Fog increases and the effects of aerosols in China

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Description

Fog is a severe weather hazard that greatly influences traffic and daily life with potentially heavy economic loss. An increasing number of traffic accidents caused by fog have been reported in China in recent years. In this study, we show that the frequencies of fog events in wintertime over eastern-central China have doubled over the past three decades. For the same period, surface wind speeds have dropped from 3.7 m/s to about 3 m/s, and the mean number of cold air outbreaks has decreased from 7 to around 5 times per winter; relative humidity and the frequency of light wind events have also increased significantly. Weakening of the East Asian winter monsoon system appears to be responsible for these changes, which is further linked to global warming and regional increases in atmospheric aerosol loading. Both trends reduce the thermal contrast between high and low latitudes, weaken the East Asian winter monsoon circulation, and favor the formation of fog. This hypothesis is tested using the National Centers for Environmental Prediction (NCEP) reanalysis data and model simulations with the National Center for Atmospheric Research Community Climate Model (NCAR/CCM3). The analyses show that the 500-mb trough in East Asia has shallowed over the past three decades. Meanwhile, the surface Siberian high has weakened, which is likely the cause for the diminishment in speed of the prevailing northwesterly winds and the reduction in intrusions of dry and cold air from the northwest. The increase in atmospheric aerosols was shown to weaken the East Asian winter monsoon as well. The reduced thermal contrast between China and Siberian regions caused by the cooling effect of aerosols over China also leads to a reduction in the influx of dry and cold air over eastern-central China. These effects are responsible for the increased convergence of water vapor there. All these changes favor the formation and maintenance of fog over this region.