Prathipati Vinay Kumar

Long‐term variations in the frequencies of different rainfall events during summer monsoon season (June through September) are analysed over a period of 38‐years (1980–2017) using the simulations of weather research and forecasting (WRF) model evaluated against the India Meteorological Department (IMD). Further, we have divided the study period into two sub‐periods such as before (1980–1998), and after the mega ENSO event (1999–2017). Our analysis suggests the WRF exhibits good skill in capturing the increased trends in the frequency of heavy and very heavy (extreme) rainfall events during the recent epoch (1999–2017).We have assessed the skill of WRF in capturing the observed probability distributions of daily rain rates over different sub‐regions of India. Our results depict that the downscaled products of WRF captured well the extreme rainfall as observed in IMD, signifying that this dynamically downscaled model with continuous 36‐hr re‐initialization makes it suitable to simulate extreme rainfall events compared to light rainfall event. However, our analysis reports that there is a slight overestimation in WRF rainfall and fails in capturing the light rainfall events compared to IMD. The overestimation of rainfall by WRF possibly be due to increased pressure gradients and enhances the convection in the upper streams and associated rainfall. Our study emphasizes that the variability in summer monsoon rainfall during the recent period is due to the changes in the monsoon circulations, particularly monsoon low‐level jet that leads to alter the moisture budget in terms of moisture convergence and transport. Moreover, the variability in frequencies of rainfall events are analysed for strong La Niña/El Niño years and observed an increase in extremes over major part of India during strong La Niña except over parts of Central India and NEI where strong El Niño dominates; which is well captured by the WRF model.

The smoothed time series of northeast monsoon rainfall over south peninsular (SP) India has revealed two extreme active epochs (1877–1887 and 2005–2015) and two extreme weak epochs (1899–1909 and 1980–1990) during 1871–2016. Only the recent two epochs 1980–1990 and 2005–2015 were chosen due to data constraints. For these two epochs, the distribution of zonal wind pattern at the 850 and 200 hPa levels, latent heat flux, vertical shear in the zonal wind between 850 and 200 hPa, sea surface temperature and outgoing long wave radiation (OLR) were studied. The vertical profile of zonal wind over the mean position of the subtropical westerly jet and the maximum wind reversal were also analysed. Further, the frequencies of dry days, little rainfall days (0–20 mm), moderate rainfall days (20–60 mm), heavy rainfall days (60–100 mm) and very heavy rainfall days (>100 mm) were evaluated. SP India has experienced active/weak monsoon conditions every one to two decades alternately. An active monsoon epoch shows a higher north–south temperature gradient over the Bay of Bengal than a weak monsoon epoch. An upper level subtropical westerly jet over north India, strong tropical easterlies in the lower troposphere and low vertical shear in the zonal wind over the Bay of Bengal were observed during the active epoch, which facilitates the transportation of good amounts of moisture to SP India. The low OLR values over SP India in the active monsoon epoch indicate more convective activity over that region. Enhanced rainfall activity over SP India in the active monsoon epoch is due to the enhanced frequency of very heavy, heavy, moderate and little rainfall events and the reduced frequency of dry days.

In global warming, India is adversely affected by weather extremes. Summer monsoon contributes about 70% of annual mean rainfall to India by mode of an ensemble of synoptic disturbances and intense events. These events bring extreme amounts of rainfall in very few days. We have studied the anomalies of rainfall events for wet days (>0 mm rainfall), dry days (=0 mm), little rainfall days (>0 to <20 mm), moderate rainfall days (≥20 mm to <60 mm), heavy rainfall days (≥ 60 to <100 mm) and very heavy rainfall days (≥ 100 mm) during the period, 1901–1930; 1931–1960; 1961–1990 and 1991–2015 using India Meteorological Department (IMD) rainfall data. In the present scenario, even though an extensive increase in frequency of lows forming in the Bay of Bengal is observed, only a few are intensified into depressions and above stages. Alternatively, monsoon circulation is trailing its strength; therefore extreme (heavy and very heavy rainfall) events came into existence to balance the mean rainfall activity. The rise in these events may be due to an increasing inconsistency of the strength of low‐level jet, an increase of sea surface temperature over the Arabian Sea and dynamic flow of moisture supply to inland from neighbourhood seas. An increase in the frequency of extreme rainfall events is seen over Konkan & Goa, Madhya Maharashtra, Jammu & Kashmir, central Northeast India (CNEI) and west central India (WCI) in the recent years (25 years) compared to previous 9 decades. Whereas, a decrease in the frequencies of little and moderate rainfall events are observed over the parts of the Western Ghats, Northeast India, WCI and CNEI. There is an increasing trend in the inconsistency of daily rainfall activity due to an increase in the frequency of extreme events in the global warming era.