Characteristics of particle size distributions in the tropical tropopause based on optical particle counter and lidar measurements

Abstract

An optical particle counter (OPC) is used in conjunction with lidar measurements to examine the characteristics of the particle size distribution in cirrus cloud at the tropical tropopause (TT) over Thailand. Of 11 OPC launches, cirrus cloud was detected at 10-15 km high on 7 occasions, cirrus was detected at the TT in 6 cases, and si-5 multaneous OPC and lidar measurements were made on two occasions. Comparison of lidar and OPC measurements reveal that the cloud height of cirrus in the TT varies by several hundred meters over distances of tens kilometers; hence the height is not horizontally uniform. The mode radii of particles constituting the clouds are estimated by lidar and OPC measurements to be less than approximately 10 µm. The regression 10 lines of the particle size distribution with and without cirrus cloud exhibit similar features at equivalent radii of <0.7 µm. Enhancement in the integrated number concentration at radii greater than 0.7 µm indicates that liquid particles tend to be frozen at a radius of 0.7 µm, with cirrus clouds above 10 km exhibiting similar features. In addition, common features of cirrus clouds at the TT include a local maximum in the particle size 15 distribution at 2.0 µm and a peak between 0.5 µm and 1.7 µm in the ratio of the standard deviation of count values to that of the Poisson distribution of the averaged count values. Each feature implies that all ice particles in the clouds may be nucleated by the same mechanism and particles in this size range are actively frozen at the TT. These parameters are thus good indicators for checking the results of cirrus cloud models in 20 the TT.