Analysis of geostrophic adjustment to frontogenesis conditions during a heavy snowstorm in the mountainous area of southern Anhui Province

Abstract

Geostrophic adaptation is another frontogenesis mechanism that forms a stationary front, which differs from the balanced front, but often leads to local catastrophic weather in the mountainous area of southern Anhui. Conducting analysis and research on the frontogenesis conditions of geostrophic adaptation is of great significance for how mountainous terrain causes catastrophic weather. Taking the formation of a stationary front on 22 February 2022, which resulted in a local blizzard as an example, the quasi-geostrophic frontogenesis function of topographic frontogenesis is calculated and analyzed based on the following dataset, including the high-resolution hourly reanalysis data from the European meteorological center ERA and conventional data from ground stations, the new generation Doppler (CINRAD/SA) weather radar data, the 3-D wind field data obtained with a 3-D wind field inversion technique and the interpolation method with the accuracy of 0.02°×0.02°. The results are as follows. (1) During this frontogenesis process, the northeast wind flow on the ground intensified, and the gradient of the flow function increased, resulting in discontinuous fluid density at the foot of the leeward slope (30.5°—31.0°N), which leads to geostrophic adjustment to frontogenesis in a narrow area at its front. Thus, a local snowstorm with a small scale and short duration occurred. The geostrophic adjustment to frontogenesis occurs in a stable atmospheric structure and an environment of wet symmetry instability. (2) By analyzing the four terms of the surface frontogenesis function and their related factors, it is shown that the contribution of vertical motion tilt term to frontogenesis is dominant, while the horizontal deformation and convergence have minor contributions. It indicates that the geostrophic adjustment to frontogenesis is mainly the result of the upward transportation of potential temperature, rather than the deformation of the wind field. (3) By analyzing the four terms of the high-altitude frontogenesis function and the mesoscale wind field retrieved by radar, during the adjusting of the wind field from a non-equilibrium state to an equilibrium state, a strong secondary circulation appears in the middle and lower layers of the atmosphere at an altitude of 1.5-3.0 km above the front zone, which leads to the cooling due to the rise of cold air over the cold zone. While above the warm zone, non-adiabatic heating released by snowfall condensation latent heat causes warming, resulting in an increase in the temperature gradient at the front zone and frontogenesis.

Keywords

• geostrophic adjustment frontogenesis
• frontogenesis function
• terrain
• secondary circulation