Long-Term Variations of Seasonal Dryness/Wetness in Beijing

The seasonal dryness/wetness in Beijing is identified and compared. The results show that the dry/wet grades derived from the Gamma probability distribution are more effective to be representative of climate states in. that area than from other distributions. It is found that the dry climate state in the four seasons during recent 30 years occurred more frequently than ever before, and the occurrence frequency varies slightly with different season. Moreover, the dry climate state in summer occurred more frequently Than in other seasons. It follows that from the long-term variations, the dryness is prone to changing into the wetness in winter and spring; the wetness is prone to changing into the dryness in summer and autumn. From a causal analysis, it is known that, the dryness in Beijing is related to the warm/cold state of the see surface temperature in the Pacific. Also, the periodic analysis of the dry/wet grade series shows that the severity of dry state in summer would be mitigated in the next 10-30 years.