Analysis on the Meteorological Causes and Environmental Characteristics of a Snowstorm Event in the Dadu River Basin

A heavy snowstorm process in the reaches of the Dadu River basin in late February 2016 was analyzed. Through synoptic diagnosis and environmental meteorological parameter refinement, it was revealed that a strong cold air outbreak is a precursor of heavy snowfall in the upper reaches of the Dadu River basin, which can be predicted 2–3 days in advance. The occurrence of a blizzard requires the cooperation of cold air with warm air. For the Dadu River basin, the primary moisture sources come from two paths: 1) The plateau shear line formed by a meeting of cold air with warm and wet air in front of the south trough, which is the dominant synoptic system at the middle level in the upper reaches of the Dadu River basin. 2) The warm and humid air stream that enters the northwestern part of the Sichuan Basin at a low level along the east of the Hengduan Mountains, providing sufficient water vapor conditions for snowfall in the region. The ascending motion driven by the dynamical configuration with convergence at the low level and divergence at the high level lifts the water vapor from the low level to the upper level. The water vapor then directly sublimates into snowflakes because of the cold environment, falling in the upper reaches of the Dadu River basin. The temperature and humidity conditions during this heavy snowstorm process can be summarized as follows: The temperature at 700 hPa is not higher than −5°C, the temperature at 500 hPa is not higher than −9°C, and the surface temperature is not higher than 0°C. The relative humidity of the ambient atmosphere is higher than 70%, and the moisture content in the air reaches above 4 g kg⁻¹.