A Dual-Frequency Microwave Technique for Measuring Rainwater Content

A. R. Jameson Applied Research Corporation, Landover, Maryland

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Abstract

While there are many microwave techniques for estimating the rainfall rate, there are presently few if any for accurately determining the rainwater content (W). This study shows that the dual-frequency (38, 25 GHZ) differential attenuation (A38−25) coefficient can provide accurate estimates of W potentially over a wide range of rainwater contents.

While measurements along a microwave link are fairly rally implemented, radar estimate of A38−25 can become clouded by differences between the radar reflectivity factors at the two frequencies (Z38, Z25). Root-mean-square deviations (ε) of the estimated W from the actual Ware calculated for a wide variety of drop-size distributions and rainwater contents. The computed ε include the effects of standard measurement errors and differences between Z38 and Z25. Accurate estimates (ε≦∼25%) appear possible using a 38–25-GHZ radar when W≧1.5−2 g m−3, depending upon the desired spatial resolution, and along a microwave link when W≧0.5 g m−3. However, extending microwave measurements to smaller W will probably require frequencies higher than 40 GHz. Consequently, it appears unlikely that microwave techniques for estimating W will ever be capable of the long penetration distances enjoyed by several methods for estimating rainfall rate.

Abstract

While there are many microwave techniques for estimating the rainfall rate, there are presently few if any for accurately determining the rainwater content (W). This study shows that the dual-frequency (38, 25 GHZ) differential attenuation (A38−25) coefficient can provide accurate estimates of W potentially over a wide range of rainwater contents.

While measurements along a microwave link are fairly rally implemented, radar estimate of A38−25 can become clouded by differences between the radar reflectivity factors at the two frequencies (Z38, Z25). Root-mean-square deviations (ε) of the estimated W from the actual Ware calculated for a wide variety of drop-size distributions and rainwater contents. The computed ε include the effects of standard measurement errors and differences between Z38 and Z25. Accurate estimates (ε≦∼25%) appear possible using a 38–25-GHZ radar when W≧1.5−2 g m−3, depending upon the desired spatial resolution, and along a microwave link when W≧0.5 g m−3. However, extending microwave measurements to smaller W will probably require frequencies higher than 40 GHz. Consequently, it appears unlikely that microwave techniques for estimating W will ever be capable of the long penetration distances enjoyed by several methods for estimating rainfall rate.

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