SODAR OBSERVATIONS OF THE NOCTURNAL BOUNDARY-LAYER AT KHARAGPUR, INDIA

Sodar has been installed at Kharagpur (22.2 degrees N, 87.3 degrees E) as a part of the MONTBLEX-90 experiment and data were collected durin g the monsoon period. The variation of the nocturnal boundary layer (N BL) during the monsoon period is discussed. The height corresponding t o the low-level wind maximum in the sodar wind profile during night ti me is identified as the NBL height. Mean monthly winds for July and Au gust, plotted as time-height cross sections, reveal the height of the ground-based stable layer. The average NBL heights in the months of Ju ly and August are found to be 324 m and 296 m respectively. It is obse rved that the NBL height is relatively high in the month of July (acti ve phase of monsoon) compared to that during August (weak phase). The months of July (total rainfall = 901 mm) and August (total rainfall = 134 mm) are associated with cloudy and relatively clear sky conditions . This indicates that clouds (through their effect on longwave cooling to space) play an important role in determining the NBL height during the monsoon.