Satellite Observed Land Surface Greening in Summer Controlled by the Precipitation Frequency Rather Than Its Total Over Tibetan Plateau

Land surface greening has substantially changed the carbon sequestration and hydrological processes over Tibetan Plateau (TP). Previous studies have revealed that the increased total precipitation (P-total) is the main driver of the enhanced peak growth in TP. However, the role of precipitation pattern, especially the frequency of precipitation (P-freq, number of rainy days [>0.1 mm]), has not been well analyzed. We used time series of satellite-derived Normalized Difference Vegetation Index (NDVI) to investigate the effect of P-freq in controlling the peak growth (the maximum of the NDVI [NDVImax]) of different vegetation types in TP for the period of 1982-2015. We found that the widespread greening trend with P-total disappeared when P-freq was introduced as a controlling variable, and that P-freq alone contributed more than P-total to the increase of NDVImax. The underlying mechanism for the higher contribution of P-freq than P-total to NDVImax is that increased P-freq significantly improved soil moisture, reduced daytime temperature while increased nighttime temperature, thereby alleviating summer drought. Our results highlight the importance of P-freq in interpreting the variation of peak growth, and these effects might be better represented in ecosystem models by considering P-freq rather than P-total alone with future climate change.