Thermal and wind structure of the monsoon trough boundary layer

Radiosonde data from Jodhpur, taken at 0530, 1730 and around 1100 hr IST during MONTBLEX 1990, reveal that the distribution of virtual potential temperature theta(v) below about 500 hPa has a structure characterized by up to three layers each of approximately constant gradient. We are thus red to introduce a characterization of the observed thermal structure through a sequence of the symbols N, S and U, standing respectively for neutral, stable or unstable conditions in the different layers, beginning with the one closest to the ground. It is found that, of the 29 combinations possible, only the seven classes, S, SS', SNS', NS, NSS', USS' and UNS are observed, where S' stands for a stable layer with a different gradient of theta(v) than in the layer S. It is also found that, in 90% of the launches at 0530 hr, 48% of the launches at 1730 hr and 69 % of the launches around 1100 hr, the first radiosonde layer near the ground is stable; the classical mixed layer was found in only 11% of the data set analysed, and if present on other occasions, must have been less than 250 m in height, the first level at which radiasonde data are available. Supplementing the above data, sodar echograms, available during 82 % of the time between June and August 1990, suggest a stable layer up to a few tens of metres 48% of the time. A comparative study of the radiosonde data at Ranchi shows that the frequent prevalence of stability near the surface at Jodhpur cannot be attributed entirely to the large scale subsidence known to be characteristic of the Rajasthan area. Further, data at Jodhpur reveal a weak low level jet at heights generally ranging from 400 to 900 m with wind speeds of 6 to 15 m/s. Based on these results, it is conjectured that the lowest layers in the atmosphere during the monsoons, especially with heavy clouding or rain, may frequently be closer to the classical nocturnal boundary layer than to the standard convective mixed layer, although often with shallow plumes that penetrate such a stable layer during daytime.