## 2 Materials and Methods  
### Introduction of Experimental Conditions  
Atmospheric exploration for snowfall was carried out in Darlag, Guoluo Tibetan Autonomous Prefecture, China (33.55 \({}^{\circ}\)E, 99.95 \({}^{\circ}\)N). Darlag is located in the QTP region, belonging to the Bayan Har Mountains, with an average elevation of 4200 m (Figure 1). It has an alpine and semi-humid climate with a diurnal temperature difference of 15-25 \({}^{\circ}\)C. The field site is located in the source area of the Three Rivers, namely, the Yangtze, Yellow, and Lancang-Mekong Rivers, with abundant water resources in the air. The potential for runoff excavation is 30-60 mm. The warm season from June to September mainly consists of liquid precipitation, accounting for 77.7% of the annual average precipitation. The snow period is from October to May of the next year. The snowfall period can be as long as 162 days, with a maximum snow depth of 120 mm, which is suitable for snow observations.  
From 19 October 2019 to 8 January 2020, 22 field tests were performed on the Qinghai-Tibet Plateau. Among them, 11 sets of atmospheric explorations of snowfall observations  
Figure 1: Geographical location of the field site of meteorological observations, (**a**–**c**) refer to position of field site, MWR and LUFFT weather station.  
and 11 sets of clear-day comparisons were collected (Table 1). During the East Asian winter monsoon (EAWM), the minimum night temperature at the field site is approximately \(-30\)\({}^{\circ}\)C. Considering personnel safety and equipment accuracy, radiosonde observations were generally carried out in the daytime.  
A ground-based MWR (manufactured by Radiometrics Corporation, model MP3000A) and weather station with intelligent weather sensors (manufactured by LUFFT measurement and Control Technology Co., LTD, model WS401-UMB) were adopted to measure zenith direction atmospheric information and near-ground rainfall information. In addition, the MWR is equipped with a variety of meteorological sensors to measure the atmospheric temperature, relative humidity, atmospheric pressure, and precipitation on the instrument surface. The existence of clouds is judged by measuring the infrared temperature in the zenith direction. MP-3000A MWR is a 35-channel microwave radiometer that consists of a temperature profile and water vapour profile subsystems. The temperature profile subsystem uses 14 high-frequency channels (51-59 GHz) for sky brightness temperature (BT) observation, while the water vapour profile subsystem accepts 21 low-frequency channels (22-30 GHz) for BT detection. In the present snowfall observation, 8 low-frequency channels were chosen for sky brightness temperature observation, namely, 22.234, 22.500, 23.034, 23.834, 25.000, 26.234, 28.000, and 30.000 GHz. The ground-based microwave radiometer completes a whole sky scanning observation every 3 min, with a detection range of 0-400 K and a resolution of 0.1. For the weather station, both liquid and solid precipitation can be identified with a sampling period of approximately 5 s, and its sampling accuracy of near-surface precipitation intensity is 0.1 mm/h. The spatial resolutions of the atmospheric profiles of the MWR at 0-500 m, 500 m-2 km and 2-10 km are 50 m, 100 m and 250 m, respectively.