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Brad W. Orr and Robert A. Kropfli

Abstract

A method is presented that estimates particle fall velocities from Doppler velocity and reflectivity measurements taken with a vertically pointing Doppler radar. The method is applicable to uniform, stratified clouds and is applied here to cirrus clouds. A unique aspect of the technique consists of partitioning the Doppler velocities into discrete cloud height and cloud reflectivity bins prior to temporal averaging. The first step of the method is to temporally average the partitioned Doppler velocities over an hour or two to remove the effects of small-scale vertical air motions. This establishes relationships between particle fall velocity and radar reflectivity at various levels within the cloud. Comparisons with aircraft in situ observations from other experiments show consistency with the remote-sensing observations. These results suggest that particle fall speeds can be determined to within 5–10 cm s−1 by means of this technique.

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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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A. Shelby Frisch, Brad W. Orr, and Brooks E. Martner

Abstract

A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s−1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical velocities were significantly downward during the late afternoon and upward at night.

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Robert M. Banta, Lisa S. Darby, Jerome D. Fast, James O. Pinto, C. David Whiteman, William J. Shaw, and Brad W. Orr

Abstract

A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north–south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s−1 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.

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Brian A. Klimowski, Robert Becker, Eric A. Betterton, Roelof Bruintjes, Terry L. Clark, William D. Hall, Brad W. Orr, Robert A. Kropfli, Paivi Piironen, Roger F. Reinking, Dennis Sundie, and Taneil Uttal

The 1995 Arizona Program was a field experiment aimed at advancing the understanding of winter storm development in a mountainous region of central Arizona. From 15 January through 15 March 1995, a wide variety of instrumentation was operated in and around the Verde Valley southwest of Flagstaff, Arizona. These instruments included two Doppler dual-polarization radars, an instrumented airplane, a lidar, microwave and infrared radiometers, an acoustic sounder, and other surface-based facilities. Twenty-nine scientists from eight institutions took part in the program. Of special interest was the interaction of topographically induced, storm-embedded gravity waves with ambient upslope flow. It is hypothesized that these waves serve to augment the upslope-forced precipitation that falls on the mountain ridges. A major thrust of the program was to compare the observations of these winter storms to those predicted with the Clark-NCAR 3D, nonhydrostatic numerical model.

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Roger F. Reinking, Roger Caiazza, Robert A. Kropfli, Brad W. Orr, Brooks E. Martner, Thomas A. Niziol, Gregory P. Byrd, Richard S. Penc, Robert J. Zamora, Jack B. Snider, Robert J. Ballentine, Alfred J. Stamm, Christopher D. Bedford, Paul Joe, and Albert J. Koscielny

Snowstorms generated over the Great Lakes bring localized heavy precipitation, blizzard conditions, and whiteouts to downwind shores. Hazardous freezing rain often affects the same region in winter. Conventional observations and numerical models generally are resolved too coarsely to allow detection or accurate prediction of these mesoscale severe weather phenomena. The Lake Ontario Winter Storms (LOWS) project was conducted to demonstrate and evaluate the potential for real-time mesoscale monitoring and location-specific prediction of lake-effect storms and freezing rain, using the newest of available technologies. LOWS employed an array of specialized atmospheric remote sensors (a dual-polarization short wavelength radar, microwave radiometer, radio acoustic sounding system, and three wind profilers) with supporting observing systems and mesoscale numerical models. An overview of LOWS and its initial accomplishments is presented.

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