2001 Volume 79 Issue 1B Pages 463-474
In order to reveal the characteristics of convective clouds over the Tibetan Plateau, we carried out Doppler radar observations of the clouds in the suburbs of Naqu city (4500 m ASL), which is located in the central part of the Tibetan Plateau, from 27 May to 19 September 1998, during the intensive observation period of the GEWEX Asian Monsoon Experiment in Tibet. Diurnal variation of the radar echo are and echo-top height were remarkable throughout the intensive observation period indicating the formation and development of convective clouds in the daytime and their decaying at night. The echo-top height and convective area, extracted objectively from grid data of radar reflectivity, were larger after the monsoon onset than before the onset. The maximum echo-top height of convective clouds (> 10 dBZ) was about 17 km ASL. Furthermore, many vortices accompanied by the clouds, identified on Doppler velocity fields, were observed in the daytime. The vortices generated in nearly all regions of the radar observation area (r = 64 km). The strongest vorticity of a given day with vortices during the observation period showed a positive correlation with the appearance of the maximum echo-top height (> 30 dBZ) of the convective clouds. The daily maximum vorticity after the monsoon onset was larger than that of the pre-monsoon period. A comparison of features of radar reflectivity and vorticity of convective clouds with sounding data indicates the presence of the following processes: formation of a strong updraft by solar heating, descending of westerly wind in the upper troposphere to the lower altitude, increase in strength of the vertical shear in low altitudes, tilting of horizontal vortex tube, and intensification of vortex in the clouds. Convective clouds developed above the melting layer at Naqu on the Tibetan Plateau were characterized by their rapid development from solar heating, by the formation of graupel particles and by the association of a strong vortex in the clouds.
