Effect of rainfall-induced diabatic heating over southern China on the formation of wintertime haze on the North China Plain

Abstract

During the winters (December–February) between 1985 and 2015, the North China Plain (NCP, 30–40.5∘ N, 112–121.5∘ E) suffered many periods of heavy haze, and these episodes were contemporaneous with extreme rainfall over southern China; i.e., south rainfall–north haze events. The formation of such haze events depends on meteorological conditions which are related to the atmospheric circulation associated with rainfall over southern China, but the underlying physical mechanism remains unclear. This study uses observations and model simulations to demonstrate that haze over the NCP is modulated by anomalous anticyclonic circulation caused by the two Rossby wave trains, in conjunction with the north–south circulation system, which ascends over southern China, moves north into northern China near 200–250 hPa, and then descends in the study area. Moreover, in response to rainfall heating, southern China is an obvious Rossby wave source, supporting waves along the subtropical westerly jet waveguide and finally strengthening anticyclonic circulation over the NCP. Composite analysis indicates that these changes lead to a stronger descending motion, higher relative humidity, and a weaker northerly wind, which favors the production and accumulation of haze over the NCP. A linear baroclinic model simulation reproduced the observed north–south circulation system reasonably well and supports the diagnostic analysis. Quasi-geostrophic vertical pressure velocity diagnostics were used to quantify the contributions to the north–south circulation system made by large-scale adiabatic forcing and diabatic heating (Q). The results indicated that the north–south circulation system is induced mainly by diabatic heating related to precipitation over southern China, and the effect of large-scale circulation is negligible. These results provide the basis for a more comprehensive understanding of the mechanisms that drive the formation of haze over the NCP.