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Keith L. Seitter
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Keith L. Seitter

Abstract

A new form of the density current speed equation is presented which uses the surface pressure rise to predict the speed of the current front. This allows the speed prediction to be made with only surface measured quantities. This form of the equation is tested on gust front observations and numerical density current simulations and found to give good results.

A simple, moist numerical model is used to simulate dry density currents and moist currents which produce an arc/rop cloud. The numerical results provide information on the lifting associated with density current passage and the forced updraft velocities at the front, and also show that use of the pressure form of the density current speed equation is especially important when condensation is occurring above the density current.

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Keith L. Seitter and Judy Holoviak
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Stephen A. Pennell and Keith L. Seitter

Abstract

An explicit solution is derived for the trajectory of a particle in an inertial flow on a rotating sphere. This solution explains many of the features of inertial trajectories that have been presented in the literature.

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Keith L. Seitter and H. Stuart Muench

Abstract

Observations are presented of a cold front which passed over the Florida peninsula and produced a rope cloud visible on satellite imagery. The low level structure of the leading edge of the front is revealed with data from the 150 m tower at Cape Canaveral Air Force Station. It is shown that the leading edge of the front is very similar to thunderstorm gust fronts and the speed is predicted well by the gust front speed equation. This indicates that the rope cloud is directly analogous to the arc cloud sometimes observed along thunderstorm gust fronts. Further, the rope cloud appears to be a less intense manifestation of the forced convection that produces the narrow cold‐frontal rainbands observed in some other cold front studies.

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Keith L. Seitter and Frank P. Colby Jr.

Abstract

A mesobeta-scale numerical model is described that is designed specifically for operational use on relatively small computers (supermicro-class computers of the MicroVAX 3000/4000 type). A major aspect of the model leading to improved computational efficiency is that rather than using many model layers near the surface to resolve the growth and decay of the boundary layer explicitly, the model treats the boundary layer as a single model layer of known structure whose depth can evolve during the integration. The model equations are recant in a coordinate system, referred to as boundary-layer coordinates, based on the depth of the evolving boundary layer. The model described here does not include condensation processes, but it does include a radiation parameterization, schemes governing the structure of the stable and unstable boundary layers and the transitions between these regimes, and parameterizations for the fluxes of heat and moisture between the boundary layer and the earth's surface. Simulations have been carried out with a prototype model that has five layers and 20- km grid spacing in the fine grid mesh of its nested domain. Results of these simulations show that the model is capable of reproducing such mesoscale phenomena as mountain lee waves and the Florida sea-breeze circulation fairly well.

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Keith L. Seitter and Hsiao-Lan Kuo

Abstract

The structure of squall-line type thunderstorms is investigated with special emphasis on the upshear sloping updraft which is often observed in these storms. To aid in the investigation, a new version of the convective Richardson number is formed which adds the negative buoyancy due to liquid water loading to the thermal buoyancy of the lifted parcel in the calculation of the buoyant energy. This modified convective Richardson number is easily calculated using a pre-storm sounding and has predictive value for thunderstorm type.

While it is widely accepted that the updraft's upshear slope is caused by the ascending parcels partially conserving their horizontal momentum toward the rear of the storm, this paper presents a new theory for the production of this slope. In this theory, the vorticity production due to the liquid water distribution leads to an upshear slope of the updraft/downdraft interface. A stable slope is reached when the loading mechanism is balanced by the forces exerted by the environmental shear.

A two-dimensional numerical model is developed which is shown to be capable of reproducing many of the observed features of squall-line type thunderstorms, including the upshear sloping updraft. The internal structure of these storms is investigated with both Boussinesq and anelastic versions of the model. Through comparative simulations, the importance of liquid water loading and evaporative cooling is demonstrated.

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Susan F. Zevin and Keith L. Seitter
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Keith L. Seitter and Hsiao-Lan Kuo

Abstract

Abstract not available.

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Keith L. Seitter and Frank P. Colby Jr.

Students in the meteorology program at the University of Lowell provide weather forecasts for the university community and for the residents of the Lowell area. The forecasting service is organized and run entirely by the students through the University of Lowell student chapter of the American Meteorological Society. Students record a forecast and discussion on a phone answering machine and deliver written forecasts to key university offices. The service recently expanded to include periodic discussions by students on the local cable-television station.

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