Thermal and wind structure of the monsoon trough boundary layer |
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| Authors: | G Rajkumar R Narasimha S P Singal B S Gera |
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| Institution: | (1) Jawaharlal Nehru Centre for Advanced Scientific Research, 560 064 Bangalore, India;(2) National Physical Laboratory, 110 012 New Delhi, India |
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| Abstract: | Radiosonde data from Jodhpur, taken at 0530, 1730 and around 1100 hr IST during MONTBLEX 1990, reveal that the distribution
of virtual potential temperature0
v below about 500 hPa has a structure characterized by up to three layers each of approximately constant gradient. We are thus
led to introduce a characterization of the observed thermal structure through a sequence of the symbolsN, S andU, 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′ andUNS are observed, whereS′ stands for a stable layer with a different gradient of0
r. than in the layerS. 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 radiosonde
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. |
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| Keywords: | Atmospheric boundary layer monsoons thermodynamic structure |
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