Microphysical Properties of Tropopause Cirrus Clouds during BATAL campaigns in India.

In-situ measurements of size and shape distributions of ice-crystals in thin cirrus clouds near the cold-point tropopause (16-18 km) over the Asian Summer Monsoon region are sparce. Under the framework of Balloon measurement campaigns of the Asian Tropopause Aerosol Layer (BATAL), number concentration, size, and shape distributions of ice-crystals in thin cirrus cloud layers located near the cold-point tropopause were measured for the first time over Tata Institute of Fundamental Research Balloon Facility in Hyderabad (17.47 N, 78.58 E), India during August 2017, August 2018, and July 2019. These measurements were carried out using a combination of balloon-borne instruments such as backscatter sondes (COBALD), an optical particle counter (Boulder counter) dedicated for ice-measurements and a miniaturized suite of aircraft instrument (Micro-COPP) for sizing and imaging ice-crystals along with temperature and water vapor measurements from radiosondes and Cryogenic Frost-Point Hygrometer (CFH), respectively. The combination of Boulder counter and Micro-COPP covered ice-crystals ranging from size 1 µm to 1 mm. All these instruments often detected thin cirrus clouds under extremely cold temperature (~-85° C) near the cold-point tropopause. Boulder counter measurements from most of the flights revealed that the ice-crystal number concentration in these clouds was less than 10 #L-1 with ice-crystals smaller than 50 µm. We also derived microphysical (effective diameter and ice-water content) and optical properties (extinction coefficient and lidar ratio) for tropopause cirrus clouds from the size-distribution. Cloud Particle Imager (CPI) onboard Micro-COPP revealed the presence of quasi-spheroid ice crystals in these clouds which is in good agreement with the aircraft measurements over the Western Pacific. The formation mechanism of these cirrus clouds is investigated using satellites, back-trajectories, and reanalysis data.