NPP/ⅦRS satellite retrieval of summer convective cloud microphysical properties over the Tibetan Plateau

The Tibetan Plateau (TP) has important influences on the formation and development of the East Asian atmospheric circulation, climate change and disastrous weathers in its downstream regions. It is very important to understand the microphysical characteristics of clouds at the TP. However, observations and researches of the microphysical characteristics at this area are insufficient because of the sparse observation stations and few cloud physical data. ⅦRS (Visible Infrared Imaging Radiometer Suite) onboard the Suomi NPP (National Polar-orbiting Partnership) satellite, has an improved imaging spectroradiometer with seventeen channels of 750 m moderate resolution and five channels of 375 m image resolution. The high-resolution satellite has an advantage for observing the small or initial convective clouds. Based on the retrieval methodologies of cloud microphysical properties by NPP satellite were proposed, an automated mapping software package named AMCC (Automatic Mapping of Convective Clouds) has been developed to retrieve properties of convective cloud at the scale of satellite swath. The properties of convective clouds are automatically retrieved by AMCC and their values are averaged in 0.33°×0.33° grids via NPP/ⅦRS satellite data over the TP in the summers from 2013 to 2017. The results show that:(1) the temperature of lifting condensation level (T_LCL) in Naqu meteorological station and the temperature of cloud base (T_b) from ⅦRS are linearly correlated with the correlation coefficient of 0.87 and the root-mean-square (RMS) error of 3.0℃; (2) convective clouds at the TP have the following microphysical properties. First, the cloud base temperature (T_b) is about -5℃, and the height of the cloud base above the ground (H_b) ranges between 1800-2200 m, and the cloud water content is small. Second, the cloud condensation nuclei concentrations (N_CCN) is between 200 and 400 mg^-1 with 0.7% of maximum supersaturation (S_max); consequently, the condensation growth rate of water cloud droplet with less N_CCN and higher S_max is fast. Third, because the precipitation initiation depth (D₁₄) varies within 1500-2000 m and 500-1000 m at the Yarlung Zangbo River Basin and the Zangnan region, respectively, and the clouds at these areas are more prone to precipitation. Fourth, the height of the cloud top above sea level (H_top) is between 10-13 km, but the cloud depth (D_cld) is rather small, which is about 5000 m at its southern part and gradually reduces to 2500 m at its northern part. Fifth, the glaciation temperature (T_g) ranges -30℃ at its central and southern parts to -25℃ at its northern part, which, combined with the high T_g and the T_b that is less than 0℃, lead to the domination of ice process in the clouds; (3) these microphysical properties of convective cloud at the TP explains why the precipitation is frequent and lasts over a short time with small amount and large rain drops.