降水观测误差修正对降水变化趋势的影响

Effects of the Bias-Correction on Changing Tendency of Precipitation over China

依据中国气象局国家气候数据中心提供的660个气象站1951—2004年逐日气温、降水、风速资料,以及雨雪类型资料,在乌鲁木齐河源降水误差对比观测试验基础上,对我国的降水资料进行了系统的修正计算.这一修正不仅使降水量在数量上增加,更接近实际值,同时由于相关气象要素的变化,还会对降水的长期变化趋势产生影响,通过对比分析原始观测降水系列和修正后的降水系列的长期变化趋势,结合理论分析估计这一影响的大小.结果表明:我国观测和修正后的全国面积加权平均降水量分别为565.2 mm和654.9 mm,平均修正量为89.7 mm(16%);修正后的降水系列,绝对趋势大于实际观测系列趋势,而相对趋势则小于观测系列.全部台站统计结果表明,实际的降水变化趋势被低估了6%,而相对变化趋势被高估了约10%.同时,由于我国大部分地区风速呈减少趋势,导致降水观测误差减小,结果实际的降水变化趋势远小于观测资料显示的趋势值,1955—2004年全国实测降水的变化趋势为-1.9 mm.(10a)-1,而修正后降水的变化趋势高达-6.0 mm.(10a)-1.目前依据实测的降水观测资料得出的降水变化趋势远远小于实际的变化趋势.

In this study, long-term monthly climatic data during 1951-2004 at 660 meteorological stations over China have been analyzed to examine the impact of bias correction on the changing tendencies of monthly and yearly precipitations. The annual area-average precipitation over China is about 565.2 mm, and the corrected one is 654. 9 mm, the corrected amount is 89.7 mm. The impact of bias correction on the tendency depends on the gauge catch efficiency, and the changes of wind speed, temperature, snow percentage, and number of trace and measurable precipitation days. The changing tendencies of corrected and measured monthly and yearly precipitations have similar regional patterns over China, although the tendencies are different quantitatively over some regions. Measured annual precipitation changing tendencies over China are -105 - ＋137 mm · （10a）^-1 （-11.75-22.9%·（10a）^-1）, and the corrected annual precipitation changing tendencies are -142 -＋146 mm· （10a）^-1 （-13.0-17.9% · （10a）^-1） ; On the other hand, the relative changing tendencies have been reduced generally due to the bias correction, because correction increases the mean precipitation. Bias correction enhances absolute monthly precipitation changing tendencies in every month by about 5 to 25%, leading to a 6% overall increases of yearly precipitation changing tendencies over China, while the relative annual precipitation changing tendencies have been underestimated about 10%. The area-average measured precipitation changing tendency over China is only -1.9 mm· （10a）^-1 during 1955-2004 and the area-average corrected annual precipitation changing tendency is up to -6.0 mm·（10a）^-1. The annual precipitation changing tendency has been strongly underestimated based on the measured precipitation data. The changing tendencies mentioned here are large enough to affect regional climate and hydrology analyses. More efforts are needed to improve the study of the impact of bias correction on precipitation change over other large countries and regions.