我国北方地区降水再循环率的初步评估

PRECIPITATION RECYCLING OVER THE NORTHERN CHINA

利用1976～1995年的20年月平均NCEP/NCAR再分析资料,对我国北方地区降水再循环率进行了评估,结果表明黄河流域总的降雨只有19%来自当地的水汽蒸发,其他81%降水来自外部的水汽流;黄河上游降水主要由来自青藏高原较强的水汽所提供,因此这里的降水再循环率小于15%。但黄河上游的降水蒸发后,在西风的输送下,为黄河下游提供了大量的水汽,通过汇合南北方向来的水汽,在黄河下游形成了降水再循环率达到30%以上的椭圆形区域。评估结果显示降水再循环率随季节变化差异较大,8月份的降水再循环率最高,达31%,而在11、12、1月份3个月的降水再循环率不到5%。结果还显示蒸发率和降水率的空间分布以及它们的年际变化都对应得很好,表明我国北方地区蒸发受降水影响较强。蒸发率、降水率和降水再循环率在20年中均有增加,表明气候变暖对我国北方地区的水汽循环的影响已经显现。

The precipitation recycling over the Northern China was evaluated using 20 years' NCEP reanalysis data. It is found that only 19% of the precipitation in the Yellow River basin comes from the local evaporation, while the other 81% comes from outside vapor. Precipitation over the upper reaches of the Huanghe River basin are mainly contributed by the strong vapor flow from the Tibet Plateau, so the recycling in the upper reaches is less than 15%. With the westerly winds, water vapor from evaporation in the upper reaches of the Huanghe River are brought into its lower reaches, combining with the moisture from the south and north, are contributed to the local precipitation, resulting in a elliptic region with high recycling rates over 30% in the lower reaches of the Huanghe River. The precipitation recycling varies obviously with seasons: it has the highest value of about 0.32 in August, while it does not reach 0.05 in November, December and January. Both evaporation and precipitation show good correspondence in spatial distribution and yearly variation, which reveals that evaporation is strongly influenced by local precipitation. Evaporation, precipitation and recycling ratios increase during the 20 years, which may be the effect of the climate warming.