一个相对精确计算沙瓦特指数方案的实现

在对流性天气的预报工作中，沙瓦特指数是一个被广泛应用的物理量，准确、快捷计算该指数一直以来都具有重要的实际意义。根据干、湿绝热过程相当位温守恒的原理，运用David Bolton给出的系列公式计算凝结温度、相当位温，利用计算机编程实现了应用探空资料自动、准确计算沙瓦特指数。11个实例的计算表明，与其他5种沙瓦特指数计算方案相比，新方案计算简洁、绝对误差的平均值与最大值均有所减小。其可能的原因是，计算中间过程避免了繁琐的数学推导和多次近似处理，且抬升凝结温度、相当位温等重要物理量计算比较准确。利用2010-2011年湖北省汉口站MICAPS格式探空资料，对比了查表法与新计算的沙瓦特指数对有、无雷暴天气的预示作用。结果表明：如果规定该指数小于零时有雷暴，则计算得出的数据可以较好地减少雷暴漏报。文中还指出了应用该指数时的局限性。

A method for accurate calculation of Showalter index

Since Showalter index is widely applied in convective weather analysis and forecast, it is meaningful to calculate it accurately and rapidly all the time. Based on the rule that equivalent potential temperature is unchangeable during both dry and moist processes, condensation temperature and equivalent potential temperature were calculated using a series of equations provided by David Bolton. Showalter index was computed automatically and accurately by a program in terms of radiosonde data. 11 cases were selected to calculate Showalter index using the new method. Compared with other 5 methods, the new method has characteristics of simple calculation, less average and maximum absolute errors. The possible reasons included two aspects. Many complex derivations and approximation treatments have been avoided. On the other hand, the deduced figures such as the lifting condensation level temperature, the equivalent potential temperature were more accurate than that from other methods. Based on the radiosonde data with MICAPS format from 2010 to 2011at Hankou station in Hubei province, the forecast accuracy of two methods, namely, a table method and the new method in this paper, was compared for thunderstorm. The results show that the missed number of thunderstorm is reduced if a negative Showalter index is used as a factor to show the possible presence of thunderstorm. In addition, a limitation of the new method is also discussed.