The Boundary Layer Structures And Formation/Dissipation Mechanism Of Winter Fog In Nanjing

A field experiment was conducted to investigate fogs in the northern suburb of Nanjing, China during the winter of 2006, and the data of planetary boundary layer(PBL) meteorological elements, fog droplet spectra and etc were collected. The data analyses suggest that on clear days in winter, the temperature inversion usually forms/dissipates in 16:30-18:50 (All times Beijing Standard Time, BST in brief)/ 09:50-11:20 BST next day, respectively, with the duration of 15-18.5h. The single-layer inversion dominates in winter, and sometimes the double-layer (one primary layer plus one secondary layer) structure also occurs for a short period. The primary character of the wind field in the formation/dissipation processes of inversion is that the wind speed increases with height in the near surface layer, and becomes uniform in the mid-upper layer. In the maintenance stage, the wind field mainly shows a unimodal, bimodal, or multimodal distribution. The specific humidity field has no obvious correspondence to the temperature field. The two types of fogs have different characters due to different influencing factors. Effects of radiation and advection are almost equal to each other in the fogs occurred both on Dec. 12 and 14, 2006, therefore they are both advection-radiation fogs. The fog from Dec. 24 to 27, 2006 is a typical advection fog relatively, wherein the warm and wet flow is responsible for the formation and maintenance of the fog, and the burst of northerly winds associated with cold advection results in the dissipation of the fog. Double-layer structure occurs when the fog top falls both during the fogs on 14 and from 24 to 27. In comparison with the advection-radiation fog, the duration of severe fog stage is longer and the temperature inversion is featured by larger intensity, larger thickness, and more stable center in the advection fog. In the advection-radiation fog event on Dec. 12 the rare isothermal vertical profile appears. The main moisture source of the advection fog is warm and wet flow, while that of advection-radiation fogs is the ground surface with flow as a secondary source. The comprehensive analyses of the three fog events show that the isolines of specific humidity are sparse in fog body, and there is a good spatial-temporal correspondence between the specific humidity and temperature fields, which is different from fog-free days. However, the temporal evolutions of the both are opposite at the fog top in the rise/fall processes. During the fog process, the wind speed shows mainly unimodal, bimodal, and multi-modal vertical distributions. The southerly flow is favorable to the formation and maintenance of fog, while the northerly flow frequently results in the fall of fog top, or even the collapse of the fog body. The existence of temperature inversion is critical for the formation and maintenance of fog. However the maximum intensity of the primary inversion always lies near or below the top of advection fog, and after the occurrence of approximately isothermal profile in the fog on 12, the fog does not disappear. The formation/dissipation of turbulent motion leads to the separation/mergence of inversion layers, and the development of turbulence at the fog top results in the fall of fog top. In the fog event during Dec. 24-27, the different variations of jets happen, such as mergence, strong-weak fluctuation or trending to be homogeneous, due to the development of turbulence between jets and the supply of momentum.