A source of simulation error for nocturnal heavy rainfall over the Sichuan Basin: The role of low‐level wind structures and dynamics

Abstract Accurate prediction of heavy precipitation over the Sichuan Basin (SCB) remains challenging due to the complexity of the surrounding mountains and their interactions with environmental airflow. Identifying key error sources in numerical weather prediction is therefore critical. This study investigates an orographic heavy rainfall event over the SCB that was largely missed by operational forecasting systems. Using multisource observations and reanalysis, we show that underestimation of precipitation is closely associated with insufficient low‐level convergence resulting from underestimated low‐level winds. A series of nudging‐based sensitivity experiments were conducted to assess the impacts of winds, temperature, and humidity on precipitation forecasts. Results indicate that the wind field exerts the dominant influence: wind nudging improves both wind and rainfall simulations, increasing SCB rainfall by nearly fourfold. Momentum analysis reveals that local variations in easterly winds are mainly governed by the Coriolis force acting on the ageostrophic winds, while northerly wind variations are determined by both the Coriolis force acting on the ageostrophic winds and frictional effects. Further regional wind nudging experiments demonstrate that the precipitation peak simulated by northeasterly wind nudging is delayed by nearly five hours, whereas easterly wind nudging captures the diurnal cycle of precipitation more accurately. These findings suggest that accurate prediction of the fine‐scale wind‐field structure is essential for improving forecasts of nocturnal heavy rainfall in the SCB.