Surendra S. Parasnis

An Overview of the Land Surface Processes Experiment (Laspex) over a Semi-Arid Region of India

To understand and quantify the land-surface-vegetation interactionwith the atmospheric boundary layer, and validate or improve upon the existing surfaceflux parameterization schemes in various weather forecast models, a LAnd SurfaceProcesses EXperiment (LASPEX), was designed and executed in the semi-arid regionof Gujarat, India during January 1997–December 1998. Micrometeorological tower observations,soil and vegetation parameters, radiation, turbulence and upper airobservations were taken continuously for two years at five sites, separated by about60–100 km from each other. Towers of 9 m height with instruments at four levels wereinstalled at sites that are agricultural fields and characterized with a variety of soilproperties, vegetation and diverse crops. An overview of the experiment is presented.Some results, such as the seasonal variation of surface energy balance and turbulence statistics,are discussed.

The Convective Boundary Layer over the Deccan Plateau, India during the summer monsoon

The thermodynamic structure of the Convective Boundary Layer (CBL) over the Deccan Plateau, India has been investigated using aerological data during the summer monsoon seasons of 1980 and 1981. Conserved-variable analysis and the saturation-point approach, which were used in this study, suggest that the top of the CBL varied between 700–600 mb during the monsoon. The air above the top of the CBL during a weak monsoon was estimated to have subsided for 4 days with a subsidence rate of 30 mb day-1.

A convectively-driven boundary layer in the monsoon trough

Characteristic features of the convectively driven monsoon-trough boundary layer have been explored using the conserved-variable method of analysis. Aerological observations during the Monsoon Trough Boundary Layer Experiment 1990 (MONTBLEX-90) during 18–20 August have been used to investigate the thermodynamic features of the Convective Boundary Layer (CBL). Thermodynamic parameters such asθe,θes have been used to study the dynamical aspects of the CBL. Also, mixed-layer heights at an inland station, in the monsoon trough region, obtained from SODAR, are used to document the saturation of the mixed layer after the onset of the monsoon.

Convective boundary layer in the region of the monsoon trough - a case study

A case study of the convectively driven monsoon boundary layer has been carried out using the acrological observations at four stations in the region of monsoon trough during Monsoon Trough Boundary Layer Experiment (MONTBLEX) 1988. The Convective Boundary Layer (CBL) in the region of monsoon trough did not show double mixing line structure. A single mixing line representing the CBL with different stabilities with respect to the convective activities was observed.

Thermodynamic structure of the atmospheric boundary layer over the Arabian sea as revealed by Monsoon-77 data

The thermodynamic structure of the Atmospheric Boundary Layer (ABL) over the Arabian sea region has been studied with the help of 135 aerological observations obtained during MONSOON-77 in the region (10–14° N, 64–68° E) by USSR research vessels. Low-level inversions were observed over the western Arabian sea region (west of 66° E) in association with suppressed convection. The different sublayers of the ABL, viz. the mixed layer, the cloud layer and the inversion/isothermal/stable layer were identified. The low-level stability analysis indicated that in the region east of 66° E, conditions were favourable for deep convection. The thermodynamic transformation of the boundary layer after precipitation was documented.

Thermodynamic structure of the boundary layer in relation to a monsoon depression over the Bay of Bengal — A case study

Aerological observations carried out on board ORV Sagarkanya at a stationary location (20° N, 89° E) over the Head Bay of Bengal during August 18–21, 1990 were analysed to study the thermodynamic structure of the marine boundary layer in relation to a monsoon depression which formedin situ with its centre at 20° N, 88° E. The q(mixing ratio) reversal observed at 850 hPa prior to formation of the low pressure area shifted to a higher level (h<700 hPa) with the formtion of the low. Positive buoyancy below 850 hPa prior to the formation of the low indicated conditions favourable for deep convection. When the low pressure area intensified into a depression, negative buoyancy was observed at lower levels.

Convective mixing in the monsoon boundary layer

Aerological observations over a tropical inland station collected during different phases of the Indian summer monsoon have been used to study the convective mixing in the monsoon boundary layer. The mixing line (ML) model for the thermodynamic structure of a partially cloudy boundary layer has been used to estimate the mixing by convection. The ML model together with the saturation point approach is able to depict different convective mixing regimes in the monsoon boundary layer.

Temperature stratification of the atmospheric boundary layer over the Deccan Plateau, India, during the summer monsoon

The vertical and horizontal temperature structure of the atmospheric boundary layer (ABL) were studied using aircraft observations made in the lowest 2.4 km above ground level during the summer monsoon.The vertical temperature structure of the ABL in the region may be classified into the following four categories.CategoryThe ABL consisted of two layers of thickness 700–900 m separated by a thin transition layer. The lapse rates in the former two layers were dry adiabatic.CategoryThe lowest layer of the ABL of thickness 400–600 m was adiabatically stratified and the overlying layer was stable with gradients of potential temperature 4–5°C km−1. The stable layer contained a thin adiabatic stratified layer of 200–300 m thickness at a height of 1.5 km.CategoryThe lowest 200–400 m layer of the ABL was adiabatically stratified and the overlying layer was stable with potential temperature gradients of 5–6 °C km1.CategoryThe ABL was mainly stable with potential temperature gradients of 6 °C km−1 or greater. Occasionally thin layers with adiabatic stratification were found embedded in the ABL.The temperature distribution of the horizontal temperature at 900 m was mainly normal. The high-frequency portion of the spectra lying between 0.05 and 0.16 Hz (corresponding to wave length 1 km to 300 m) oscillated around the −\2/3 power law line. The spectral curve showed a significant peak at 0.011 Hz having a wave-length of 5 km.

Stability of the sub-cloud layer during the summer monsoon

Stability of the sub-cloud layer has been studied by examining the difference between the temperature of the lifting condensation level (LCL) and that of a meteorological sounding at the LCL. The difference in these temperatures was used as an indicator of the stability of the sub-cloud layer. When the temperature at LCL was warmer/cooler than its surroundings, it indicated the unstable/ adiabatic/stable conditions of the sub-cloud layer. It was observed that when conditions were unstable or adiabatic, there was less monsoon activity. Stable conditions were associated with active monsoon periods. The results are discussed with the enhancement/decrease of convective activity on days of weak/active monsoon conditions.

On the thermodynamics of downdrafts in the monsoon trough region

A technique, “saturation-point analysis’, has been used to study convective-scale down-drafts driven by the evaporation of rain in the monsoon trough region during the passage of a disturbance. The aerological soundings collected during the passage of a depression within a monsoon trough were used for this study. The results showed a characteristic mixing line in the lower troposphere ahead of the depression while a distinct evaporation line structure was observed during passage of the depression.