Polarization Radar Measurements in Rain at 5 and 9 GHz

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

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Abstract

There is increased interest in dual-polarization radar measurements at 9 and 5 GHz not only for scientific reasons but also because such radars are less expensive to build and are easier to transport than 3-GHz S-band radars. Unfortunately, measurements at 5 and 9 GHz are affected by attenuation, particularly in rain. In this paper, potential techniques for measuring rainfall rate R and rainwater content W at these frequencies are explored. While several are promising, their success depends critically on how well the effects of attenuation can be removed from measurements of differential reflectivity ZDR and radar reflectivity factor at horizontal polarization ZH.

This study explores a technique proposed by others to use the polarization propagation differential phase shift with increasing distance from the radar (ΦH–V)to estimate and remove the effects of specific and polarization differential attenuation from ZH and ZDR, respectively. It is shown that in rain this technique is sensitive to variations in the drop-size distribution. Nevertheless, this analysis confirms that ΦH–V, can be used to extend the distance over which useful measurements of ZH and ZDR can be obtained. Unfortunately, residual errors introduced by attenuation and the “noise” from the correction scheme itself eclipse the potential of many possible techniques for quantitative rainfall measurements at these frequencies. On the bright side, however, the attenuation recovery scheme appears adequate for obtaining useful polarization microphysical measurements particularly above the melting level, even after encounters with rain.

Abstract

There is increased interest in dual-polarization radar measurements at 9 and 5 GHz not only for scientific reasons but also because such radars are less expensive to build and are easier to transport than 3-GHz S-band radars. Unfortunately, measurements at 5 and 9 GHz are affected by attenuation, particularly in rain. In this paper, potential techniques for measuring rainfall rate R and rainwater content W at these frequencies are explored. While several are promising, their success depends critically on how well the effects of attenuation can be removed from measurements of differential reflectivity ZDR and radar reflectivity factor at horizontal polarization ZH.

This study explores a technique proposed by others to use the polarization propagation differential phase shift with increasing distance from the radar (ΦH–V)to estimate and remove the effects of specific and polarization differential attenuation from ZH and ZDR, respectively. It is shown that in rain this technique is sensitive to variations in the drop-size distribution. Nevertheless, this analysis confirms that ΦH–V, can be used to extend the distance over which useful measurements of ZH and ZDR can be obtained. Unfortunately, residual errors introduced by attenuation and the “noise” from the correction scheme itself eclipse the potential of many possible techniques for quantitative rainfall measurements at these frequencies. On the bright side, however, the attenuation recovery scheme appears adequate for obtaining useful polarization microphysical measurements particularly above the melting level, even after encounters with rain.

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