Heavy air pollution suppresses summer thunderstorms in central China

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Description

50 years (1951-2005) of rainfall, thunderstorms, temperatures, winds and visibility data have been analyzed at the heavy polluted Xian valley, a nearby Mount Hua in central China, for assessing the impact of the increasing air pollution on convective precipitation. Humidity-corrected visibility is used as a proxy for aerosol optical depth (AOD) which is often so heavy that stabilizes 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 weak decreasing trend in thunderstorm frequency. This decrease was contributed from 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, rainfall amount there appears to have little diurnal cycle, and respectively shows little trend with the increasing aerosol amounts. Only small fraction of the rainfall in Xian is generated by local instability. The finding may be extended to other areas where local surface heating dominates rainfall amount.