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WILLIAM F. BROWN

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Stephen A. Cohn, Vanda Grubiššićć, and William O. J. Brown

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A network of three boundary layer radar wind profilers is used to study characteristics of mountain waves and rotors and to explore the utility of such a network. The data employed were collected as part of the Terrain-Induced Rotor Experiment (T-REX), which took place in Owens Valley, California, in early 2006. The wind profilers provide a continuous time––height representation of wave and rotor structure. During intensive observing period 3 (IOP 3), the profiler network was positioned in an L-shaped configuration, capturing key features of the mountain waves and rotor, including the boundary layer vortex sheet (or shear layer), turbulence within this shear layer, the classical lower turbulence zone (LTZ), and wave motion above the LTZ. Observed features were found to be in good agreement with recent high-resolution numerical simulations. Using the wind profiler with superior time resolution (Multiple Antenna Profiler Radar), a series of updraft––downdraft couplets were observed beneath the first downwind wave crest. These are interpreted as signatures of subrotors. Such detailed observations of subrotors are rare, even though subrotors are believed to be a common feature of rotor circulations in Owens Valley. During IOP 6, the network was repositioned to form a line across the valley. A simple algorithm was used to determine the amplitude, wavelength, and phase of the primary wave over the valley and to observe their changes over time and height. In the IOP-6 case, the wavelength increased over time, the phase indicated an eastward-shifting wave crest, and the amplitude increased with height and also varied over time.

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Michael J. Naughton, Gerald L. Browning, and William Bourke

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The convergence of spectral model numerical solutions of the global shallow-water equations is examined as a function of the time step and the spectral truncation. The contributions to the errors due to the spatial and temporal discretizations are separately identified and compared. Numerical convergence experiments are performed with the inviscid equations from smooth (Rossby-Haurwitz wave) and observed (R45 atmospheric analysis) initial conditions, and also with the diffusive shallow-water equations. Results are compared with the forced inviscid shallow-water equations case studied by Browning et at. Reduction of the time discretization error by the removal of fast waves from the solution using initialization is shown. The effects of forcing and diffusion on the convergence are discussed. Time truncation errors are found to dominate when a feature is large scale and well resolved; spatial truncation errors dominate-for small-scale features and also for large scales after the small scales have affected them. Possible implications of these results for global atmospheric modeling are discussed.

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Michael J. Brown, S. Pal Arya, and William H. Snyder

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The vertical diffusion of a passive tracer released from surface and elevated sources in a neutrally stratified boundary layer has been studied by comparing field and laboratory experiments with a non-Gaussian K-theory model that assumes power-law profiles for the mean velocity and vertical eddy diffusivity. Several important differences between model predictions and experimental data were discovered: 1) the model overestimated ground-level concentrations from surface and elevated releases at distances beyond the peak concentration; 2) the model overpredicted vertical mixing near elevated sources, especially in the upward direction; 3) the model-predicted exponent α in the exponential vertical concentration profile for a surface release [C̄(z) exp(−z α)] was smaller than the experimentally measured exponent. Model closure assumptions and experimental shortcomings are discussed in relation to their probable effect on model predictions and experimental measurements.

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Shaunna L. Donaher, Bruce A. Albrecht, Ming Fang, and William Brown

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Observations of 14 stratiform periods in outer tropical cyclone rainbands are used to evaluate wind structure using a velocity–azimuth display (VAD) technique applied to KAMX (Miami) Weather Surveillance Radar-1988 Doppler (WSR-88D) data. These 14 cases occurred over land in southern Florida from Tropical Storm Fay and Hurricanes Gustav and Ike during 2008. Profiles show a maximum horizontal wind speed between 1000 and 1500 m in height, with occasional evidence of a secondary horizontal wind maximum near 3500–5000 m. Storm-relative wind components are calculated, and radial wind profiles show a mean transition from radial inflow at low levels to radial outflow around 2500–3000-m altitude. The radial inflow maximum is around 500 m, while maximum outflow is more variable. These profile characteristics are consistent with previous wind observations in rainbands over land and water. Changes in wind structure within one 4-h period are examined, with changes seen linked to the environmental influence on the rainband. All rainbands show a logarithmic wind speed decrease below 200 m. This layer is studied in detail using a log-wind fit method and a ratio method to calculate aerodynamic roughness length. Much lower ratios of surface to higher-level winds were found than in previous studies over open oceans. Another significant finding of this work is the lack of a constant aerodynamic roughness length despite similar storm wind profiles. These results are useful in broadening the understanding of low-level impacts of landfalling rainbands far from the storm center.

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Tanya M. Brown, William H. Pogorzelski, and Ian M. Giammanco

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A series of thunderstorms on 24 May 2011 produced significant hail in the Dallas–Fort Worth (DFW) metroplex, resulting in an estimated $876.8 million (U.S. dollars) in insured losses to property and automobiles, according to the Texas Department of Insurance. Insurance claims and policy-in-force data were obtained from five insurance companies for more than 67 000 residential properties located in 20 ZIP codes. The methodology for selecting the 20 ZIP codes is described. This study evaluates roofing material type with regard to resiliency to hailstone impacts and relative damage costs associated with roofing systems versus wall systems. A comparison of Weather Surveillance Radar-1988 Doppler (WSR-88D) radar-estimated hail sizes and damage levels seen in the claims data is made. Recommendations for improved data collection and quality of insurance claims data, as well as guidance for future property insurance claims studies, are summarized. Studies such as these allow insurance underwriters and claims adjusters to better evaluate the relative performance and vulnerability of various roofing systems and other building components as a function of hail size. They also highlight the abilities and limitations of utilizing radar horizontal reflectivity-based hail sizes, local storm reports, and Storm Data for claims processing. Large studies of this kind may be able to provide guidance to consumers, designers, and contractors concerning building product selections for improved resiliency to hailstorms, and give a glimpse into how product performance varies with storm exposure. Reducing hail losses would reduce the financial burden on property owners and insurers and reduce the amount of building materials being disposed of after storms.

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David O. Blanchard, William R. Cotton, and John M. Brown

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The hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems (MCSs) is explored by sampling environments that supported the growth of MCSs in the Preliminary Regional Experiment for STORM (Stormscale Operational and Research Meteorology) (PRE-STORM) network with high quality special soundings. Secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the PRE-STORM field program. Additional examples of MCS environments were examined using data from the Mesoscale Analysis and Prediction System. Results show strong divergence and cross-stream accelerations occurred at upper-tropospheric levels where inertial instabilities were present. These accelerations were not uniform over the domain but were focused in the regions of instability. Also, the analyses of these data showed that regions of inertial instability may be more commonplace than is typically assumed.

The Regional Atmospheric Modeling System was used to increase the understanding of the basic processes and secondary circulations that enhance MCS growth in inertially unstable environments. Model results indicate that the strength of the divergent outflow was strongly linked to the degree of inertial stability in the local environment. The results also showed a strong dependence on the magnitude of the Coriolis parameter. Finally, experiments using varying degrees of vertical stability indicated that there was also significant sensitivity to this parameter.

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Stephanie P. Browner, William L. Woodley, and Cecilia G. Griffith

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An analysis of 16 days from eight Atlantic storms, two in 1974 and six in 1975, objectively quantified a suspected diurnal oscillation of tropical storm cirrus cloud cover. The oscillation shows a maximum area at approximately 1700 local mean solar time and a minimum area at 0300 local mean solar time. The average ratio of the maximum area to the minimum area is 1.65.

SMS infrared imagery was analyzed with a scanning false-color densitometer to obtain area measurements of the cloudiness associated with the storms. These measurements were made approximately every 1½ h at three temperature thresholds: 253, 239 and 223 K.

Two tests were performed to rule out the possibility of the oscillation being due either to the satellite sensor or to image processing. Measurement of the ocean surface temperature, was made with SMS-I to determine whether the sensor detected a constant ocean temperature. The second test compared simultaneous area measurements obtained by SMS-I and SMS-II. The results of these tests support the storm oscillation detected.

Two other related phenomena were also observed: 1) the amplitude of the area oscillation is apparently inversely proportional to the intensity of the storm, and 2) a time-dependent, shorter period oscillation is superimposed on the daily oscillation. Inferences of causality are made.

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Anthony G. Barnston, William L. Woodley, John A. Flueck, and Michael H. Brown

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The Florida Area Cumulus Experiment (FACE) is a single area, randomized experiment designed to assess the ground-level rainfall effects of dynamic cloud seeding in summer on the south Florida peninsula. The second phase of FACE (FACE-2), an attempt to confirm the indication of seeding-induced rain increases in FACE-1, has been completed. A description of the FACE-2 program design and how well it was implemented in the summers of 1978, 1979 and 1980 is provided. The data reduction process and its rationale are described both for the basic rainfall data and for the predictor variables to be used in the covariate analyses. The resulting FACE-2 rainfall and covariate data are presented for each of the 61 days of experimentation without knowledge of whether actual seeding (using silver iodide) took place. (Part II will contain the confirmatory and replicated analyses of the effects of seeding, and Part III will present a number of exploratory analyses of the FACE-1 and FACE-2 data.)

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Rodger A. Brown, William C. Bumgarner, Kenneth C. Crawford, and Dale Sirmans

Single Doppler radar measurements were made in a squall line that formed in southern Kansas during the afternoon of 2 June 1971 and moved south-southeastward through central Oklahoma. During the period of data collection, a pronounced hook echo, having at least one funnel cloud associated with it, developed. Preliminary analyses of these first Doppler velocity measurements within a radar hook echo in the tornado belt are presented.

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