Tendencies of Tropical Cloud Clusters Transformation Into Tropical Cyclones

Abstract Tropical cloud clusters (TCC) play a vital role in earth’s climate by not only releasing a large amount of latent heat into the atmosphere but also by forming the basis for development of tropical cyclones (TC). However, not all the TCCs can be developed into cyclones and only a few of them develop into TC, selectively. There are large uncertainties in present understanding on why only certain TCC develops into a TC and others don't? The present study employs the global TCC observations generated by GridSat and IBTrACS datasets from 1980 to 2009 to investigate the TCC distributions over the various Oceanic basins such as North Atlantic (NA), South Atlantic (SA), East-West and South Pacific (EP, WP and SP) and North Indian (NI) and South Indian (SI) basins. The central objective of the present study is to characterize the size spectrum of TCCs and to investigate their potential transformation into TC. The TCCs are identified based on the different IR temperature thresholds in each basin. The present results suggest that overall ~ 5.5% of TCCs were developed into TCs per year across the globe and there is an increasing trend in number of TCCs that were grown into TCs during the study period. The size spectrum of TCCs showed a dominant peak at 100-200 km2. About 48% of TCCs transform into TCs within 24 hr of being identified. Furthermore, 85% of TCCs develop in to TCs within 84 hr of the first identification and only 5% of TCCs develop into TCs after 84 hr. Further, we have also analysed the background environmental conditions such as low-level wind speed, vorticity, divergence, vertical shear, upper level relative humidly and latent heating (LH) for developing and non-developing TCCs over NI basin. It is noted that the relative humidity in the developing composite is around 10-20% higher than that in non-developing TCCs and LH in developing TCCs is 0.15 K/hr larger than that in non-developing TCCs. The significance of the present study lies in investigating the developing TCCs as function of their size and lifetime including their long-term trends and bringing out the favourable environmental conditions for developing TCCs in the NI Ocean.