Dennis Miller Hydrometeorology Group Hydrology Laboratory Office of Hydrologic Development

1. Mitigating Effects ofTerrain Beam BlockageOn Radar QPE Estimates – Dual-pol and Horizontal-pol solutions Dennis Miller Hydrometeorology Group Hydrology Laboratory Office of Hydrologic Development NOAA/National Weather Service

2. Study: • Many areas have problems with inconsistent radar QPE due to partial beam blockage • Current default assumption is to use Z-R estimates, corrected for known terrain blockage, up to 50% blockage • Rainrates from Kdp (specific differential phase) are to be used in dual-pol QPE in the presence of hail • Kdp is not believed to be sensitive to beam blockage • Test assumptions on how much blockage can be accounted for in Z-R estimates, and if Kdp is a superior rainrate estimator in partially-blocked situations

3. Figure 1a. Beam blockage at 0.50 for Colorado on S-Pol grid centered at S-Pol site east of Boulder. Figure 1b. Same as Fig 1a, but on ~4x4 km2 HRAP gridS-Pol grid.

4. a b c d e Figure 2. 1-hour accumulations at 0.50 for Colorado on ~4x4 km2 HRAP grid centered at S-Pol site east of Boulder, ending 07/04/2006, 00z: a) R(Z0); b) R(Z90); c) R(n-kdp); d) R(Kdp-h); also e) Stage IV (verification).

5. a b c d e Figure 3. Long-term, summer-season accumulations(161 hours during June, July, August 2006) at 0.50 for Colorado on ~4x4 km2 HRAP grid centered approx. at the S-Pol site: a) R(Z0); b) R(Z90); c) R(n-kdp); d) R(Kdp-h);also e) Stage IV (verification). Grid points determined by subjective QC analysis to be substantially contaminated by clutter were removed (appear as background black) and not included in statistical analyses. Note, in comparison to Fig. 1, the general correspondence of the greater accumulations with the higher terrain/partially blocked regions of the Rockies Front Range to the north and south of the radar location, and of the lesser accumulations with the Plains/unblocked regions to the east. The high Rockies to the west of the radar are totally blocked and not included in the analysis.

6. Figure 5. S-Pol to Stage IV ratios, stratified by blockage category.

7. a Figure 3b. Same as Fig. 3a but for Correlation Coefficient. b Figure 6. S-Pol to Stage IV correlation coefficients, for (a) rainfall total over all 161 h combined, and (b), all nonzero hourly totals considered individually. Correlations in (a) are essentially those for the spatial pattern of total precipitation

8. Conclusions • In warm-season convective environments, correcting horizontal-pol reflectivity for beam blockage up to at least 75% is highly effective (current default for using next-higher beam is 50%) • R(Kdp) differential phase QPE are about as good, overall, as Z-R QPE, but might be affected by beam blockage • Results might be specific to S-band radar, and possibly the NCAR S-pol unit, which has some differences with WSR-88D

9. a b Figure 4. Average precipitation as a function of blockage category,by various QPE methodologies (blue: R(Z0); red: R(Z90); white: R(n-Kdp); turquoise: R(Kdp-h)) and Stage IV verification (brown or purple). (a) Average, areal seasonal totals (0-25% blockage: 30,728 HRAP grid boxes; 25-50%: 5,006 boxes; 50-75%: 8,587 boxes; 75-90%: 4,107 boxes; all cases together: 48,428 boxes); (b) average non-zero hourly amounts (0-25%: 1,464 HRAP grid boxes; 25-50%: 148 boxes; 50-75%: 196 boxes; 75-90%: 79 boxes; and all cases together: 1,887 boxes).

10. a b Figure 7. As in Fig. 6, except for Error Standard Deviation.