【摘要】 为了更好地理解西北干旱区大气混合层(ML)厚度的变化特征及其对当地干旱气候形成的影响,我们利用张掖和民勤站夏季及相关月的实测探空资料及T-logP图解法,首先计算了该两地逐日ML厚度,然后分析并讨论了它的时空间变化特征、与干湿天气气候的关系,以及夏半年的深厚ML对加剧当地干旱气候的影响。结果表明:(1)河西中东部ML厚度的年变化及地区差异明显。冬季最浅薄,夏半年深厚(特别是5、6月),4月及10月分别是ML急剧增厚及变薄的过渡期;同时,更靠近西北干旱区中心的河西西部及北部的ML更深厚。(2)夏季干(湿)天气通过加强(减弱)地气间的感热交换和干对流,而明显影响当地的ML厚度。平均而言,以高温日最深厚,干日次之,小雨日再次之,而中强雨日最浅薄。干年夏季的ML厚度平均比湿年的对应值增厚300 m左右。夏季典型干日的ML厚度比雨日厚3000 m,典型干日的ML厚度昼亱变化不大。(3)反过来夏半年深厚的ML也通过增加雨滴蒸发损耗,减少了干旱区的降水,加剧了当地干旱的程度,因此夏半年深厚的ML也是形成干旱气候背景的成因之一。更多还原

【Abstract】 To better understand the features of distribution and variation of the atmospheric mixed layer depth(hereafter ML) and the impact of the deep ML on the dry climate over Northwest China(NWC),in this paper,the ML thickness,its change,and the impact of it on the NWC dry climate formation have been calculated and analyzed,utilizing the observed sounding data in Zhangye and Minqin in summer and related months in more years.The main conclusions are as follows:(1) There are clear annual changes and inter-local differences for the ML thickness in Zhangye and Minqin.On the average,the ML depth is the shallowest with just less than 1 km in winter;the deepest with 3~4 km,in summer half year(especially in May and June before the local relative rainy period).The ML depth is much higher than that in humid-and semi-humid areas in East China,and also is equivalent to or even higher than that in Qinghai-Xizang Plateau in summer.Further,the ML depth are higher in far north and west in Hexi Corridor of Gansu,where are near to the center of dry area of NWC.(2) The dry(wet) weather has largely influence on the ML depth in arid area over NWC summer through intensifying(weakening) the exchange of sensible heat and dry convection.The statistic shows that the deepest averaged ML depth can be as high as 3413 m on the high temperature days;the next with 2838 m,on the dry days;the third with 2261 m,on the light rain ones;the shallowest with just 1942 m,on the medium-and strong-rain days.Consequently,in comparison to wet summers,the ML depth in dry summers is about 300 m highers.(3) Not only the dry and wet weather in dry area summer can considerably influence the ML depth,but also through the evaporation depletion of rain drop,the deep ML over the arid land in summer half year reduce,in turn,precipitation and intensify its dry climate.Thus,the deep ML over arid area in summer half year is one of factors of NWC drought climate formation.Some existed evidences show the conclusions above have the representative for entire NWC(even China-Mongolia) dry land for whole summer half year(May~September,not just June-August).Certainly some further investigation is still needed as well.更多还原