Impact of Orographic Gravity Wave Drag Parameterization on Northeast China Cold Vortex Heavy Rainfall Simulated at a Convection-Allowing Resolution

Abstract The Northeast China cold vortex (NECV) is a major weather system producing heavy rainfall in northern China, yet the influence of complex terrain, especially orographic gravity waves (OGWs), on such heavy rainfall remains poorly understood. This study investigates the impact of OGW drag (OGWD) parameterization on an NECV heavy rainfall event over the southern Yanshan Mountains on 6 July 2011, using the Weather Research and Forecasting Model at 3-km resolution. Results show that the OGWD parameterization can weaken the NECV circulation and diminish the orographic lifting and moisture transport over the southern slope of the Yanshan Mountains given the decelerated upslope flow. Therefore, the overestimation of the heavy rainfall intensity in the absence of OGWD parameterization was alleviated significantly, indicating the importance of OGWD parameterization even in high-resolution numerical models. However, the parameterization of OGWD introduced weak but widespread spurious rainfall ahead of the Taihang Mountains, as it decelerated the northwesterly downslope winds on the southeastern slope of the Taihang Mountains which enhanced the upslope moisture transport. This spurious rainfall was mitigated significantly when using a revised OGWD scheme accounting for the nonhydrostatic effect (NHE) on the surface momentum flux of vertically propagating OGWs. The NHE more notably attenuated the OGWD over the Taihang Mountains than over the Yanshan Mountains, which strengthened the NECV northwesterly flow downgliding the Taihang Mountains and inhibited the moisture transport.