The role of cloud radiative effect in the maintenance and development of persistent heavy fog events in northern China

Observations have shown that persistent heavy fog events in northern China are usually accompanied by the presence of clouds above the fog and characterized by co-existing clouds and fog. Combined with satellite, radiosonde and ground observations, tethered balloon soundings, and microwave radiometer data, the mesoscale model WRF is applied to investigate the role of cloud radiative effect in the maintenance and development of two persistent fog events occurred on 3-6 December 2011 and 28-31 January 2013, respectively. Horizontal distributions of fog and clouds as well as vertical profiles of temperature and relative humidity from the WRF simulations are compared with and verified against observations. The results indicate that the persistent fog processes lasted for more than 48 hours and accompanied by southward warm moist advection at lower levels near the ground. During the development of heavy fog events, the single-layer fog gradually developed into a cloud-fog coexisting state. The middle and high clouds with ice and snow particles covered the foggy area 24 hours after the fog formed. The cloud base was above 3 km height, and the cloud thickness was larger than 3.5 km. During the daytime, due to the co-existence of clouds and fog and enhanced solar radiation reflection by the clouds and fog, the solar radiation at the surface was greatly reduced by 71%-84%. This weakened the surface warming and prevented the fog from dissipating. Meanwhile, the greenhouse effect caused by the clouds and fog enhanced the turbulent mixing, which promoted the fog to extend upward and maintain in the stable layer. At the nighttime, longwave radiative cooling at the surface was reduced due to the greenhouse effect of the clouds and fog, and the surface net radiation increased by more than 70 W/m². This was not conducive to further development of the fog event. However, the enhanced turbulent diffusion process due to the greenhouse effect of clouds and fog was favorable for the development of the fog event in the vertical direction. Thus, the presence of clouds above the fog plays an important role in the maintenance and development of the long-lasting fog event through cloud radiative effect.