Particle reflectivity and movements in cirrus clouds over Beijing from four years of Ka radar measurements

Abstract

Cirrus clouds are ice clouds. Our understanding of the basic microscale physics of ice clouds is not yet adequate. In this study, interior physical properties of cirrus clouds over Beijing are investigated based on four years of continuous measurements of a Ka-band polar Doppler radar, which performs long-term observations with higher temporal resolution than the aircraft and the satellite. The probability distribution functions of radar reflectivity at various temperatures are presented and a positive correlation relationship between reflectivity and temperature is also found. In addition, the distributions of the particle Doppler velocity and particle movements in cirrus clouds are also revealed. According to the cloud-base temperature (CBT), all cirrus clouds are divided into three types [Cih: CBT < 273 K and CBH (cloud-base height) ≥ 258 K; Cim: CBT < 258 K and CBT > 235 K; Cil: CBT ≤ 235 K] in order to understand their formation mechanisms and possible differences. The results of investigation show that Cil has the lowest mean depth, the lowest mean and maximum reflectivity, and the lowest inhomogeneity among the three types of cirrus clouds, while Cih has the highest and Cim lies between the two. The reflectivity–temperature relationship is also different in the three types. In particular, for Cil, reflectivity varies little with the variation in temperature. The significant difference in microphysical properties and interior structures between Cil and Cih proves their different formation mechanisms. Cil, illustrating higher vertical uniformity, is likely generated by radiation cooling, while the other cirrus cloud types are generated differently. The results presented and analyzed in this study provide new knowledge regarding cirrus clouds in Beijing, a mid-latitude region, and a reference for related parameterization in global climate models.