UHF wind profiler observations of monsoon low level jet over Pune

Data from a UHF (404 MHz) wind profiler during the active monsoon periods in 2003, 2004 and 2005 have been analyzed. Details of July 2003 as representative have been presented, which clearly show existence of jet stream over the wind profiler site at Pune (18.3 °N, 73.5 °E) throughout the day and night. The paper presents observed occurrence statistics of low level jet (LLJ) during monsoon month of July 2003. Wind speeds in excess of 20 ms-1 are frequently observed during active monsoon phase and peak found around a height of 2-3 km. Wind speed then decreases on either side of the peak without more than 40° change in the prevailing wind directions. The evolution of monsoon LLJ, particularly its diurnal variation and its difference with respect to nocturnal LLJ phenomenon have been brought out. The observed vertical velocities are predominantly upward, up to 4 km in the presence of LLJ with mean downward momentum flux. This upward directed clear air vertical velocities probably help in developing convective systems. The energy dissipation rates are also estimated and shown to be maximum near the LLJ height.