1Dept. of Earth and Ocean Sciences, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan.
2Dept. of Information and Systems Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
3Dept. of Earth System Science, Fukuoka University, 8-19-1 Nanakuma, Jonan, Fukuoka 814-0180, Japan
4Inst. of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka 237-0061, Japan.
5Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan
6Graduate School of Earth and Environmental Science, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan
7National Inst. for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
8National Research Inst. for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba 305-0006, Japan
9Graduate School of Science, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan
10Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Patumwan, Bangkok 10330, Thailand
11School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo 113-0033, Japan
12Inst. of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro 153-8505, Japan
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 simultaneous 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 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 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 the TT.