Measuring Rainwater Content by Radar Using Propagation Differential Phase Shift

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  • 1 Applied Research Corporation, Landover, Maryland
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Abstract

While radars measure several quantities closely coupled to the rainfall rate, for frequencies less than 15 GHZ, estimates of the rainwater content W are traditionally computed from the radar reflectivity factor Z or the rate of attenuation A—quantities only weakly related to W. Consequently, instantaneous point estimates of W using Z and A are often erroneous.

A more natural, alternative parameter for estimating W at these frequencies is the specific polarization propagation differential phase shift ΦDP, which is a measure of the change in the difference between phases of vertically (V) and horizontally (H) polarized waves with increasing distance from a radar. It is now well known that W is newly linearly related to ΦDP divided by (1 − Я), where Я is the mass-weighted mean axis ratio of the raindrops. Unfortunately, such relations are not widely used in part because measurements of ΦDP are scarce but also because one must determine Я. In this work it is shown that this parameter can be estimated using the differential reflectivity (ZH/ZV) at 3 GHz. An alternative technique is suggested for higher frequencies when the differential reflectivity becomes degraded by attenuation.

While theory indicates that it should be possible using ΦDP to estimate W quite accurately, measurement errors increase the uncertainty to ±18%–35% depending on Я. While far from ideal, it appears that these estimates are likely to be considerably more accurate than those deduced using currently available methods.

Abstract

While radars measure several quantities closely coupled to the rainfall rate, for frequencies less than 15 GHZ, estimates of the rainwater content W are traditionally computed from the radar reflectivity factor Z or the rate of attenuation A—quantities only weakly related to W. Consequently, instantaneous point estimates of W using Z and A are often erroneous.

A more natural, alternative parameter for estimating W at these frequencies is the specific polarization propagation differential phase shift ΦDP, which is a measure of the change in the difference between phases of vertically (V) and horizontally (H) polarized waves with increasing distance from a radar. It is now well known that W is newly linearly related to ΦDP divided by (1 − Я), where Я is the mass-weighted mean axis ratio of the raindrops. Unfortunately, such relations are not widely used in part because measurements of ΦDP are scarce but also because one must determine Я. In this work it is shown that this parameter can be estimated using the differential reflectivity (ZH/ZV) at 3 GHz. An alternative technique is suggested for higher frequencies when the differential reflectivity becomes degraded by attenuation.

While theory indicates that it should be possible using ΦDP to estimate W quite accurately, measurement errors increase the uncertainty to ±18%–35% depending on Я. While far from ideal, it appears that these estimates are likely to be considerably more accurate than those deduced using currently available methods.

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