Acts Of Warming And Water Vapor Content On Decrease In Light Rain In The Summer Half Year Over Eastern China

Based on daily rainfall data collected at395gauge stations over eastern China (east of105°E) during1979-2009, the variation of light rain events with intensities of0.1-10mm/day in summer half years (from May to October) were analyzed. Results indicated that both the light rain amount and light rain events had declined notly in summer half years, with the trend of-4.89mm/decade,-2.48day/decade, respectively. The reduction of light rain amount is primarily caused by the decrease of light rain events, which dropped off remarkably before1989and decreased rapidly after1994.The first two principal components of EOF analysis on light rain events not only showed a long time decrease, but depicted regional differences, that light rain events declined more distinctly in northeastern and southern region than the central region. Spatial and temporal features, as well as the periods derived from EOF analysis on temperature, precipitable water content and relative humidity at low troposphere are coincide with those of light rain events. Composite analysis also suggested that there are more (less) light rain events in the year with higher(lower) relative humidity and higher (lower) precipitable water content at low atmosphere, while there are less (more) light rain events in the year with higher (lower) troposphere. According to the Tetens’s formula and relative humidity formula, relative humidity decreased by5.5%due to the low troposphere warming, and decreased by0.16%because of the specific humidity. Both the warming and water-vapor content are the reason for the reduction of light rain events, but warming is deemed as the major account, for its impact on light rain events is more obvious than that of water-vapor content.Light rain change characteristics under different weather conditions in summer half year have been further analyzed. It found that the decrease of light rain was mainly attributed to the reduction of light rain in cloudy days. The trend coefficients of light rain amount, light rain days in cloudy days are4.12mm/decade and2.09d/decade, respectively. The first mode of EOF analysis on light rain in cloudy days depicted the reduction of light rain throughout eastern China. The maximum entropy spectrum analysis on the1st principal component showed the significant periods of30and2.73 years of light rain amount in cloudy days. There are distinct relationship between light rain and low tropospheric relative humidity, moisture, and temperature in cloudy days. Component analysis suggested that in years with higher (fewer) relative humidity and precipitable water vapor content, there are much more (less) light rain days and amount, and this effect is more obvious on day scale. While for years with higher (lower) temperature, light rain days and amount are much less (more), but in days with higher (lower) temperature, light rain intensity is more strong (weak), which is according to the Clausius-Clapeyron equation, that the water holding capacity of air increases by-7%per1℃of warming, which show that light rain events restrained by warming reflected on climate scale. First modes of EOF analysis on relative humidity, precipitable water vapor content and temperature present their similar pattern with light rain amount. And their1st principal components showed the same periods with light rain amount in cloudy days. Based on the Tetens’s formula and relative humidity formula, it found that the decrease of relative humidity during1979-2009in cloudy days is caused by warming, and the decrease of relative humidity during2000and2009is mainly attributed to the reduction of water vapor content. Relative humidity reducing over northern region was mainly caused by the water vapor content combined with the water vapor transport analysis; and in southern region, its reduction was contributed to the warming and reduced water vapor.Under the condition of reduced relative humidity in dull days, light rain increased over most eastern China except for the Yangtze River. Light rain increased over northern China is correlated to the increased CAPE during1990-1999, while light rain increased over Huang-Huai basin is related with the increased relative humidity during2000-2009. Composite analysis between CAPE and light rain in dull days suggested that CAPE impact on light rain with regional features, that major concentrates in northern China, northeastern China and southern China. While for years with higher CAPE (lower CAPE), light rain are much more (much less). Increased low air temperature and water vapor contributed to the increased CAPE in dull days, and the impacts of geopotential hight has few impacts on CAPE compared with temperature and water vapor.According to the above analysis, it concluded that decrease of light rain in summer half year over eastern China is attributed to the reduction of light rain frequency, and the decrease is mainly attributed to the reduction of light rain in cloudy days. There are close relationship between light rain and low tropospheric relative humidity, precipitable water vapor content and temperature. The temperature and water vapor affect light rain variation through relative humidity. According to the Clausius-Clapeyron equation analysis, it suggested that warming is deemed as the main reason for light rain decrease under all weather conditions, with little change of moisture; while under the condition of cloudy days, the decrease of light rain is caused by reduced water vapor in the north, and affected by warming and decreasing water vapor in the south. Light rain increased in dull days are mainly caused by increased CAPE, which are affected by warming and increased water vapor.