Abstract:

Using conventional meteorological observations, intensive automatic stations, visibility observation data, and NCEP-FNL reanalysis data, the cause of a rare long-term large-scale foggy weather in Liaoning province was analyzed. The results showed that the process of heavy fog on November 20 to 22, 2014 is divided into two phases. After the explosive development of the fog at 14-16 o'clock on November 21, the extremely dense fog lasts 12 hours. This situation is rare in Liaoning province for nearly 20 years. The first stage of heavy fog is radiation fog. The weak warm advection in the lower layer before the fog is conducive to warming up. The weak cold advection in the middle layer during the fog is conducive to the occurrence of clear sky radiation conditions. Under the action of radiation cooling at night, a temperature inversion is formed below 975 hPa. The temperature drops, the temperature dew point difference decreases, and the relative humidity increases. The near-surface breeze is beneficial to reduce the temperature and the water vapor is not easy to flow out. The temperature inversion effect makes it difficult for the water vapor to diffuse to the upper layer, and the concentration of water vapor near the surface layer increases, resulting in the explosive development of the first stage of fog. The second stage of heavy fog is frontal fog. During the explosive development stage of heavy fog, there is no temperature inversion. A weak cold front passes, and convergence near the front causes water vapor to rise and cool and condense. At the same time, low clouds near the front cause raindrops to evaporate in the dry and cold air, which is conducive to water vapor saturation and condensation near the ground. This is the cause of the explosive development of heavy fog. It is relatively rare in Liaoning province.

Key words: Temperature inversion, Radiation fog, Frontal fog