Thermodynamic structure of the convective boundary layer (CBL) over the Indian monsoon region during CAIPEEX campaigns

Abstract

Spatial and temporal variability in the convective boundary layer (CBL) height for the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) study period are examined using the data collected from high-resolution radiosondes during May–September 2009 over the Indian monsoon region. In total, 57 radiosonde launchings were carried out at ∼ 11:00–17:00 IST over six different stations covering a large geographical region, ranging from latitude ∼ 13 to 32° N and longitude 73 to 92° E. Of the total 57 launchings, 17 were made during cloudy conditions during which relative humidity (RH) was found to be greater than 83 % for an ∼ 1.0 km layer at various altitudes below 6 km. Within the layer the difference between saturated equivalent potential temperature and equivalent potential temperature is small, and it satisfies the condition that RH > 83 % for about 1 km is considered as the cloudy layer. There are eight cases when the cloud-topped boundary layer (CTBL) and 19 cases when fair-weather boundary layer (FWBL) is observed. The CBL heights are obtained using thermodynamic profiles, which vary from ∼ 0.4 to 2.5 km a. g. l. The formation of the cloud layers above the boundary layer generally lowers the CBL height and is responsible for its day-to-day variability. The development of the cloud beneath the boundary layer generally elevates the CBL, which is also responsible for the large day-to-day variability in the CBL. The FWBL identified using relative invariance of the thermodynamic profiles varies from ∼ 2.0 to 5.5 km, which is clearly marked by a local minimum in the refractivity gradient. During cloudy days, the CBL is found to be shallow and the surface temperature lower when compared to clear-sky days. The CBL and the lifting condensation level (LCL) heights are randomly related and are found to be at a lower height during cloudy days when compared to clear-sky days. Finally, the typical comparison between the CBL height obtained using thermodynamic profiles and backscattering profiles using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) is examined.