Observational Study On The Formation Mechanism Of Radiation Fog In The Winter Canopy Of Xishuangbanna Tropical Rainforest

This paper uses the gradient fog observation data obtained in the tropical rain forest of Xishuangbanna from November 22,2019 to February 28,2020,and analyzes the droplet spectrum and microphysical data such as number concentration,number density,liquid water content(LWC),number density(N),dissipation(ε)and volume diameter(D_v)of a gradient fog process in Xishuangbanna.The analysis shows that the formation of the upper layer of fog is very short,the formation time of the lower layer of fog is later than that of the upper layer,and the duration of the formation stage is longer,but the size of each microphysical quantity is more consistent with that of the upper layer.In the development stage,the droplet number concentration and particle size increase rapidly.Compared with the upper layer fog,the lower layer droplet number concentration increases less but the droplet spectrum is wider.During the entire fog process,the microphysical quantities of the lower fog have a larger standard deviation(σ).The cause of the large standard deviation is the spatial non-uniform structure of the fog.Due to the protection of the vegetation canopy,the wind speed of the lower fog is low and the turbulence is not obvious,so the fog can not diffuse through the turbulence after it is formed,so that the fog tends to be uniform.At each stage of fog development,the droplet spectrum patterns of the two layers of fog are different in the peak position and peak size.The droplet spectrum of the two layers of the formation stage are both single-peak distribution,and the development,maturity,and dissipation stages are all bimodal distributions.Colliding plays a dominant role in fog,and the enhanced effect of gravitational colliding effect will enhance the correlation between microphysical quantities.This is because colliding to become large fog drops will consume small fog drops,and the small fog drops generated by nucleation are not enough to compensate for the part collided.When the threshold function(T)>0.6 is automatically transformed,the number concentration of large fog drops increases significantly.During a fog on January 14,2020,D_vandεshowed non-monotonic changes.In the rising stage of LWC,D_vis positively correlated withε.At this time,the microphysical processes in the fog are nucleation and condensation.The correlation between N and D_vdepends on whether the small droplets generated by nucleation can make up for the small droplets consumed in the condensation process;if the droplets can be supplemented,the correlation is positive;otherwise,it is negative.In the stable stage of LWC,D_vis negatively correlated withε.At this time,the microphysical processes in the fog are condensation and coalescence,accompanied by continuous nucleation process.The correlation between LWC andεstill depends on the competition between small droplets generated by nucleation and those consumed by condensation/coalescence.In the descent stage of LWC,D_vandεas positive correlation,while there is evaporation,denuclearization and touch and process of evaporation can make the droplets N was positively associated with D_v,the wind speed up the evaporation process,on the one hand,on the other hand can make the droplets are more likely to happen to touch and resulting in the emergence of large droplets,this part of the large droplets can make N positive correlations with D_vweakened and even become negative correlation.The higher the coverage rate of tropical rain forest in Xishuangbanna,the higher the LWC,the lower the N and the higher the MD in the fog.The droplet spectra of winter radiation fog in tropical rain forests in Xishuangbanna have obvious bimodal distribution characteristics and can be described by the G-exponential distribution function.