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TAKASHI NITTA and JOHN B. HOVERMALE

Abstract

A technique of initialization for the primitive forecast equations is presented. The method consists of a marching prediction scheme performed in such a manner that the large-scale solution remains approximately steady, and high-frequency modes created in the adjustment process are damped selectively by time differencing with the Euler-backward method. The scheme places no restriction on the wind divergence field and ensures truncation error consistency between initialization and forecast equations.

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Kenneth E. Mitchell and John B. Hovermale

Abstract

The structure of the thunderstorm gust front is investigated by a nonhydrostatic, two-dimensional (x z/) numerical model. In the model, which is dry, the production of negatively buoyant air by evaporation is parameterized via an externally imposed, local-cooling function. This parameterization sustains a steady cold downdraft, which drives the surface outflow and associated gust front.

It is shown that two dominant factors influencing gust front structure in the vertical plane are the solenoidal field coincident with the front and surface friction, modeled by means of a simple bulk aerodynamic drag formulation. The circulation theorem is invoked to illustrate how solenoidal accelerations oppose the deceleration by surface friction. After the onset of a downdraft in the model, these opposing tendencies soon reach a balance. Thus, following a brief transient stage, the model gust front exhibits a persistent configuration as it propagates rapidly forward. The essential features of this configuration are examined and compared with both tower observations of gust fronts and laboratory models of gravity currents.

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Frederick G. Shuman and John B. Hovermale

Abstract

In mid-1966 a new baroclinic numerical weather prediction model became operational at the National Meteorological Center, an event made possible by advances in computer and communications technology. The new model integrates directly the primitive (hydrostatic) hydrodynamic and thermodynamic equations, a departure from previous operational models whose central dynamic equation was that of conservation of vorticity. In its first fourteen months of operational use, it has resulted in highly significant improvements in the Center's products.

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