Spatial and Temporal Changes of Atmospheric Precipitable Water in Tibet Region in Recent 30 Years

The spatial and temporal distributions of atmospheric precipitable water(APW) and precipitation conversion efficiency(PCE) in Tibet region were investigated using NCEP/NCAR reanalysis data with monthly precipitation at 34 meteorological stations from 1980 to 2009.The results show that:(1) APW is gradually decreasing from southeast to northwest parts of Tibet region,APW during the recent 30 years has decreasing and its interannual variability is relatively small,which appeared an obvious seasonal difference,the maximum value was in summer and the minimum value was in winter.There is no more difference for the amount of APW between more and less rain years indicating that the water content in the air is relatively stable in Tibet region,which is beneficial to its reasonable exploitation and utilization.(2) The highest PCE in Tibet region is in the middle and the eastern parts of Nagqu and the southeast part of Nyingchi,while the lowest PCE is in the northwest of Tibet.PCE appears the increasing trend during the recent 30 years and the interannual variability is relatively large while it has same seasonal variation as APW.The amount of the PCE determines the year will be more rain year or less rain year to some extent.(3) The spatial distribution of APW is quite similar with the distribution of natural precipitation,but it has opposite temporal variation trend to the increasing trend of precipitation while PCE has basically same variation trend as natural precipitation during the recent 30 years.The ascending(descending) of the PCE is corresponded to the increase(decrease) of precipitation.