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一种垂直分层方案及其在地形复杂区域预报中的应用

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

  • 摘要: 为了提高CMA-MESO模式在高分辨率和复杂地形区域的预报能力,基于连续函数开发了一种适用于高度地形追随坐标的垂直分层新方案(简称xu方案)。该方案允许灵活设置垂直层数和模式顶高,并通过分层参数调整最低层高度。新方案稳定,经各种调节后,仍能保持分层厚度的连续、平滑和上疏下密的特点。新方案在CMA-MESO模式(6.0版本)进行应用试验,与现有业务分层(71层)方案(meso_71)对比,相同层数的新方案(xu_71)总体分布与业务方案相似,连续性更好。在批量预报对比试验中,评估了等压面要素的RMSE、2 m温度预报的RMSE及累积降水的TS评分,结果表明,xu_71方案略优于meso_71方案。进一步分析了分层参数对2 m温度预报的影响,发现稳定和超稳定下相似函数的不确定性会增加2 m温度的诊断误差,尤其是在最低层高度较大的情况下;相比于平坦区域,复杂地形更容易放大这种影响。通过新方案进行强降水预报的加密分层数值试验表明,加密至91层的xu_91方案在预报中效果较好,主降水区超过100 mm的预报结果最接近实况。与分层最疏的xu_51方案相比,xu_91方案预报出更深厚的上升区、更明显的垂直环流和配合更好的水物质分布,以及更高的边界层高度和更强的感热通量。总体来看,本研究的垂直分层方案效果较好,应用简便,还可结合物理参数化等研究进一步提升应用潜力。

     

    Abstract: To enhance the forecasting capability of the China Meteorological Administration Mesoscale (CMA-MESO) model in high-resolution domains and complex terrain, a vertical layering scheme (xu scheme) was developed. The scheme is designed for height-based terrain-following coordinates using continuous functions. It allows flexible adjustment of vertical layering and model top height and introduces a parameter b to control the bottom-layer thickness. The scheme is numerically stable and maintains continuous and smooth layer thickness, with denser layers near the surface. The xu_71 scheme (implemented 71 layers using xu scheme) was implemented in CMA-MESO V6.0 and showed characteristics similar to those of the meso_71 scheme (operational 71 layers scheme), but with better vertical profile continuity. Its performance was evaluated through batch forecast comparisons using key metrics, including the root-mean-square error on isobaric surfaces, 2-m temperature forecasts, and threat scores for accumulated precipitation. Results indicated that the xu_71 scheme slightly outperformed the meso_71 scheme across these key metrics. Additionally, the influence of b on 2-m temperature forecasts was analyzed using surface-layer parameterization, revealing that uncertainties inherent in stable similarity functions can increase diagnostic errors in 2-m temperature forecasts, especially when combined with thicker lowest model layer height (controlled by b) and complex terrains. Numerical analyses of dense layering during heavy rainfall forecasts showed that the xu_91 scheme (91 layers) performed better than coarser configurations. Specifically, the xu_91 scheme’s forecasts were closer to the actual observations for accumulated-precipitation areas exceeding 100 mm. Compared with the xu_51 scheme (51 layers), xu_91 predicted deeper updraft zones, more distinct vertical circulations, better vertical hydrometeors coordination, 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 in physical parameterization studies.

     

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