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John W. Diercks and Richard A. Anthes

Abstract

This paper investigates the generation and propagation of spiral bands on an axisymmetric base-state vortex. A linear model is used to study the formation of bands from internal gravity-inertia waves in a barotropic atmosphere. Spiral bands form random perturbations placed on a vortex with unstable static stability that is equal for ascending and descending motion. This growing mode assumes the characteristics of pseudoadiabatic motion in a conditionally unstable atmosphere due to the coarse vertical resolution of the linear model. Implicit diffusion from the centered finite-difference scheme shifts the preferred growth modes from infinite wavenumbers, characteristic of inviscid analytical solutions, to 4Δλ and 4Δr,ar wavelengths in numerical experiments. Here Δλ and Δr are the angular and radial distances between grid points. Explicit diffusion representing subgrid-scale eddies shifts preferred modes to longer wavelengths. Rotation in the basic state is a necessary condition before the unstable gravity-inertia waves form spiral bands. Rotation also organizes stable perturbations into a banded pattern. Inertial instability and the Coriolis parameter are unimportant for band formation in these linear experiments. The distance between bands increases and the growth rate decreases in experiments in which adiabatic warming occurs with descent and warming due to latent heat release occurs with ascending motion.

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John W. Diercks and Richard A. Anthes

Abstract

Results from diagnostic studies of a nonlinear hurricane model support the conclusion that internal gravity-inertia waves are responsible for hurricane rainbands. The mean relative vorticity differed little between the bands and their environment, a characteristic of gravity waves modified slightly by the earth's rotation. Small differences in mean radial and tangential velocity components, divergence, and the radial pressure gradient force were noted between the bands and their environment. The upper layers of the bands were responsible for a small increase in the model storm's kinetic energy due to a net convergence of kinetic energy flux from the environment into the bands. A large net convergence of cyclonic angular momentum flux into the bands occurred in the boundary layer. Conversion of available potential energy to kinetic energy was not significant in the model bands. Finally, latent heating in the bands did not play an important role in the maintenance or propagation of the bands at large radii.

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Dale C. Barnum and John W. Diercks

Abstract

The Winter Trajectory Test Program was conducted as part of the Air Weather Service Forecaster Assistance Program. The objective of this test was to determine if three-dimensional trajectories derived from the output of the Air Force Global Weather Central's six-level model are useful in the preparation of terminal forecasts. Headquarters Air Weather Service personnel evaluated forecasts prepared at detachments located in the central and eastern United States and modified some of these forecasts on the basis of trajectory information. The application of trajectory data improved the four-month verification of the terminal forecasts by 3.1%. Test procedures are discussed, and objective rules developed for the evaluation are presented.

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Jack E. Huntley and John W. Diercks

Abstract

Developing tropical cyclones are often observed with significant displacements between their surface and upper level circulation centers. The slope is in the direction of the convective cloud mass which also is displaced from the surface center during the early stage of development. As the cyclone intensifies, the surface and upper level centers become vertically aligned. Three representative tropical cyclones in the western North Pacific with extensive aircraft reconnaissance are discussed to illustrate this phenomenon.

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Jack E. Huntley and John W. Diercks

Abstract

No abstract available.

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George M. Dunnavan and John W. Diercks

Abstract

Super Typhoon Tip was an eventful tropical cyclone which developed in the western North Pacific in early, October 1979. Besides establishing the world's record for the lowest minimum sea level pressure ever measured in a tropical cyclone, Tip also possessed the largest surface, circulation pattern ever observed for a tropical cyclone. The development cycle of Super Typhoon Tip from a weak disturbance to a mature typhoon to an extratropical system is discussed in view of the record breaking performance of this typhoon.

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