Observations and modeling of fog by cloud radar and optical sensors

Fog is a significant factor affecting the public traffic because visibility is reduced to a large extent. Therefore the determination of optical visibility in fog from radar instruments has received much interest. To observe fog with radar, high frequency bands (millimeter waves) have the best option. A 35 GHz cloud radar at the Cabauw Experimental Site for Atmospheric Research in the Netherlands has been used to make fog measurements in the “fog mode”. Meanwhile, in-situ visibility sensors (Biral SWS-100) and FSSP (Forward Scattering Spectrometer Probe) installed on a mast at Cabauw are also in use which provide visibility and drop size distribution (DSD) respectively. The synergy of remote sensing instruments at Cabauw is an essential prerequisite for fog observations. A fog case is analyzed in terms of the relations between radar reflectivity, visibility, liquid water content, and effective radius which depend on the DSD. In this paper, the modeling of fog is based on the assumption of gamma distribution as fog DSD, and is compared with the fog measurements. The results indicate the reliability of the model.