高分辨率土壤湿度状态对南非上空对流开始的短期数值天气预报的影响
The Impact of High-Resolution Soil Moisture States on Short-Term Numerical Weather Prediction of Convective Initiation over South Africa
摘要：
地球表面与大气之间的相互作用在积云对流的形成中起着关键的作用。在陆地表面，解决陆-气相互作用的一个必要的边界条件是土壤湿度。这项研究的目的是双重的。一个，通过面向对象的和传统的验证技术，确定如何更高的分辨率土壤湿度初始条件影响对流开始（CI）的位置和时间内的对流允许，业务数值预报模式在南非的预测。二是研究模拟的午后雷暴过程中CI与土壤湿度的关系。该研究报告的结果，从66统一模式模拟（在4.4公里网格分辨率）的9个夏季下午CI事件在天气良性条件下在南非。更高分辨率的土壤湿度条件使观测到的风暴和预报的风暴之间的质心距离平均减少了7公里（提高了9%），质心距离的最大减少发生在最短的提前时间，减少了12公里。定位误差的大部分改善发生在纬向方向。然而，很少或没有差异，发现在CI的时间，最有可能是由于模型网格大小CI时间的主导作用，掩盖了土壤水分异常的影响。CI的概率是最高的干旱和中度土壤和地区沿不同的土壤水分梯度。得出的结论是，模拟CI在南非优先发生在周边的湿土壤水分补丁，那里有风和较高的感热通量的表面辐合增加。
英文摘要：
The interaction between the Earths surface and the atmosphere plays a key role in the initiation of cumulus convection. Over the land surface, a necessary boundary condition to consider for resolving land-atmosphere interactions is soil moisture. The aim in the study is twofold. One, through object oriented and traditional verification techniques determine how higher resolution soil moisture initial conditions influences the prediction of the location and timing of convective initiation (CI) within a convective permitting, operational NWP model over South Africa. Two, to study the modelled CI-soil moisture relationship during real afternoon thunderstorm events. The study reports the results from 66 Unified Model simulations (at 4.4km grid resolution) for nine summer afternoon CI events during synoptically benign conditions over South Africa. The higher resolution soil moisture conditions reduce centroid distance between observed and forecast storms on average by 7km (9 percent improvement), with the most decrease in centroid distance occurring at the shortest lead times, by 12km. Most improvement in location error occurs in the zonal directional. However, little to no difference is found in the timing of CI, most likely attributable to the dominant effect of model grid size on CI timing, overshadowing the influence from soil moisture anomalies. Probability of CI is highest over dry and moderate soils and areas along distinct soil moisture gradients. The conclusion is that modelled CI over South Africa preferentially occurs on the periphery of wet soil moisture patches, where there is increased surface convergence of wind and higher sensible heat flux.
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