一种基于多物理量阈值定义的热带气旋生成预报方法及在ECMWF全球模式中的应用验证

TROPICAL CYCLONE GENESIS FORECASTING BASED ON THRESHOLDS OF MULTIPLE PHYSICAL PARAMETERS AND VERIFICATION OF PERFORMANCE USING ECMWF MODEL

对于热带气旋的路径与强度预测，数值天气预报模式提供了相当有用的参考或指导预报。为进一步了解数值模式对掌握热带气旋生成的特性，基于近年国内外对热带气旋生成预报的研究结果，构造了一套热带气旋生成的多阈值组合判据系统，并利用该判据和ECMWF模式资料对2013-2015年于西太平洋海域生成的热带气旋进行了预报试验。结果表明，对于热带气旋生成的判据，使用不同百分位的历史统计阈值会对预报效果造成显著影响。在特定区间对某个物理量阈值进行合理的调整可以有效改善预报效果。在西太平洋海区，850hPa相对涡度阈值的微幅调整对结果的影响最为敏感。最优阈值组合方案的预报命中率检验结果表明，在数值模式起报时间为低压生成时间的前12—72小时时效曲线上，24和48小时时效是拐点，24小时时效之前，命中率基本维持在0．7以上的高位，随预报时间的降幅很小；之后预报质量陡降，48小时时效后预报效果趋向稳定。此外，不同年份的预报效果存在明显差异。在西太平洋不同海域，ECMWF模式对热带气旋的生成预报质量差异明显。菲律宾以东的洋面在不同起报时间的预报能力明显高于南海海域。西太平洋海区存在两个明显的高错报率区域：一个位于南海中、北部至菲律宾群岛近海，一个位于西太平洋中部。在不同大洋之间，ECMWF模式的热带气旋生成预报性能也存在显著分别。起报时间在热带气旋生成前24小时内，西太平洋海区对热带气旋生成的预报要明显优于大西洋海区。

Numerical weather prediction （NWP） models provide useful reference and guidance in the track and intensity forecasts of tropical cyclones （TC）. This study reviews the recent research results and derives a set of criteria for the formation of tropical cyclones using historical data in order to understand the characteristics of TC genesis forecast by NWP models. Verification of the criteria for the western North Pacific based on ECMWF model data between 2013 and 2015 are conducted. The results indicate that the performance of algorithm is significantly affected by the percentile values taken as the criteria thresholds. It can be improved by adjusting the thresholds of certain fields in certain ranges of values. In the western North Pacific, the forecast results are most sensitive to small changes in the relative vorticity on the 850 hPa level. The combination of parameters and thresholds are optimized for the overall forecast performance. In terms of the hit rate in the 12 - 72 hours prior to the formation of TC, the turning point of performance lies between 24 and 48 hours. For lead time less than 24 hours, the hit rate maintains around 0.7 or above and does not fall appreciably. However, the performance shows a sharp drop before stabilizing beyond 48 hours. The results vary significantly from year to year and also from one geographical region to another. The algorithm performs far better for the TC forming to the east of the Philippines than that over the South China Sea. In contrast, high false alarm rates are found in the northern and central parts of the South China Sea up to the waters around the Philippine islands and the central part of the western North Pacific. Such a discrepancy in ECMWF＇s performance can also be observed between different basins-the forecasts within the 24 hours prior to the genesis of a tropical cyclone for the western North Pacific verify better than those for the Atlantic.