Ground-Based Observations and Modeling of the Visibility and Radar Reflectivity in a Radiation Fog Layer

The development of a radiation fog layer at the Cabauw Experimental Site for Atmospheric Research(51.97°N, 4.93°E) on 23 March 2011 was observed with ground-based in situ and remote sensing observationsto investigate the relationship between visibility and radar reflectivity. The fog layer thickness was less than200 m. Radar reflectivity values did not exceed 225 dBZ even with visibilities less than 100 m. The onset andevaporation of fog produce different radar reflectivity-visibility relationships. The evolution of the fog layerwas modeled with a droplet activation model tha used the aerosol size distribution observed at the 60-m altitude tower level as input. Radar reflectivity and visibility were calculated from model drop ize spectra using Mie scattering theory. Since radiative cooling rates are small in comparison with cooling rates due to adiabatic lift of aerosol-laden air, the modeled supersaturation remains low so that few aerosol particles are activated to cloud droplets. The modeling results suggest that the different radar reflectivity-v sibility relationshipsare the result of differences in the interplay between water vapor and cloud droplets during formation and evaporation of the fog. During droplet activation, only a few large cloud droplets remain after successfully competing for water vapor with the smaller activated droplets. These small droplets eventually evaporate (deactivate) again. In the fog dissolution/evaporation stage, only these large droplet need to be evaporated.Therefore, to convert radar reflectivity to visibility for traffic safety products, knowledge of the state of local fog evolution is necessary. © 2013 American Meteorological Society.