Application of a Scale-Aware Gravity Wave Drag Scheme in High-Resolution Numerical Weather Prediction

As the resolution of numerical prediction systems increases, the subgrid-scale orographic gravity wave drag (GWD) and low-level blocking parameterization become less suitable. To address this issue, WRFv4.3 has introduced two additional orographic drag suites: small-scale GWD and turbulent orographic form drag, thus evolving into a scale-aware scheme. To evaluate the performance of this high-resolution model, application tests were conducted over North China. Three schemes were designed: a model with GWD shut down, a model with the original GWD activated, and a model with the new scale-aware GWD activated. The results reveal that the original GWD, which contains only subgrid-scale orographic GWD and low-level blocking without varying resolution, significantly affects wind at all levels in the model; however, the effect is not always positive. In contrast, the scale-aware GWD scheme minimizes the aforementioned factors to zero while maintaining small-scale GWD and turbulent orographic drag in the 3-km model. Therefore, the drag force exists only in the planetary boundary layer, and the low-level wind can be effectively improved. Statistical results show that the scale-aware GWD scheme can significantly decrease the positive bias and root mean square error of near-surface wind forecasts. However, for other variables, the improvement was not significant.