P2.7 ATTENUATION OF RADAR SIGNAL IN MELTING HAIL AT C BAND L. Borowska (1)* , A. Ryzhkov (2,3) , D. Zrnic (3) , P. Zhang (2,3) , J. Gu (4) , P. Neilley (5) , M. Knight (5) , R. Palmer (6,7) , B. Cheong (7) , A. Battaglia (1) , C. Simmer (1) (1) Meteorological Institute at the University of Bonn, Germany (2) Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma, USA (3) National Severe Storms Laboratory, USA (4) Korea Meteorological Administration, Seoul, Republic of Korea (5) Weather Services International / Enterprise Electronics Corporation, USA (6) School of Meteorology, University of Oklahoma, USA (7) Atmospheric Radar Research Center at the University of Oklahoma, USA 1. INTRODUCTION Anomalously high attenuation and differential attenuation at C band have been observed in recent studies (Ryzhkov et al. 2007; Tabary et al. 2008; Vulpiani et al. 2008). The questions are: (1) what is the cause of anomalous attenuation and (2) how to correct for it? It is not clear yet if it is caused by heavy rain containing large drops or melting hail, or both. High specific differential attenuation A DP has often been manifested by large negative Z DR in the rear side of strong convective cells. It is obviously caused by non- spherical oriented hydrometeors. Thus the key question here concerns its relation to the specific attenuation at horizontal polarization A h ; i.e., are the two correlated? Everything else being the same, large differential attenuation favors the use of Z v rather than Z h in regions where it is observed. In this paper, attenuation and differential attenuation at C band are estimated in storm cells suspect of having hail or rain/hail mixture. The estimates are made by comparing Z and Z DR measured with a nearly collocated S band radar in Norman OK. 2. EXPERIMENTAL SETUP The C-band radar (OU Prime) is located in Norman, Oklahoma and belongs to the University of Oklahoma. Its main characteristics are listed in Table 1. The nearly collocated S band radar is the NOAA’s research version WSR-88D (designated as KOUN) that has dual polarization (Melnikov et al. 2003). Both radars operated in the so-called SHV mode, i.e. the horizontally and vertically polarized fields are transmitted and received simultaneously. Besides the difference in * Corresponding author address: L. Borowska, Meteorological Institute at the University of Bonn, Germany; e-mail: borowska@uni-bonn.de wavelengths, the two radars differ in beamwidth (0.5 deg for OU Prime vs ~ 1 deg for KOUN), resolution volume depth (125 m vs 250 m) and peak power (1 MW vs ~ 750 kW). Parameter Value Wavelength 5.44 cm Antenna beam width 0.5 deg Peak transmitter power 1 MW Pulse depth (variable) nominal 125 m Pulse repetition time (variable) nominal 0.8 ms TABLE 1: Characteristics of the OU Prime radar. The radars are separated by about 6 km. This separation adds to the uncertainty of comparisons. Nonetheless, there are no closer collocated stationary dual polarization weather radars anywhere that we know of for similar comparisons. 3. MEASUREMENTS On several occasions, polarimetric variables have been recorded quasi-simultaneously by both radars. The radars operated in surveillance scans and the beginnings of volume scans were not synchronized but the differences in time did not exceed 3 min. Cells for comparisons were chosen based on the reflectivity factor Z(S) of KOUN (11 cm wavelength) and similarity of structure observed by the two radars. In this paper, data from the cells with Z h > 55 dBZ are analyzed. We have chosen two good cases for comparison of S- and C-band data: 03/10/2009 (squall line) and 03/27/2009 (strong isolated hailstorms). In the first case, hail was likely aloft but not on the ground (according to the Storm Data). In the second case, hail of 3/4'' diameter was reported on the ground. Both cases reveal anomalously high attenuation and differential attenuation. For the second case the drop in Z DR at C-band (down to -10 dB) was particularly larger and connected also to a larger