T213L31全球模式云和对流参数化方案改进试验

Forecast experiments of revised cloud and convection schemesin NMC T213L31 global model

中文关键词:  T213全球模式，云和对流参数化方案，降水

英文关键词:T213L31 global model, Cloud and convective parameterization schemes, Precipitation

中文摘要:

      针对中国业务中期数值预报模式T213对中雨量级以上的降水预报空报比较明显的问题，文中对此模式预报的降水进行了诊断分析。发现T213模式预报的总降水分布主要是由可分辨尺度降水决定的，且在降水偏多最明显的地区，可分辨尺度的降水即超过或达到了观测的总降水，表明降水空报的主要原因在于可分辨尺度降水偏多。可分辨尺度降水偏多的可能原因有：土壤湿度初始化、云变量的初始化和直接产生降水的云与对流参数化方案存在一定的缺陷。鉴于前两者是目前国际上的难点，文中针对第3个方面的可能原因进行了分析和相应的改进。包括在对流方案之前增加一次云方案的调用；对流参数化方案的闭合由“动力型”改为对流有效位能调整闭合；更复杂的对流触发机制；改进冰沉降和降水通量计算。改进的主要目的是使对流参数化方案更活跃，从而减少格点尺度对流的发生。采用改进的方案，进行了敏感性试验和2005年夏季的连续滚动同化预报试验，并与中国区域400个标准站的降水观测和GPCP的全球降水观测进行了比较。结果表明，改进的方案无论是对中国区域还是全球夏季平均的降水分布预报都好于业务，但四川省和赤道东太平洋降水偏多的问题依然存在。中国区域的降水统计检验还表明，除小雨外，其他量级在大部分时效上降水的TS评分增加，预报偏差降低。

英文摘要:

      In order to solve the obvious underestimation of moderate to extraordinary heavy rainfall in the T213L31 operational global model of China, this paper diagnoses and analyzes the precipitation forecast performance of the model in 2005. The results show that the predicted total precipitation is largely composed of grid scale rainfall which can reach and even exceed the strength of observed total precipitation in regions where overestimation is the most remarkable. This implies that the main reason for inaccurate precipitation forecast is the overestimation/underestimation of resolvable/unresolvable scale rainfall. Causes for overestimation of resolvable scale rainfall include flaws in the initialization of soil moisture and cloud related variables, and in the cloud and convective parameterizations which directly produce precipitation. This paper is devoted to analyzing and improving the cloud and convective parameterizations. Specific measures include: adding one more calling of the cloud scheme before the convection scheme is called; changing the convective closure from “dynamical” to “Convective Available Potential Energy (CAPE) adjustment”; using more complex convection triggering mechanisms; and improving calculations of ice sedimentation and precipitation flux. The main purpose is to make the convective scheme more active, thus reducing the grid scale convection. With these improvements, sensitivity experiments and continuous assimilation and forecast experiments for the summer of 2005 are carried out, and the results are compared with rainfall observations at 400 standard stations in China and GPCP global observations. Comparative results show that revised schemes have a better capability than the original operational schemes in forecasting both China and global summer rainfall, except that the overestimation of rainfall still exists in the Sichuan Province and the equatorial eastern Pacific. In addition, statistical verifications of precipitation forecasts also indicate that the threat scores of precipitation forecasts for most rainfall categories except light rainfall, are increased, and biases are reduced, for most of the forecast leading times.