Terrain‐Induced Contrasts in the Diurnal Variations of Low‐Level Jets Over the Middle Reaches of the Yangtze River During the 2010 Mei‐yu Field Campaign

Abstract The diurnal cycle of low‐level jets (LLJs) plays a crucial role in severe convective weather during the Mei‐yu season over the middle reaches of the Yangtze River (MRYR). However, previous studies have focused on individual sites or large‐scale climatic contrasts, leaving sub‐regional variations in LLJ diurnal behaviors and their governing mechanisms insufficiently understood. Using intensive radiosonde observations from three topographically distinct sites during the 2010 Mei‐yu field campaign, combined with ERA5 reanalysis, we partially address this knowledge gap. Results show that (a) LLJ occurrence exhibits a single peak between 1.5 and 2.5 km AGL in terrain‐affected regions, dominated by synoptic‐system‐related LLJs (SLLJs) with higher core altitudes, whereas a double‐peak structure appears over plains, where both boundary‐layer jets (BLJs) and SLLJs frequently occur. (b) In terrain‐affected regions, SLLJ frequency and intensity peak at night, governed by nocturnal inertial oscillation with deeper boundary layers. In contrast, over plains, SLLJs peak during the daytime due to northward advancement of the Western Pacific Subtropical High (WPSH). Nocturnal BLJs over plains are influenced by both inertial oscillation and WPSH westward extension. (c) In terrain‐affected regions, SLLJ core heights descend from 20:00 to 02:00 LST and then ascend, modulated by the boundary layer via capping inversions that inhibit vertical momentum transfer and maintain a strong momentum layer aloft. Over plains, this descending phase persists until 08:00 LST, associated with intensifications of Mei‐yu fronts at 800 hPa. Derived from a single Mei‐yu period, results may not be representative of multi‐year climatological conditions. These findings advance dynamical understanding of terrain‐induced contrasts in LLJs over the MRYR and provide a benchmark for evaluating numerical simulations.