中文摘要：

为进一步检验东南沿海、孟加拉湾、青藏高原、两风带等4个水汽源区对我国西北区夏季降水影响的相对重要性，本文利用MM5模式，以1979年7月为模拟对象，以NCEP再分析值作初、侧边界条件，继本文上篇作了该模式对该月的模拟性能检验后，又分别作了检验减、增各源区水汽影响的11个敏感性试验。下篇的结果表明：（1）夏季，东南沿海源区减（增）水汽后，主要通过西伸的西太副高西南侧的东南风和西侧的偏南风气流的“干（湿）平流”输送，第2天将直接影响西北区东部；继之，若遇河西走廊低层较强偏东风，前述“于（湿）平流”还可再继续影响河西及南疆降水的减（增）。即东南沿海是影响西北区的主要水汽源区。（2）孟湾源区水汽的减（增）变化常经北路和东路共同影响西北区的降水。它主要影响青海及河西走廊中、东部的降水，也间接地影响西北区东部及南疆的降水。（3）高原（特别是西风带）源区因本身水汽少，它们的影响要小些。高原源区主要影响青海及河西中、东部的降水，也部分影响西北区东部和南疆的降水；而西风带区主要影响北疆及南疆的降水。（4）作西北区夏季降水预报时，要关注其上游孟湾，特别是东南沿海源区水汽、沿途天气系统及低空急流的变化。

英文摘要：

To objectively test the relative importance of the impact on summer precipitation in Northwest China （NWC） of the water vapor in various source areas, in these companion papers, first, a typically wet month over nearly NWC around July 1979 was chosen; then the likely water vapor source areas of NWC such as Off the Southeast Coast of China（SE）, The Bay of Bengal（BB）, Qinghai-Xizang Plateau（QXP） and Westerly Zone（WZ） were drawn; and again then, a set of thirteen numerical experiments totally, including two contrast ones testing MM5 modeling performance and eleven sensitive ones changing respectively water vapor over each source area, have been designed and integrated for whole July of 1979, utilizing the PSU/NCAR MM5V3.7 model and the NCEP reanalyzed values data as the initial and lateral boundary conditions. The main conclusions for the part Ⅱ of the papers are as follows： （1） With decreasing （increasing） the water vapor over SE source area, the ＂dried （wetted） advection＂ of water vapor through the southeasterly on the SW side of and the southerly on the W side of West Pacific Subtropic High extended westward, the major precipitation decreasing （increasing） regions in east part of NWC （including east Qinghai） will be occurred in the second day. After then, the rainfall decreased （increased） regions can be extended westward further if there exists the easterly in lower-levels in Gansu Corridor. Thus, the SE source area is the most important one for summer precipitation in NWC. （2）After decreasing （increasing） the water vapor over the BB source area, the ＂dried （wetted） advection＂ effects go in two ways both the north-and east-ward, then influence the precipitation of NWC. The BB source area affects mainly the precipitation in Qinghai and Gansu Corridor; partly, the one in east part of NWC; occasionally, the one in South Xinjiang. （3） Because the QXP and WZ source areas （especially the latter） have a little water vapor, the former affects ma