driven entrainment and turbulence in a smoke cloud, as simulated by different numerical models. Q.J Royal Met. Soc. 125 , 391–423 (1999). De Lozar, A. & Mellado, J. P. Mixing Driven by Radiative and Evaporative Cooling at the Stratocumulus Top. Journal of the Atmospheric Sciences 72 , 4681–4700 (2015). Seo, E. & Dirmeyer, P. A. Understanding the diurnal cycle of land–atmosphere interactions from flux site observations. Hydrology and Earth System Sciences 26 , 5411–5429 (2022). Vilà-Guerau de Arellano, J. et al. The role of boundary layer dynamics on the diurnal evolution of isoprene and the hydroxyl radical over tropical forests. Journal of Geophysical Research: Atmospheres 116 , (2011). Miralles, D. G., Teuling, A. J., Van Heerwaarden, C. C. & De Arellano, J. V. G. Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation. Nature Geoscience 7 , 345–349 (2014). Kochanski, A. K. et al. Modeling Wildfire Smoke Feedback Mechanisms Using a Coupled Fire-Atmosphere Model With a Radiatively Active Aerosol Scheme. Journal of Geophysical Research: Atmospheres 124 , 9099–9116 (2019). Wilmot, T. Y., Mallia, D. V., Hallar, A. G. & Lin, J. C. Wildfire plumes in the Western US are reaching greater heights and injecting more aerosols aloft as wildfire activity intensifies. Sci Rep 12 , 12400 (2022). Cruz, M. G., Alexander, M. E. & Fernandes, P. M. Evidence for lack of a fuel effect on forest and shrubland fire rates of spread under elevated fire danger conditions: implications for modelling and management. Int. J. Wildland Fire 31 , 471–479 (2022). Li, J., Chu, Y., Li, X. & Dong, Y. Long-term trends of global maximum atmospheric mixed layer heights derived from radiosonde measurements. Environ. Res. Lett. 15 , 034054 (2020). Perkins-Kirkpatrick, S. E. & Lewis, S. C. Increasing trends in regional heatwaves. Nat Commun 11 , 3357 (2020). Additional Declarations There is NO Competing Interest. Supplementary Files complementarymaterials3.0.docx Complementary Tables and Figures ifmanresa2.mp4 Dry pyrocloud detail lakefire.mp4 Dry pyrocloud injectionn height increase Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4053550","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":279332377,"identity":"54b2f633-daae-454e-a196-4670a26d2f22","order_by":0,"name":"Marc Castellnou Ribau","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8klEQVRIiWNgGAWjYPACCxkgwfiAeA0HGCR4gBSzAZjHBiISiNPCJkGUFt32w48/f6iQ4DFn7zGr/FJxR55/fgPjh58/cGsxO5NmJnHgjASPZc8Zs9syZ54ZzjjGwCzZg8cWsxsMZgwH2yR4DG7kmN2WbDucwHAM6DQevFrYP384+A+ipVjy3+EEeaAWxj94tfAYSBxsgGhh/NhwOMEAqIUZry1ncsokzhwDajlzrFia4dhhw43HEpulZdLwaDl+fPOHihobOYPjzRs//qg5LC93+PDBj29scGtBAcw8YIqxgUj1ILV4YmMUjIJRMApGMAAAS65STEyKgwAAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-7893-6744","institution":"Bombers Generalitat de Catalunya","correspondingAuthor":true,"prefix":"","firstName":"Marc","middleName":"Castellnou","lastName":"Ribau","suffix":""},{"id":279332378,"identity":"704c9cdd-45ac-4981-9648-fbbc11ed8448","order_by":1,"name":"Mercedes Bachfischer","email":"","orcid":"","institution":"Bombers Generalitat de Catalunya","correspondingAuthor":false,"prefix":"","firstName":"Mercedes","middleName":"","lastName":"Bachfischer","suffix":""},{"id":279332379,"identity":"e7e1e354-358d-4623-a953-4ea9b2326b0b","order_by":2,"name":"Marta Miralles Bover","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Marta","middleName":"Miralles","lastName":"Bover","suffix":""},{"id":279332380,"identity":"d5ab2219-b67d-466f-8a64-28a4ccd2daaf","order_by":3,"name":"Jordi Vila-Guerau de Arellano","email":"","orcid":"https://orcid.org/0000-0003-0342-9171","institution":"Wageningen University Research","correspondingAuthor":false,"prefix":"","firstName":"Jordi","middleName":"Vila-Guerau","lastName":"de Arellano","suffix":""}],"badges":[],"createdAt":"2024-03-09 08:35:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4053550/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4053550/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":52675698,"identity":"87359a3a-3768-4e7e-9576-37a52b2fc949","added_by":"auto","created_at":"2024-03-14 11:16:36","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":246370,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe extreme wildfire events dataset is \u003c/strong\u003echaracterized by daily spread, stability in the vertical profile of potential temperature, and moist pyrocloud type\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003ea)\u003c/strong\u003e 185 observed fires are classified by daily burn magnitude (ha·day\u003csup\u003e-1\u003c/sup\u003e). The dot size indicates the maximum spread timeframe to visualize the degree of optimal burning conditions (OBC). With dot color we consider the midday timeframe (0800-1800 LT) as the OBC, DtN transition (1900 to 2400 LT) as decaying OBC, and night (2400 to 0800 LT) as poor OBC. \u003cstrong\u003eb\u003c/strong\u003e) Correlation of significant ROS\u003csub\u003eb\u003c/sub\u003e events (ROS\u003csub\u003eb\u003c/sub\u003e dobles modeled ROS) with atmospheric stability in the lower and FT. Stability is quantified in the ABL by the potential temperature gradient δθ/δz (K·m\u003csup\u003e-1\u003c/sup\u003e) and in the FT by the lapse rate ꙋθ (K·m\u003csup\u003e-1\u003c/sup\u003e). The lower the gradient, the more thermodynamically unstable atmospheric conditions. The dots are colored using the ABL height (m) based on ERA5. We overlap the deep pyroCb, pyroCu, and shallow opyroCu\u003csup\u003e15\u003c/sup\u003e moist pyrocloud type occurrence. We found three regimes based on pyrocloud types and stability conditions. Square I have vertical instability in ABL and FT, which corresponds to the \u003cem\u003epyroconvection regime\u003c/em\u003e. Square II, only is unstable in the lower atmosphere, and corresponds to the extreme wind-driven \u003cem\u003e'shallow pyroconvection regime\u003c/em\u003e’. Square III is unstable in the FT, corresponding to the \u003cem\u003e'deep pyroconvection regime\u003c/em\u003e.'\u0026nbsp; ' \u003cstrong\u003ec) \u003c/strong\u003eDetail of\u003cstrong\u003e \u003c/strong\u003epyrocloud type event density distribution by stability in the lower atmosphere\u003cstrong\u003e \u003c/strong\u003eδθ/δz (K·m\u003csup\u003e-1\u003c/sup\u003e) and FT ꙋθ (K·m\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/beaa61677d483fb8dbf9a7eb.png"},{"id":52675703,"identity":"6e9e4ba7-5a7d-4528-8970-d880d8304847","added_by":"auto","created_at":"2024-03-14 11:16:38","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":421741,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eConceptual evolution of dry pyrocloud hypothesis for the fire‒atmosphere coupling and decoupling processes\u003c/strong\u003e. Under the convective unstable boundary layer’s (CBL) daytime cycle in the lower atmosphere, fire‒atmosphere coupling produces pyroconvection. After the fire initiation, the CBL (green line) is modified into a fireABL (dashed red line) by the dry pyrocloud’s turbulent deepening (legend scheme), enhancing entrainment from the FT. A dry pyrocloud can become a moist pyrocloud if the fire plume reaches the LCL level (blue line). Under decaying evening transition conditions (orange dashed line) onset, the fireABL can decouple from surface conditions. A residual fireABL persists (violet shadow), keeping an unstable layer. This facilitates unexpected extreme fire behaviour. The different coupling stages (letter a) and decoupling (letter b) were added to the conceptual central scheme. \u003cstrong\u003eA)\u003c/strong\u003e \u003cstrong\u003eCoupling evolution (before evening transition): \u003c/strong\u003eThe fireABL forms a well-mixed layer downwind from the head fire and below the plume umbrella. The fire intensity allows the plume to reach the CBL top (a1). The plume turbulence creates a dry pyrocloud that entrains warm and warm air down, further enhancing fireABL growth (a2). With a higher fireABL, plume overshooting reaches the LCL level and evolves to moist pyrocloud as shallow pyroCu (a3) or even deep pyroCu/Cb (a4). \u003cstrong\u003eB) Decoupling evolution (during and after evening transition): \u003c/strong\u003eWe\u003cstrong\u003e \u003c/strong\u003ehypothesize that once the fireABL has been established (b1), during the DtN, the fireABL remains as an actively well-mixed residual layer under the pyrocloud umbrella (b2). The residual fireABL facilitates extreme fire behaviour, enabling the deepening of the plumes during the night. The nocturnal decrease of the LCL, closer to the residual layer height, leads to optimal conditions for forming moist pyroclouds (b3).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/0a7812a46b67954f81d6654e.png"},{"id":52676316,"identity":"783e6392-2c48-4bca-ba8c-7f6cee21b0ec","added_by":"auto","created_at":"2024-03-14 11:24:37","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":208367,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAssessing the importance of dry pyrocloud in the fireABL and the (observed fire ROS/modeled fire ROS)\u003c/strong\u003e. The fireABL is modeled using a conceptual model that couples ERA5 atmospheric vertical profiles of potential temperature, and specific humidity with the kinematic fireline intensity (K·m\u003csup\u003e2\u003c/sup\u003e·s\u003csup\u003e-1\u003c/sup\u003e) from observed fires. We analyze the contribution of moist vs. dry convection regimes to fireABL formation. Our metric is the ratio of LCL height with the boundary layer (ABL or fireABL). An LCL/ABL height ratio \u0026lt; 1 (above the dotted line) suggests moist convection, and an LCL/ABL height ratio \u0026gt; 1 (below the dotted line) suggests dry convection. The hourly spread data is classified by\u003cstrong\u003e \u003c/strong\u003eROS\u003csub\u003eb\u003c/sub\u003e (size of the bubble) and Local Time (LT)\u0026nbsp; as in Figure 1 (colour of the bubble). \u003cstrong\u003ea1) \u003c/strong\u003eLCL height correlation with ABL height for hourly ROS\u003csub\u003eb\u003c/sub\u003e. \u003cstrong\u003ea2) \u003c/strong\u003eLCL height correlation with fireABL height for hourly ROS\u003csub\u003eb\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/e4abba1d183e937bdfb0122f.png"},{"id":52675699,"identity":"ff6a60a8-df5d-4aff-ab02-8320297a97d0","added_by":"auto","created_at":"2024-03-14 11:16:37","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":188571,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCorrespondence of ABL and fireABL depth daily cycles with observed ROS\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eb\u003c/strong\u003e\u003c/sub\u003e. The averaged 24-hour daily cycle (LT) of the boundary layer height (ABL, expected fireABL, and observed fireABL) is compared with the ROSb events (box plot on the secondary axis). The figure shows that the observed fireABL preserves a height significantly different from the ABL and expected fireABL from 18 LT to 3 LT (p=0.05). The observation indicates that the fireABL height achieved during daytime is preserved during DtN and the early night hours independently from the ABL cycle and fire-induced plume injection height. The hours of fireABL maintenance correspond to the extended cycle of extreme fire indicated by ROSb higher than 3 (dashed line). To highlight the fireABL preservation dependence of the maximum fireABL height achieved during daytime, we add the observed fireABL interpolated mean line during the 14 to 07 LT hours for filtered ROSb \u0026gt; 1, \u0026gt;2, and \u0026gt; 4. The ABL is computed using the ERA5 ECMWF modeled vertical profile. The expected fireABL is calculated using the ABL and the resulting heat flux from simulated fire behavior using surface weather conditions. The observed fireABL is computed using ABL and the heat flux from observed extreme fire behavior. The ABL, the observed fireABL, and the expected fireABL show the mean value, the 95% confidence interval, and the interpolated line joining the means for each hour. The LT hours are colored according to the day (yellow), DtN (red), and night (blue).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/b13709bc30efdfd9271844f3.png"},{"id":52675702,"identity":"5e8a797a-e87e-4161-a9d4-38e292bc2e58","added_by":"auto","created_at":"2024-03-14 11:16:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":516099,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eConceptual summary of the vertical and evolution of the main research findings\u003c/strong\u003e: The use of sketched puzzle pieces provides an intuitive visualization of the fireABL dependence on concatenated processes during the day and night. Solid vertical lines represent the turbulence enhanced by the fire that governs the initial coupling between fire and the atmosphere. Vertical dashed lines represent the dry pyrocloud mixing turbulences formed once the fire plume reaches the ABL top. Double lines represent layer depth. The figure shows the schematic ABL, fireABL, SBL, and LCL levels as horizontal dashed lines. Plume created dry pyrocloud enhances the entrainment of warm and dry air from the free troposphere, as highlighted by the white circle detail, complementing the deepening of the fireABL. At night, the SBL formation indicates fireABL is no longer connected to the surface and relies solely on pyrocloud mixing processes. We included two sequences to show the effect of our findings on the fire ROS. The upper sequence depicts the expected fire behaviour enhancement under a dry pyrocloud-modified fireABL. The lower sequence shows the anticipated ROS only considering the fire plume turbulence without considering fireABL. Comparing both sequences visually estimates the ROS\u003csub\u003eb \u003c/sub\u003eas a consequence of the\u003csub\u003e \u003c/sub\u003efireABL.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/c13cbc6e672169d4ccd009d2.png"},{"id":57621781,"identity":"dd4bbc0e-e861-46e3-b46d-25bca83a9d34","added_by":"auto","created_at":"2024-06-03 13:10:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2283119,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/2cda3623-3e4f-4165-9bcf-581e43ad2306.pdf"},{"id":52675700,"identity":"374d1b98-50f7-447a-bb8f-758b47ac104b","added_by":"auto","created_at":"2024-03-14 11:16:37","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5129913,"visible":true,"origin":"","legend":"Complementary Tables and Figures","description":"","filename":"complementarymaterials3.0.docx","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/6f7d8bd139d175d6e0ab57e2.docx"},{"id":52675704,"identity":"1b97aa23-9433-4797-9f2d-d7931db2d618","added_by":"auto","created_at":"2024-03-14 11:16:38","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":37385311,"visible":true,"origin":"","legend":"Dry pyrocloud detail","description":"","filename":"ifmanresa2.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/010d7e7f3e69c7b934fd8a4e.mp4"},{"id":52675705,"identity":"db048b96-e3a7-4360-9078-6b9dc124e3c7","added_by":"auto","created_at":"2024-03-14 11:16:38","extension":"mp4","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":48703510,"visible":true,"origin":"","legend":"Dry pyrocloud injectionn height increase","description":"","filename":"lakefire.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4053550/v1/57ba6cbfe03bf3668af6ae29.mp4"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Dry pyroclouds promote longer-lasting extreme wildfire events intensification.","fulltext":[{"header":"One-sentence summary","content":"\u003cp\u003eDry pyrocloud shifts the driving turbulent conditions from the surface to the atmosphere, deepening a fire-modified atmospheric boundary layer and facilitating its maintenance when decoupled during the day-to-night transition, resulting in intensified and longer wildfires under pyroconvective conditions.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003ePyrocumulonimbus (pyroCb)\u003csup\u003e1\u003c/sup\u003e driven wildfires cause extreme wildfire events (EWE) to spread day and night regardless of the surface weather and fuel conditions, triggering unprecedented fire storms\u003csup\u003e2\u003c/sup\u003e. Such wildfires are increasing in occurrence\u003csup\u003e3\u003c/sup\u003e and have been reported worldwide, including in Canada, California, Chile, Amazonia, \u0026nbsp;Australia, \u0026nbsp; Portugal, Spain, and Greece (Table S1). The spread of extreme fire at night raises questions about the dependence of fire‒atmosphere coupling on high fireline intensity (FLI) during optimal burning conditions\u003csup\u003e4\u003c/sup\u003e. Their unpredictability and sustained FLI well over 1·10\u003csup\u003e4\u003c/sup\u003e kW·m\u003csup\u003e-1\u003c/sup\u003e compromise the suppression capacity\u003csup\u003e5\u003c/sup\u003e, and firefighter safety\u003csup\u003e6\u003c/sup\u003e. Therefore, unstoppable EWEs driven by pyroCu/Cb\u003csup\u003e2\u003c/sup\u003e are shockingly impacting lives, property, and biodiversity\u003csup\u003e7\u003c/sup\u003e (Table S1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePyroconvection intensifies wildfire’s behaviour\u0026nbsp;\u003csup\u003e8\u003c/sup\u003e. First, the coupling of FLI to the above atmosphere enhances the boundary layer turbulence and atmospheric thermodynamic instability\u003csup\u003e9\u003c/sup\u003e. Second, under unstable ABL conditions, convective plume dynamics form a moist pyrocloud\u003csup\u003e10,11\u003c/sup\u003e, characterized by accelerating plume updrafts\u003csup\u003e12\u003c/sup\u003e and downdrafts once it reaches the stage of a mature pyroCb. Finally, the pyroconvective turbulent atmosphere creates events dominated by coherent vortical structures such as tornadoes\u003csup\u003e13\u003c/sup\u003e. The physical explanation of this concatenation of processes is still poorly understood\u003csup\u003e14\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe ROS bias (ROS\u003csub\u003eb\u003c/sub\u003e) is a metric to quantify the impact of those pyroconvective phenomena on a fire\u003csup\u003e15,16\u003c/sup\u003e. The metric compares, in the form of a ratio, the observed wildfire ROS with the modeled one only accounting for weather conditions at the surface. ROS\u003csub\u003eb\u003c/sub\u003e \u0026gt; 3 indicates our incapacity to explain fire-weather interaction with the current theory. Pyroconvective fires have previously been associated\u0026nbsp;with optimal burning conditions (OBC) linked with diurnal dry and warm weather, facilitating fuel load availability\u003csup\u003e17\u003c/sup\u003e. The OBC facilitating extreme fires correlates more strongly with Tª and RH than wind\u003csup\u003e18\u003c/sup\u003e. Even when mesoscale intense wind events spread unusually large extreme fires day and night, such large extreme fires require those winds to be warm and dry\u003csup\u003e19\u003c/sup\u003e, thus confirming fuel availability as the core of OBC. When fuel availability at central daytime hours coincides with an unstable atmosphere\u003csup\u003e8\u003c/sup\u003e, it creates optimal conditions for fire‒atmosphere coupling\u003csup\u003e10,11,14\u003c/sup\u003e, resulting in pyroconvection and ROS\u003csub\u003eb\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eThese more optimal burning conditions are increasingly frequent under climate change conditions\u003csup\u003e20\u003c/sup\u003e, facilitating a trend to EWE intensification\u003csup\u003e21\u003c/sup\u003e. Global warming significantly increases fuel load through enhanced aridity\u003csup\u003e22\u003c/sup\u003e and fire weather episodes intensification that extend its duration with warmer and drier nights\u003csup\u003e23\u003c/sup\u003e. Strikingly, the observed increase in EWEs goes beyond the scope of intensified midday OBC or warmer and drier nights. More specifically, sustained pyroconvection and extreme ROS\u003csub\u003eb\u003c/sub\u003e last longer and emerge during day-to-night transitions (DtN)\u003csup\u003e24\u003c/sup\u003e. Nocturnal EWEs started to be recorded within the fire community after 2017’s Las Maquinas fire in Chile and the Serta-Arganil fire in Portugal. ROS of 8-13 km/h and ROSb of 14-16 were observed between 20:00-06:00 LT (Table S1). These phenomena contradict the evening decaying of unstable atmospheric thermodynamic conditions\u003csup\u003e25\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eUnderstanding the process underlying the extreme fire spread is of pivotal importance\u003csup\u003e26\u003c/sup\u003e. Efforts to advance our comprehension of pyroconvection intensification of fire spread have focused on fire physics\u003csup\u003e11,27\u003c/sup\u003e. Nevertheless, during DtN pyrocloud-dominated wildfire events, prediction uncertainties on ROSb persists\u003csup\u003e28\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eHere, we propose a new physical explanation for the intensification of the longer-lived wildfire event by integrating a global dataset of 182 EWEs with detailed 849 hours of extreme spread rates and a conceptual fire-atmosphere model. We analyze the interrelationships between day, day-to-night transition, and night ROS\u003csub\u003eb\u003c/sub\u003e events with lower and free troposphere (FT) stability and pyrocloud type occurrence\u003csup\u003e15\u003c/sup\u003e. We hypothesize that during extreme wildfires, fire-atmosphere interaction shifts from turbulence driven by surface-fire conditions to turbulence governed by the ABL entrainment-pyrocloud conditions.\u003c/p\u003e"},{"header":"2. Global links relating abnormal fire spread to atmospheric thermodynamics","content":"\u003cp\u003eThe Observed EWE worldwide (Figure 1a) exhibits regional differences when analyzed in terms of daily spread (ha\u0026middot;day\u003csup\u003e-1\u003c/sup\u003e) and time of maximum spread. In short, local fuel, landscape morphology, and weather shape the spread of wildfires. However, we have identified a counterintuitive new worldwide pattern of extreme ROS events outside optimal burning conditions. Timeframes serve here as a surrogate for the burning conditions. Maximum fuel availability is expected during daylight (0600 and 1800 local time, LT), aligning with daily cycles of solar radiation, temperature and humidity\u003csup\u003e30\u003c/sup\u003e,\u0026nbsp;and satellite-reported peak fire\u0026nbsp;activity\u003csup\u003e31\u003c/sup\u003e. Reduced fuel load availability is assumed at day-to-night transition (DtN, 1800 to 2400 LT), and poor fuel availability at night (2400 to 0600 LT) due to cooler, moister, and darker conditions. Notably, our data set shows that up to 42% of the maximum ROS events are outside of daytime (DtN, night), thus outside optimal burning conditions.\u003c/p\u003e\n\u003cp\u003eSuch a new extreme fire spread pattern during DtN and night also contradicts the need for ABL instability as a necessary condition to create fire-atmosphere coupled extreme fires and moist pyroconvection.\u0026nbsp;Using the vertical profiles of the state variables in ERA5, with a spatial resolution of 30 x 30 km\u003csup\u003e2\u003c/sup\u003e, we calculate a combined dual gradient classification of hourly ROS\u003csub\u003eb\u003c/sub\u003e events according to the ABL and FT thermodynamic stability\u003csup\u003e32\u003c/sup\u003e (Figure 1b). As a metric within the ABL, we use the potential temperature gradient \u0026delta;\u0026theta;/\u0026delta;z (K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e). Our criteria are the following: \u0026delta;\u0026theta;/\u0026delta;z\u0026lt; 0.001 K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e\u0026nbsp; K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e as an unstable convective boundary layer (CBL), and \u0026delta;\u0026theta;/\u0026delta;z \u0026gt; 0.001 K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e as a stable boundary layer (SBL). To describe the FT, we use the\u0026nbsp;potential temperature lapse rate ꙋ\u003csub\u003e\u0026theta;\u003c/sub\u003e (K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e), considering ꙋ\u003csub\u003e\u0026theta;\u003c/sub\u003e \u0026gt; 0.004 K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e an indication of thermal stable stability.\u003c/p\u003e\n\u003cp\u003eOur analysis, therefore, links the global ROS\u003csub\u003eb\u003c/sub\u003e distribution to moist pyroclouds, both governed by thermodynamic instability in the atmospheric vertical profile (Figure 1b).\u0026nbsp;Most ROS\u003csub\u003eb\u003c/sub\u003e fire spreads in our dataset concentrate under ABL \u0026delta;\u0026theta;/\u0026delta;z \u0026lt; 0.003 K\u0026middot;m-1 and FT ꙋ\u0026theta; \u0026lt; 0.005 K\u0026middot;m-1 (square I in Figure 1b). They are driven by a pyroconvective regime, under unstable to weakly stable ABL and FT conditions, mostly linked to a deep ABL (\u0026gt; 2000 m AGL). It physically describes the unstable ABL conditions that facilitate the intensification of plume-dominated\u003csup\u003e16\u003c/sup\u003e wildfires producing moist pyroclouds\u003csup\u003e10\u003c/sup\u003e. It is here worth stressing that this \u003cem\u003epyroconvection regim\u003c/em\u003ee goes beyond the prototypical CBL conditions\u003csup\u003e33\u003c/sup\u003e (\u0026delta;\u0026theta;/\u0026delta;z \u0026lt; 0.001 K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e) and includes fire-extended CBL up to \u0026delta;\u0026theta;/\u0026delta;z \u0026lt; 0.003 K\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e. From now on, we will refer to it as eCBL. Our interpretation is that fire-driven turbulence at the surface triggers the mixing from the eCBL into CBL conditions, driving the fire-atmosphere coupling.\u003c/p\u003e\n\u003cp\u003eThe eCBL also facilitates pyroclouds and ROS\u003csub\u003eb\u003c/sub\u003e under strong stable FT (Figure 1b, square II). Wildfires observed under these conditions are convective wind-driven\u003csup\u003e16\u003c/sup\u003e under a \u0026lsquo;\u003cem\u003eshallow pyroconvection regime.\u0026rsquo;\u003c/em\u003e They are strongly influenced by FT ꙋ\u0026theta;, abruptly limiting plume\u0026apos;s deepening. To integrate eCBL conditions into CBL, strong wind mechanical\u0026nbsp;turbulence\u003csup\u003e34\u003c/sup\u003e \u0026nbsp;is crucial in enhancing the plume\u0026apos;s\u0026nbsp;ability to entrain geostrophic winds. The resulting acceleration of the ABL wind leads to an extreme spreading fire that can persist throughout the day and night (Figure S1).\u003c/p\u003e\n\u003cp\u003eRemarkably, the explanation of extreme fire events\u003csup\u003e10,11,14,35\u003c/sup\u003e driven by moist pyroclouds, based on ABL thermodynamic instability, falters when confronted with ROS\u003csub\u003eb\u003c/sub\u003e under strong ABL thermodynamic stable conditions (Figure 1b, square III). Strikingly, PyroCb-driven spreads occur in those highly stable ABL. These events occur in shallow ABL (\u0026lt;1000 m AGL) during the evening and night when fuel availability is reduced and turbulence decays in the lower\u0026nbsp;atmosphere\u003csup\u003e25\u003c/sup\u003e. Vertical pyrocloud deepening in this deep pyroconvective regime is governed by FT instability. Our observations confirm that fire turbulence from an ongoing fire transitioning from day to night can penetrate the shallow SBL formed on the surface. After that process, the fire plume reaches the weaker stability on top and deepens suddenly. This process leads to extreme fire behaviour in otherwise decaying surface burning conditions. Two representative examples of this phenomenon are The Ateca fire of 2022 (Spain) and the Santa Ana fire of 2023 (Chile) (Figure S1).\u003c/p\u003e\n\u003cp\u003eOur observations reveal new insights into the fire-atmosphere coupling processes. Enhanced ABL instability facilitates pyroconvective conditions, but the FT stability determines the type of pyrocloud (Figure 1c), as it controls its vertical growth capacity. In shallow pyroconvection, a stable FT constraint overshooting and wind-driven fires produce shallow oPyroCu. A weak stable FT modulates the deepening of shallow and deep pyroCu, while a weaker stable FT promotes extreme pyroCb in a deep pyroconvective regime. In the \u003cem\u003edeep pyroconvection regime\u003c/em\u003e, we notice a plume-deepening process primarily driven by the FT stability conditions rather than the fire itself. We refer to such events as decoupled pyroconvection. The findings suggest a shift in the fire-atmosphere coupling driving turbulence towards the top of the ABL when the plume begins to deepen in the FT.\u003c/p\u003e\n\u003cp\u003eBased on these findings, we propose a unified double process that currently drives extreme wildfire events worldwide: a concatenation of fire-atmosphere coupling-decoupling processes governed by FT stability. In Figure 2, we provide visual evidence of the coupling process during diurnal conditions and the decoupling process during the evening transition conditions.\u003c/p\u003e\n\u003cp\u003eFirst, during the day (Figure 2a), we show the different coupling stages based on a combination of real cases that occurred in California (USA) and Catalonia (Spain). The plume penetration\u003csup\u003e35\u003c/sup\u003e through the ABL leads to a dry pyrocloud formation (Figure 2.a1) in the transition layer between the ABL top and the LCL (video S3). Such a layer separates the regimes of moist cloud convection above from those of dry convection and mechanical mixing below\u003csup\u003e36\u003c/sup\u003e. The dry pyrocloud turbulence in this layer enhances a combination of processes: first, the entrainment of the FT dry and warm air into the ABL, second, the subsiding shells from cloud evaporative cooling\u003csup\u003e37\u003c/sup\u003e, and third, the plume mass flux transport\u003csup\u003e38\u003c/sup\u003e. This combined mechanism gradually deepens and mixes the ABL (Figure 2.a1 to 2.a4) in a relatively large area and, therefore, forms a fire-induced ABL or fireABL (video S4). We estimate an increase of up to 2000 m fireABL from our observation\u0026apos;s statistics. Key in our study is these processes\u0026apos; impact on the ROS metric, with an increase of up to 20 times. Although the dry pyrocloud can evolve to a moist pyrocloud thousands of meters deep through the latent heat release\u003csup\u003e10,11\u003c/sup\u003e, we consider the fireABL as the layer below the plume injection height, visually identified by the plume umbrella. We have observed the dry pyrocloud turbulence in a wide range of convective plumes by in-plume radiosondes, confirming that it is a well-established process in extreme wildfires (Figure S5).\u003c/p\u003e\n\u003cp\u003eDuring the evening transition (Figure 2b), the fireABL decoupling sequence is key in the fire spread intensification. The fireABL, once formed under diurnal conditions, is maintained downwind below the plume umbrella (Figure 2.b2). This occurs since the fireABL mixing is produced by combining the fire turbulence at the surface and the dry pyrocloud entraining turbulence on top of the fireABL. During the DtN transition,\u0026nbsp;as the surface cools down, an SBL is formed, leading to an inhibition of the vertical mixing\u003csup\u003e39\u003c/sup\u003e and a decoupling between the SBL and the above fireABL (Figure2.b3). Under those conditions, the fire plume deepens and forms pyroclouds through the night after easily overcoming the shallow SBL, as discussed in Figure 1. As such, this process prolongs the daytime fireABL conditions and extreme fire behaviour conditions towards the DtN and night hours.\u003c/p\u003e\n\u003cp\u003eThe best quantification of the process by which the dry pyrocloud turbulent dynamics exacerbates fire behaviour and facilitates moist convection is obtained by analyzing the change from ABL to fireABL height with the LCL during ROS\u003csub\u003eb\u003c/sub\u003e events (Figure 3a1-a2). By combining a conceptual energy balance coupled plume model\u003csup\u003e40\u003c/sup\u003e with the kinematic fireline intensity (K\u0026middot;m\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e) retrieved from the observed fire spread, we estimate fireABL by the resulting plume injection height. Our criterion is that moist convection occurs when ABL height \u0026gt; LCL height, being dry convection otherwise. While fires around the black dashed line in Figure 3a1 correspond to the moist convection ROS\u003csub\u003eb\u003c/sub\u003e cases, it is essential to notice the set of fires that fall within dry convective ABL with a height below 2000 m AGL (dashed square in Figure 3a1). This same set situates above 2000 m AGL when we correlate their fireABL with the LCL (Figure 3a2). They cluster with the original moist convection cases around the LCL level and its overshooting buffer zone (violet shadow). Due to the fireABL deepening in dry convection situations, the events with moist convection favorable conditions (violet shadow) increases from 42,1 % of ABLs (Figure 3a) to 76,4 % of fireABLs (Figure 3b). This finding reveals that dry pyroclouds are a key phenomenon in intensifying extreme fire behaviour by increasing the probability of moist pyroclouds occurrence.\u003c/p\u003e\n\u003cp\u003eThe group of fire events in Figure 3 that change fireABL position and get closer to LCL depth contain the extreme fire events occurring at DtN and night (red and blue dots), supporting the hypothesis of fireABL relevance on expanding the CBL conditions during the DtN timeframe as suggested in our hypothesis in Figure 2B. (Figures S6).\u003c/p\u003e\n\u003cp\u003eThe local expansion of fireABL convective conditions beyond the prototypical CBL cycle (Figure 4) becomes clear when we compare the ABL height against the fireABL height daily cycle. To highlight the dry pyrocloud turbulence effect on fireABL formation we base our quantification using both, the \u0026lsquo;observed fireABL\u0026rsquo; based on the real observed fire behavior and the \u0026lsquo;expected fireABL\u0026rsquo; based on the simulated fire behavior using surface burning conditions. Our statistics show an \u0026lsquo;observed fireABL\u0026rsquo; 17 h long cycle of 2000 m deep (black dashed line) CBL conditions (11:00 to 03:00) in contrast of the classic\u003csup\u003e33\u003c/sup\u003e 8 hours long (11:00 to 19:00 LT) CBL depth 8 h cycle for ABL and \u0026lsquo;expected fireABL\u0026rsquo;. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe observed fireABL staying significantly deeper (p=0.005, Table S2) than the ABL and expected fireABL after its afternoon \u0026nbsp;maximum contradicts the ABL depth decrease below 2000 m during the evening transition\u003csup\u003e41\u003c/sup\u003e. Filtering the extreme events by ROSb (grey lines in Figure 4), the process of fireABL maintenance is even pronounced. The expected fireABL and ABL failure to capture the vertical development due to plume injection and its maintenance in the DtN and night hours reinforces the dry pyrocloud importance on complementing the fire fire-atmosphere coupling.\u003c/p\u003e\n\u003cp\u003eThe fireABL expansion coincides with 16 continuous hours of ROS\u003csub\u003eb\u003c/sub\u003e \u0026gt; 3 (boxplot in Figure 4), confirming the correlation between the strength of the plume updraft and fire ROS\u003csup\u003e15\u003c/sup\u003e. The prolonged and coinciding fireABL and ROS\u003csub\u003eb\u003c/sub\u003e cycles support the hypothesis of extended extreme fire behaviour events by coupling-decoupling processes by fire plume and dry pyrocloud hypothesized in Figure 2.\u003c/p\u003e\n\u003cp\u003eData from a radiosone launched at 23:52 LT below the \u0026lsquo;umbrella\u0026rsquo; smoke-cloud during the Santa Ana fire in Chile 2023 (Figure S7), directly observed a 2800 m AGL nocturnal residual fireABL layer (0,003 k\u0026middot;m\u003csup\u003e-1\u003c/sup\u003e) on top of a 700 m AGL SBL maintaining the height of the diurnal smoke injection height layer. This observation confirms the nocturnal residual fireABL dependence on the daily fireABL formation.\u003c/p\u003e"},{"header":"3. Towards new dynamics of the fire-atmosphere system","content":"\u003cp\u003eOur results confirm that wildfires need a pyroconvective regime based on weak stability in the lower and upper atmosphere to produce pyroCu/Cb\u003csup\u003e10,11\u003c/sup\u003e and intensify\u0026nbsp;ROS\u003csub\u003eb\u003c/sub\u003e events. However, the need for weak stability in the ABL is challenged when we observe extreme events outside the \u003cem\u003epyroconvection regime\u003c/em\u003e (Figure 1b) in the stable ABL of both the \u003cem\u003ewind-driven regime\u003c/em\u003e and the shallow nocturnal SBL of the \u003cem\u003epyroCb regime\u003c/em\u003e. The ABL stability\u003csup\u003e4,35\u003c/sup\u003e importance in promoting extreme fire behaviour is diminished by the capacity of fire or wind turbulence to mix eCBL \u0026delta;\u0026theta;/\u0026delta;z values up to 0.006 K\u0026middot;m-1 into CBL conditions. It is the FT stability that modulates the pyrocloud depth, facilitating\u0026nbsp;pyrocloud events independently of the CBL daily cycle. As a result, pyrocloud events concatenate during the day and night,\u0026nbsp;allowing abnormal long-lasting firestorms. This phenomenon has been observed in Australia 2020, Portugal 2017 and Canada 2023 (Figure S8).\u003c/p\u003e\n\u003cp\u003eSuch FT-driven firestorms are paradoxically facilitated by two dry convection processes in the ABL: an initiation process driven by fire-buoyant turbulence, mixing the ABL from the bottom, and an intensification process by dry pyrocloud entrainment turbulence, mixing the ABL from the top. These two simultaneous mixing turbulence processes, the fire plume, and dry pyrocloud modify the ABL\u0026rsquo;s depth into a fire-induced ABL or fireABL. They resemble the direct heating and entrainment heating cycles in the land-atmosphere system\u003csup\u003e42\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe fire plume turbulence mixing capability is fueled by FLI from the surface. It drives fireABL depth increase, creating fire-atmosphere coupling in slightly stable ABL (eCBL conditions).\u0026nbsp;The ABL mixing is driven by the fire thermals\u0026rsquo; buoyancy, as with CBL in the classical mixing layer theory\u003csup\u003e33\u003c/sup\u003e, and the plume detrainment of its core temperature\u003csup\u003e11\u003c/sup\u003e. \u0026nbsp;The process is described as the penetration and deepening stages\u003csup\u003e8\u003c/sup\u003e through the atmospheric vertical profile. Those are the fundamental processes in the extreme pyroCb theory, governing the plume\u0026rsquo;s capacity to reach the free convection level and subsequently trigger (very) rapid growth\u003csup\u003e10,11,28\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe dry pyrocloud turbulence enhances the entrainment of dry and warm air from the ABL top, mixing it into the already existing fireABL. It accelerates ABL depth increase\u003csup\u003e43\u003c/sup\u003e, intensifying fire-atmosphere coupling. The dry pyrocloud turbulence is observed to expand the cloud driven mixing process for the fireABL also horizontally (figure S9). It extends downwind from the fire , with the evolution of the pyrocloud into a thick cloud resembling a stratocumulus-like smoke cloud\u003csup\u003e44\u003c/sup\u003e (Figure 2). We name such cloud pyro-stratocumulus (pyroStrCu). It continuously contributes to the entrainment of air through radiative and evaporative cooling turbulence\u003csup\u003e45,46\u003c/sup\u003e, typical of such cloud type. The process introduces a new fire‒atmosphere coupling effect in a much broader area than the plume itself. As a process it expands further into the night than the observed time lag (2 to 5 hours) between the maximum surface heat flux and the CBL\u0026rsquo;s deepening peak\u003csup\u003e41\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe PyroStrCu plays a crucial role in preserving fireABL depth and facilitating its decoupling from ABL daily cycle (Figure4), thus facilitating extreme fire behaviour. The process is even more important when fireABL will become a residual layer by the formation of nocturnal surface stability. The residual fireABL persistence relies on the radiative and evaporative cooling turbulence from pyroStrCu. Is this residual layer that facilitates the plume deepening into FT, triggering moist convection when the LCL depth declines at night\u003csup\u003e47\u003c/sup\u003e (Figure S6).\u0026nbsp;Our explanation is related to the identified role played by residual layers in enhancing the penetration of plumes\u003csup\u003e48\u003c/sup\u003e. Overall, the residual fireABL resembles the deep residual layer cycle created by the deep hot and dry CBL during persistent mega-heatwaves\u003csup\u003e49\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eThe pyroStrCu enhancement of fire spread is opposed to the reported fuel availability reduction by persistent smoke cover\u003csup\u003e50\u003c/sup\u003e. However, our observations indicate pyroStrCu turbulence downwind from the head fire within the immediate distance (\u0026lt; 2 to 5 km), eventually evolving into a thin smoke layer in the fire region.\u003c/p\u003e\n\u003cp\u003eThe observed cloud-driven mixing processes redefine our understanding of fire-atmosphere coupling, shifting the focus from surface to top ABL mixing processes. The surface and pyrocloud turbulence combination reveals diverse, unaccounted fireABL mixing processes (Figure 5). This introduces a new framework explaining various observed phenomena. It supports the generalization of moist pyrocloud events, as the increased plume injection height\u003csup\u003e51\u003c/sup\u003e\u0026nbsp; facilitates LCL reach. It explains the observed longer-lasting extreme fire behaviour, as the deep fireABL extends from day to night involving diurnal coupling and nocturnal decoupling processes. The observed stable plume injection and pyrocloud depth despite strong surface fire changes\u003csup\u003e10\u003c/sup\u003e can be attributed to pyrocloud-governed mixing processes. The framework also highlights extreme fire events (Table S1) driven by FT conditions entrainment.\u003c/p\u003e\n\u003cp\u003eTogether, the plume and pyrocloud mixing turbulence drive \u0026nbsp;a concatenation of coupled and decoupled processes (Figure 4) that supports a higher importance of the \u0026lsquo;weather hypothesis\u0026rsquo; in explaining the behaviour of extreme fires, to the detriment of the fuel hypothesis\u003csup\u003e52\u003c/sup\u003e. Our research connects both explanations. First, the bottom-up-driven formation of fireABL explains the need for fuel availability and OBC for fire-atmosphere coupling and fireABL formation. Once fireABL is formed, dry pyrocloud modulated by the influence of the FT stability facilitates the fireABL deepening during the day and its maintenance, regardless of fuel conditions, when decoupled at night.\u003c/p\u003e\n\u003cp\u003eOur findings provide us with observational evidence of how events will continue to unfold. As the climate change increases aridity\u003csup\u003e22\u003c/sup\u003e and ABL height\u003csup\u003e53\u003c/sup\u003e, we will experience deeper dry pyroclouds. This will result in the enhancement of moist pyroconvection and ROS\u003csub\u003eb\u003c/sub\u003e late in the day when the decoupled fireABL is closer to LCL. The more frequent heat waves\u003csup\u003e54\u003c/sup\u003e already promote a positive feedback on creating and preserving the residual fireABL. Moreover, warmer nights\u003csup\u003e23\u003c/sup\u003e during the heat waves make it easier for the fires to erode nocturnal SBL and access the residual fireABL. Overall, we can expect an increase in abnormal deep and long-lasting pyroconvection events at night.\u003c/p\u003e\n\u003cp\u003eThe findings critically compromise suppression capacity, calling for revising fundamental concepts, enhancing observation techniques, and developing new simulation strategies for wildfire management.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthors contribution\u003c/strong\u003e: Marc Castellnou conceived the research and hypothesis, wrote the article with support from Marta Miralles and Mercedes Bachfischer.\u003c/p\u003e\n\u003cp\u003eMarc Castellnou and Mercedes Bachfischer designed the data gathering.\u003c/p\u003e\n\u003cp\u003eJordi Vila-Guerau de Arellano supervised the research and deeply involved in the writing task.\u003c/p\u003e\n\u003cp\u003eAcknowledgments:\u0026nbsp;This paper and its research would not have been possible without the support of fire Analysts from different Fire and Rescue Services in complementing fire spread data: The Bombers GRAF Generalitat de Catalunya, Jaume Cendra from Diputaci\u0026oacute;n General Aragon, Paco Senra from INFOCA, Miguel Angel Botella from GVA, Fernando Chico from INFOCAM, Gerard Grau from Aude Sapeurs-Pompiers, Fabio Silva from GAUF in Portugal Civil Protection, Jorge Saavedra from CONAF in Chile, Luca Tonarelli from antincendi boschivi della Regione Toscana in Italia, Juaquin Ramirez from Technosylva in California and Sisoula Tanidou from Civil Protection Greece. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe thank Andrea Duane, Rut Domenech, Laia Estivill, and Brian Verhoeven for their expertise and assistance throughout our study. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOpen data: \u003c/strong\u003e\u003c/p\u003e\n\u003cp skip=\"true\"\u003eAll data used in this work is published at: https://doi.org/10.5281/zenodo.10799438\u003c/p\u003e\n\u003cp\u003eReanalysis data used in the research can be found at: https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe general fire spread information data can be obtained from: https://www.earthdata.nasa.gov/learn/find-data/near-real-time/firms\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interest statement\u003c/strong\u003e: The authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAMS. 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Increasing trends in regional heatwaves. \u003cem\u003eNat Commun\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, 3357 (2020).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4053550/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4053550/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Global occurrence of pyroclouds leads to an accelerated wildfire rate-of-spread (ROS), creating extreme wildfire events (EWE). Pyroclouds form during wildfires under unstable atmospheres. Recent EWEs unexpectedly created deep moist pyrocloud and accelerated ROS through the night, beyond the unstable atmosphere’s daily cycle. Here, we analyzed the dependence of the ROS acceleration on pyroclouds and atmospheric instability. We used 190 EWEs observed worldwide, supporting their analysis with a coupled fire‒atmosphere model. We find that accelerated ROS depends on dry pyroclouds processes driven by atmospheric instability, particularly that of the free troposphere (FT). Dry pyroclouds form when plume deepening reaches the transition layer between the atmospheric boundary layer (ABL) top and the lifting condensation level (LCL). The depth of dry pyrocloud turbulence, modulated by the FT stability, plays a leading role in further enhancing the downward entrainment of warm and dry air. This entrainment process mixes ABL to form a deeper fireABL, optimizing conditions for moist pyrocloud events. A novel finding is the role of the dry pyrocloud entrainment process in maintaining a decoupled fireABL from surface conditions during the day-to-night transition. This dynamical transition explains the recently observed nocturnal extreme fire events that lasted up to 17 hours, a phenomenon present in 40.2% of observed EWEs worldwide. We argue that during wildfires, the dry pyrocloud persistence shifts the turbulent driving conditions from surface to atmosphere, disrupts the daily cycle of the lower atmosphere, and increases the number of longer-lasting pyroconvective EWEs.","manuscriptTitle":"Dry pyroclouds promote longer-lasting extreme wildfire events intensification.","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-14 11:16:31","doi":"10.21203/rs.3.rs-4053550/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"de19a809-ca62-4093-969e-809a1776bc1c","owner":[],"postedDate":"March 14th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":29419121,"name":"Earth and environmental sciences/Natural hazards"},{"id":29419122,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science/Atmospheric dynamics"},{"id":29419123,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling"}],"tags":[],"updatedAt":"2024-06-03T13:01:57+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-14 11:16:31","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4053550","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4053550","identity":"rs-4053550","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}