Analysis on the causes of a continuous clear-sky gale event in winter in the Three Gorges Dam Area

Abstract

A continuous clear-sky gale event (hereinafter referred to as "21·1" clear-sky gale in the dam area) occurs in the Three Gorges Dam Area in the middle January of 2021, which seriously affected the navigation safety of the Yangtze River near the dam area. Using routine upper-air and surface observations, mesoscale observational data and other data, we have applied the weather analysis and dynamic diagnostic methods to analyze the spatial-temporal distribution characteristics and the causes of "21·1" clear-sky gale in the dam area. The results show that this clear-sky gale event occurred in the valley under the leeward slope, and there are four wind speed fluctuations during the whole event, whose wind speed at night is greater than that in the daytime. The ground gale is caused by the momentum downward-transportation in the boundary layer, which is derived from the low-level jet. The aforementioned descending motion causing the ground gale is related to the lee wave with large amplitude. During this clear-sky gale event, the steady vertical wind shear and inversion layer in the boundary layer make the lee wave last for a long time, and the greater vertical declining rate of temperature under the inversion layer further strengthens the descending motion, which is the main reason why the "21·1" clear-sky gale in the dam area forms and continues.

Keywords

• clear-sky gale
• momentum downward-transportation
• leeward wave
• inversion layer
• three gorges dam area