Responses of hail and storm days to climate changes in the Tibetan Plateau

There is increasing concern that local severe storm occurrence may be changing as a result of climate change. The Tibetan Plateau (TP), as one of the world's most sensitive areas to climate change, became significantly warmer during recent decades.Based on the surface observations over TP during the warm season (from June to September), it is found that since 1960 (1980), storm (hail) days have been decreasing by 6.2%/decade (18.3%/decade) in the region. However, what cause the frequency changes of storm and hail in the TP is largely unknown. Based on 53-year continuous weather records at 48 TP stations and reanalysis data, we show here for the first time that the consistent decline of storm days is strongly related to a drier mid-troposphere since 1960. Further analysis demonstrated that fewer hail days are driven by an elevation of the melting level (thermodynamically) and a weaker wind shear (dynamically) in a warming climate. These results imply that less storm and hail may occur over TP when climate warms.