A Vertical Layering Scheme and Its Application in Forecasting Over Complex Terrain Areas

To enhance the forecasting capabilities of CMA-MESO in high-resolution and complex terrain areas, a layering scheme suitable for height based terrain-following coordinates was developed based on continuous functions. This scheme allows for easy adjustment of vertical layers and model top height, and includes a parameter b to adjust the bottom layer thickness. The scheme is stable and maintains continuous and smooth layer thickness, with denser layers near the surface. Tested in CMA-MESO V6.0, the new xu_71 scheme (71 layers) showed similar characteristics to the existing meso_71 scheme but with better continuity. Batch forecast comparisons evaluated RMSE of isobaric surface elements, 2-meter temperature forecasts, and TS for accumulated precipitation. Results indicated that xu_71 performed slightly better than meso_71. The influence of parameter b on 2 m temperature forecast was analyzed with surface layer parameterization, revealing that uncertainty in stable similarity functions can increase diagnostic errors, especially with thicker bottom layers and in complex terrain. Numerical experiments for dense layering in heavy rainfall forecasts showed that the xu_91 scheme (91 layers) had better performance, with forecasts closer to actual observations for areas exceeding 100 mm. Compared to the xu_51 scheme (51 layers), xu_91 predicted deeper updraft zones, more distinct vertical circulation, better water distribution, higher boundary layer heights, and stronger sensible heat fluxes. Overall, the designed vertical layering scheme is effective, easy to apply, and has the potential for enhanced application with physical parameterization studies.