Comparative Study On The Microphysical Characteristics Of Dense Fog At Ground Level And 20m Height In Shouxian County

Fog is a weather phenomenon with vertical structure characteristics.Most studies of fog microphysics were limited to the ground.In January 2019 and January 2020,the comprehensive observation experiments of fog microphysics were conducted in Shouxian at surface and the height of 20 m above the surface.Combined with the data of the 20 m meteorological tower,the tethered sonde system,the automatic weather station and the eddy covariance system,the microphysics of the fog processes at the ground level and 20 m height were analyzed,and the influences of turbulence,water vapor flux and other factors on the vertical structures of fog microphysics were also discussed.The results show that,in the early stage of the three radiation fog processes,there were continuous precipitation processes,and a large amount of water vapor accumulated near the surface.Under the condition of the long wave radiation cooling and breeze after sunset,fog first formed on the surface and rapidly developed into dense fog.The temperature rose near the ground around sunrise,the first fog process dissipated rapidly,and the second and third fog further strengthened until noon.Three fog processes were typical radiation fog processes,in which the duration of dense fog accounted for more than 80%of the whole fog process.After sunrise,the stronger temperature inversion continued to maintain,which was conducive to the accumulation of water vapor and particulate matter in the near surface layer,and promoted the further enhancement of fog.During the second and third fog,the temperature inversion began to rise with the increase of solar radiation,and the near surface fog further developed after sunrise.In the formation stage,fog first formed on the ground and developed to the height of 20m with the cooling of the upper air layer.When the fog developed to the height of 20 m,it was unstable.With the thickening of the fog layer,the temperature of each gradient layer near the ground tended to be consistent,and the microphysics at the two heights gradually approached and changed quasi-synchronously with time.The fog near the ground developed stably and evenly.During the first fog process,the two heights of fog dissipated simultaneously;During the second fog dissipation,the surface fog dissipated later than 20 m fog for about 10 min due to the maintenance of small particles near the ground;During the third fog dissipation period,the surface temperature was slightly higher than the upper air temperature,resulting in faster evaporation of surface fog droplets,and the surface fog dissipation was about 16 minutes earlier than 20 m fog.In the early stage of 20 m fog,the droplet size of 20 m fog was concentrated in 3～6μm,the spectrum was characterized by a narrow width and most of the droplets were smaller than6μm.With the vertical transport of vapor,the condensation process was active,while the number of droplets larger than 6μm increased,the drop spectrum broadened and closed to the ground spectrum width.In the mature and dissipation stage,the distribution of droplet spectrum at two heights evolved synchronously.Because the vapor content at 20 m height was always less than the vapor content near the ground,the spectral width of 20 m droplet was always less than that of the ground.The change of turbulence state near the ground affected the evolution of the vertical structure of fog microphysics.In the early stage of fog,the variance of vertical velocity(σ_w~2)was less than 0.001 m~2/s~2,it was conductive to maintain the vapor in the surface layer.In the formation stage,σ_w~2 fluctuated around 0.001～0.002 m~2/s~2.In the development stage,σ_w~2obviously increased.In the mature stage,it was relatively stable around 0.01 m~2/s~2.Whenσ_w~2reached around 0.02 m~2/s~2,the fog started to dissipate.The stable state before fog was conducive to the maintenance of vapor which is required for the occurrence of radiation fog.Under the condition of weak flux of vapor,the condensation of droplets led to the decrease of vapor density in the surface layer.Moderate turbulence and flux of vapor were conducive to the uniform development and stable maintenance of the near-surface radiation fog.