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Brooks E. Martner

Abstract

Radar reflectivity factors determined from disdrometer measurements of drop spectra are compared with simultaneous WSR-57 radar measurements in two Oklahoma thunderstorms. The possibility of using a disdrometer for an in-field calibration check of a radar is examined and found to have limited usefulness for convective precipitation sampled at long ranges.

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Brooks E. Martner

Abstract

Continuous vertically pointing measurements of a thunderstorm outflow, including its gust front, were obtained with a Doppler radar near New Salem, North Dakota. The measurements provide a high-resolution depiction of the vertical structure of reflectivity and vertical velocity within the gust front, the outflow, and the parent storm. Earlier gust front remote sensing studies have used Doppler observations obtained with low-elevation-angle scans to accurately measure the horizontal flow pattern from which vertical velocities were subsequently estimated by integrating the continuity equation. In contrast, the New Salem case provides direct, rather than derived, Doppler measurement of vertical velocities with better vertical resolution and vastly superior temporal resolution. The gust front’s vertical structure is in general agreement with earlier observations and numerical simulations, except that the transition from strong upward to strong downward motion was more abrupt. The maximum updraft, of almost 10 m s−1, was measured in the gust front at 1.35 km above ground level and was followed by equally strong downward motion only 1 min later at a slightly higher altitude.The observations support the earlier use of the continuity method for deriving the basic pattern of vertical motions in density currents from quasi-horizontal scan data.

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James E. Dye and Brooks E. Martner

Abstract

Data from the hailpad network of the National Hail Research Experiment were examined in relation to the equivalent radar reflectivity factors recorded in the lowest level sweeps of the radar beam over the pads during hailstorms in 1972 and 1976. The relationship between hail detected at the ground and reflectivity factor was examined for both areal coverage and on a point-by-point basis for each hailpad. The comparisons show that reflectivity factors of 55 dBZ are often measured when no hail is observed at the ground. Rain alone can give rise to reflectivities of this magnitude. The results of the study show that in northeastern Colorado low-level equivalent radar reflectivity factors alone cannot be used to determine the region of hailfall at the ground, nor are they likely to augment quantitative measurements by a ground network of hail sensors. The results found in northeastern Colorado are compared to results from other geographical regions.

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Brooks E. Martner and John D. Marwitz

Abstract

Measurements of wind from a network of surface anemometers and a 107 m tower have been analyzed for southern Wyoming where a project for large-scale generation of electricity from wind power is underway. Topographically forced channeling of stable air flow across a low region of the Continental Divide is mainly responsible for very high mean wind speeds especially in winter. The seasonal cycle of wind speed exhibits a maximum in winter and minimum in summer. Mean wind speeds are approximately 50% greater in winter months than in summer, and the available wind power density is a factor of ∼4.0 greater in winter than in summer. The diurnal cycle is characterized by minimum speed near sunrise and maximum in afternoon hours. Wind directions are narrowly confined from the west-southwest by topographic channeling of the flow, particularly in winter. Wind speed increases sharply with height at night but the profile becomes much more uniform during daylight hours in response to mixing of the lower atmosphere initiated by surface heating.

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Brooks E. Martner and Louis J. Battan

Abstract

The radar reflectivity factors, the reflectivity-weighted mean terminal velocities (V T), and the standard deviations (σ v) of the resulting Doppler spectra, were computed for specified size distributions of rain, dry and wet ice spheres (taken to be hailstones), and rain with hail. Unambigous estimates of the mean velocity and standard deviation can be obtained from a radar measurement of reflectivity for rain alone and for dry ice spheres as a function of maximum sphere size. The results for wet ice spheres are strongly dependent on the thickness of the liquid water coating on the ice core. When rain and hail coexist, large values of reflectivity are associated with large ranges of V T and σ v. If the shape of the hail size distribution is known, an independent measurement of the maximum hailstone diameter or a knowledge of the standard deviation of the observed Doppler velocity spectrum can reduce the uncertainty in estimates of V V T.

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Roger F. Reinking and Brooks E. Martner

Abstract

Questions of delivery, transport, and dispersion of cloud seeding aerosol in a convective feeder cloud are addressed by using radar chaff as a surrogate for aerosol and tracking it with circular-polarization radar. In a case study, a line source of chaff was released by an aircraft at the roots of a growing cloud flanking and feeding into a thunderstorm line. The chaff was tracked as it dispersed in the boundary layer and rose more than 3 km from the cloud base at +14°C to levels cold enough to nucleate ice-forming seeding aerosols. Quantitative measures of the rates of loft and dispersion, and the volume filling and dilution were obtained. The measurements permit examination of the hypotheses and potential efficacy of cloud-base seeding to increase rain and suppress hail. Notably, the problem of delivery, transport, and dispersion of cloud seeding aerosol is much the same as the air quality question of the nature and effect of cloud venting of the boundary layer, and the findings here apply in that context as well.

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Marcia K. Politovich, B. Boba Stankov, and Brooks E. Martner

Abstract

Methods by which attitude ranges of supercooled cloud liquid water in the atmosphere may be estimated are explored using measurements from a combination of ground-based remote sensors. The tests were conducted as part of the Winter Icing and Storms Project that took place in eastern Colorado during the winters of 1990, 1991, and 1993. The basic method augments microwave radiometer measurements of path-integrated liquid water with observations from additional remote sensors to establish height limits for the supercooled liquid. One variation uses a simple adiabatic parcel lifting model initiated at a cloud-base height determined from a ecilometer, temperature and pressure from a radio acoustic sounding system or rawinsonde, and combines these with the radiometers total liquid measurement to obtain an estimate of the liquid cloud-top height. Since it does not account for liquid loss by entrainment or ice-liquid interaction processes this method tends to underestimate the true liquid cloud top; for two cases examined in detail, 54% of icing pilot reports in the area were from above this estimated height. Some error is introduced due to differences in sampling locations and from horizontal variability in liquid water content. Vertical cloud boundaries from a Ka-band radar were also used in the study; these often indicated thicker clouds than the liquid-layer depths observed from research aircraft, possibly due to the ambiguity of the ice-liquid phase distinction.

Comparisons of liquid vertical profiles are presented, using normalized profile shapes based an uniform, adiabatic, and aircraft-derived composite assumptions. The adiabatic and climatological profile shapes generally agreed well with measurements from a research aircraft and were more realistic than the uniform profile. Suggestions for applications of these results toward a red-time aviation hazard identification system are presented.

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Sergey Y. Matrosov, Robert A. Kropfli, Roger F. Reinking, and Brooks E. Martner

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K DP and δ o, respectively) in rain are discussed for X- (λ ∼ 3 cm) and Ka-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K DP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h−1 is generally small and should not pose a serious problem when estimating K DP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ ∼ 10–11 cm) wavelengths is an increase in K DP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from Ka-band measurements of the differential phase.

Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4–5 mm h−1. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K DPR relation derived for the mean drop aspect ratio (R = 20.5K0.80DP) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

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Sergey Y. Matrosov, Kurt A. Clark, Brooks E. Martner, and Ali Tokay

Abstract

A combined polarimetric estimator for rainfall rate (R) retrievals from polarimetric radar measurements at X band is proposed. This estimator uses the horizontal polarization radar reflectivity Z e, differential reflectivity Z DR, and specific differential phase shift K DP, and it intrinsically accounts for changes in how drop oblateness increases with size. Because this estimator uses power measurements (i.e., Z e and Z DR), a procedure for correcting these measurements for effects of partial attenuation and differential attenuation using the differential phase measurement is suggested. An altitude correction for estimates of rainfall rates is also suggested. The proposed combined polarimetric estimator that uses K DP, Z DR, and Z e, an estimator that uses K DP alone for equilibrium drop shapes, and different Z eR relations were applied to the 15 rain events observed with the NOAA X-band transportable polarimetric radar during the eight-week field campaign at the NASA Wallops Island facility in Virginia. The observed rains ranged from very light stratiform events to very heavy convective ones with cells producing rainfall rates in excess of 100 mm h−1. The three different ground validation sites were equipped with high-resolution (0.01 in.) tipping-bucket rain gauges. One of these sites also was equipped with disdrometers. In terms of the relative standard deviation, the combined polarimetric estimator provided the best overall agreement with gauge data (22%), closely followed by a case-tuned Z eR relation (23%) that was determined for each observational case from drop size distributions (DSD) measured in situ by a disdrometer and was available only a posteriori. The use of the K DP-only estimator and a mean Z eR relation resulted in 30% and 32% relative standard deviations, correspondingly. The combined polarimetric estimator, the K DP-only estimator, and the case-tuned Z eR relation estimator provided about a 6%–9% negative bias in comparison with the gauge data; the mean Z eR relation estimator provided a larger negative bias (18%).

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Sergey Y. Matrosov, Roger F. Reinking, Robert A. Kropfli, Brooks E. Martner, and B. W. Bartram

Abstract

An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45° linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45° linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory’s Ka-band radar. In comparison with the true circular and slant-45° linear polarizations, the discussed elliptical polarizations provide a stronger signal in the “weak” radar receiver channel; however, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation angles that do not show much sensitivity to particle orientations, the available quasi-circular polarization provides a better depolarization contrast between nonspherical and spherical particles than does the available quasi-linear slant-45°polarization. The use of the proposed approach is illustrated with the experimental data collected during a recent field experiment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and graupel. When a reasonable assumption about particle bulk density is made, quantitative estimates of particle aspect ratios from radar depolarization data are in good agreement with in situ observations. Uncertainties of particle aspect ratios estimated from depolarization measurements due to 0.1 g cm−3 variations in the assumed bulk density are about 0.1.

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