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 confirmed both by theoretical models and experimental measurements. An illustrative example is the often observed lighthouse effect when a storm passes the radar station. For qualitative work, such as detecting and tracking precipitation 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 rather difficult task. Thus theoretical values of radome attenuation have to be compared with measurements. This paper estimates radome attenuation under natural conditions using radar measurements of precipitation. Seven precipitation systems observed with the Swiss C-band Doppler radars have been analysed. Reflectivity measurements before, during and after rain on the radome are compared. The average signal loss within the precipitation area gives a quantitative estimate of the phenomenon. Complementary information is obtained from investigating reflectivity probability distributions and reflectivity transformation matrices. Radome attenuation ranges up to 5.4 dBZ two-way in moderate rain, and thus reduces the precipitation estimates to less than half. Results of theoretical models lead to the same order of magnitude. It has not yet been shown to what extent errors can be reduced. Since radome 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 phenomenon.