Advection Fog And Its Aerial Microphysical Properties In Tianjin

Fog layer has a certain thickness and vertical structural features.Traditional field observations of fog microphysics are usually carried out on the ground,so only the knowledge about the bottom of the fog layer is obtained.In the winter of 2016 / 2017,a field experiment focused on the microphysical properties of the advection fog was conducted in Tianjin.With the data set of fog droplet spectrum at a height of 66 m above the ground and the atmospheric boundary layer observation from a 255 m meteorological tower,the life cycle phases of two advection fog events occuring successively after severe haze were objectively differentianted by the use of non-parametric change-point detection and trend tests.The main objective is to reveal the observational characteristics of the microphysics and size distribution of fog droplets at a certain height inside the fog layer,and the relation with fog evolution.The results show that the non-parametric test methed about multiple elements combined with subjective discrimination can objectively determine the exact moment of the phase change points of advection fog evolution.The saturated layer firstly forms in the air and expands to the ground,and then the microphysical parameters at 66 m height changed in quasi-synchronism with the ground visibility.The mature phases of the fog events were accompanied by the southwest warm and moist air,and the temperature increased within the tower height,and the fog layer is under weak unstable or nearly neutral conditions.The continuous advection input and turbulence enhancement can weakening the positive correlation between the concentration of droplets and liquid water content.During the entire fog period,although the characteristics parameters were relatively low compared to other ground relative clean environment observations,large droplets had always existed.The numbers of droplets with different size decreased synchronously because of fog droplet evaporation,the reduction rate of small droplets is greater than big droplets and the effect of droplet evaporation is obvious in the dissipation phase,also show that liquid water content and surface average diameter decreased slowly with the increasing of net radiant flux density.The liquid water content increased sharply to the extreme value with the continuous increase of the concentration,and the fog spectrum broadened significantly.The high concentration ofparticulate matter caused by heavy pollution will lead to an increase in the fog droplets concentration,and the size of droplets will be smaller,at the same time,the advection fog has a significant wet removal effect on the particulate matter.With the change of concentration about ground particulate matter at different sites,can judge the horizontal development of the fog from southwest to northeast.At the sampling height,the high concentration of fog droplets appeared at the beginning of the mature phases,the liquid water content and characteristic diameter reached the maximum value before the end of mature phases,but at this time,the concentration of the droplets decreased and the surface visibility slightly increased.