Cloud macrophysical properties from KAZR at the SACOL

Abstract

Clouds are one of the most important components in the climate system through their profound effects on the radiative energy budget. However, due to the lack of accurate representation of clouds in global climate models, they are the major source of uncertainties for the future climate projections. Millimeter wavelength Cloud Radars (MMCRs) are powerful instruments that can resolve vertical structure as well as microphysical properties of clouds. The wavelength of MMCRs, which is shorter than those of weather radars, allows an excellent sensitivity to cloud droplets and ice crystals while being much longer than lidar wavelengths enables MMCRs to penetrate multiple cloud layers. Ka-Band Zenith Radar (KAZR), which has been deployed at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) since July 2013, has been continuously operated for three years. By applying a cloud mask algorithm that is operationally employed in the Atmospheric Radiation Measurement Program to distinguish radar echoes from noise, we analyze the macrophysical properties of clouds above the SACOL site by using the KAZR observations through 2014. It is shown that the occurrence of cloud base peaks at both 1.5 and 5.5 km while the cloud top peaks at 2.5 and 8.5 km. About 67% of the cloud thickness is within 2 km. The heights of cloud base and top in spring and summer are higher than autumn and winter. The diurnal cycles of cloud top and base are significant in summer and autumn but not in spring and winter. In summer, both the cloud base and top heights increase after sunrise and then decrease from mid-afternoon to midnight, while the cloud base and cloud top heights increase from noon to midnight in autumn. These seasonal features of diurnal changes may be closely related to the cloud formation mechanism and seasonal variations of solar radiation. Clouds over the SACOL can be classified as low, middle and high clouds based on the cloud height and thickness. The monthly mean occurrence of all clouds varies between 44% and 76%. The occurrence of low, middle and high clouds is 13%, 30%, 34%, respectively. The occurrence of middle and high clouds does not show a significant difference except for spring when the high clouds occur more often than middle clouds. Low clouds, however, occur less frequently throughout the year than the other two cloud types. Clouds can also be classified based on the number of cloud layers. Clouds with layers less than three contribute to ~98% of the total clouds and multi-layer clouds occur frequently in summer and autumn. This may be a result of the convection that is more likely to happen in summer and autumn and will induce a vertical distribution of clouds. The cloud base and top heights detected by KAZR, CloudSat and CALIPSO are also compared in this study. We find that the differences in cloud base and top heights between CloudSat and KAZR are less than 0.12 km. The reflectivities detected by CloudSat are comparable with those from KAZR. CloudSat cannot detect clouds with weak signal less than −30 dBZ due to the limitation of its sensitivity. This part of missing clouds can be captured by CALIPSO but CALIPSO lidar signals can be attenuated in thick clouds leading to an overestimation of cloud base.