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Path- and Area-Integrated Rainfall Measurement by Microwave Attenuation in the 1–3 cm Band

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  • a National Center for Atmospheric Research, Boulder, Colo. 80303
  • | b Department of Physics and Astronomy, Clemson University, Clemson, S. C. 29631
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Abstract

The reasons for the linear relationship between microwave attenuation A and rainfall rate R near 1 cm wavelength are explained. This linearity also implies independence of the A-R relationship from the drop size distribution (DSD), thus making attenuation measurements near this wavelength attractive for path-averaged rainfall. Regression equations of the form A = KRα are calculated for four radar wavelengths from 0.86 to 3.2 cm from drop size spectra. As predicted, α increases from about 1.04 to 1.16 and average errors of estimate of R from the regression equations increase from about 9 to 21% from 0.86 to 3.2 cm, respectively. The larger errors at 3.2 cm reflect the increased dependence on the form of DSD. Even at 3.2 cm, the errors are typically less than half those incurred from the use of reflectivity factor Z and a priori Z-R relations.

Various methods of measuring path- and area-averaged R are studied. Radar methods using standard targets fail over 30 km paths at wavelengths of 0.86 and 1.25 cm at R greater than about 9 and 20 mm h−1, respectively, because of excessive attenuation but are operative to larger mean rates at 1.78 and 3.2 cm. One-way methods between transmitter and receiver are the most suitable in terms of maximum measurable R. A wavelength between 1.5 and 2 cm provides a reasonable compromise between maximum measurable R and minimum errors. Proper measurements require the use of both vertical and horizontal polarization. Prior experiments are reviewed and explanations offered for both the large scatter in the results of some experiments and the occasional excess attenuation over theoretical prediction.

Abstract

The reasons for the linear relationship between microwave attenuation A and rainfall rate R near 1 cm wavelength are explained. This linearity also implies independence of the A-R relationship from the drop size distribution (DSD), thus making attenuation measurements near this wavelength attractive for path-averaged rainfall. Regression equations of the form A = KRα are calculated for four radar wavelengths from 0.86 to 3.2 cm from drop size spectra. As predicted, α increases from about 1.04 to 1.16 and average errors of estimate of R from the regression equations increase from about 9 to 21% from 0.86 to 3.2 cm, respectively. The larger errors at 3.2 cm reflect the increased dependence on the form of DSD. Even at 3.2 cm, the errors are typically less than half those incurred from the use of reflectivity factor Z and a priori Z-R relations.

Various methods of measuring path- and area-averaged R are studied. Radar methods using standard targets fail over 30 km paths at wavelengths of 0.86 and 1.25 cm at R greater than about 9 and 20 mm h−1, respectively, because of excessive attenuation but are operative to larger mean rates at 1.78 and 3.2 cm. One-way methods between transmitter and receiver are the most suitable in terms of maximum measurable R. A wavelength between 1.5 and 2 cm provides a reasonable compromise between maximum measurable R and minimum errors. Proper measurements require the use of both vertical and horizontal polarization. Prior experiments are reviewed and explanations offered for both the large scatter in the results of some experiments and the occasional excess attenuation over theoretical prediction.

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