Comprehending the Attributes and Strengthening of Mesoscale Convective Systems over South Asia

Mesoscale Convective Systems (MCSs) account for over 50% of annual rainfall across the tropics and many regions of the subtropics and midlatitudes. They are often associated with strong lightning and extremely heavy rainfall, which can lead to floods. Consequently, comprehending the spatio-temporal attributes and long-term trends of MCSs is crucial for better preparedness to avoid natural hazards in current and future climatic conditions. In contrast to other MCS hotspots around the globe, investigations on the long-term alterations of MCSs in South Asia are still scarce. Our work employs high-resolution satellite brightness temperature and precipitation data, together with a novel MCS tracking method (PyFLEXTRKR), to generate a database of MCS occurrences in South Asia during the last two decades (2001-2020), and then perform a detailed analysis to understand their climatological characteristics, the environmental conditions favouring them and the long term trend. Bay of Bengal, Western Ghats, and Southern Peninsula of India are the sites where the highest number of MCSs develop. Additionally, there is a distinct seasonality in MCS activity, with the summer monsoon season seeing the highest formation of convective systems. During this season, especially over the Bay of Bengal, we observe the strong characteristics of the MCS, such as its larger area, longer duration, and greater rain rate. To a certain degree, the formation of small (<10⁴ km²) and medium-sized (10⁴ km²–4.4 x 10⁴ km²) MCSs is equally dispersed between the ocean and land, whereas the large (4.4 x 10⁴ km²–1.6 x 10⁵ km²) MCSs form mostly across the oceans. On the other hand, the Super (> 1.6x10⁵ km²) MCSs are exclusively found over the Bay of Bengal, primarily during the monsoon season. The percentage of rainfall contributed by MCSs over South Asia varies with the seasons, and the highest amount is received during the monsoon season. There is also a disparity over land and ocean, with land areas receiving 40%-50% and oceans receiving 55%-65% of their annual mean rainfall. Compared to the other types of convections, the MCSs contribute to the major fraction of total rainfall produced over both land and ocean, especially towards the higher magnitudes of rainfall. The ability to produce strong rainfall by any MCS strongly depends on its spatial extent (r_corr = 0.83 (0.88) over land (ocean)), followed by lifetime (r_corr = 0.48 (0.65) over land (ocean)), whereas the brightness temperature is negatively correlated weakly (r_corr = -0.24 (-0.37) over land (ocean)). The analysis of the large-scale environmental conditions reveals a gradual build-up of favorable conditions six hours before the initiation, with a noticeable increase within a 100-kilometer radius just three hours before the initiation. Our analysis shows that within the last 20 years, MCSs have increased in frequency and spatial extent. Furthermore, the precipitation associated with MCS has shown a notable upward trend. The rising frequency and severity of MCSs are propelled by increasingly conducive water vapor rich environment, which are expected to escalate with global warming. This could significantly impact the hydroclimate of South Asia, particularly the probability of severe events.