A Novel Radar-Based Visibility Estimator

Remote visibility (Vis) estimation by radar is of interest to aviation, road traffic, and other fields. Millimeter-wave radars are suitable candidates because of such advantages as high spatial resolution and sensitivity to small droplets in reflection and attenuation. To investigate remote Vis estimation and to develop physics-based models and algorithms, a 35 GHz cloud radar at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the western part of the Netherlands has acquired data during fog periods in 'fog mode.' The advantage in using millimeter-waves for remote sensing of fog is that they interact strong enough with fog for sensing it, but not too strong so that they can penetrate the fog allowing to sense fog top. Simultaneously, fog drop size distribution (DSD) and Vis are continuously automatically measured by the in situ optical sensors at CESAR. Radar reflectivity (Z) and Vis can be linked theoretically since they are related to the sixth and the second moments of an assumed Gamma-shaped DSD. However, in reality the fog DSD is not always Gamma-shaped, leading to errors in the Vis estimation. A further development of the Vis-Z model includes the attenuation factor (La), which is proportional to the liquid water content at a given radar wavelength. This improves the estimated accuracy of Vis, in the theoretical moments-based model. Finally, we were able to arrive at a higher accuracy by introducing an empirical exponential model, estimating Vis from Z and La. A test based on DSD data sets for various fog types in the literature showed robust performance of the Vis-Z-La model for large variations in DSD. The Vis-Z-La model is also validated with the actual fog data sets that were collected by the in situ and remote sensing instruments synergy at CESAR.