Radar observations and modeling of fog at 35 GHz

Millimeter-wave radars are becoming prevalent in fog detection because their advantages over the other in situ and remote sensing instruments. The evaluation of a millimeter-wave radar's performance as a weather remote sensing tool must rely on assessment of the physical conditions governing scattering and absorption of millimeter waves which are determined by the wavelength, ambient temperature and particle properties (size, shape, dielectric properties etc.). Scattering and absorption characteristics of fog droplets are first investigated over a wide frequency range and temperature scale using Mie scattering theory. An assumption of modified gamma distribution represented as fog droplets size distribution is applied to retrieve radar reflectivity factor (often referred as reflectivity) and visibility respectively. A modeling relationship between radar reflectivity and visibility is then derived. A fog event at the Cabauw Experimental Site for Atmospheric Research during 17-18 February 2012 was observed with a synergy of ground-based in situ and remote sensing instruments, and radar reflectivity and visibility were measured with a 35 GHz cloud radar and in situ sensors respectively. Meanwhile, the information of fog droplets size distribution on those days was also given by a FSSP (Forward Scattering Spectrometer Probe), which is used to retrieve radar reflectivity and visibility as well. Both measurement results are compared with the model, and it indicates that the model basically coincide with the observations and can reflect the general case of fog layers at the Cabauw. This paper also illuminates a new way of developing a radar-based visibility estimator in terms of the link between radar reflectivity and visibility.