Variability of drop size distribution over a mid-latitude regime and application to radar calibration

Polarimetric radar variables of rainfall events, like differential reflectivity (ZDR), or specific differential phase (KDP), are better suited for estimating rain rate (R) than just the reflectivity factor for horizontally polarized waves (ZH). A variety of physical and empirical approaches exist to estimate the rain rate from polarimetric radar observables. This relationship varies considering a wide range of variables, location and the weather conditions. In this study, polarimetric radar variables were calculated for C- band in order to obtain adequate radar rainfall estimations for the central area of Argentina.

The radar network over Argentina is sparse but there is growing interest in raising awareness of the atmosphere through this information. Two C-band radar operates over central Argentina since 2004 (Ezeiza and Pergamino) but recently (January 2009) two polarimetric radars has been deployed over the area (Paraná and Anguil).

Two Thies disdrometers were installed over the coverage of Parana and Ezeiza radars during September 2009 to January 2010 collecting a 4-month summer drop size distribution (DSD) over the area. In the present study, the polarimetric radar variables: ZH, ZDR, and KDP were calculated for DSD information available. The sensitivity of the calculated variables is investigated considering different rain drop shape models and possible errors in the DSD sample due to oversampling of large drops. The relationship between the rainfall and radar ZH and KDP is investigated during convective and stratiform periods over two Thies sites. Comparations between both variables and measurements from the radars were performed showing important differences over both radars, specially where polarimetric information is available. A Z-R and KDP–R relationship is proposed for each radar in order to obtain correct precipitation estimations over the area.