Study of the Microphysical Characteristics of Weak Precipitation in Mêdog, Southeastern Tibetan Plateau Using Ka-Band Cloud Radar

Mêdog, located in the southeastern Tibetan Plateau (TP) and the valley of the lower reaches of the Yarlung Zangbo River, is the main water vapor channel from the Indian Ocean to the TP. Mêdog is also an important part of the TP precipitation system because it has the largest annual average precipitation amount over the TP. Based on the Ka-band cloud radar (KaCR) observation data of the Mêdog National Climate Observatory in 2020, this study first preprocessed the power spectrum data of the KaCR, which were verified by comparing with measurements from a collocated precipitation phenomenometer. Then, two weak stratiform precipitation processes that occurred on March 6 and August 24, 2020, were selected, and the raindrop size distribution (RSD) was retrieved from the power spectrum data of the KaCR to explore the microphysical characteristics of weak precipitation in the dry and rainy seasons in Mêdog. Results showed that the systematic error of the reflectivity factor reached approximately 12 dB between KaCR measurements and theoretical values of KaCR calculated from the observations using the precipitation phenomenometer. Good consistency between the two datasets is evident after KaCR was corrected. Furthermore, the near-surface RSDs retrieved from KaCR was close to those observed from the precipitation phenomenometer. The heights of the bright band in Mêdog varied with the seasons and were low in the dry season (i.e., approximately 1.5 km above ground level) and high in the rainy season (i.e., approximately 4 km above ground level). The spectral width of the RSD of the weak stratiform precipitation cases was narrow, and the diameter of the raindrop did not exceed 3 mm in Mêdog. Above the bright band, the diameter of small ice particles gradually increased with the decrease in height according to the spectrum skewness and kurtosis. However, the growth of ice particles in the dry season is more obvious than that in the rainy season. Below the bright band, the ice particles converted into liquid water drops, whose concentration decreased as the height decreased in the process of falling, probably due to the coalescence and evaporation of raindrops. The smaller the diameter is, the faster the concentration of raindrops decreases. Near the ground, the significant decrease in the concentration of raindrops can be attributed to enhanced evaporation.