Influencing Factors Of Cloud And Fog Visibility In Huangshan Mountains And Its Microphysical Process

Low visibility in fog seriously affects land,sea and air transportation.Accurate forecast of fog visibility is an important support to reduce the loss of fog disaster.Therefore,it is particularly important to improve the parameterization scheme of fog visibility.In this study,the influencing factors of cloud/fog visibility and related microphysical processes are analyzed based on the observation data at Guangming peak of Mount Huangshan.Firstly,by comparing previous visibility parameterizations,it is verified that number concentration and liquid water content are important influencing factors of fog visibility.The parameters in previous parameterizations are often taken as constants,but studies have found that these coefficients are closely related to the microphysical quantities themselves.A new visibility parameterization scheme is obtained,by establishing the relationship between the coefficients and the number concentration,which improves the previous parameterizations.The universality of the new parameterization is tested by using independent data.Secondly,the theoretical relationship between visibility and cloud/fog microphysical characteristics is derived.It is found that the spectral width of cloud droplet spectrum is also an important factor affecting visibility,in addition to number concentration and liquid water content.The spectral width can be expressed by the dispersion or the ratio of effective radius to volume mean radius.Comparison of this new parameterization scheme with added spectral width and the existing visibility schemes based on the observational data verifies the reliability and superiority of this parameterization scheme.In addition,comprehensive observational data and theoretical analysis using Gamma distribution found that the relative deviation between the visibility calculated with constant extinction efficiency factor of 2 and with the Mie theory first increases and then decreases with the increase of visibility.The relative deviation is positively correlated with the first bin strength of droplet size distributions(the first bin number concentration divided by the total number concentration).Finally,since relative dispersion of droplet size distributions is important to visibility,it is revealed that collision-coalescence is the main microphysical process affecting relative dispersion by analyzing the relationship between relative dispersion and volume-mean radius.With the increase of the volume-mean radius,the relationship between relative dispersion and volume-mean radius changes from positive to negative,which is mainly related to activation,condensation,evaporation and deactivation,consistent with previous studies.On this basis,it is further found that the negative correlation is relatively weak,mainly due to collisioncoalescence.In addition,can still well distinguish their positive and negative correlations.the greater(smaller)the first bin strength of droplet size distributions,the more positive(negative)correlation between relative dispersion and volume-mean radius.These results enhance the theoretical understanding of the influencing factors of relative dispersion,which is conducive to the improvement of relative dispersion parameterizations and visibility parameterizations.