The Enhancing Effect of Double Low‐Level Jets on Heavy Rainfall Over the Northern South China Sea in Autumn

Abstract

Low‐level jets (LLJs), including boundary‐layer jets (BLJs) and synoptic‐system‐related low‐level jets (SLLJs), are key factors influencing autumn rainfall over the northern South China Sea (NSCS). This study investigates LLJ characteristics and their effects on non‐typhoon heavy rainfall over the NSCS during September–October 2014–2023. Results show that LLJ‐related precipitation accounts for 61.6% of the total, including 26.0% associated with pure BLJ and 33.2% with double LLJs (BLJs coexisting with SLLJs). Both BLJs and SLLJs are characterized by a dominant geostrophic wind component, suggesting an important role of synoptic‐scale pressure‐gradient forcing in their development and maintenance. In pure BLJ cases, the jet is primarily driven by northeasterly winds from the southeast flank of the continental cold high (CCH). Maximum precipitation occurs over the offshore region southeast of Hainan Island, near the BLJ exit region, where boundary‐layer convergence favors convection. When double LLJs occur, the BLJs intensify and SLLJs form under the combined influences of the strengthened CCH and the active intertropical convergence zone near 12.5°N. The Annamite Range further strengthens the intensity of the SLLJs through the speed‐up effect. The SLLJ‐related divergence at 600–800 hPa and strengthened BLJ‐induced convergence together favor convection. Consequently, precipitation is concentrated over the offshore region south of Hainan Island, where the left side of the BLJ exit overlaps with the SLLJ entrance. This study clarifies the precipitation mechanisms under different LLJ conditions over the NSCS and highlights that double LLJs facilitate more intense precipitation over the offshore region south of Hainan Island.