Using Simulated Radiances to Understand the Limitations of Satellite-Retrieved Volcanic Ash Data and the Implications for Volcanic Ash Cloud Forecasting

Volcanic Ash Advisory Centres (VAACs) have generated volcanic ash forecasts for the aviation industry since the mid-1990s. The excellent spatial and temporal coverage of satellite data make them critical to the validation of ash dispersion model forecasts. This study investigates the limitations of satellite-retrieved volcanic ash data through the production of simulated radiances for a range of ash cloud properties encompassing the satellite retrieval’s sensitivity. We run a detection and retrieval algorithm (Francis et al., 2012, https://doi.org/10.1029/2011JD016788) on these simulated clouds and assess the sensitivity and performance of the algorithms. Expected limitations are highlighted, including a lack of sensitivity to particles larger than ~10 µm in radius and challenges in accurately retrieving heights in the stratosphere. However, other previously poorly-defined limitations are also constrained, such as the reduction in sensitivity as ash column loading increases in optically-thick clouds and increasing underestimations for ash column loadings >~7 g m-2. We consider the implications of the identified limitations when using satellite-retrieved ash column loadings to verify dispersion model output. We show that, accounting for the limitations of the satellite retrieval, a significant proportion of mass in the model output can lie outside the sensitivity range of the satellite detection and retrieval. This demonstrates the importance of understanding observations’ limitations when comparing to model output, knowledge which should be used when verifying operational volcanic ash cloud forecasts.