Time evolution of monsoon low-level jet observed over an Indian tropical station during the peak monsoon period from high-resolution Doppler wind lidar measurements

Abstract

Doppler wind lidar measurements of horizontal winds at an Indian tropical station, Mahbubnagar (16.73°N, 77.98°E, 445 m above mean sea level), were used to investigate the time evolution of the monsoon low-level jet (MLLJ) during the southwest monsoon season. Vertical profiles of zonal wind in the altitude range of 100 to 3000 m above surface (at every 50 m height interval and 5 min time averaged) obtained during the period 25 July to 23 August 2011 are considered for the analysis. The zonal winds in the altitude up to 3000 m above ground are predominantly westerly throughout the period and on almost all the days there is a westerly wind speed maximum around 500 m above ground during nighttime. Soon after local sunrise, the core of this wind speed maximum (jet) gets lifted up and by afternoon, the westerly wind maximum is shifted to a higher altitude of 2000 m-2500 m without much change in its magnitude. Analysis of the high-resolution lidar data strongly indicates that the same nocturnal LLJ seems to be moving up and evolving into a daytime westerly MLLJ reported in several previous studies. Wind speed and direction derived from the wind lidar agree reasonably well with simultaneously observed GPS upper air sounding wind measurements. Further analysis shows that the time-height evolution of the jet core is closely associated with daytime convection and boundary layer growth. The presence of clouds over the region seems to inhibit this type of time evolution.

Variability of monsoon low‐level jet and associated rainfall over India

“…An analysis of the diurnal variations of MLLJ during the individual monsoon months (Figure S5) indicates that the strength of the MLLJ is higher and exhibits larger diurnal variations during the peak monsoon months of July and August than in June and September. The influence of the pressure gradient and the resulting large‐scale southwesterly flow is stronger during the daytime, while the effect of mountain gaps is predominant during night‐time where the secondary flow of the MLLJ winds is found to be stronger (Ruchith et al ., ; Du et al ., ). The diurnal changes in the winds can alter the humidity gradients in the LLJ outflow region, which then triggers moisture convergence and favours the formation of convection systems (Chen and Tomassini, ).…”

Aerosol indirect effects from ground‐based retrievals over the rain shadow region in Indian subcontinent

“…During monsoon months, westerly/southwesterly winds prevail over the region. Ruchith et al [] observed wind lidar‐derived vertical distribution of zonal winds over the region. They found that the upward lift of the core of westerly maximum wind speed after the local sunrise is due to the growth of the convective boundary layer, and during the late afternoon it collapsed to lower levels.…”

Contrasting Features of Monsoon Precipitation Around the Meghalaya Plateau Under Westerly and Easterly Regimes

“…Therefore, the speed of the LLJ increases, particularly around 950–925 hPa, because the strong low‐level southeasterly wind from the Bay of Bengal can flow over the flat lowland without being slowed down greatly by surface friction. The inertial oscillation can also enhance the nocturnal LLJ (Blackadar, ; Rife et al, ; Van de Wiel et al, ; Ruchith et al, ). The acceleration of the jet occurs above the neutrally stratified layer above 950 hPa.…”