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M. A. Nelson, E. R. Pardyjak, J. C. Klewicki, S. U. Pol, and M. J. Brown

Abstract

Velocity data were obtained from sonic anemometer measurements within an east–west-running street canyon located in the urban core of Oklahoma City, Oklahoma, during the Joint Urban 2003 field campaign. These data were used to explore the directional dependence of the mean flow and turbulence within a real-world street canyon. The along-canyon vortex that is a key characteristic of idealized street canyon studies was not evident in the mean wind data, although the sensor placement was not optimized for the detection of such structures. Instead, surface wind measurements imply that regions of horizontal convergence and divergence exist within the canopy, which are likely caused by taller buildings diverting the winds aloft down into the canopy. The details of these processes appear to be dependent on relatively small perturbations in the prevailing wind direction. Turbulence intensities within the canyon interior appeared to have more dependence on prevailing wind direction than they did in the intersections. Turbulence in the intersections tended to be higher than was observed in the canyon interior. This behavior implies that there are some fundamental differences between the flow structure found in North American–style cities where building heights are typically heterogeneous and that found in European-style cities, which generally have more homogeneous building heights. It is hypothesized that the greater three-dimensionality caused by the heterogeneous building heights increases the ventilation of the urban canopy through mean advective transport as well as enhanced turbulence.

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Rodger A. Brown, Donald W. Burgess, John K. Carter, Leslie R. Lemon, and Dale Sirmans

Some results of the first 10 cm dual-Doppler radar measurements in a tornadic storm are presented. A mesoscale cyclonic circulation confirms proposed single Doppler vortex signature and indicates that the curved reflectivity hook echo is around the periphery of the circulation. The interpolated tornado position is within the mesocyclone where high-variance Doppler velocity spectra suggest strong velocity gradients.

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Matthew D. Biddle, Ryan P. Brown, Charles A. Doswell III, and David R. Legates

Abstract

Previously published claims of large regional (northern vs southern states) differences in risks of fatality associated with tornadoes in the United States are reexamined. This new study extends earlier claims to include 1) data from a much longer time frame, 2) injuries as well as fatalities, and 3) more precise estimates of meteorological features of tornado events (specifically, a precise calculation of daytime vs nighttime and pathlength). The current study also includes formal mediation analyses involving variables that might explain regional differences. Results indicate that significant increases in the risk of fatality and injury do occur in southern states as compared with northern states. Mediation models show that these regional differences remain significant when meteorological factors of nocturnal occurrence and pathlength are included. Thus, these meteorological factors cannot explain regional differences in risk of fatality and injury, a failure that is unlikely to reflect a lack of data or a lack of precision in the measurement of potential mediators.

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Ilson C. A. da Silveira, Glenn R. Flierl, and Wendell S. Brown

Abstract

In this work, Pratt and Stern’s quasigeostrophic, 1½-layer, infinite jet model is connected to a western boundary by a system of two converging boundary currents. The model has a piecewise constant potential vorticity structure and the departing jet has a zonal cusplike profile in the ocean interior. The relative strengths of the coastal jets can be varied and the coastline can be tilted relative to north. The coastline tilt and the coastal current asymmetry cause an alongshore momentum imbalance that creates a spatially damped, quasi-stationary wave pattern. The presence of the boundary favors the long waves in the model, which behave fairly linearly in all study cases. The effects of the coastline tilt and the coastal current asymmetry are varied to reinforce or cancel each other. In the former case, a retroflection type of boundary current separation, like the one observed in most Southern Hemisphere western boundary currents, is obtained. In the latter case, a much smoother separation results, as when the Gulf Stream leaves the North American coast. In order to comply with the piecewise constant potential vorticity constraint, the β effect is included in the model only very crudely. The “beta” term in the potential vorticity relationship is totally compensated for by a steady flow pattern similar to the edge between two Fofonoff gyres. It is found that when β is nonzero, the wavelengths are somewhat shorter than those of f-plane cases.

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P. J. Minnett, R. O. Knuteson, F. A. Best, B. J. Osborne, J. A. Hanafin, and O. B. Brown

Abstract

The Marine-Atmospheric Emitted Radiance Interferometer (M-AERI) is described, and some examples of the environmental variables that can be derived from its measurements and the types of research that these can support are briefly presented. The M-AERI is a robust, accurate, self-calibrating, seagoing Fourier-transform interferometric infrared spectroradiometer that is deployed on marine platforms to measure the emission spectra from the sea surface and marine atmosphere. The instrument works continuously under computer control and functions well under a very wide range of environmental conditions with a high rate of data return. Spectral measurements are made in the range of ∼3 to ∼18 μm wavelength and are calibrated using two internal, National Institute of Standards and Technology–traceable blackbody cavities. The environmental variables derived from the spectra include the surface skin temperature of the ocean, surface emissivity, near-surface air temperature, and profiles of temperature and humidity through the lower troposphere. These measurements are sufficiently accurate both to validate satellite-derived surface temperature fields and to study the physics of the skin layer.

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Rodger A. Brown, Vincent T. Wood, Randy M. Steadham, Robert R. Lee, Bradley A. Flickinger, and Dale Sirmans

Abstract

For the first time since the installation of the national network of Weather Surveillance Radar-1988 Doppler (WSR-88D), a new scanning strategy—Volume Coverage Pattern 12 (VCP 12)—has been added to the suite of scanning strategies. VCP 12 is a faster version of VCP 11 and has denser vertical sampling at lower elevation angles. This note discusses results of field tests in Oklahoma and Mississippi during 2001–03 that led to the decision to implement VCP 12. Output from meteorological algorithms for a test-bed radar using an experimental VCP were compared with output for a nearby operational WSR-88D using VCP 11 or 21. These comparisons were made for severe storms that were at comparable distances from both radars. Findings indicate that denser vertical sampling at lower elevation angles leads to earlier and longer algorithm identifications of storm cells and mesocyclones, especially those more distant from a radar.

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R. J. Barthelmie, P. Crippa, H. Wang, C. M. Smith, R. Krishnamurthy, A. Choukulkar, R. Calhoun, D. Valyou, P. Marzocca, D. Matthiesen, G. Brown, and S. C. Pryor

The 3D wind and turbulence characteristics of the atmospheric boundary layer experiment (3D Wind) was conducted to evaluate innovative remote sensing and in situ platforms for measurements of wind and turbulence regimes. The experiment is part of a planned series that focuses on quantifying wind and turbulence characteristics at the scales of modern wind turbines and wind farms and was conducted in northern Indiana in May 2012. 3D Wind had the following specific objectives: (i) intercomparison experiments evaluating wind speed profiles across the wind turbine rotor plane from traditional cup anemometers and wind vanes on a meteorological mast and from a tethered balloon, sonic anemometers (mast mounted and on an unmanned aerial vehicle), three vertical-pointing (continuous wave) lidars and a pulsed scanning lidar, and (ii) integrate these measurements and output from 3-km-resolution (over the inner domain) simulations with the Weather Research and Forecasting Model to develop a detailed depiction of the atmospheric flow, upwind, within, and downwind of a large, irregularly spaced wind farm. This paper provides an overview of the measurement techniques, their advantages and disadvantages focusing on the integration of wind and turbulence characteristics that are necessary for wind farm development and operation. Analyses of the measurements are summarized to characterize instrument cross comparison, wind profiles, and spatial gradients and wind turbine wakes.

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L. A. Vincent, X. Zhang, R. D. Brown, Y. Feng, E. Mekis, E. J. Milewska, H. Wan, and X. L. Wang

Abstract

Trends in Canada’s climate are analyzed using recently updated data to provide a comprehensive view of climate variability and long-term changes over the period of instrumental record. Trends in surface air temperature, precipitation, snow cover, and streamflow indices are examined along with the potential impact of low-frequency variability related to large-scale atmospheric and oceanic oscillations on these trends. The results show that temperature has increased significantly in most regions of Canada over the period 1948–2012, with the largest warming occurring in winter and spring. Precipitation has also increased, especially in the north. Changes in other climate and hydroclimatic variables, including a decrease in the amount of precipitation falling as snow in the south, fewer days with snow cover, an earlier start of the spring high-flow season, and an increase in April streamflow, are consistent with the observed warming and precipitation trends. For the period 1900–2012, there are sufficient temperature and precipitation data for trend analysis for southern Canada (south of 60°N) only. During this period, temperature has increased significantly across the region, precipitation has increased, and the amount of precipitation falling as snow has decreased in many areas south of 55°N. The results also show that modes of low-frequency variability modulate the spatial distribution and strength of the trends; however, they alone cannot explain the observed long-term trends in these climate variables.

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Josephine R. Brown, Scott B. Power, Francois P. Delage, Robert A. Colman, Aurel F. Moise, and Bradley F. Murphy

Abstract

Understanding how the South Pacific convergence zone (SPCZ) may change in the future requires the use of global coupled atmosphere–ocean models. It is therefore important to evaluate the ability of such models to realistically simulate the SPCZ. The simulation of the SPCZ in 24 coupled model simulations of the twentieth century is examined. The models and simulations are those used for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). The seasonal climatology and interannual variability of the SPCZ is evaluated using observed and model precipitation. Twenty models simulate a distinct SPCZ, while four models merge intertropical convergence zone and SPCZ precipitation. The majority of models simulate an SPCZ with an overly zonal orientation, rather than extending in a diagonal band into the southeast Pacific as observed. Two-thirds of models capture the observed meridional displacement of the SPCZ during El Niño and La Niña events. The four models that use ocean heat flux adjustments simulate a better tropical SPCZ pattern because of a better representation of the Pacific sea surface temperature pattern and absence of cold sea surface temperature biases on the equator. However, the flux-adjusted models do not show greater skill in simulating the interannual variability of the SPCZ. While a small subset of models does not adequately reproduce the climatology or variability of the SPCZ, the majority of models are able to capture the main features of SPCZ climatology and variability, and they can therefore be used with some confidence for future climate projections.

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Leslie R. Lemon, Ralph J. Donaldson Jr., Donald W. Burgess, and Rodger A. Brown

Significant advances in single-Doppler radar application to severe storm study and identification have been made since 1965. Mesocyclones have been detected by Doppler radar and found to precede severe weather occurrence by several tens of minutes. A typical mesocyclone evolution leading to tornado development has also been documented. The tornado vortex itself has a revealing signature in Doppler radar data, the tornadic vortex signature (TVS). Statistics of both mesocyclone and TVS association with confirmed severe weather are presented in this paper. Doppler radar provides the potential for improving severe thunderstorm warnings. Experiments are underway to test the operational use of this new tool in storm warning and flight advisory services.

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