Influences of Precipitation on Atmospheric Extinction Coefficient and Visibility

The variation of visibility is mainly influenced by the scattering and absorption of atmospheric particles at visible wavelengths. The principle of visibility sensors are based on the relationship between visibility and atmospheric extinction coefficient, which is integrated by scattering coefficient of atmospheric particles within a volume. The influence of aerosol, dust particles and smoke are usually taken into account in the traditional visibility analysis. However, as one important factor affecting visibility, precipitation particles are usually neglected, causing obvious inaccuracy when precipitation occurs. Physical characteristics (size, velocity, density, shape, size distribution) of precipitation particles are analyzed and then their influences on atmospheric extinction coefficient and visibility are discussed based on scattering characteristic of raindrops and snowflakes. The relationships of rainfall and snowfall with visibility are obtained by numerical simulation based on fitting size distribution parameters, which are validated by observation data of precipitation and visibility obtained by Parsivel precipitation particle spectrometer in Nanjing. The comparisons show that the numerical simulations are in good agreement with the observation data. The conclusions are as follows: The visibility decreases exponentially as the rainfall/snowfall intensity increases; however, the influence of rainfall and snowfall on visibility is different, because the atmospheric extinction is mainly affected by the types, density, velocity, sizes and distribution of the precipitation particles; with the same precipitation intensity, smaller particles leads to lower visibility; the theoretical relationship of precipitation intensity to visibility takes on a good accordance with the observation data while considering the size distribution and ice crystal types; and the influences of snowfall on visibility are more complex because the atmospheric extinction snowfall are related to the snow/crystal types. Finally the influences of precipitation on visibility are confirmed by the combination of observation data and theoretical analysis. The influences of precipitation particles on atmospheric extinction coefficient and visibility are discussed on the assumption that the aerosols are totally cleared by rain or snow in this model, which results in the overestimation of visibility. To obtain more accurate relationship of visibility and precipitation, the influence of aerosols and its variation with time should be considered, which deserves further investigation.