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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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Brooks E. Martner
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Brooks E. Martner

Abstract

Observations of mammatus-like cloud features associated with a convective rain shower were obtained using a vertically pointing 8-mm-wavelength Doppler radar. The radar's excellent sensitivity and resolution allowed even very weak, finescale features of the cloud to be resolved. The mammatus were located 3–5 km above ground along the underneath side of an anvil-like stratiform region of the storm cloud and preceded the arrival of precipitation. Both reflectivity and velocity exhibited strongly periodic patterns, with larger downward motions and larger reflectivities occurring together. The mammatus lobes were separated by about 90 s or approximately 1 km, horizontally. The mammatus features could be traced more than 500 m upward into the cloud echo interior where the amplitude of the vertical velocity oscillations was greatest. The measured vertical velocities of particle motions ranged from +0.5 to −3.0 m s−1. The observed reflectivity and velocity patterns suggest that although evaporation was acting to shape the mamma as they descended toward the echo bottom, other mechanisms may have been responsible for their initial formation in the interior region of the cloud echo.

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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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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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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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Brad W. Orr and Brooks E. Martner

Abstract

Recent studies have demonstrated that the 404-MHz wind profilers of the National Oceanic and Atmospheric Administration WPDN (Wind Profiler Demonstration Network) can detect precipitation under most circumstances. Their ability to detect nonprecipitating and weakly precipitating clouds, however, has remained the subject of debate. To address this question, a 35-GHz Ka-band cloud-sensing radar was operated side by side, with a WPDN profiler in Colorado during the winter of 1993. The short wavelength (0.87 cm), finescale resolution, and excellent sensitivity of the Ka-band system to small hydrometeors make it very well suited for detailed measurements of clouds and weak precipitation. Comparisons of data from the two instruments show that in addition to detecting precipitation, the profiler did indeed detect nonprecipitating ice clouds under some circumstances that can be approximately delineated by profiler reflectivity and vertical velocity thresholds. These thresholds are a function of height for the case examined. A weak cloud of given intensity is easier for the profiler to detect if it is located high in the troposphere rather than close to the ground, because it contrasts more strongly against the background of clear-air reflectivity, which generally decreases sharply with height The apparent mechanism of cloud detection by the profiler is Rayleigh backscattering from ice crystals that are larger than typical cloud droplets but have minimal fall speeds.

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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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Roger M. Wakimoto and Brooks E. Martner

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An integrated analysis of photographic and Doppler radar observations of a tornadic storm during the Convection Initiation and Downburst Experiment (CINDE) is presented. High-resolution single-Doppler radar measurements are combined with cloud photogrammetry to reveal the detailed structural relationship of the hook echo and the misocyclone with visual features of the tornado. Vertical cross sections of dual-Doppler winds in the plane of the photographs were also examined to determine the complex motions within and surrounding the vortex. The tornado was found to be within a weak-echo hole of the hook echo. The hole progressed upward above cloud base as the tornado matured. An annulus of higher reflectivity that formed a book echo is hypothesized to have been composed of sparse but large raindrops. The airflow fields suggest that vortex breakdown and axial downdrafts were present near the ground at early stages shortly after the tornado became visible. Later, axial upward flow dominated at all levels until the collapse of the vortex.

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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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