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Russell L. Elsberry

The extensive research in tropical cyclone modeling during the 1960s and 1970s has resulted in a number of applications for real-time track prediction. A review of the characteristics of these 3-dimensional dynamical models is given, including a discussion of procedures for initializing and tracking the model storm. Some limited verifications of track forecasts are described. An outlook for the future is presented; both in terms of numerical model improvements, and for large-scale and inner-scale data required to implement the improved models.

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Russell L. Elsberry
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Russell L. Elsberry

Four separate (but coincident in time) field experiments to study tropical cyclones in the western North Pacific area will be carried out during August/September 1990 by the United States, the Economic and Social Commission for Asia and the Pacific/World Meteorological Organization (ESCAP/WMO) Typhoon Committee, the USSR, and by Taiwanese scientists. The objective of the U.S. experiment is to improve basic understanding of tropical cyclone motion. The focus of the ESCAP/WMO Typhoon Committee experiment is to improve operational track prediction. A multi-ship and aircraft expedition is planned by the USSR to understand the effects of the ocean inhomogeneities on tropical cyclones, and the response of the ocean to typhoon passage. Researchers in Taiwan will organize special observations of typhoons approaching Taiwan to understand the wind and precipitation distributions. The combined observations from these experiments should result in a comprehensive dataset for the study of western Pacific tropical cyclones.

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David Adamec and Russell L. Elsberry

Abstract

Shifts in location and strength of an intense oceanic flow such as the Gulf Stream to a cross-stream gradient in cooling are studied using two-dimensional numerical simulations. The gradient in cooling is imposed by removing more heat from the warm side of the associated baroclinic zone than is removed from the cold side. The initial flow is assumed to be in geostrophic balance. When only a vertical heat exchange associated with the convective overturning induced by surface cooling is allowed, the magnitude of the horizontal pressure gradient is reduced and the flow becomes supergeostrophic. The resulting cross-stream velocity will tend to shift the front toward the region of larger upward surface heat fluxes. When a vertical exchange of momentum is also allowed in the convective adjustment, the reduction of the initial surface velocities due to turbulent momentum exchange is not balanced geostrophically by a reduction in the horizontal pressure gradient. The flow becomes subgeostrophic and a cross-stream flow is produced that shifts the front toward the region of smaller upward surface heat fluxes. Although the along-stream current decreases near the surface, the current below the mixed layer is strengthened due to the exchange with relatively high momentum from above. The additional response due to an increase in the southward and eastward wind stress is compared to the response due to cooling only. Small changes in the temperature and flow fields occur when a southward wind stress is included. An eastward wind stress of 0.2 N m has a greater effect on the position of the simulated Gulf Stream than does a very strong gradient in cooling.

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David Adamec and Russell L. Elsberry

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No abstract available.

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David Adamec and Russell L. Elsberry

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The three-dimensional response of strong currents to cross-stream gradients of surface cooling is studied using numerical simulations. In particular, surface cooling is explored as a possible mechanism for explaining an observed 100 km southward shift in the mean position of the Gulf Stream during winter. The cooling increases in the downstream direction and in the direction of highest sea surface temperatures. In the immediate vicinity of the concentrated horizontal temperature gradient associated with the strong current system, most of the flow changes are induced by the cross-stream cooling gradient. The magnitude and direction of the cross stream circulation is highly dependent on whether or not a vertical mixing of momentum occurs when the water column convectively adjusts in response to the surface cooling. A weak cross-stream flow toward the higher sea surface temperatures occurs in the surface layer if momentum mixing does not occur, whereas a stronger flow toward lower sea surface temperatures results if momentum mixing does take place. In regions where the vertical shear is not large, the responses in the flow fields are due solely to the alongstream pressure gradient induced by the prescribed alongstream cooling gradient. The cross-stream response due to horizontal cooling gradient is not large enough to displace the Gulf Stream appreciably southward in any of the numerical simulations. By contrast, a moderate increase in the zonal wind stress is more effective in displacing the core of a strong current system than are very strong gradients in the surface cooling.

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David Adamec and Russell L. Elsberry

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Simulations of the oceanic mixed layer at Ocean Weather Ship Papa are used to study the sensitivity of the 30-day predictions of mixed-layer depth and temperature to the time resolution and averaging of the atmospheric forcing during spring, summer and autumn. The model simulations are sensitive to the length of the averaging window applied to the atmospheric forcing. Both the detail and trends in the mixed-layer depth and temperature deviate more from the control run when the length of the averaging window is increased. The effect of averaging the meteorological observations prior to calculating the surface fluxes is examined separately from the case in which the fluxes are calculated prior to the averaging. The cases which use forcing calculated from averages of the actual observations better simulate the detail and trend in mixed-layer depth of 30-day windows in the spring, summer and autumn than cases which use forcing based on the average of the calculated fluxes. By contrast, forcing based on an average of the calculated fluxes leads to better predictions of the detail and tend in the sea-surface temperature than the cases which use average observations to compute an average forcing.

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David Adamec and Russell L. Elsberry

Abstract

The effect of errors and biases in the atmospheric forcing for oceanic mixed layer model predictions is studied using data sensitivity techniques. First the bulk model of Garwood is used to predict 17 years of mixed layer evolution and temperature structure at Ocean Station Papa using forcing derived from the 3 h atmospheric observations. The model is then integrated again varying, one at a time, each atmospheric forcing variable by a Gaussian error whose spread is proportional to the standard deviations of that variable during late winter or midsummer. The results of those integrations are then compared with the control run to assess the effects of the added random errors or biases. A positive or negative bias in the atmospheric forcing is much more detrimental to the ocean prediction than is a random error with zero mean. The predicted mixed layer depths are more sensitive to errors introduced in the forcing in winter than in summer. Conversely, the mixed layer temperature is more sensitive to errors in summer than in winter. For both winter and summer, the wind speed is the most critical factor in predicting mixed layer depth and temperature. Dew point temperature is an important variable for mixed layer predictions during the winter. During summer, cloud cover becomes an important variab1e. The results of this study are compared with errors in mixed layer depth and temperature predictions that are due to errors in the initial profile. The errors in the predictions which are due to errors in the atmospheric forcing are comparable in magnitude to those errors which are due to imperfect initial conditions.

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Michael Fiorino and Russell L. Elsberry

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A two-dimensional Fourier decomposition procedure is used to isolate small (≤500 km), medium (500< λ ≤ 1500 km) and large (>1500 km) scale components of some typical tangential wind profiles used in theoretical studies of tropical cyclone motion. The contribution of these scales to the vortex motion is studied in a nondivergent barotropic model with no initial basic flow by selectively retaining or deleting different scales. Transfer of energy between wave groups due to nonlinear scale interaction occurs slowly in this model so that a scale group that is removed in the initial conditions is not restored by 72 h.

The largest scales, which account for a significant fraction of the vortex structure, primarily determine the speed of motion. That is, the speed of motion is proportional to the percentage of the total vortex that projects onto the largest scales. The medium and small males that contain less energy (because of the assumed vortex structure parameters) have a significant effect on the direction of motion by influencing the rotation of the asymmetric gyres that are induced primarily by the largest scales. The most important implication of these tests for dynamical tropical cyclone forecast models is that the initial vortex specification will project energy onto longer wavelengths that significantly affect track prediction. The agreement between the scales in the initial structure of the vortex and in the environmental analysis needs to be carefully considered.

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