Multiscale Processes Modulating the Duration of Extremely Persistent Heavy Rainfall: A Comparative Study of Two Winter Events

Abstract Previous studies have paid little attention to how multiscale system interactions affect the maintenance of extremely persistent heavy rainfall (EPHR) events. Using observational analysis and numerical simulations, this study explores the main reasons for the differences in the duration of two EPHR events that occurred in similar regions, at similar times, and under comparable low‐level background circulation, but with different durations. Both EPHR events took place in the afternoon over southeast Hainan Island. The development of strong convection is closely related to the convergence between the northeasterly offshore boundary layer jet (BLJ) and the northerly bypass flow generated by the topographic blockage of the Wuzhi Mountain. In Case 1, the offshore BLJ undergoes constant clockwise deflection, gradually shifting from northeast to southeast winds, which enables the strong convergence zone to persist for an extended period. However, in Case 2, the BLJ undergoes constant counterclockwise deflection, evolving from northeast to north winds, which causes the strong convergence zone to gradually disappear. The difference in the direction of deflection of the offshore BLJ is the key reason for the variation in duration between the two EPHR events. Diagnostics revealed that the differences in offshore BLJ deflections are primarily caused by variations in meridional pressure gradient forces. These are influenced by differences in divergence and latent heat release associated with the upper‐level circulation system. The results of this study are crucial for helping forecasters gain a deeper understanding of the maintenance mechanisms of local EPHR events and improve the forecasting capability for local extreme precipitation.