1.Anhui Institute of Meteorological Sciences/Key Laboratory of Atmospheric Sciences and Satellite Remote Sensing of Anhui Province;2.Key Laboratory for Atmospheric Physics and Environment,Nanjing University of Information Science and Technology;3.Shouxian National Climatology Observatory/Huaihe River Basin Typical Agro-Ecosystems Meteorology Field Experiment Station of CMA
Fog has adverse effects on transportation, especially extremely dense fog. In this paper, the fog droplet spectrum data measured by FM-100 fog drop spectrometer at Shouxian National Climate Observatory in January 2019, together with the contemporary conventional meteorological observation data, were used to investigate the microphysical characteristics of fog with different intensities. Based on the analysis of the relationships between visibility (V) and liquid water content (L), number concentration (N) of fog droplets and relative humidity (RH), various visibility parameterization schemes were established. The results show that: (1) With the increase of fog intensity, the water content in fog increased significantly, with average values of 0.003, 0.01 and 0.09 g·m-3 during the periods of fog, dense fog and extremely dense fog, respectively. When the L was greater than 0.02 g·m-3, the proportion of extremely dense fog reached 95%. (2) The N and droplet size increased with the increase of fog intensity. From fog to dense fog, the N increased significantly (increased by 67%), while from dense fog to extremely dense fog, the droplet size increased significantly, and the average diameter (D) and effective radius (Re) increased by 62% and 135% respectively. When the Re was greater than 4.7 μm, the proportion of extremely dense fog reached 95%. (3) All the spectra distributions of droplet number concentration for fog, dense fog and extremely dense fog were bimodal structure, with the major peaks close to the end of small particles. The spectrum type of extremely dense fog was Deirmendjian distribution; while it was Junge distribution for dense fog and fog. As for fog water mass concentration spectrum it was characterized by multi peaks for extremely dense fog with the maximum peak appeared at 21.5 μm, bimodal distribution and single peak type for dense fog and fog respectively, with the maximum peak at 5 μm. (4) Both L and N were inversely correlated with visibility, and L showed the highest correlation coefficient with visibility. Four kinds of visibility parameterization schemes were established by using full sample and segmented method respectively, and the test results indicated that the visibility subsection fitting scheme based on L was the best.