1.上海市浦东新区气象局，上海 200135;2.云南大学大气科学系，昆明 650091
1.Shanghai Pudong Meteorological Bureau, Shanghai 200135;2.Department of Atmosphere, Yunnan University, Kunming 650091
本文通过对1980～2013年华东区域447站各等级降水的时空分布特征进行分析，从低空气温、水汽、相对湿度入手，采用经验正交函数分析（Empirical Orthogonal Function，简称EOF）、合成分析等诊断方法揭示小雨与低空气温、水汽、相对湿度之间的关系；根据克劳修斯-克拉珀龙（Clausius-Clapeyron）方程，从物理角度对温度和水汽含量变化对小雨减少的相对贡献进行分析，并选取长三角区域利用能见度资料分析小雨减少的特征。研究主要得出以下结论：（1）华东区域1980～2013年总降水日和总降水量分布呈现南多北少随纬度增加递减趋势，总降水日主要以小雨日为主，小雨日占总降水日的71.84%。除暴雨外，其它各等级降水均呈现出减少趋势，以小雨的减少最为显著，小雨日和小雨量区域平均减少趋势为－3.37 d (10 a)-1、－6.52 mm (10 a)-1。小雨日EOF分解的第一模态呈现了华东区域小雨日一致减少的分布特征；第二模态则体现了小雨日变化的不均匀性，具有明显的区域差异。（2）小雨日的减少是伴随着低空增温发生的，低空温度上升了0.32 K (10 a)-1，同时可降水量呈现微弱的减少趋势。小雨日的变化与低空相对湿度有很好的正相关关系，EOF分解结果也揭示了低空相对湿度具有与小雨一致的空间分布特征。合成分析表明低空气温偏高（低）年，小雨日偏少（多），可降水量偏多（少）年，小雨日偏多（少），相对湿度偏高（低）年，小雨日偏多（少）。（3）根据Clausius-Clapeyron方程和相对湿度公式，低空增暖使饱和水汽压增加6% K-1～7% K-1，饱和水汽压增加在水汽微弱减少的前提下，导致了低空相对湿度的显著减少，从而导致了小雨的减少。对低空温度，比湿变化对小雨的影响进行分析，发现单纯由温度变化引起的相对湿度减少为－4.83%，而单纯由于比湿变化引起的相对湿度变化为－1.91%，分析表明了低空增暖是引起小雨日减少的主要原因。（4）能见度较差的区域，年均小雨日和小雨量偏少，但是小雨的长期减少趋势主要还是由低空增暖引起的相对湿度的减少导致的。
Daily precipitation data from 447 stations in eastern China were used to discuss the spatio-temporal characteristics of different grades of precipitation during the period of 1980-2013. Correlations between light rain and lower tropospheric temperature (LTT), lower tropospheric water vapor content (LTW), and lower tropospheric relative humidity (LTRH) were subjected to EOF and composite analyses. The contributions of warming and the variation in relative humidity to the decrease in light rain were clarified by using the Clausius-Clapeyron equation. The characteristics of the reduction in light rain over the Yangtze River Delta were also analyzed on the basis of daily visibility data. The following results were obtained: (1) Annual mean precipitation days and precipitation amount over eastern China declined with increasing latitude, and light rain days (LRD) provided the major contribution to all rainfall days, and accounted for 71.84% of all rainfall days. Different grades of rain, except for storms, all tended to decrease, and LRD and light rain amount (LRA) distinctly decreased at the rates of -3.37 d (10 a)-1 and -6.52 mm (10 a)-1, respectively. The first mode of EOF for LRD showed the reduction of LRD over the whole study region, whereas the second mode showed distinct spatial differences. (2) The reduction in light rain was accompanied by lower tropospheric warming. LTT increased at the rate of 0.32 K (10 a)-1. By contrast, LTW slightly decreased. The positive correlations of the variation in light rain with LTRH indicated that the spatial characteristic of LTRH was similar to that of light rain revealed by EOF analysis. The results of composite analysis suggested that light rain was low (high) in the year of high (low) LTT, and high (low) in the year of high (low) LTRH and high (low) LTW. (3) Calculations obtained by using the Clausius-Clapeyron equation and the relative humidity equations revealed that the saturation vapor content increased by 6% K-1 to 7% K-1 as a result of lower tropospheric warming, which induced the distinct decrease in the LTRH given the negligible change in lower tropospheric specific humidity and ultimately reduced light rain. The effects of LTT and lower tropospheric specific humidity on the changes in light rain suggested that the 4.83% reduction in light rain was caused by LTT only, whereas the 1.91% reduction in light rain was caused by lower tropospheric specific humidity only. (4) Although annual mean LRA and LRDs were considerably low in regions with low visibility, the long-term reduction in light rain induced by the decrease in LTRH was attributed to lower tropospheric warming.