Retrieval of fog microphysical properties from cloud radar and optical sensors

Among the in-situ and remote sensing instruments, millimeter-wave radars are becoming much prevalent in remote fog detection attributed to their advantages in high spatial resolution and sensitivity. A 35 GHz cloud radar at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the western part of the Netherlands has been used in “fog mode” for the first time. In the meantime, visibility has been measured by in-situ visibility sensors fitted at manifold levels of a 213-m mast at Cabauw. Since radar reflectivity (Z), visibility (Vis), and the microphysical properties of fog are related to the moments of fog drop size distribution (DSD), therefore they can be linked via an assumption of DSD. Assuming the fog DSD follows gamma distribution, we derive a universal Vis-Z model which depends on the shape and scale parameter of the gamma distribution. The parameters can be estimated by fitting the gamma distribution to the fog DSD measurements. In this paper, fog DSD is measured by a Forward Scattering Spectrometer Probe (FSSP) fitted at 60-m level of the Cabauw mast. Ultimately, the entire radar signals can convert to visibility information based on the Vis-Z model, and the microphysical properties of fog can be retrieved from the measured Z and the estimated Vis. The retrieval method is demonstrated with the fog data sets obtained at Cabauw.