Diurnal cycle of summer season thunderstorm activity in India

The diurnal cycle of tropical convection over the Indian region has been analysed in this study for the period of March to June from the thunderstorm reports of ground-based observatories throughout India during 2016–2020. The analyses indicate that during this period of the year, when the land progressively heats up diurnally, under conditions of deficient moisture, synoptic systems interact with the semi-permanent features of the atmosphere over the region to define areas of moisture and wind convergence, which in turn determine the frequency and diurnal cycle of thunderstorm activity. While the easterly and westerly waves are the major synoptic scale weather systems that affect the Indian region during March to May, the atmospheric changes are associated with the onset of the monsoon season in June. Amongst the semi-permanent features, the gradual intensification and migration of the shallow heat low to northwest India during May and June reinforce the western disturbances over this region, thereby intensifying the thunderstorm activity during the afternoon to evening hours over the western Himalayas and northwest Indian region. The role of the low-level anticyclones over the Arabian Sea and the Bay of Bengal is seen in the east–west-oriented moisture gradient across the Indian subcontinent which makes the east Indian subcontinent generally more prone to thunderstorm activity during this season. The east–west-oriented discontinuity line across north India is particularly intense during the morning hours along the foothills of the Himalayas. Its location directs moisture from the Bay of Bengal into the Himalayas causing early initiation of thunderstorm activity over the Himalayas. The discontinuity line moves southwards to the north Indian plains later in the day, although the western end becomes less marked. The north–south-oriented discontinuity line across the Indian subcontinent between the two anticyclones intensifies during the afternoon hours due to land heating and combines with the east–west wind discontinuity to become a T-shaped maximum convergence zone for thunderstorm activity during the afternoon hours over the Indian region, which intensifies as the months progress. With the onset of the southwest monsoon over the south peninsula and east Indian regions in June, the change in wind pattern from an easterly to a southwesterly flow regime over the south peninsula is reflected in an abrupt shift of the afternoon maximum of thunderstorm activity over the inland regions of the south peninsula to an early morning maximum over the southwest peninsular coast. Simultaneously, with the weakening of the anticyclone over the north Bay of Bengal and gradual strengthening of the southerly moisture flow into east and northeast India, there was in-phase increase in thunderstorm activity during the afternoon hours over the plains of east-central India, east India, and adjoining southern parts of northeast India during June. The diurnal pattern of tropical convection significantly affects human lives over the Indian subcontinent.