西安夏季降水的日变化特征研究

Study of the Daily Variation of Summer Precipitation in Xi'an

利用西安市1961—2010年夏季逐时自记降水资料，分析了西安市夏季降水日变化特征及其年代际变化趋势．结果表明：近50a来西安市夏季降水日变化在降水量和降水频次上呈现出不一致的变化特征，夏季降水日峰值出现在下午至傍晚，逐时降水量以15：00一20：O0时为高值区，以17：0（]时的累积降水量最大；逐时累积降水频次呈现出双峰型分布，以04：00—10：00时为降水频次峰值区，以17：0020：OO时为频次的次峰值区；降水强度以15：00—20：00时时段小时降水强度大，04：00—13：00时小时降水强度较小．降水持续时间超过6h的长持续性降水的最大降水量通常出现在清晨和午后，而持续时间在1～6h之间的短持续性降水易于在傍晚达到降水量峰值．

Variation characteristics of summer daily precipitation and its decadal trend are analyzed using the hourly recorded data during the summer from 1961 to 2010 in Xi＇an City. It is found that the amount and frequency of summer daily precipitation have inconsistent change in the 50 years from 1961 to 2010. Usually, the summer daily precipitation peak takes place between afternoon and evening with a maximum hourly rainfall amount between 15：00 and 20：00, a maximum hourly cumulative rainfall amount at 17：00, a secondary high value between 15：00 and 20：00 and a minimum between 22：00 and 03：00. The hourly cumulative rainfall frequency, however, differs from the cumulative rainfall amount as follows： the cumulative precipitation frequency has a doublespeak pattern with a peak between 04：00 and 10：00 and a peak between 17：00 and 20：00. Low frequencies are observed between 19：00 and 24：00, indicating that rain fall is more frequent during the day than at night. Therefore, the summer daily precipitation in Xi＇an is characterized by light rain during daytime （high frequency but small amount） and heavy rain in late afternoon （low frequency but large amount） with less frequency and amount at nighttime. Apparent decadal variations in the amount and frequency of the hourly precipitation are observed.