六盘山地区空中水资源特征及水凝物降水效率研究

Study of the Characteristics of Atmospheric Water Resources and Hydrometeor Precipitation Efficiency over the Liupan Shan Area

为了利用人工增雨技术合理开发六盘山地区空中水资源,首先需要了解该地区水汽场、地形对当地降水的影响和空中水资源的特征及典型降水过程中云系的降水效率。本文采用欧洲中期天气预报中心(ECMWF)发布的高时空分辨率ERA5再分析数据集和中分辨率成像光谱仪(MODIS)数据,通过统计分析研究了该地区水汽的输送、地形强迫作用下的辐合抬升状况和地形云参量特征,并分别利用WRF模式数值模拟的输出结果和ERA5再分析数据,估算2016~2017年夏季自西向东移经该山区的多次混合降水云系的水凝物降水效率。研究结果表明:位于西北地区东部的六盘山地区具有较为丰沛的大气可降水量和更强的水汽输送。受亚洲季风影响,夏季偏南风向六盘山地区输送了丰沛的水汽,山区成为相对湿度高值区;春、夏、秋季午后山区云量(CF)达70%及以上,夏季云水路径(CWP)和云光学厚度(COT)均明显大于周边地区。在夏季降水过程中,地形引起的动力场对降水有明显的影响,在日降水量5 mm以上强度的过程中,气流遇迎风坡地形产生明显辐合抬升,且辐合抬升越强时降水强度越大。夏季典型降水系统中,山区水凝物降水效率平均约为48.1%,空中还有较大部分的水凝物未能成为降水。因此作为水源涵养地的六盘山地区夏季空中水资源相对丰富而降水量不足,空中水资源具有一定开发空间。

To develop atmospheric water resources reasonably over the Liupan Shan area based on precipitation enhancement techniques,it is necessary to first understand the influence of the water vapor field and topography on local precipitation,the characteristics of water resources in the air,and the hydrometeor precipitation efficiency of clouds in typical precipitation processes over this area.On the basis of the high spatial and temporal resolution reanalysis dataset issued by ECMWF(European Centre for Medium-Range Weather Forecasts)and MODIS(moderate resolution imaging spectrometer)data,the features of water vapor transport,the convergence and ascending motion of water vapor flux forced by topography,and the orographic cloud characteristics were investigated using statistical analysis methods.Moreover,during several precipitation events with stratus clouds embedded in convective systems moving from west to east in the summer of 2016 and 2017,the hydrometeor precipitation efficiency was calculated on the basis of the simulation results of the WRF model and ERA5 reanalysis dataset.The results are as follows.In the eastern part of Northwest China,abundant atmospheric precipitable water and strong water vapor transport are observed over the Liupan Shan area.Under the influence of the Asian monsoon,abundant water vapor is transported by the southerly wind to this mountain area in summer,thereby making it a high relative humidity zone.The CF(cloud fraction)reaches 70%over the Liupan Shan in the afternoons of spring,summer,and autumn.Moreover,CWP(cloud water path)and optical thickness(COT)in summer are considerably larger than those over the surrounding areas.The dynamic field caused by topography has a considerable impact on summer precipitation.Obvious convergence and ascending motion of water vapor flux forced by topography occurs in the process with daily precipitation exceeding 5 mm.Furthermore,when the convergence and ascending motion are more intense,the corresponding rainfall intensity is stronger.In typical precipitation systems in summer,the average hydrometeor precipitation efficiency is approximately 48.1%and a considerable amount of hydrometeor in the air does not undergo precipitation.Therefore,in terms of water conservation,the Liupan Shan area has relatively abundant atmospheric water resources;however,precipitation in summer is insufficient,which indicates that potential exists for developing atmospheric water resources in this area.