Vertical Structure Characteristics of Precipitation in Mêdog Area of Southeastern Tibet During the Monsoon Period

Abstract

Precipitation is particularly important for the earth's climate system. Understanding the structural characteristics, microphysical processes and drop size distribution (DSD) of precipitation is very important for quantitative precipitation estimation with radar and improving microphysical parameter schemes of numerical weather prediction models. With the launch of the Second Tibet Plateau Scientific Expedition and Research(STEPS), Chinese Academy of Meteorological Sciences has deployed Ka-band cloud radar (KaCR), X-band dual polarization phased array radar (X-PAR), disdrometer, micro rain radar (MRR) and other detection equipment in Mêdog, filling the gap of cloud and precipitation observation in this area and provides data basis for studying the physical characteristics of clouds and precipitation. Mêdog is located at Yarlung Zangbo Grand Canyon, the entrance of the water vapor channel in southeastern Tibet. Influenced by the warm and humid airflow brought by the Indian Ocean monsoon, the precipitation of Mêdog during the monsoon period exceeds 60% of the annual precipitation. MRR is a low-cost, miniaturized vertical directional Doppler rain radar that can more accurately analyze the vertical structural changes of precipitation. Based on observation of the rain gauge, MRR and disdrometer set up at Mêdog National Climate Observatory from 1 June to 30 September in 2021, the consistency of different instruments is studied. The observed rainfall is classified into convective, stratiform and shallow precipitation types, and the average vertical distribution characteristics of different precipitation types are studied from the aspects of raindrop size distribution, falling speed, rain rate, liquid water content and radar reflectivity. The results show that the measurement of rain gauge, MRR and distrometer are highly consistent. The correlation coefficient of daily rainfall is above 0.89, and the highest correlation coefficient between MRR and rain gauge is 0.96. However, MRR overestimates weak precipitation and underestimate strong precipitation. There are significant differences in the vertical structure of different precipitation types during the monsoon period of Mêdog. Values of each microphysical quantity of convective precipitation are larger. The collision and growth process of raindrop is significant during the falling process below 3 km height, and the raindrop number concentration increases rapidly. There is significant updraft at a height of 1-2 km. The echo intensity of stratiform cloud precipitation is weak below the height of the melting layer. The radar reflectivity, rain rate and liquid water content increase with altitude decrease, the falling speed of raindrops remains basically stable in the vertical direction. The concentration of medium-sized raindrops remains constant with height, and the evaporation, fragmentation, and coalescence processes are in a relatively balance. Values of each microphysical quantity of shallow precipitation are relatively small but vary significantly with height and show negative slops in the vertical direction. The shallow precipitation is dominated by the collision process of raindrops.

Keywords

• micro rain radar
• raindrop size distribution
• vertical structure
• precipitation classification