39Hydro The Mechanism of Low-Level Jet Formation during the Rainy Season in the Yangtze River Basin in 2024

The low-level jets (LLJs) that appeared in the middle and lower reaches of the Yangtze River and Jiangnan area on June 28 were caused by a combination of an increase in the daily mean geostrophic winds and diurnal cycle in the ageostrophic winds. To be specific, a) On June 27, relative to the location of the Meiyu front and precipitation belt, there was a low-pressure (trough) to the northwest and a high-pressure (western Pacific subtropical high (WPSH)) to the southeast. On June 28, the WPSH enhanced while moving southward, meanwhile trough strengthened while moving eastward along and at the south side of the Yangtze River, leading to a significant decrease in geopotential height (>50 gpm) in central China. As a result, the air pressure within low-pressure area decreased more, while within high-pressure area reduced less, triggering the enhancement of southeast-northwest air pressure gradient, resulting in increased southwesterly geostrophic winds. b) The ageostrophic winds influenced by inertial oscillations (surface thermal differences) and other influences were predominantly southeasterly in the area south of the Yangtze River in China in late June. They were more pronounced in areas with higher topography. Along the Yangtze River and in the northern part of Jiangnan area, where the terrain is dominated by plains, LLJs were dominated by geostrophic winds, and ageostrophic winds located to the left of the geostrophic winds increased the southerly component of the LLJs. However, in the southern part of Jiangnan area, where the terrain is more complex, the ageostrophic winds were of comparable magnitude to the geostrophic winds, resulting in the LLJs turning southerly. The large values region of ageostrophic winds was mainly located in the overlapping zone of complex topography and significant warming in the daytime boundary layer on the previous day. This indicates that the daily variation of ageostrophic winds due to the interaction of topography with the thermal changes associated with the inertial oscillations is also an important factor in the formation of LLJs on the morning of the 28th