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. During the winters (December–February) between 1985 and2015, the North China Plain (NCP, 30–40.5∘ N,112–121.5∘ E) suffered many periods of heavy haze, and theseepisodes were contemporaneous with extreme rainfall over southern China;i.e., south rainfall–north haze events. The formation of such haze eventsdepends on meteorological conditions which are related to the atmosphericcirculation associated with rainfall over southern China, but the underlyingphysical mechanism remains unclear. This study uses observations and modelsimulations to demonstrate that haze over the NCP is modulated by anomalousanticyclonic circulation caused by the two Rossby wave trains, inconjunction with the north–south circulation system, which ascends oversouthern China, moves north into northern China near 200–250 hPa, and thendescends in the study area. Moreover, in response to rainfall heating,southern China is an obvious Rossby wave source, supporting waves along thesubtropical westerly jet waveguide and finally strengthening anticycloniccirculation over the NCP. Composite analysis indicates that these changeslead to a stronger descending motion, higher relative humidity, and a weakernortherly wind, which favors the production and accumulation of haze overthe NCP. A linear baroclinic model simulation reproduced the observednorth–south circulation system reasonably well and supports the diagnosticanalysis. Quasi-geostrophic vertical pressure velocity diagnostics were usedto quantify the contributions to the north–south circulation system madeby large-scale adiabatic forcing and diabatic heating (Q). The resultsindicated that the north–south circulation system is induced mainly bydiabatic heating related to precipitation over southern China, and theeffect of large-scale circulation is negligible. These results provide thebasis for a more comprehensive understanding of the mechanisms that drivethe formation of haze over the NCP.

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