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An optical particle counter (OPC) is used in conjunction with lidarmeasurements to examine the characteristics of the particle sizedistribution in cirrus cloud in the tropical tropopause layer (TTL) overThailand where the TTL is defined as the height at which temperature islower than −75°C in this paper. Of 11 OPC launches, cirrus cloud wasdetected at 10–15 km high on 7 occasions, cirrus was detected in the TTL in6 cases, and simultaneous OPC and lidar measurements were made on twooccasions. Comparison of lidar and OPC measurements reveal that the cloudheights of cirrus in the TTL varies by several hundred meters over distancesof tens kilometers; hence the height is not always horizontally uniform. Themode radii of particles constituting the clouds are estimated by lidar andOPC measurements to be less than approximately 10 μm. The regressionlines of the particle size distribution with and without cirrus cloud exhibitsimilar features at equivalent radii of <0.8 μm. Enhancement in theintegrated number concentration at radii greater than 0.8 μm indicatesthat liquid particles tend to be frozen at a radius of 0.8 μm, withcirrus clouds above 10 km exhibiting similar features. On the other hand,enhancement in the particle size distribution at radii greater than 0.9 μm anda peak at around 0.8 μm in the ratio of the standard deviationof count values to that of the Poisson distribution of the averaged countvalues are common features of cirrus clouds in the TTL, where the ratioshows the vertical homogeneity of the particle number. These typicalfeatures suggest that the transition from liquid, sulfuric acid aerosol, toice is more observable in the TTL and the timing of freezing may vary withheight in the TTL

Characteristics of aerosol and cloud particle size distributions in the tropical tropopause layer measured with optical particle counter and lidar