Heavy air pollution suppresses summer thunderstorms in central China

Time series of rainfall, thunderstorms, temperatures, winds and aerosols of 50 years have been analyzed at the Xian valley (1951-2005, rain rates data are only available for the period of 1961-2000 for Xian) and the nearby Mount Hua (1951-2005) in central China, for assessing the impact of the increasing air pollution on convective precipitation. Adding aerosols to pristine air initially increases convective rainfall. However, aerosol amounts in the Xian valley (represented by large AOT and significant decreasing trend in visibility in the study area) have been shown to be sufficiently high so that added aerosols suppress convection and precipitation, by both radiative and microphysical effects, even at the starting of the analysis period in the 1950s. It was found that the aerosol's negative radiative forcing stabilized the lowest troposphere. The stabilization resulted in less vertical exchanges of air, which caused reduction in the lowland (Xian) surface winds and increase in the highland (Mount Hua) wind speeds. The decreased instability caused a decrease in the frequency of the thunderstorm normalized by rainfall amount in the lowland due to the thick aerosol layer above, but not at the highland, above which the aerosol layer was much thinner. The indicated decreasing trend of highland precipitation was associated with a similar size decreasing trend in thunderstorm frequency. This decrease was contributed by light and moderate (< 25 mm day(-1)) rainy days. These patterns of rainfall changes at the highland are consistent with the microphysical suppressive effects of aerosols. Despite the dramatic relative decrease in the already originally scarce thunderstorm activity in the Xian valley, the rainfall amount there appears to have little diurnal cycle, and shows little trend with the increasing aerosol amounts. Because only small fraction of the rainfall in Xian is generated by local instability, as indicated by the flat diurnal cycle, it appears to be a condition which is unsuitable for quantifying the impact of heavy aerosols on rainfall amounts. However, the dramatic relative decrease of the scarce thunderstorms in Xian suggests that aerosol's radiative effect can be substantial. Such study should be extended to other areas where local surface heating dominates rainfall amount. (C) 2013 Elsevier Ltd. All rights reserved.