Rainfall attenuation of centimeter waves: Comparison of theory and measurement

This paper is a critical survey of the measurement of rainfall by remote sensors. It is shown that single‐parameter radar rainfall measurements are limited because both reflectivity and rain rate are dependent on at least two parameters of the drop size distribution, viz., representative raindrop size and number concentration. Simulations are presented which use experimental raindrop size spectra and show the improvement in accuracy attainable through the use of combinations of two and three remote measurables. The spectrum of remote measurables is then reviewed. These include path integrated techniques of radiometry and of microwave and optical attenuation. A carefully designed short‐path microwave attenuation experiment which employs these techniques is sufficiently persuasive to show that the disappointing results achieved in many others was due largely to a combination of rain sampling problems and vertical air motions between the path and the gages. Other experiments reviewed show that when paths are colinear, attenuation deduced from radar and radiometry is in good agreement with that measured directly. Several dual‐wavelength radar methods are described which were aimed at improved measurements in small range increments but have produced generally disappointing results. However, when the total path attenuation estimated in this way, or by radiometry, is used as a constraint on the retrieval of rain profiles from the radar, the results are more promising. Selected experiments involving combinations of two or more of the three measurables, radar reflectivity Z, attenuation, and/or radiometry, show considerable promise when adequate account is taken of the sampling and air motion problems. The best results in gate‐by‐gate measurements have been achieved with dual polarization or differential reflectivity (ZDR). However, even these are subject to potentially large errors when the rainfall does not behave according to a priori assumptions embodied in the technique. An accompanying paper in this issue shows that the use of a third remote parameter in addition to Z and ZDR offers great promise. With a growing appreciation of the needs and the capabilities of the various techniques, the future of highly improved remote rainfall measurements seems bright.

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“…7) show much greater scatter and are more typical of the results reported in Medhurst's (1965) review.…”

Section: Ground To Groundsupporting

confidence: 60%

“…In an early study comparing theory and measurement Medhurst (1965) reported disappointing results. He found very wide scatter and a clear tendency for measured attenuations to exceed maximum theoretical values (i.e.. those corresponding to monodisperse drop sizes having the maximum extinction cross-section).…”

Section: Ground To Groundmentioning

confidence: 99%

The multiparameter remote measurement of rainfall

1984

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“…The results reported by Gunn and East (1954), Wexler and Atlas (1963) and Medhurst (1965) suggest that the attenuation of millimeter waves is too serious to be ignored in the observation of the precipitation in the case that the precipitation rate is greater than a few mm/hr. On the contrary, if the precipitation is weak as in the stratiform precipitations, the attenuation of X-and C-band rader waves by the precipitation particles is insignificant.…”

Section: Introductionmentioning

confidence: 96%

Two-Wavelength (0.86cm and 3.2cm) Radar Observation of Microphysical Process in Stratiform Precipitation

Yokoyama

1985

Journal of the Meteorological Society of Japan

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Strong attenuation of millimeter wave due to precipitation particles is the most serious problem when we use K-band (0.86cm wavelength) radar for the observation of the precipitation.In this paper, we calculated the attenuation of K-, X-(3.2cm) and C-band (5.6cm) waves based on a model of precipitation, for three precipitation intensities, i, e., heavy (32mm/hr), ordinate (4mm/hr) and weak (0.5mm/hr) precipitations. And, we surveyed any possibility of the observation with weak affection of the attenuation using the bands. The observation of the precipitation was executed in Part II.The results show that the attenuation coefficient is larger in the melting layer by one order or two orders than in the lower raindrop region.The results also indicate that the affection of the size distribution to the attenuation abruptly increases as the attenuation is enhanced.The attenuations of X-and C-band waves are small enough to be neglected except for the heavy precipitation.In the case of heavy precipitation, K-band wave is severely attenuated, especially in the melting layer, so that the signals cannot be detected by the radar receiver.In the cases of ordinary and weak precipitations, however, we can obtain true radar reflectivity factor of K-band wave by corrections of the attenuation. In addition, the correction method using the radar reflectivity factor of the X-band radar was investigated.We can mention conclusively that after the correction the precipitations weaker than about 4*7mm/hr can be safely observed within an error of 2dB by the twowavelength radar technique using K-and X-band radars in the raindrop region.In the melting layer, the correction method can be utilized without large error when the precipitation is weaker than about 2*4mm/hr.

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“…(27) and (28). Since a steady signal and constant noise power are assumed, all the vi<s and at's in these equations are equal; hence for M pulses integrated the results are vM= M t (30) and…”

Section: I=l1mentioning

confidence: 99%

“…This is the half-width of the absorption line at the half-maximum point. 30,000 to 80,000 foot region. The increase at 10,000 feet is about 3 percent, and of course there is no change at zero altitude where the absorption is greatest.…”

Section: Propagation-medium Losses Tropospheric Absorptionmentioning

confidence: 99%

A Guide to Basic Pulse-Radar Maximum-Range Calculation. Part 2. Derivations of Equations, Bases of Graphs, and Additional Explanations

Blake¹

1969

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Rainfall attenuation of centimeter waves: Comparison of theory and measurement

Cited by 176 publications

References 13 publications

The multiparameter remote measurement of rainfall

The multiparameter remote measurement of rainfall

Two-Wavelength (0.86cm and 3.2cm) Radar Observation of Microphysical Process in Stratiform Precipitation

A Guide to Basic Pulse-Radar Maximum-Range Calculation. Part 2. Derivations of Equations, Bases of Graphs, and Additional Explanations

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