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Radar Issues

This technical document discusses the challenges of beam filling and attenuation related to precipitation for WSR-88D and WSR-74C radars. It reviews scan strategies and methodologies for mitigating issues arising from wet radomes and intervening precipitation. The study presents empirical models to understand the relationship between rainfall rate and radar signals, including results from significant events. It concludes that while beam filling is seldom a critical issue, wet radome attenuation significantly affects WSR-74C, whereas WSR-88D operates efficiently in such conditions.

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Radar Issues

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  1. Radar Issues Francis J. Merceret NASA/YA-D 12 November 2002

  2. Beam Filling Scan Strategy Attenuation Wet Radome Intervening Precipitation Issues and Instruments • WSR-88D (NEXRAD) • 10 cm • Doppler • NWS/MLB • WSR-74C • 5 cm • PAFB

  3. Beam Filling – Radar Characteristics

  4. Beam Filling – Equivalent Attenuation, WSR-88D Size Range Eff. Attn. as function of feature size (Km) and range (Km)

  5. Beam Filling – Equivalent Attenuation, WSR-74C Size Range Eff. Attn. as function of feature size (Km) and range (Km)

  6. Scan Strategy – WSR-88D

  7. Scan Strategy – WSR-74C (old)

  8. Scan Strategy – WSR-74C (new)

  9. Wet Radome Attenuation - Methodology • Literature search (print and electronic) • Compile into single database • Fit empirical formula: L=C*R*tanh2(F/10) R = rainfall rate (mm/hr) F = frequency (GHz) C = 0.165 (standard), 0.0575 (hydrophobic) L = two-way loss (dB)

  10. Rain Rate (mm/Hr) S-Band Hydrophobic S-Band Standard C-Band Hydrophobic C-Band Standard 1 0.01 0.03 0.03 0.10 2 0.02 0.06 0.07 0.19 5 0.05 0.14 0.17 0.48 10 0.1 0.28 0.33 0.95 20 0.2 0.56 0.66 1.9 50 0.49 1.4 1.66 4.8 100 0.98 2.8 3.32 9.5 200 1.95 5.6 6.63 19 Wet Radome Attenuation - Results

  11. Wet Radome Attenuation – Example: 24 June 2001 WSR-88D WSR-74C

  12. Precipitation Attenuation - Methodology • Literature search for relationships among R(mm/hr), Z(mm6/m3) and M(g/m3) • Stratiform rain, convective rain, snow, and Marshall-Palmer precipitation types • Select worst case relationship and worst case type (worst case = highest predicted attenuation for given measured Z) • Empirical Model: A(dB/Km)=a*10^(b*dBZ) where a and b are wavelength-dependent constants

  13. Precipitation Attenuation - Results

  14. Conclusions • Beam filling is rarely a concern • Each radar has scan pattern gaps that might be significant in a particular location, but use of both radars together can mitigate this • Wet radome attenuation is a major problem for the WSR-74C, but not for the WSR-88D • Precipitation attenuation can be a major problem for quantitative measurements

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