An Analytic and Numerical Study on the Physical Meaning Of Pan Evaporation and Its Trend in Rencent 40 Years

Abstract

The physical meaning of pan evaporation is analyzed based on the gradient transport theory in boundary layer meteorology and energy conservation principle, and verified by the correlation analyses between pan evaporation and environmental factors with the data from 62 Chinese routine weather stations at which the data of long-term solar radiation is available. The trends of pan evaporation and environmental factors in the last 40 years are analyzed. The result indicates that the long-term trend of pan evaporation cannot be explained by the change of a environmental factor, such as, it is more reasonable to attribute the decrease of pan evaporation to the global radiation reduction in eastern China than in western China. The data observed during the International Satellite Cloud Climatology Project (ISCCP) from 1983 to 2001 demonstrates the slight decrease of total cloud amount and the obvious increase of total cloud water path in most of China, which means the cloud property has changed, and the cloud becomes more opaque. These cloud changes show the precipitable water in atmosphere has an increasing trend, and the earth-atmosphere system becomes moister year by year. Based on the hydrological cycle, it is proved that the variation of water vapor in the earth-atmosphere can make the relative humidity, the global radiation and the potential evaporation in the boundary layer change evidently, as shown by numerical experiments with an atmospheric general circulation model.