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C. David Whiteman and J. Christopher Doran

Abstract

The relationship between winds above and within the Tennessee Valley is investigated climatologically and with an atmospheric numerical model. For the climatological analyses, winds above the valley were determined by interpolation from four surrounding rawinsonde stations, while winds within the valley were measured on four 100-m towers. Tennessee Valley winds are generally weak and bidirectional, oriented along the valley's axis. The valley wind direction depends strongly on the component of the synoptic-scale pressure gradient that is superimposed along the valley's axis at ridge-top level, with winds blowing along the valley's axis from high toward low pressure. This relationship between winds above and within the valley can result in countercurrents similar to those observed in the Rhine Valley. While winds in the Tennessee Valley are driven primarily by this pressure-driven channeling mechanism, downward momentum transport can cause afternoon winds within the valley to approach the wind directions aloft when winds at ridge-top level are strong, and thermally driven valley circulations can appear at night when winds at ridge-top level are weak. A hydrostatic numerical model was used to provide additional insight into the physical processes governing the near-surface winds in the Tennessee Valley. The results support the identification of pressure-driven channeling, downward momentum transport, and thermal forcing as the principal mechanisms determining valley wind directions. They also illustrate the importance of topographical features in producing deviations from simple pressure-driven channeling. The relative importance of thermally driven and pressure-driven winds is examined, and guidelines are presented for estimating when one or the other process will dominate.

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William J. Shaw and J. Christopher Doran

Abstract

Data from several surface meteorological networks in the vicinity of the U.S. Department of Energy’s Southern Great Plains Cloud and Radiation Testbed were used to investigate the relationship between boundary layer circulations, as reflected in composited divergence fields, and variations in vegetation, surface temperature, and topography. The study is unique in using data from a dense collection of surface meteorological instruments that are distributed over a region comparable in size to a GCM grid cell in a region of sharply varying land use. These land use differences provide variations in surface heat flux on a scale O(100 km) that has been postulated to produce the strongest surface-induced mesoscale circulations in the boundary layer. This paper details the first signature in data of a boundary layer circulation that can be attributed to land use differences at this scale. It is found, however, that in the composited fields the majority of the divergence extrema persist over seasons, are present in all observed wind conditions, are geographically fixed, and are more likely related to gentle topographic features rather than to land use differences.

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John W. Nielsen-Gammon, Christina L. Powell, M. J. Mahoney, Wayne M. Angevine, Christoph Senff, Allen White, Carl Berkowitz, Christopher Doran, and Kevin Knupp

Abstract

An airborne microwave temperature profiler (MTP) was deployed during the Texas 2000 Air Quality Study (TexAQS-2000) to make measurements of boundary layer thermal structure. An objective technique was developed and tested for estimating the mixed layer (ML) height from the MTP vertical temperature profiles. The technique identifies the ML height as a threshold increase of potential temperature from its minimum value within the boundary layer. To calibrate the technique and evaluate the usefulness of this approach, coincident estimates from radiosondes, radar wind profilers, an aerosol backscatter lidar, and in situ aircraft measurements were compared with each other and with the MTP. Relative biases among all instruments were generally less than 50 m, and the agreement between MTP ML height estimates and other estimates was at least as good as the agreement among the other estimates. The ML height estimates from the MTP and other instruments are utilized to determine the spatial and temporal evolution of ML height in the Houston, Texas, area on 1 September 2000. An elevated temperature inversion was present, so ML growth was inhibited until early afternoon. In the afternoon, large spatial variations in ML height developed across the Houston area. The highest ML heights, well over 2 km, were observed to the north of Houston, while downwind of Galveston Bay and within the late afternoon sea breeze ML heights were much lower. The spatial variations that were found away from the immediate influence of coastal circulations were unexpected, and multiple independent ML height estimates were essential for documenting this feature.

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Walter F. Dabberdt, Mary Anne Carroll, William Appleby, Darrel Baumgardner, Gregory Carmichael, Paula Davidson, J. Christopher Doran, Timothy S. Dye, Susan Grimmond, Paulette Middleton, William Neff, and Yang Zhang
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Walter F. Dabberdt, Jeremy Hales, Steven Zubrick, Andrew Crook, Witold Krajewski, J. Christopher Doran, Cynthia Mueller, Clark King, Ronald N. Keener, Robert Bornstein, David Rodenhuis, Paul Kocin, Michael A. Rossetti, Fred Sharrocks, and Ellis M. Stanley Sr.

The 10th Prospectus Development Team (PDT-10) of the U.S. Weather Research Program was charged with identifying research needs and opportunities related to the short-term prediction of weather and air quality in urban forecast zones. Weather has special and significant impacts on large numbers of the U.S. population who live in major urban areas. It is recognized that urban users have different weather information needs than do their rural counterparts. Further, large urban areas can impact local weather and hydrologic processes in various ways. The recommendations of the team emphasize that human life and well-being in urban areas can be protected and enjoyed to a significantly greater degree. In particular, PDT-10 supports the need for 1) improved access to real-time weather information, 2) improved tailoring of weather data to the specific needs of individual user groups, and 3) more user-specific forecasts of weather and air quality. Specific recommendations fall within nine thematic areas: 1) development of a user-oriented weather database; 2) focused research on the impacts of visibility and icing on transportation; 3) improved understanding and forecasting of winter storms; 4) improved understanding and forecasting of convective storms; 5) improved forecasting of intense/severe lightning; 6) further research into the impacts of large urban areas on the location and intensity of urban convection; 7) focused research on the application of mesoscale forecasting in support of emergency response and air quality; 8) quantification and reduction of uncertainty in hydrological, meteorological, and air quality modeling; and 9) the need for improved observing systems. An overarching recommendation of PDT-10 is that research into understanding and predicting weather impacts in urban areas should receive increased emphasis by the atmospheric science community at large, and that urban weather should be a focal point of the U.S. Weather Research Program.

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