1Department of Earth and Ocean Sciences, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
2Department of Information and Systems Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
3Department of Earth System Science, Fukuoka University, 8-19-1 Nanakuma, Jonan, Fukuoka 814-0180, Japan
4Institute 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 Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
8National Research Institute 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
12Institute 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 in the tropical tropopause layer (TTL) over Thailand where the TTL is defined as the height at which temperature is lower than −75°C in this paper. Of 11 OPC launches, cirrus cloud was detected at 10–15 km high on 7 occasions, cirrus was detected in the TTL in 6 cases, and simultaneous OPC and lidar measurements were made on two occasions. Comparison of lidar and OPC measurements reveal that the cloud heights of cirrus in the TTL varies by several hundred meters over distances of tens kilometers; hence the height is not always 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 <0.8 μm. Enhancement in the integrated number concentration at radii greater than 0.8 μm indicates that liquid particles tend to be frozen at a radius of 0.8 μm, with cirrus clouds above 10 km exhibiting similar features. On the other hand, enhancement in the particle size distribution at radii greater than 0.9 μm and a peak at around 0.8 μm in the ratio of the standard deviation of count values to that of the Poisson distribution of the averaged count values are common features of cirrus clouds in the TTL, where the ratio shows the vertical homogeneity of the particle number. These typical features suggest that the transition from liquid, sulfuric acid aerosol, to ice is more observable in the TTL and the timing of freezing may vary with height in the TTL.
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Iwasaki, S., Maruyama, K., Hayashi, M., Ogino, S.-Y., Ishimoto, H., Tachibana, Y., Shimizu, A., Matsui, I., Sugimoto, N., Yamashita, K., Saga, K., Iwamoto, K., Kamiakito, Y., Chabangborn, A., Thana, B., Hashizume, M., Koike, T., and Oki, T.: Characteristics of aerosol and cloud particle size distributions in the tropical tropopause layer measured with optical particle counter and lidar, Atmos. Chem. Phys., 7, 3507-3518, doi:10.5194/acp-7-3507-2007, 2007.