• B S Gera

      Articles written in Journal of Earth System Science

    • Atmospheric boundary layer studies at Jodhpur during MONTBLEX using sodar and tower

      B S Gera S P Singal Neeraj Saxena Y S Ramakrishna

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      A monostatic sodar was set up at Jodhpur, near the western end of the monsoon trough, to investigate the atmospheric boundary layer dynamics. A 30 m instrumented tower was also located close to the sodar antenna. Data were collected from June to August during the monsoon period of 1990, as also from July 1992 to September 1993.

      Thermal plumes, surface-based stable layers (both flat or short spiky top and tall spiky top), elevated/multi-layers with or without undulations and dot echo structures were seen; however, erosion of the morning inversion layer in the form of a rising layer with growing thermal plumes under it was rarely seen, and that too only during the winter period. The observed structure of the stable layer with tall spikes and its depth have been found to be correlated with the intensity of the monsoon spell; the dot echoes have been found to be correlated with the approach of a monsoon depression near Jodhpur; and the elevated/multilayers have been attributed to the formation of a subsidence (shear instability).

    • Thermal and wind structure of the monsoon trough boundary layer

      G Rajkumar R Narasimha S P Singal B S Gera

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      Radiosonde data from Jodhpur, taken at 0530, 1730 and around 1100 hr IST during MONTBLEX 1990, reveal that the distribution of virtual potential temperature0v 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 of0r. 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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