Research On Characteristics Of Water Vapor And Summer Effective Precipitation Conversion Rate Over The Yellow River Basin

Water vapor is the source of precipitation, while the abnormal of moisture transport has direct impact on the drought or the flood, and it is significant of revealing the water vapor flux divergence and its spatial and temporal distribution to a regional climatic and hydrologic characteristics.Based on the NCEP/NCAR reanalysis data, the NCEP final analysis data and the observed rainfall data, the characteristics such as the spatial and temporal distribution of vertically integrated water vapor content,the water vapor flux and the water vapor budget of annual average and different seasons over the Yellow River basin are studied firstly, and then the concepts of the effective layer and the effective precipitation conversion rate are put forward, and the situation in summer is deeply researched. Finally,numerical experiments are designed to verify that summer precipitation in the Yellow River basin is mainly from water vapor of the effective layers and summer effective layers of climate scale are also applicable to those of extreme weather case of heavy rainfall.The simulation ability of WRF model about an extreme weather case of heavy rainfall over the Yellow River basin is tested as a prerequisite for sensitivity analysis.The main conclusions are as follows:(1) The annual average water vapor content is5—27mm of the Yellow River basin. The water vapor content reaches the minimum in winter (January) and the maximum in summer (July), and it is the lowest over the Tibetan plateau and the highest over the lower Yellow River, while it is in the middle over the westerly belt. By means of REOF(rotated empirical orthogonal function) method, the atmospheric precipitable water of annual average, January, April, July and October can be divided into two, four, four, four and four sub-regions over the Yellow River basin,respectively. The water vapor content over the whole basin and the middle and lower Yellow River basin tends to decrease, while the trends are different over other subareas.(2) The water vapor transport is insignificant in January,while in July the water vapor is transported from Southwest, Southeast and Northwest to the Yellow River basin, and the lower Yellow River of multi-year mean is mainly influenced by the first two flows. In the flood years, the southwest and southeast water vapor flows have obvious impact on middle and lower reaches of the Yellow River basin, while the upper reaches is rarely influenced; In the drought years, water vapor flows transport to neither the upper nor middle reaches of the Yellow River basin, only the southwest water vapor flow transports to the lower reaches. Spatial and temporal variations of water vapor budget correspond to that of precipitation over every subarea respectively, while the net meridional water vapor flux is the main supply for water vapor budget and precipitation. The net water budget and the water vapor input to every boundary in flood years are significantly larger than those of in drought years. In January, the weak water vapor input from the west and the north boundary is much smaller than the output from the east and the south boundary, leading to moisture divergence; In July, huge amounts of water vapor comes from the west and the south boundary, it results in the water vapor convergence over most subareas of multi-year mean. But specifically speaking, it results in the water vapor convergence over all subareas in the flood years, while results in the water vapor divergence in the drought years.(3) summer precipitation conversion rate of the whole layer over the Yellow River basin is relatively low, since considering that there should be a certain pressure level which has the most favorable conditions of water vapor transportation, lifting condensation and vertical motion during the entire layer, thus this pressure level is defined as the layer of effective precipitation conversion rate (referred to as the effective layer), and the ratio of precipitation to the effective layer’s precipitable water is called the effective precipitation conversion rate;Through analyzing the average factors such as vertical velocity and water vapor in every subarea in summer, the summer effective layers of the upper, middle and lower reaches are identified as ground—500hPa,600—400hPa and850—600hPa respectively; It’s proven that the evolutional trends of summer mean water budget and precipitation conversion rate in the whole layer and in effective layer are consistent, peak and valley corresponding, and the values of precipitation conversion rate of the effective layer are greater than that of the whole layer during the whole49years. The summer effective precipitation conversion rate reaches the highest over the Tibetan Plateau and the eastern part of the middle Yellow River but the lowest over the northeast side of Tibetan Plateau separately, and it’s distribution forms are similar in drought/flood years and multi-year mean, while the values in flood years are significantly higher than those in drought years.(4) WRF model has a good simulation ability to an extreme weather case of heavy rainfall over the Yellow River basin; The sensitivity experiments of removing the non-effective layers’water vapor and removing the effective layers’ water vapor of initial fields with WRF model from both the positive and the negative aspects explain that summer precipitation in the Yellow River basin is mainly from water vapor of the effective layers, and the conclusion that summer effective layers of climate scale are also applicable to those of extreme weather case of heavy rainfall is illustrated.