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R. Rabin and I. Zawadzki

Abstract

The relationship between convective scales in the pre-storm environment and Doppler measurements of divergence is discussed with attention to problems inherent in radar measurements. A method is suggested to minimize the effects of beam smoothing and reflectivity weighting. A case study indicates the possibility of operational short-term forecasting of the onset time of precipitation from the radar measurements of divergence.

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R. M. Rabin, R. J. Doviak, and A. Sundara-Rajan

Abstract

Dual Doppler-radars are used to obtain momentum flux and turbulence intensities in the cloudless convective boundary-layer (CBL) containing roll vortices. Momentum flux from small scales unresolved by the radar is determined from the difference between the measured flux and the flux implied by the velocity defect profile. While the small-scale flux indicates a reasonable eddy diffusivity profile, the measured flux is of the opposite sign expected from K theory. Two terms in the turbulent kinetic energy budget, viz, the shear production and the divergence of the energy flux, are calculated for measurable scales. It is proposed that the tilting of convective roll circulations by cross-shear produces the observed flux in the upper part of the CBL.

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Albert J. Koscielny, R. J. Doviak, and R. Rabin

Abstract

Methods of statistical regression have been applied to single-radar radial velocity fields to map certain mesoscale (20–100 km) kinematic properties (e.g., divergence) of the convective boundary layer (CBL). Several methods, previously proposed, were found to produce estimates that were biased or whose variances were too large. When wind fields are linear on the meso- or larger scale, then single-Doppler velocity accuracies allow the estimation of horizontal divergence with an accuracy of about 4 × 10−5 s−1 and a resolution of ∼30 km, which may be sufficient to sense pre-thunderstorm convergence

A case study for 19 June 1980 suggests that single-Doppler weather radars of modest sensitivity can map the mesoscale divergence patterns within the cloud-free CBL. For this day, convergence zones to the northeast seem to precede cloud development by 1–2 h, and to the west precede thunderstorms by 3–4 h.

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R. M. Rabin, D. E. Engles, and A. J. Koscielny

Abstract

Mean kinematic wind properties are obtained from the analysis of clear-air Doppler radar data along circles, a few hours prior to thunderstorm development. A variety of information is found to complement synoptic analysis. Besides updated profiles of mean wind above the radar, the analysis provides horizontal and vertical gradients of wind over a wide area, which determine the position, slope, and intensification of a front. Destabilization of air above the frontal zone, which leads to storms, is diagnosed from an initial sounding of temperature and humidity, and areal averaged vertical air velocity deduced from the radar.

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M. Segal, R. W. Arritt, C. Clark, R. Rabin, and J. Brown

Abstract

The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.

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M. Segal, C. Anderson, R. W. Arritt, R. M. Rabin, and D. W. Martin

Abstract

Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.

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R. A. Maddox, D. S. Zaras, P. L. MacKeen, J. J. Gourley, R. Rabin, and K. W. Howard

Abstract

The new Doppler radars of the National Weather Service (i.e., the WSR-88D radars) are operated continuously in a volume scanning mode (called Volume Coverage Pattern, VCP) with the elevation tilt angles fixed for several VCPs. Because of the fixed VCPs, the radar data can be used to determine the heights of precipitation echo features only to limits of accuracy that depend upon the elevation angles used in the VCP, the radar beamwidth, and the range of echoes. Data from adjacent WSR-88D radars, if used simultaneously, could reduce significantly the height uncertainties inherent in single radar measurements. This is illustrated for idealized situations and also for an event involving a long-lived, tornadic thunderstorm. The use of coordinated scan strategies and combined data analysis procedures for adjacent WSR-88D radars during significant thunderstorm events should be considered for operational applications.

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