Microphysical Characteristics and Precipitation Formation Mechanisms of Convective Clouds over the Tibetan Plateau in Summer

Abstract

Tibetan Plateau (TP) has high impact on weather, climate, and water cycle of China, and it also affects the flood and drought in south China by modulating the onset and retreat of the Asian monsoon. However, owning to the lack of direct observations, the knowledge of microphysical characteristics and mechanisms inside the clouds over TP is still seriously lacking. During the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-Ⅲ), field observations is carried out in the summer of 2014, which employed ground-based and airborne instruments. By using the aircraft measurements collected during the TIPEX-Ⅲ, the microphysical characteristics and precipitation formation mechanisms of summertime clouds are studied. The results show that clouds detected by the aircraft are mainly newly born or developing mixed-phase convective clouds, as well as some residual clouds. The maximum and average concentrations of cloud drops are 1.1×105 L-1 and (9±10)×103 L-1, respectively, and the order of magnitude is 104 L-1, which is lower than clouds of plain and maritime regions by 1-2 orders. The maximum concentration for larger cloud particles is 28.82 L-1, and the order of magnitude is 100-101 L-1, which is also lower than other regions. The maximum liquid and total water content are 0.25 g·m-3 and 1.33 g·m-3, respectively, and the order of magnitude is 10-1-100 g·m-3, with abundant supercooled liquid water content in the clouds. The uplifting velocity distributes mainly in the range of 1-4 m·s-1 with a maximum of 4.3 m·s-1, indicating the convective clouds over the TP are weaker than other regions. The cloud drop size distributions (DSD) are mostly bimodal with different second peaks at the larger end, and some of the DSDs are unimodal, which are mainly found in newly borne clouds. There are more large cloud drops and drizzles in the clouds over the TP, which is the result of active warm rain processes. And the ice particles mainly consist of opaque and dense graupels as well as some needles and plates, indicating active rimming processes. The warm rain processes do not generate rain directly, but contribute to the subsequent glaciation and rimming processes, leading to the quick formation of precipitation over the TP. The residual clouds show similar ice characteristics with convective clouds, but much drier and weaker, and they also maintain small amount of supercooled liquid water.

Keywords

• aircraft measurements
• cloud microphysical structure
• characteristics of cloud and precipitation formation