Role of Pre-Monsoon Showers in the Evolution of Indian Heatwaves

Heatwaves constitute one of the most lethal weather phenomena, presenting substantial risks to millions of individuals. Characterized by extended periods of extreme temperatures, these events significantly impact ecosystems, economies, and human mortality rates. When coupled with high humidity, these events pose high heat stress over the heatwave domain. India, being one of the significant hotspots, experiences heatwaves during the pre-monsoon season. These heatwaves are associated with both moist and dry mechanisms. Moist heatwaves have high wet bulb temperatures and cause high fatalities among humans and mammals. With high population loading and the context of climate change, the origin or source of these moist heat waves has not been examined thoroughly till now. 

In the study, we investigate the precursors of the moist and dry heat waves in the Indo-Gangetic Plains using the Eulerian temperature decomposition equation to find out the dominant processes responsible for the formation of these events. The past literature says that advection is the major component in triggering these events, but our analysis proves that the effect of advection is minimal and supports the weak temperature gradient (WTG) theory in the tropics. To study the precursors, we extend our analysis from the pre-heatwave time to the onset of the heatwaves. Our analysis shows that pre-monsoon showers are responsible for forming moist heat waves. These showers are associated with nighttime low-level clouds that trap the outgoing long-wave radiation, further accumulating the heat content and causing the temperatures to rise. Further, these rainfall activities must be supported by the mid-tropospheric dryness (MTD) for it to be sustained throughout the period. The MTD helps the low-level clouds resulting from shallow convection remain as they are and does not promote deep convection. We emphasize the importance of local atmospheric conditions along with large-scale activities (that trigger anticyclones in the upper troposphere) in sustaining the heatwave intensity. The findings of this study will help in developing heatwave early warning systems at localized scales.

Keywords: Moist heatwaves, Pre-Monsoon showers, Mid Tropospheric Dryness, Weak Temperature Gradient, Advection