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David P. Dempsey and Richard Rotunno

Abstract

The fundamental assumptions underlying mesoscale mixed-layer models of the atmospheric boundary layer are that 1) the flow is hydrostatic, and 2) the Reynolds-averaged potential temperature and horizontal velocity are vertically well mixed. These assumptions determine completely the Reynolds-stress profile, which to a good approximation is a quadratic function of height, and has curvature proportional to the horizontal buoyancy gradient. The only significant source of the vertical component of vorticity in mesoscale mixed-layer models is the curl of the divergence of the Reynolds stress, which can generate quasi-stationary vortices downstream of three-dimensional topography in flow containing buoyancy gradients. We provide quantitative guidance about conditions sufficient for these vortices to form under the mixed-layer modeling assumptions.

We caution that observations do not appear to support strongly the assumption that velocity is vertically well mixed in baroclinic, convective boundary layers. We also caution that, while sufficient conditions exist under the mixed-layer modeling assumptions for quasi-stationary vortices to form, these conditions are not necessary. Sufficient conditions for such vortices to form also exist under other, completely independent modeling assumptions. Hence, the mechanism by which vorticity is generated in mixed-layer models has uncertain relevance to the atmosphere.

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Clifford F. Mass and David P. Dempsey

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No abstract available.

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David P. Dempsey and Clifford F. Mass

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Clifford F. Mass and David P. Dempsey

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This paper describes a one-level, sigma-coordinate, mesoscale model suitable for diagnosing surface winds in mountainous and coastal regions. The model requires only modest computer resources and needs little data for initialization. Energy and momentum conservation equations are integrated under steady, specified synoptic-scale height and temperature fields to a steady state to diagnose surface wind and temperature fields forced by complex terrain. If diabatic forcing is desired, the model uses the steady state results as an initial state from which the model is integrated, with varying diabatic forcing, to the verification time. The model has no mass budget, but under the hydrostatic assumption the mass field (and therefore the surface pressure field) is determined by the vertical temperature structure, which in the model is parameterized in terms of surface temperature.

Four model runs and corresponding observed wind fields are presented. They suggest that the model can diagnose many details of mesoscale flow in complex terrain for a variety of flow directions and diabatic forcings. It is suggested that adiabatic warming and cooling play a crucial role in producing topographic deflection and channeling. Recommendations of possible improvements to the model are given.

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David H. Richter, Anne E. Dempsey, and Peter P. Sullivan

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A common technique for estimating the sea surface generation functions of spray and aerosols is the so-called flux–profile method, where fixed-height concentration measurements are used to infer fluxes at the surface by assuming a form of the concentration profile. At its simplest, this method assumes a balance between spray emission and deposition, and under these conditions the concentration profile follows a power-law shape. It is the purpose of this work to evaluate the influence of waves on this power-law theory, as well as investigate its applicability over a range of droplet sizes. Large-eddy simulations combined with Lagrangian droplet tracking are used to resolve the turbulent transport of spray droplets over moving, monochromatic waves at the lower surface. The wave age and the droplet diameter are varied, and it is found that droplets are highly influenced both by their inertia (i.e., their inability to travel exactly with fluid streamlines) and the wave-induced turbulence. Deviations of the vertical concentration profiles from the power-law theory are found at all wave ages and for large droplets. The dynamics of droplets within the wave boundary layer alter their net vertical fluxes, and as a result, estimates of surface emission based on the flux–profile method can yield significant errors. In practice, the resulting implication is that the flux–profile method may unsuitable for large droplets, and the combined effect of inertia and wave-induced turbulence is responsible for the continued spread in their surface source estimates.

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Jian Zhang, Kenneth Howard, Carrie Langston, Steve Vasiloff, Brian Kaney, Ami Arthur, Suzanne Van Cooten, Kevin Kelleher, David Kitzmiller, Feng Ding, Dong-Jun Seo, Ernie Wells, and Chuck Dempsey

The National Mosaic and Multi-sensor QPE (Quantitative Precipitation Estimation), or “NMQ”, system was initially developed from a joint initiative between the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory, the Federal Aviation Administration's Aviation Weather Research Program, and the Salt River Project. Further development has continued with additional support from the National Weather Service (NWS) Office of Hydrologic Development, the NWS Office of Climate, Water, and Weather Services, and the Central Weather Bureau of Taiwan. The objectives of NMQ research and development (R&D) are 1) to develop a hydrometeorological platform for assimilating different observational networks toward creating high spatial and temporal resolution multisensor QPEs for f lood warnings and water resource management and 2) to develop a seamless high-resolution national 3D grid of radar reflectivity for severe weather detection, data assimilation, numerical weather prediction model verification, and aviation product development.

Through about ten years of R&D, a real-time NMQ system has been implemented (http://nmq.ou.edu). Since June 2006, the system has been generating high-resolution 3D reflectivity mosaic grids (31 vertical levels) and a suite of severe weather and QPE products in real-time for the conterminous United States at a 1-km horizontal resolution and 2.5 minute update cycle. The experimental products are provided in real-time to end users ranging from government agencies, universities, research institutes, and the private sector and have been utilized in various meteorological, aviation, and hydrological applications. Further, a number of operational QPE products generated from different sensors (radar, gauge, satellite) and by human experts are ingested in the NMQ system and the experimental products are evaluated against the operational products as well as independent gauge observations in real time.

The NMQ is a fully automated system. It facilitates systematic evaluations and advances of hydrometeorological sciences and technologies in a real-time environment and serves as a test bed for rapid science-to-operation infusions. This paper describes scientific components of the NMQ system and presents initial evaluation results and future development plans of the system.

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Melanie Wetzel, David Dempsey, Sandra Nilsson, Mohan Ramamurthy, Steve Koch, Jennie Moody, David Knight, Charles Murphy, David Fulker, Mary Marlino, Michael Morgan, Doug Yarger, Dan Vietor, and Greg Cox

An education-oriented workshop for college faculty in the atmospheric and related sciences was held in Boulder, Colorado, during June 1997 by three programs of the University Corporation for Atmospheric Research. The objective of this workshop was to provide faculty with hands-on training in the use of Web-based instructional methods for specific application to the teaching of satellite remote sensing in their subject areas. More than 150 faculty and associated scientists participated, and postworkshop evaluation showed it to have been a very successful integration of information and activities related to computer-based instruction, educational principles, and scientific lectures.

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