Long-term trends of instability and associated parameters over the Indian region obtained using a radiosonde network
Long-term trends of the parameters related to convection and instability obtained from 27 radiosonde stations across six subdivisions over the Indian region during the period 1980–2016 are presented. A total of 16 parcel and instability parameters along with moisture content, wind shear, and thunderstorm and rainfall frequencies have been utilized for this purpose. Robust fit regression analysis is employed on the regional average time series to calculate the long-term trends on both a seasonal and a yearly basis. The level of free convection (LFC) and the equilibrium level (EL) height are found to ascend significantly in all Indian subdivisions. Consequently, the coastal regions (particularly the western coast) experience increases in severe thunderstorms (TSS) and severe rainfall (SRF) frequency in the pre-monsoon period, while the inland regions (especially Central India) experience an increase in ordinary thunderstorms (TSO) and weak rainfall (WRF) frequency during the monsoon and post-monsoon periods. The 16–20-year periodicity is found to dominate the long-term trends significantly compared to other periodicities and the increase in TSS, and convective available potential energy (CAPE) is found to be more severe after the year 1999. The enhancement in moisture transport and associated cooling at 100 hPa along with the dispersion of boundary layer pollutants are found to be the main causes for the increase in CAPE, which leads to more convective severity in the coastal regions. However, in inland regions, moisture-laden winds are absent and the presence of strong capping effect of pollutants on instability in the lower troposphere has resulted in more convective inhibition energy (CINE). Hence, TSO and occurrences of WRF have increased particularly in these regions.