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晴雨(雪) 和气温预报评分方法的初步研究
Preliminary Study on the Scoring Methods of Cloud-free Rainfall/Snowfall and Air Temperature Forecasts
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摘要: 利用全国逐日天气预报产品和对应实况数据, 分析了目前普遍使用的晴雨(雪) 和气温预报评分方法存在的问题, 并进行了改进尝试和研究。结果表明:由于没有考虑降水概率的影响, 在降水概率全国差异较大的多数月份, 晴雨(雪) 预报正确率与单站无降水频率表现为正相关, 具有无降水频率越大评分越高的趋势; 采用绝对标准值(1 ℃或2 ℃) 作为阈值进行气温预报准确率评分, 评分结果与气温日际变化呈明显负相关, 气温日际变化偏小则评分值偏高的趋势比较明显。该文提出的晴雨(雪) 和气温预报改进评分方法能有效减少降水概率和气温日际变化对晴雨(雪) 和气温预报评分的影响, 提高不同气候背景地区天气预报评分结果的可比性, 在天气预报质量检验和评估业务中具有一定的应用和推广价值。Abstract: Scientific and reasonable forecast scoring method is fundamental for evaluating the efficiency of the weather forecast objectively.By comparing forecast results with the corresponding observed data of 373 stations in China, the problems in existing forecast scoring method of the cloud-free rainfall/snow fall and temperature are investigated. Experimental amendments are made to the method, too. As a widely used method, forecast accuracy of the cloud-free rainfall/snowfall proves to be simple and practical in judging the effects of forecast to some extent.But without considering influence of rainfall probability, the forecast accuracy cannot distinguish blind prediction or persistent prediction accuracy effectively, and may bring abnormal high value even some mistakes.Skill-score of the cloud-free rainfall/snowfall forecast accuracy is positively correlated to the no-rainfall frequency at a single station during the months when regional rainfall probability difference is significant. In terms of temperature forecast, daily variation of the air temperature is a significant factor that affects the skill-score.When adopting 1℃ or 2℃ as the absolute standard value threshold, the skill-score of temperature forecasting is negatively correlated to the daily temperature variation.PF method and Index Threshold method are proposed in order to reduce the influences of rainfall probability and daily temperature variation.The results indicate the forecast score of precipitation by PF method is not closely correlated with rainfall frequency at single station during the months when regional rainfall probability difference is dominant in China. When adopting 2/3 or 1/2 as the index threshold, the linear regression coefficient between the daily temperature variation and the temperature forecast score can be significantly reduced.The correlation coefficient also decreases obviously with the value down to below 0.15, which is clearly lower than adopting the absolute standard value threshold. In other words, skill-score of temperature forecast using Index Threshold method is less sensitive to the daily temperature variation than using forecast accuracy method. Above all, the new methods proposed can effectively reduce the influences of rainfall probability and daily temperature variation on the skill-score of the cloud-free rainfall/snowfall and air temperature. It also improves the comparability of the weather forecast scores in the regions with different climate background. Therefore, it can be applied to the quality test and assessment on the weather forecast.
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图 1 全国373站2007年7月最低、最高气温预报评分值和气温日际变化之间的线性趋势
Fig. 1 Comparison of the daily changing of air temperature and the forecast scores of daily minimum/ maximum temperature of 373 stations over China in July 2007
表 1 预报次数n=60时不同降水次数a或无降水次数b的减分值d(d2, d3分别为m=2, m=3时的计算值)
Table 1 Relationship between the score-d and the different rainfall frequency (a or b) (d2 computed by m=2, d3 computed by m=3)
表 2 各评分方法统计结果与单站无降水概率相关系数检验
Table 2 The correlation coefficient of comparing rainfall frequency with the forecast score of precipitation by different method
表 3 不同阈值下最低、最高气温预报评分结果与最低、最高气温日际变化平均值直线回归斜率(A) 和相关系数(R)
Table 3 The linear regression coeff icient (A) and the correlation coefficient (R) between the daily changing of the air temperature and the forecast score of daily minimum/maximum temperature by adopting the different standard value as the threshold
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