The Role of Local Moisture Recycling Evaluated Using Stable Isotope Data from over the Middle of the Tibetan Plateau during the Monsoon Season

Abstract

Both meteorological data and stable isotope data were used to investigate the role that local moisture recycling plays in maintaining moist land surface conditions over the middle of the Tibetan Plateau during the summer monsoon season. Past studies have shown that precipitation events of the summer monsoon season can be categorized according to synoptic conditions as east-migrating trough types, heat low types, and regional circulation types. Precipitation events during an intensive observation period from 13 to 27 August 2004 were therefore classified into these three types. The contributions of locally recycled moisture in each precipitation type were investigated using isotopic features. The isotope data include precipitation, near-surface atmospheric moisture, and evapotranspiration. First, using a simple Rayleigh distillation model, the isotopic content of the moisture source of the rainfall was estimated from observed precipitation isotope data. The contribution of lower-atmospheric moisture to the precipitation was then evaluated by comparing isotopic values. Next, when rainfall was mainly fed by lower-atmosperic moisture, the contribution of evapotranspirated water in the lower atmosphere was assessed by considering the factors controlling the isotopic variability of lower-atmospheric moisture. The results show that, in the case of trough-type rainfall, moisture flux convergence occurred in this area and a remarkable increase in precipitable water was observed when a trough approached the site. Thus, observed large isotopic variation associated with the passage of a trough reflects the isotopic content of moisture advected from the surrounding areas. With the exception of trough-type rainfall, the simulated isotopic values agreed well with the isotopic value of lower-atmospheric moisture. This finding indicates that lower-atmospheric moisture is the dominant source of such rainfall. In these periods, temporal isotopic variation in lower-atmospheric moisture showed gradual increases accompanied by an increased contribution of evapotranspirated water that had relatively heavy isotopic values. In particular, when the regional circulation type of rainfall was observed, the local recycling ratio, which is the contribution of locally evapotranspirated water in the boundary layer, increased from 30% to 80%. Locally recycled moisture thus plays an important role in precipitation associated with regional circulation. Active moisture recycling contributes to a high frequency of precipitation events so that the moist land surface is maintained during the summer monsoon period in this region.