Boundary layer jet on the lee side of Western Ghats during southwest monsoon as revealed by high resolution sodar winds

Abstract

High resolution winds measured by SOund Detection And Ranging (SODAR) in the atmospheric boundary layer (ABL) over Pune, located on the lee side of the Western Ghats (WG) reveal the presence of Low Level Jet (LLJ) in the atmospheric boundary layer (hereafter called as boundary layer jet, BLJ) during southwest monsoon (June–August) for three consecutive years, viz., 2009, 2010 and 2011. Its prevalence only during the monsoon period, even at monthly scale indicates a possible interconnection to another LLJ, Somali jet, present during this period. An investigation is done to look into the dominant mechanisms those could result in such an occurrence in the lee side of the WG. Baroclinicity, inertial oscillation, upstream topographic blocking and variability of Somali jet are the mechanisms considered. Baroclinicity due to east–west temperature gradient on the leeside to induce thermal wind and inertial oscillation appear to have a little role in LLJ formation. However, Somali jet seems to govern the BLJ on the lee side through its dynamical interaction with the WG causing flow reversal and wave breaking above WG and inducing acceleration of downslope winds. The height and sharpness of Somali jet core on the windward side and its magnitude as well as direction with respect to WG are observed to determine the strength of BLJ on the lee side illustrating the signature of the summer monsoon in the boundary layer over Pune.