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Russell L. Elsberry
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Russell L. Pfost

This paper reexamines two historic South Florida hurricanes, the “Miami” Hurricane of 1926, and the “Okeechobee” Hurricane of 1928. These storms are frequently cited for their disastrous impacts, but the casualty figures currently associated with them are low due to underreporting of nonwhite persons and other sociological factors. More accurate information is available, and to put the impact of these storms in a better historical perspective, the casualty figures associated with them should be corrected.

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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

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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Gary L. Russell

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Starting with Arakawa and Lamb’s second-order C-grid scheme, this paper describes the modifications made to the dynamics to create a C-grid atmospheric model with a variable number of cells for each vertical column. Where mountains exist, grid cells are discarded at the bottom of the column so that the mass per square meter of retained cells is more nearly equal to that of horizontally adjacent cells. This leads to the following chain of causes and effects: decreased mass variations reduce the numerically induced alternating patterns in the horizontal velocity components, which reduce erroneous vertical mass fluxes, which reduce erroneous precipitation. In addition, horizontal flows above mountains are smoother, the Ferrel cell is stronger, and the polar cell is better organized. The C-grid performs geostrophic adjustment best among the gridpoint schemes, being the most sensitive to condensation-released heating perturbations. It also overreacts more egregiously to numerical errors, particularly with respect to the vertical mass flux, and consequently is often not used. Mesinger et al. applied the step-mountain (eta coordinate) technique to an E-grid scheme with excellent results. Its application to the C-grid reduces numerical errors in the vertical mass flux resulting in improvements in precipitation and other quantities.

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L. F. HUBERT and RUSSELL KOFFLER

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No Abstract Available.

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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

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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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David Adamec and Russell L. Elsberry

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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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