Interactions of the land-surface with the atmospheric boundary layer: Case studies from LASPEX

The daytime interaction of the land-surface with the atmospheric boundary layer (ABL) is studied for two case study days representing pre-monsoon and monsoon conditions during the Land Surface Processes Experiment (LASPEX) field program which was carried out in the tropical semi-arid region in the northwest Indian state of Gujarat. In this study, a one-dimensional (column) ABL model which has a land-surface scheme that interacts with the ABL is used. Results indicate that in coupled land-atmosphere simulations, realistic daytime surface fluxes and atmospheric profiles are produced, though improvement is needed, particularly in the transpiration and soil heat flux formulation

Dynamic responses of warm monsoon clouds to salt seeding

High resolution temperature measurements during single-level air- craft penetrations through warm monsoon clouds before and after salt seeding had a significant wave-length of about 2 km. The slope of the spectra relating to not-seeded traverses followed a -5/3 power law. The slope of the spectra relating to seeded traverses increased when liquid water content increased and rain formed. The temperature spectra of the seeded traverses showed a net energy gain in the larger wave-lengths ( >540 m) and a net energy loss in the shorter wavelengths. The net-energy gain could be due to condensation of water vapor on the salt parti- cles, the net energy loss to the decrease in the small scale turbulence resulting from the invigoration of the updraft. These features could be manifestations of the alteration of the dynamics of the cloud through salt seeding

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

Wind characteristics in the lowest 340 m of the atmospheric boundary layer at Pune, India

Wind characteristics in the lowest 340 m (agl) of the atmospheric boundary layer at Pune (18°32′N, 73°51′E, 559 m ASL) have been investigated using the pilot balloon wind observations obtained during the summer monsoon seasons of 1976, 1979 and 1980. Variations in the zonal and meridional components of wind at the surface, 40, 150 and 340 m (agl) have been described. Frequency distribution of the zonal component showed an unimodal character at the surface, which gradually approached a multimodal character at 340 m. The distribution pattern of the wind components was, by and large, normal. Spectral analysis of the wind components showed that the spectral energy was predominantly shared by 5–7 and 2–3 day period

Observations of reversal of mixing ratio in the atmospheric boundary layer over the Arabian sea

Analysis of the radiosonde observations over the Arabian sea region during MONSOON-77 and MONEX-79 has revealed a reversal in the mixing ratio (QR) above the inversion/stable layer in the atmospheric boundary layer. The reversal could be attributed to the detrainment of cloudy air originating in the deep moist convection which has penetrated the inversion layer in nearby areas. It was noticed that the soundings in which a reversal of mixing ratio was not observed, were associated with meteorological conditions favourable for deep convection

Thermodynamic features of the atmospheric boundary layer during the summer monsoon

Characteristic variations in the thermodynamic parameters of the boundary layer at Pune (18°32′N, 73°51′E, 559m a.s.l.) have been studied using the aerological observations collected during the summer monsoon seasons of 1980 and 1981, and the temperature observations from aircraft during the summer monsoons of 1976, 1979–1981. This study showed suppression of the mixed layer, absence of inversion/stable layers and decreased convective instability in the lower layers during the period of active monsoon conditions. The reverse was observed during the periods of weak/break monsoon conditions. Temperature stratification of sub-cloud layer has been classified into four different categories depending on the extent of the mixed layer and the gradient of potential temperature in the overlying stable layer. It was observed that these categories cover all types of weather, conditions which prevailed during the monsoon season. The results are discussed with the possible association of the weather conditions prevailed during the active and break monsoon period

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