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M. A. Rennick

Abstract

The response of a linearized numerical model of African waves to changes in the specification of the zonally-averaged flow is investigated. The growth rate and wavelength of the most unstable mode are found to depend on both the magnitude of the maximum shear and the relative magnitudes of horizontal and vertical shears. Attention is paid particularly to the mechanisms by which the wave energy grows. For jets similar to the observed African jet, the wave becomes more unstable as baroclinic processes are suppressed. Suppression of barotropic processes stabilizes the wave. These results help to clarify discrepancies in previous model results, and to explain some regional differences in the observed waves.

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M. A. Rennick

Abstract

The U.S. Navy’s operational implementation of the hurricane prediction system developed at the National Oceanographic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory is described, and the performance of the model during the 1996 western North Pacific tropical cyclone season is analyzed.

The model was highly reliable in terms of maintaining and tracking tropical cyclones, maintaining 96%, 93%, and 93% of all tropical storms and typhoons at 24-, 48-, and 72-h forecast periods. Subsequent model improvements raised these percentages to 96%, 93%, and 100%. Overall track errors were 176, 316, and 466 km for the same periods. Errors for tropical storms and typhoons were 75–150 km smaller than those for tropical depressions. The difference generally grew with forecast length. Large track errors were generally associated with a sheared environment, spurious interactions with elevated terrain, or poorly timed recurvature. On average, the model slightly underforecast intensity, but intense systems were significantly underforecast due to the inability of the model to resolve the eyewall.

For the entire season, tropical cyclone track errors are very similar to those of the navy’s global model, Navy Operational Global Atmospheric Prediction System. However, significant differences are found in individual forecasts. Further study is required to identify environmental features that lead to systematic differences in model performance.

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M. A. Rennick and R. L. Haney

Abstract

The linear stability of two coupled shallow water models of the equatorial atmosphere and ocean is investigated analytically. The ocean-to-atmosphere coupling is parameterized in terms of the sea surface temperature anomaly,T. In one model T is generated by zonal advection, while in the other model it is parameterized in terms of the thermocline depth anomaly, h. In both models, the behavior of a small amplitude wave disturbance is found to be extremely sensitive to the atmospheric first baroclinic mode Kelvin wave speed, C a and mean zonal wind, Ū Most importantly, the growth rates, phase speeds and meridional structures of the disturbances (and their dependence on the above basic state parameters) are sensitive to the form of the atmosphere-ocean coupling. This sensitivity is due to the fact that, for an oceanic Kelvin wave, the two methods of computing T result in different feedback effects. According to the simple analytic models used in this study, in which the meridional component of motion is neglected in both the atmosphere and ocean, equatorially trapped unstable (growing) modes occur only when the ocean-to-atmosphere coupling is parameterized in terms of the advectively produced sea surface temperature anomaly. The resulting growth rates and phase speeds of the growing modes can perhaps account for the onset of an ENSO (El Niño-Southern Oscillation) event, but only in the unusual case in which C a∼|Ū|.

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J. L. Hayes, R. T. Williams, and M. A. Rennick

Abstract

The growth of synoptic scale cyclones imbedded in a baroclinically unstable zonal flow over a long straight mountain range is investigated. Two different analytical models of the phenomenon are used.

The first model uses the linearized quasi-geostrophic equations. It allows a simple superposition of a steady state mountain forced solution and a transient Eady wave. There is no dynamic interaction between the two solutions, but the time evolution of the combined solution reproduces many characteristics of a disturbance passing over the Rocky Mountains.

The semigeostrophic equations are used in the second model. These equations allow a linear solution in transform space, but the transformation of the solution to physical space is nonlinear. This allows an interaction between the mountain forced and transient solutions. The minimum pressure developed by the semigeostrophic system is the same as that of the quasi-geostrophic system. However, the shape of the wave is distorted. This effect is caused by the divergent part of the mean flow over the mountain ridge.

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J. L. Hayes, R. T. Williams, and M. A. Rennick

Abstract

The effect of topography on the evolution of a disturbance in a baroclinically unstable mean flow is studied using a three-dimensional primitive equation model. A procedure is developed to compare control integrations with no topography with integrations that contain topography. It is found that lee cyclogenesis is caused primarily by the superposition of a growing baroclinic wave with a steady, orographically forced wave of the same scale. Some additional lee growth is found that may be orographically enhanced, or it may be related to certain small problems in the experimental set up. As the disturbances move over the ridge, they are deflected to the north on the upwind side Wore returning to their original latitudes on the lee side. The numerical results in this paper are in general agreement with the authors' previous analytic study.

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