Sensitivity of Coastal Rainfall Simulation to the Assimilation of Wind Profiles Near the Marine Boundary Layer Jet: An Observing System Simulation Experiment

An Observing System Simulation Experiment was conducted to assess the impact of assimilating oceanic wind profiles on the simulations of warm‐sector heavy rainfall along the South China coast. This rainfall event was strongly influenced by an upstream boundary layer jet (BLJ) over the northern South China Sea (NSCS). Using a 21‐member WRF ensemble initialized with Global Ensemble Forecast System data, the best‐performing member was selected as the nature run. Simulated wind profiles over the NSCS from the nature run were assimilated into the worst‐performing member at the initial time step, improving the representation of the BLJ structure and strength. The data assimilation experiments reveal a significant sensitivity of simulated rainfall intensity and spatial distribution to both the number and location of assimilated wind profile observations. A strengthened marine BLJ with a more southerly wind component extending toward the coast enhanced low‐level convergence, upward motion, and moisture transport at its terminus, thus creating a more favorable environment for convection. However, momentum budget analysis indicates that, despite wind profile assimilation, the nearshore BLJ remained too weak to sustain upscale convective growth following convective initiation.