The Changing Characteristics Of Water Cycle Over The Qinghai-Xizang Plateau In Last40Years

Water cycle is a main subject in atmospheric circulation. It not only linked to the major weather phenomena, such as rainfall, but also to the atmospheric energy budget. It has considerable impacts on the climate change. During the past decades, especially when the temperature increased significantly, extreme weather and climate events frequently occur, such as extreme precipitation, frozen rain, droughts etc. Such extreme events all have close relationship with the atmospheric water budget. In this paper, we analyzed the spatial and temporal characters of the water cycle over the Qinghai-Xizang Plateau (QXP) as well as its adjacent regions. The main focus is on the characteristics of the precipitation recycling over the QXP.By using reanalysis data, we diagnosed the characteristics of different types of the large scale water vapor transportation. In tradition, water vapor transport is always considered to be entirety. But strictly speaking, water vapor transport can be divided into three parts:mean meridional transport, constant vorticity transport and transient vorticity transport. The results show that in summer, water vapor in the low latitudes is transported northward as far as the southern QXP in the lower troposphere by mean meridional circulation (Hadley circulation) and constant vorticity (such as monsoon circulation). Very little water vapor can be transported onto the QXP by the mean meridional circulation in the upper troposphere. Transient vorticity transport in summer is northward, which is seen all over the QXP. This means that the water vapor over the QXP is mainly transported by the short wave trough and ridge. In winter, mean meridional circulation and constant vorticity is southward in the lower troposphere. Thus, the water vapor, assume there is, will be transported to the low latitudes. The transient vorticity transport is very strong to the south of the QXP, while little over the main body of the QXP. In order to investigate the impact of the warming climate on the meridional water transport, we divided the whole period into1961-1980and1981-2001. It is found that there is no significant change between these two periods while the transient vorticity transport change a lot. In summer the water transport by transient vorticity was weakened while it was strengthened in winter.Water vapor in the atmosphere is not totally converted into precipitation. By using reanalysis data, the proportion of the atmospheric water vapor that converted into precipitation is evaluated. Results show that with the increasing of the surface temperature, the precipitation conversion rate increased during the past40years. The middle-east QXP has the fastest increasing rate compared to other regions, which can be0.87%per decade. On average, the precipitation conversion rate is larger over the moist regions such as the south east QXP, where the rate has apparent interannual and seasonal variation. The rate in winter is higher and increases faster than in summer. The moist regions has higher conversion rate than in dry regions, while in the moist season such as summer, there large conversion rate center in the dry regions. That means the peak value in the moist regions occur in dry seasons, like winter, while the peak value in the dry regions occur in the moist seasons, like winter.Regional rainfall is decided by both the large scale water vapor transport and the local evaporation. In order to evaluate the relative contribution of the two components in precipitation, we calculated the precipitation recycling rate over the QXP in the past30years. The results show that, the precipitation recycling rate increased in the past decades. And in summer it increases the fastest in a year. From annual variation, the precipitation recycling rate has clear seasonal character. Summer has the highest precipitation recycling rate, approximately62%, while in winter it can reach0%. Those indicate that even the summer monsoon brings considerable amount of water vapor, while the large scale precipitation over the QXP rely mainly on the local water recycling. And in the past30years, the local water recycling over the QXP is strengthened...