Some aspects of the convective boundary layer over the Bay of Bengal

A study has been carried out to investigate the thermodynamic characteristics of the Convective Boundary Layer (CBL) over the Bay of Bengal region. For this purpose aerological observations collected from MONSOON-77 utilizing the erstwhile USSR research vessels (i. e. Shirshov, Priboy, Shokalsky and Okean) in the region 15-19°N, 87-91°E during 11 - 18 August 1977 and BOBMEX-98 onboard Sagar Kanya at 13°N 87°E have been considered. The results of the analysis showed that no marked differences are found in the Convective Boundary Layer characteristics over the four ships during the period under consideration. 6-hourly vertical profiles of the thermodynamical paramenters showed the evolution of the CBL structure during daytime at all the locations. The top of the CBL is found between 700-580 hPa and it varied with time. The daily 6-hourly values of surface meteorological parameters viz, p, T and Td showed diurnal variation. Also, it is evident that the surface meteorological parameters as well as structure of the CBL over the region mentioned above is influenced by the prevailing weather conditions and associated synoptic/transient scale disturbances

Marine boundary layer characteristics during a cyclonic storm over the Bay of Bengal

During the period 12-16 June 1996 a tropical cyclonic storm formed over the southwest Bay of Bengal and moved in a north-northeasterly direction. The thermodynamic characteristics of this system are investigated by utilizing the surface and upper air observations collected onboard ORV Sagar Kanya over the Bay of Bengal region. The response of the cyclonic storm is clearly evident from the ship observations when the ship was within the distance of 600-800 km from the cyclonic storm. This study explores why (i) the whole atmosphere from surface to 500 hPa had become warm and moist during the cyclonic storm period as compared to before and after the formation of this system and (ii) the lower layer of the atmosphere had become stable during the formative stage of the cyclonic storm

Thermodynamic structure of the marine atmosphere over the region 80-87°E along 13°N during August (phase II) BOBMEX-99

Thermodynamic structure of the marine atmosphere in the region between 80 and 87°E along 13°N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface -400 hPa collected onboard ORV Sagar Kanya during the period 27th-30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved structure in this layer. Further, seldom showed any inversions in this region. The profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (c-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels

Nature of light rain during presence and absence of bright band

This paper reports the evolution of rain drop size distribution (DSD) during bright band (BB) and no-BB (NBB) conditions of low intensity rainfall events as observed by a vertically pointing Micro Rain Radar (MRR) over Pune (18.58°N, 73.92°E), India. The BB is identified by enhanced radar reflectivity factor Z (dBZ) at the 0°C isotherm. The gradient of hydrometeor fall velocity is found to be a good indicator in identifying the melting layer when enhanced radar reflectivity at melting layer is not prominent. The storm structures as observed by the MRR are compared with CloudSat observations that provide evidence of ice hydrometeor at ~ −60°C with clear indication of BB at 0°C. Storm heights at warmer than 0°C are evident during NBB conditions from CloudSat. This suggests that warm rain processes are responsible for producing rain during NBB conditions. During BB conditions, bimodal DSDs below the melting layer are observed at lower altitudes. The DSDs of shallow warm precipitating systems of NBB conditions are monomodal at all the altitudes. Significantly, normalized DSDs are found to be bimodal for BB conditions, and monomodal for NBB conditions which confirm different dominant microphysical processes. It is found that the observed bimodal DSDs during BB conditions are mainly due to the collision, coalescence and break-up processes. During NBB conditions, number and size of large raindrops grow while reaching the ground without much breakup. The radar reflectivity and rainfall intensity R (mmh − 1) relationship of the form Z = aR b are found out for BB and NBB conditions. Existing different microphysical processes lead to large coefficient in the Z–R relationship with small exponent during BB conditions while during NBB conditions the coefficients are small with large exponents

The cloud aerosol interaction and precipitation enhancement experiment (CAIPEEX): Overview and preliminary results

While the demand for enhancing rainfall through cloud seeding is strong and persistent in the country, considerable uncertainty exists on the success of such an endeavour at a given location. To understand the pathways of aerosol-cloud interaction through which this might be achieved, a national experiment named Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) in two phases, was carried out. The rationale of CAIPEEX, the strategy for conducting the experiment, data quality and potential for path-breaking science are described in this article. Pending completion of quality control and calibration of the CAIPEEX phase-II data, here we present some initial results of CAIPEEX phase-I aimed at documenting the prevailing microphysical characteristics of aerosols and clouds and associated environmental conditions over different regions of the country and under different monsoon conditions with the help of an instrumented research aircraft. First-time simultaneous observations of aerosol, cloud condensation nuclei (CCN) and cloud droplet number concentration (CDNC) over the Ganges Valley during monsoon season show very high concentrations (> 1000 cm-3) of CCN at elevated layers. Observations of elevated layers with high aerosol concentration over the Gangetic valley extending up to 6 km and relatively less aerosol concentration in the boundary layer are also documented. We also present evidence of strong cloud- aerosol interaction in the moist environments with an increase in the cloud droplet effective radius. Our observations also show that pollution increases CDNC and the warm rain depth, and delays its initiation. The critical effective radius for warm rain initiation is found to be between 10 and 12 μm in the polluted clouds and it is between 12 and 14 μm in cleaner monsoon clouds

Impact of low level clouds on convective boundary layer equilibrium structure

A study has been carried out using the concepts of saturation point and conserved variable plots utilising the aerological observations collected at an inland station, Pune (18°32N, 73°51E, 559 m a.s.l.) during the summer monsoon seasons of 1980 and 1981 to investigate the impact of low level clouds on equilibrium structures in the atmospheric boundary layer (ABL). The days have been categorized according to the distribution of clouds vis-a-vis equilibrium structure of ABL. The study suggested a close association between amount of clouds in the lower atmosphere and the thermodynamical characteristics of ABL. The results of the study 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

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

Mixing processes in the atmospheric boundary layer during the summer monsoon

The saturation point concept and mixing line approach have been used in this study. During break conditions of monsoon the mixing line was cooler than the atmospheric stratification and suggested that mixing of cloud base air and inversion top air would produce downdrafts to the base of inversion. During the active conditions of monsoon the mixing line was warmer than the environmental stratification, indicating that the mixing of environmental air and cloud base air may produce warmer updrafts

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 as θe, 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