The effect of a wet radome on dualpol data quality

Operational radar systems typically are equipped with a radome. A radome protects the radar system and allows continuous operation under all weather conditions. The potential disadvantage of using a radome is that it may affect the outgoing and incoming microwave signal, in particular when the radome is wet. Generally, radomes for weather radars are optimized to minimize the transmission loss and to avoid azimuthal variations of the signal due to radome seams. Furthermore, the radome material is chosen to be hydrophobic, in order to reduce additional attenuation due to a wet radome surface. Radome attenuation has been discussed in a number papers where the effect typically is analyzed under idealized conditions and mainly for non-dualpol applications.

The level of attenuation also depends on rain fall intensity and type. The dependence on rainfall intensity has been investigated recently in Kurri and Huuskonen (2008).Under operational conditions, the attenuation of the radar signal may depend on elevation and azimuth.

For example, attenuation may be quite heterogeneous under strong wind conditions because the radome is not equally wet (e.g. Germann, 2000)

In this work we investigate the effect of radome attenuation on polarimetric moments. Data are taken from the dualpol C-band radar at the Hohenpeissenberg meteorological observatory. This radar is the quality assurance radar of the German radar net work. Operational data from 11 months are analysed and statistics are computed to identify the attenuation due to a wet radome and its variability depending on rain fall intensity and type. Volume scans with 5 minute resolution are available in STAR-Mode (10 elevation angles) and in H-mode (one elevation). Latter mode provides measurements of the linear depolarization ratio LDR, while the STAR mode volume provides copolar moments. The radar is protected by a 11-year old radome of orange-peel type. No dedicated maintenance has been applied to this radome. The independent reference precipitation measurement is a rain gauge at the radar site. These data with one-minute resolution are used to define the onset and end of a precipitation event over the radar site. Furthermore, the independent precipitation data allow the classification of the attenuation effect with respect to precipitation intensity and type. We also evaluate attenuation as a function of elevation angle.

The most prominent and systematic effect can be seen in the differential reflectivity where a bias up to 0.8 dB. Considering a target accuracy of 0.1-0.2 dB, such an attenuation error is critical for example for quantitative applications which rely on differential reflectivity as an input. We can also quantify the elevation dependence of attenuation which slightly increases at low elevation, as expected. The analysis also allows to quantify the typical drying time of the radome once the precipitation stops. The drying time is on the order of 30 minutes for our radome.

The wet radome effect on other dualpol moments (rho_hv, L_dr, Phidp) are shown in this work and implications for operational applications are discussed.

This analysis will serve as a reference data set against which the new radar system at Hohenpeissenberg will be evaluated. The new radar system will be installed within the radar replacement project RadSys-E, where the German weather radar network will be replaced by C-band dual polarization systems until 2013. In particular the performance of the new radome and its aging under operational conditions will be investigated and monitored using the quality assurance radar in order to optimize preventive maintenance plans which will be introduced with the new radar systems.