Study On The Atmospheric Moisture Cycling Over Continental China

The atmospheric moisture cycling plays an important role in the weather and climate systems.To enhance the understanding of atmospheric moisture cycling over continental China,this paper studies the atmospheric moisture cycling over China based on the Dynamic Recycling Model(DRM)and the NCEP CFSR reanalysis during the period of 1979-2010.The research contents mainly include precipitation recycling,moisture sources and transport paths for precipitation,moisture sinks of evaporation,moisture residence times,and moisture transport distances.For the precipitation recycling,we propose a method to present the 2-dimensional distribution of precipitation recycling ratio of an identical spatial scale at the given grids,by which we can produce a gridded analysis of the local and regional precipitation recycling ratios of certain scales of interest.The regional and local precipitation recycling ratios are found to be highly consistent with each other in both spatial patterns and variabilities,and further investigation indicates that regional precipitation recycling ratio is the monotonically increasing function of local precipitation recycling ratio under the ideal condition.The regions of Southwest,Northeast,and part of Northwest China show relatively high precipitation recycling ratios,which indicates strong land-atmosphere moisture feedback over those regions.Generally,precipitation recycling and precipitation show positive correlation in low-precipitation North China regions and negative correlation in high-precipitation South China regions.The possible reason is that evaporation is controlled by soil moisture and atmospheric condition in the low-and high-precipitation regions,respectively.For the precipitation over continental China,results indicate that the main moisture sources are China,the Eurasia,the Indian Ocean,the South China Sea,and the western Pacific Ocean.The moisture contributions of terrestrial and oceanic areas for annual precipitation are 50.8% and 44.4%,respectively,after 30 days' backward tracking.The moisture sources of precipitation over different regions in different seasons are different.Generally,each region and the adjacent regions contribute much moisture to its precipitation,especially in summer when the evaporation rate over land is relatively high.The contribution of terrestrial(oceanic)areas is relatively high(low)for precipitation over North China,and is relatively low(high)for precipitation over South China.For the mid-lower reaches of the Yangtze River Basin(YRB),oceanic areas are found to be the main moisture sources for summer(June 1–August 31)precipitation,and the previously overlooked southern Indian Ocean,as a source region,is found to contribute more moisture than the well-known Arabian Sea or Bay of Bengal.However,terrestrial evapotranspiration appears to be also important for summer precipitation,especially in early June when moisture contribution is more than 50%.The Indian Ocean is the most important oceanic source before mid-July,while the Pacific Ocean becomes the more important oceanic source after mid-July.To quantitatively analyze paths of moisture transport to YRB,the Trajectory Frequency Method(TFM)is proposed.The most intense branch of water vapor transport to YRB stretches from the Arabian Sea through the Bay of Bengal,the Indochina Peninsula,the South China Sea,and South China in summer.The other main transport branches are westerly moisture fluxes to the south of the Tibetan Plateau,cross-equatorial flows north of Australia,and separate branches located in the north and equatorial Pacific Ocean.Additionally,the South China Sea is very import for moisture transport to YRB,as quantitatively estimated by the TFM.For the evaporation from continental China,the main moisture sinks are China,the Eurasia north to China,and the northwestern Pacific Ocean.After 30 days' tracking,the percentage of annual evaporation that precipitates in the terrestrial areas,the oceanic areas,China,and the Pacific Ocean are 55.7%,42.8%,36.7%,and 38.9%,respectively.The moisture sinks of evaporation from different regions in different seasons are different.Generally,the percentage of evaporation precipitating in terrestrial areas in summer is higher than that in winter,and the percentage of evaporation from West China precipitating in terrestrial areas is higher than that from East China.Terrestrial and oceanic areas are the main moisture sinks of evaporation from China in summer and winter,respectively.For the moisture residence time over continental China,both of the residence times for atmospheric moisture of precipitation and evaporation show significant spatial and seasonal variations,with the local precipitation-moisture residence time generally longer than the evaporation residence time over China,which corresponds to area-averaged values of 8.3 and 6.3 days,respectively.Correspondingly,the area-averaged backward and forward tracking times at the 90% threshold(i.e.,when 90% of initial moisture is attributed for tracking)are approximately 22 and 15 days,respectively.Moreover,under the ideal situation,we theoretically deduced the explicit expressions of residence and tracking times,and obtained the proportional relationship between these times,which was further verified against the DRM-derived values.This proportional relationship provides a theoretical basis for the choice of moisture tracking time.For the moisture transport distance over continental China,the local precipitation-moisture transport distance is generally longer than the evaporation transport distance over China,with significant spatial and seasonal variations of both,and the area-averaged values of curve(straight-line)transport distances of 4874.8 and 4414.2(3169 and 2864.4)km.We found that there exists a certain proportional relationship between the moisture curve transport distance and path(or tracking)distance under the ideal situation.The moisture curve and straight-line transport distances show highly similar spatial patterns,which means that there also exists a similar proportional relationship between moisture straight-line distance and the distance starting from the precipitating(evaporating)point to the location that a trajectory reaches.Such a proportional relationship can provide some theoretical guidance for the choice of the areas of potential moisture sources or sinks.The results and conclusions of this paper may theoretically contribute to the study of atmospheric moisture cycling,and also provide some scientific basis for hydro-climate projections and assessments over continental China under the background of climate change.