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K. Aydin
and
T. A. Seliga

Abstract

Conical graupel is modeled using sphere-cone-oblate spheroidal shapes for the purpose of computing their backscattering properties at a wavelength of 10 cm in terms of the radar polarimetric observables, reflectivity factors, differential reflectivities and circular depolarization ratios. A shape distribution based on in situ measurements is used together with gamma (m=0, 2) size distributions in the computations; both wet and dry graupel are considered. Significant differences in the radar observables between the wet and dry cases are noted and the effects of canting on the radar observables are considered. The implications of these results on the differentiation of hydrometeor phase with radar are discussed.

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T. A. Seliga
and
V. N. Bringi

Abstract

The potential use of differential reflectivity measurements at orthogonal polarizations to determine rain-fall rate is examined. The method involves measurements of ZH and ZV , the radar reflectivity factors due to horizontally and vertically polarized incident waves respectively. The differential reflectivity, ZDR = 10 log (ZH /ZV ), which should be precisely determinate, occurs as a result of the distortion of raindrops as they fall at terminal velocity. The approximate theory of Gans for electromagnetic scattering by spheroids is applied to the distorted raindrops. Assuming a general exponential form for the raindrop size distribution, equations are derived relating the distribution parameters to the measurements. The determination of rainfall rate follows directly. Finally, the sensitivity of the distribution parameters to radar inaccuracies is examined, and several methods of implementing the measurements are suggested. It is concluded that good estimates of rainfall rate using a single non-attenuating wavelength radar are possible under ideal conditions.

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K. Aydin
,
T. A. Seliga
, and
V. Balaji

Abstract

A technique for the remote sensing of hail with an S-band dual linear polarization radar is described. The method employs a new hail signal HDR , which is derived from disdrometer measurements of raindrop size distributions. Experimental measurements, made in Colorado with the National Center for Atmospheric Research's (NCAR) CP-2 radar system, are used to demonstrate the technique in two major hailstorms.

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T. A. Seliga
,
K. Aydin
, and
H. Direskeneli

Abstract

Empirical relationships for estimating rainfall rate, liquid water content and median volume diameter from radar measurements of reflectivity factor and differential reflectivity are derived from a disdrometer record of a highly variable, heavy rainfall event in central Illinois. Comparisons with relationships representing exponential and gamma model drop-size distributions are made. Simulations, employing these and Z-R relationships for rainfall estimation, are performed. Statistical measures are tabulated for comparing results. These show an excellent agreement between the disdrometer- and radar-derived rainfall parameters when the latter are obtained from the empirical relationships.

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K. Aydin
,
Y. Zhao
, and
T. A. Seliga

Abstract

A differential reflectivity radar technique for observing hailstorms is demonstrated using measurements obtained during the 13 June 1984 Denver hailstorm. The hail regions of the storm are identified with the differential reflectivity hail signal. Histograms of ZH and ZDR are generated for different heights in the hail regions and the relative variation of these parameters is also determined. It is observed that due to melting, the mean values of ZH and ZDR increase with decreasing height below the 0°C level (which is around 2.6 km above ground level). Furthermore, at the lower levels ZDR varies between −1 and +2 dB and ZH is generally greater than 50 dBZ. The value of ZH peaks at around 60 dBZ or more when ZDR is in the range −0.5 to 0 dB and 1.5 to 2 dB at 1.5 km and 2 km below the 0°C level, respectively. These and other features of ZH and ZDR are interpreted in terms of the size, shape and fall behavior of the hailstones using the dual wavelength ratio and the linear depolarization ratio radar measurements together with results from Battering computations. The negative ZDR , regions in this storm are inferred to be most likely composed of melting hailstones with sizes predominantly in the 12 to 40 mm range, which fall with their larger dimensions aligned (on the average) vertically. The positive ZDR values greater than 1 dB are concluded to be due to melting hailstones with sizes less than 12 mm, which fall with their larger dimensions aligned (on the average) horizontally.

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G. Scarchilli
,
E. Gorgucci
,
T. A. Seliga
, and
K. Aydin

Abstract

Dual linear polarization weather radars measure as primary observables the mean power H, and V, corresponding to returns at horizontal and vertical polarizations, respectively. Differential reflectivity Z DR is defined as the ratio between these two measurements. Under the assumption of an exponential drop-size distribution, characterized by the two parameters N 0 and D 0, it has been shown that Z DR may be used to estimate the median volume diameter D 0, following which the parameter N 0 and, therefore, other drop-size distribution-dependent quantities, may be determined from the horizontal reflectivity Z H.

In this paper the effects of reflectivity gradients, due to the variation of the drop-size distribution within the radar scattering volume, on the radar observables (Z H, Z DR) and derived rainfall rates are examined for radar observations with a stationary antenna. The bias of the estimates, their standard errors, and the optimum receiver response are computed for power law and logarithmic receivers. Finally, for the special case of the square law receiver, the contrasting effects due to either similar or opposing signs of the gradients of the parameters N 0 and D 0 are evaluated.

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T. A. Seliga
,
V. N. Bringi
, and
H. H. Al-Khatib

Abstract

Radar measurements of average rainfall rate over two separate 550 km2 areas am compared with raingage measurements in the same areas over time intervals of ∼1 h. The measurements were performed to test the differential reflectivity (ZDR ) technique of Soliga and Bringi (1976) which provides a means of estimating rainfall rate (R)by combining measurements of radar reflectivity factors at horizontal (ZH ) and vertical (ZV ) polarizations; ZDR (dB) is defined as the ratio of these reflectivities, i.e., ZDR = 10 log(ZH /ZV ). Results from an experiment performed near Chicago, Illinois, on 9 August 1978, using the University of Chicagco-Illinois State Water Survey (CHILL) radar and the Illinois State Water Survey's Chicago Hydrometeorological Area Project (CHAP) raingage network are presented. ZDR estimates of rainfall rate compared very favorably with the raingages measurements and were significantly better than estimates obtained from two Z-R relationships, one of which was obtained by raingage calibration. Over one of the 550 km2 areas the ZDR method measured 86% of the rainfall recorded on 26 raingages during an observational period of 80 min, and over the other, 117% of the rainfall recorded on 27 raingages was obtained during a 40 min period.

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D. Giuli
,
M. Gherardelli
,
A. Freni
,
T. A. Seliga
, and
K. Aydin

Abstract

The presence of undetected mixed-phase precipitation or superimposed intense clutter can cause serious errors in the estimation of rainfall rate and other parameters of precipitation occurring in the radar scattering volume. To reduce or avoid these errors it is necessary to distinguish between the rain echo, and that due to other types of precipitation, and between precipitation radar echoes and ground clutter. Multiparameter radar measurements may be exploited for this discrimination. In particular, it is demonstrated that dual-linear polarization measurements may play a major role in this process. Sample radar data are employed to illustrate several different tests to classify radar data: the results refer to comparisons of dual-polarized echoes due to precipitation (rain or mixed-phase event) and land with echoes from land alone. This is illustrated by example through the application of a series of tests on a clutter-contaminated dual-polarized dataset obtained during the May Polarization Experiments (MAYPOLE) 1984 field program in Colorado.

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