Radome attenuation - a serious limiting factor for quantitative radar measurements?

A cover of water, wet snow and/or ice on the radome, the protection sphere around the radar antenna, may cause serious signal attenuation. This is con firmed both by theoretical models and experimental measurements. An illustr ative example is the often observed lighthouse effect when a storm passes t he radar station. For qualitative work, such as detecting and tracking prec ipitation areas, the reduction of the signal may be acceptable, whereas for quantitative precipitation estimation it may have disastrous consequences. Modeling the complicated structure of the water cover on the radome is a r ather difficult task. Thus theoretical values of radome attenuation have to be compared with measurements. This paper estimates radome attenuation under natural conditions using rada r measurements of precipitation. Seven precipitation systems observed with the Swiss C-band Doppler radars have been analysed. Reflectivity measuremen ts before, during and after rain on the radome are compared. The average si gnal loss within the precipitation area gives a quantitative estimate of th e phenomenon. Complementary information is obtained from investigating refl ectivity probability distributions and reflectivity transformation matrices . Radome attenuation ranges up to 5.4 dBZ two-way in moderate rain, and thu s reduces the precipitation estimates to less than half. Results of theoret ical models lead to the same order of magnitude. It has not yet been shown to what extent errors can be reduced. Since radom e attenuation shows a complex spatial and temporal variation its correction will be difficult. We propose to carefully interprete data measured while the radome may be covered by water and to further investigate the phenomeno n.