Abstract:

Using surface and upper-air observations as well as the ERA5 hourly reanalysis data (0.25° × 0.25°), the study analyzes the causes of the first heavy snowfall event over the Loess Plateau on November 20, 2020, and investigates the lower-level cooling mechanisms responsible for the rain-to-snow transition during the initial phase of precipitation.Results show that the heavy snowfall event is supported by two moisture transport pathways, i.e., one from the southwest and the other from the southeast.The eastward merging and deepening of the westerly trough and plateau trough guide the eastward movement of the shear line, resulting in moisture convergence and uplift in northern Shaanxi province.The divergence of an upper-level jet stream further enhances the upward motion.Conditional symmetric instability characterizes the mid-atmosphere, and the cold front triggers the release of unstable energy, forming a slantwise secondary circulation along the front.Northern Shaanxi is located in the ascending branch of this circulation, with local maximum snowfall intensity exceeding 4 mm·h^-1.The rapid temperature drop near the surface during the initial precipitation phase is the key factor in the rain-to-snow transition, while the temperature forecast biases are identified as the main cause of errors in model-based rain-snow phase predictions.Lower-level abnormal cooling due to evaporative cooling facilitates the rain-to-snow transition, which deserves emphasis in associated predictions.

Key words: Frontogenesis function, Symmetric instability, Diabatic heating, Latent heat of vaporization