Existence of low level jet during pre-monsoon period over eastern India and its role in the initiation of nocturnal thunderstorms

During the pre-monsoon period, the frequency of occurrence of low level jet (LLJ) and of nocturnal thunderstorms (NCTS) over the Gangetic Plain of Wes t Bengal, a region in the eastern part of India have been studied from both pilot balloon observations and numerical simulations. Observational study indicates that the formation of LLJ and NCTS is significant over inland sta tions. Composite diagrams for the vertical structure of LLJ show that the c ore wind speed is observed within the layers between 300 m and 600 m above ground. Sodar observations reveal that vertical shear of horizontal wind ha s positive lower value below the axis of LLJ whereas it has negative and hi gher values above the axis. This conclusion differs slightly from those obt ained from pilot balloon data because of poor resolution in the later data. Model study also supports the formation of LLJ in the evening because of la nd/sea contrast. Model-derived vertical velocities are positive over the pl aces of occurrence of nocturnal thunderstorms. The horizontal shear of wind speed beneath the core of LLJ indicates strong relative vorticity and henc e convergence at lower levels. This production of convergence by LLJ helps the formation of nocturnal thunderstorms.