Topographic Radar Beam Blockage and its Effects on Radar Rainfall Estimates: The BREAM Model Timothy A. Coleman Atmospheric Science Department The University of Alabama in Huntsville

1. Introduction Radar beam may be completely blocked by mountains, eliminating useful data beyond that range. Radar beam may be partially blocked by terrain, introducing errors into all radar data beyond the blocking terrain feature. Assumption is made that the radar beam loses transmitted, and thus returned power, proportional to the percentage area of the beam that is blocked. Using GLOBE Digital Elevation Model Data, along with radar propagation equations, to estimate beam blockage at each point. Similar to the approach by Kucera et al. (2004).

2. Radar beam propagation Assuming standard refraction and earth curvature: (Rinehart 1997) Also, for simplicity, assuming a uniform beam, and recognizing that the area of the beam blocked by terrain of height h is given by: H = (r 2 + R’ 2 + 2rR’sin  ) 1/2 – R’ + H o A B = a 2 arccos((a-h)/a) – (a-h) (a 2 -(a-h) 2 ) 1/2

3. Topographic beam blocking

4. Modeling topographic beam blocking - BREAM GLOBE DEM terrain data after conversion to polar coordinates around BMX WSR-88D Azimuth 15 degrees from WSR-88D at BMX, 46.3% of beam blocked beyond 20 km range, resulting in 2.7 dBZ reflectivity error

BREAM model beam blocking (dB) by azimuthal direction Actual ground clutter observed at BMX radar. Note good agreement with BREAM model. 4. Modeling topographic beam blocking - BREAM

BREAM can also compute, by azimuth, the minimum beam elevation required for a radar to completely avoid beam blocking

5. Beam Blocking Effects – Light Rain Rain-gauge measurements superimposed on radar-estimated precip. Note that, despite heavier rain in Jefferson County, radar shows lighter rain.