Elucidating cloud vertical structures based on three-year Ka-band cloud radar observations from Beijing, China

Abstract

Elucidating the vertical structures of clouds is key to reducing the uncertainty in cloud-induced climate forcing; however, we do not yet know explicitly about the cloud structures that form over Beijing, China, and how they vary. In this paper, we document vertically resolved cloud structures and their climatological variation based on three-year (1 January 2015 to 31 December 2017) ground-based Ka-band (35 GHz) millimeter-wave cloud radar (MMCR) measurements. Cloud top height (CTH) and radar reflectivity are higher in summer and autumn than in spring and winter. We verified the MMCR measurements of the CTH and cloud base height (CBH) independently based on observations from the Himawari-8 and CL51 ceilometer at the same station, respectively. The CTH decreased as the thickness of the cloud layer increased. This may have been due to the mechanism used to determine the CTH, which was based on data from the satellite infrared channel. There were two peaks in the CBH distribution, located at 0–1 km and 5–6 km, whereas the frequency of the CTH peaked at an altitude of 9–10 km, between 1200 and 1800 LST. Precipitating clouds tended to be located at lower altitudes and exhibited stronger radar reflectivities than non-precipitating clouds. The cloud frequency tended to peak in autumn and reached a minimum in winter. In terms of the seasonality of clouds at various levels, middle- and high-level clouds were dominant over Beijing, with high and low (precipitating) clouds concentrated between May and October. Mid-altitude clouds occurred more frequently in winter and spring. CTH peaked in summer due to the strong solar radiation received by the surface. In contrast, minimum CBH occurred in winter. The cloud heights were higher and radar reflectivities larger than reported in previous studies. These results are correlated with urban warming effects. Understanding the properties of clouds will not only be of benefit to researchers carrying out cloud forcing studies, but will also provide key validation data for climate model simulations.