Seasonal variation and vertical characteristic of cirrus clouds at a midlatitude monsoon site from Ka-band cloud radar observation

Based on vertically pointing Ka-band MMCR observation in 2020-2021, we investigate the vertical structure and diurnal and seasonal variations of cirrus clouds at a midlatitude monsoon site in central China. Statistical analysis shows that the cirrus clouds occur mostly at 7-15 km with the reflectivity of-42-7 dBZ in summer, and at 5-12 km with the reflectivity of-47-- 2 dBZ in winter. From spring to winter, the maximum cirrus occurrence frequency (COF) arises at about-36,-34,-37 and-37 dBZ in the reflectivity domain, at 10.23, 12.00, 10.41, and 8.79 km in the spatial dimension, and at-46,-52,-39 and-43 degrees C in the temperature domain, respectively. In both the spatial and temperature domains, the COF distribution is approximately consistent with the Gaussian distribution. The seasonal mean ice water content (ice effective radius) is mainly distributed in 1 x 10-3-7 x 10-3gm-3 (34-46 mu m). There is an obvious diurnal change in the cirrus activities. In summer, the COF is relatively small from 5:00 to 12:00, and reaches the maximum at about 20:00, while in autumn, the cirrus clouds are rare between 4:00 and 14:00, and have a significant COF from 19:00 to 24:00. The COF reaches the maximum of 20.03 % in July in 2020 but only 16.16 % in June in 2021. In 2020, the monthly mean cloud base and top heights show a first increase and then decrease, whereas, their evolution exhibits large fluctuations in 2021. The COF has a correlation coefficient of 0.61 with oceanic Nino index (ONI), and the difference of cirrus clouds between 2020 and 2021 is closely associated with the ONI evolution.