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Robert A. Cohen and Carl W. Kreitzberg

Abstract

Distinct airstreams, separated by sharp boundaries, are present in numerical weather simulations and can be used to identify characteristic structures in baroclinic storms. To allow objective comparisons between different analyses, a rigorous treatment of airstream boundaries is performed based upon a numerical procedure in which the uncertainty of individual trajectory paths is related to the strength of an airstream boundary, as defined and quantified herein. The properties of the procedure are then investigated via application to a numerical simulation of the ERICA IOP 4 storm. The work not only provides the necessary framework within which future analyses of airstreams can be interpreted but also provides insights into quantitative properties of atmospheric flows.

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Donald J. Perkey and Carl W. Kreitzberg

Abstract

Before high-resolution numerical models can be of use operationally, they must be restricted to a limited domain, thus necessitating lateral boundary conditions which allow the changes outside the limited domain to influence the results while not contaminating the forecast with spurious boundary-reflected energy. Such a set of time-dependent lateral boundary conditions are presented in this paper. This boundary condition set is investigated using the linear analytic and finite-difference advection equations, the non-linear finite-difference shallow-water equations, and the hydrostatic primitive equations.

The results illustrate how the boundary condition transforms long- and medium-length interior advective and gravity waves into short waves which can then be removed by a low pass filter, thereby giving the appearance that the exiting wave simply passed through the boundary. The results also indicate that large-scale advective and gravity waves enter the forecast domain with little degradation. Thus, from the tests performed, the described boundary condition scheme yields a practical solution for prescribing time-dependent lateral boundaries for a limited-area model.

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Paul J. Neiman, M. A. Shapiro, Evelyn G. Donall, and Carl W. Kreitzberg

Abstract

On 25–27 January 1988, the National Oceanic and Atmospheric Administration's Wave Propagation Laboratory, Drexel University, and the Office of Naval Research carried out a combined pre-ERICA research aircraft investigation of a major marine cyclone moving northeastward over the Canadian Maritime Provinces. Flight-level and dropwindsonde observations documented the diabatic modification of the cyclone's warm sector marine boundary layer (MBL) as it moved out over cold underlying water. These observations and results from the Blackadar one-dimensional boundary layer model both show that heat fluxes were directed downward from the warm sector MBL into the cold ocean. Vertical gradients of these downward heat fluxes diabatically cooled the lower portion of the warm sector MBL and generated large static stability within the entire layer. The increase in stable stratification allowed large vertical wind shear to exist within this layer and strong wind speeds to exist at its top. The increase in static stability within the warm sector MBL acted to concentrate isentropic potential vorticity in this layer, but these changes also weakened the horizontal gradients of temperature, moisture, and wind velocity within the adjacent warm- and cold-frontal zones at the surface.

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