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WALTER R. DAVIS

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Donald R. Davis and Soronadi Nnaji

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To evaluate the worth of uncertain information one must obtain three types of evaluative information: 1) statistical measures of the uncertainty of the information and of its likely occurrence; 2) the decision rule (how the information is used) and measures of the effectiveness for the decision action; and 3) the costs of obtaining the information. The steps in information evaluation are illustrated by an example where rainfall information is used to provide flood warning. The results illustrate how both information uncertainty and the decision rule jointly affect the value of the information and that in some circumstances information can have a negative value. In view of the increasing cost of information, it would seem prudent to utilize information evaluation in actions designed to eliminate the collection of information of low value and to efficiently use the information and data that is collected.

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D. R. DAVIS and C. E. DEAN

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Paul L. Moore and Walter R. Davis

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The occurence of the May 1951 hurricane of subtropical origin in the western Atlantic before the beginning of the usual tropical storm season was precedent-setting. Through an analysis of the hurricane an attempt is made to explain (1) the unusually early occurrence, (2) the difference between this hurricane of subtropical origin and the usual tropical storm, and (3) the movement as related to vertical structure, upper air flow, and distribution of ocean surface temperatures. The analysis suggests that the following factors contributed to the intensification of the incipient storm which began in connection with a cold high-level Low: (1) superposition of a divergent upper-wind field; (2) heating of the surface layers of the air mass by the Gulf Stream; (3) occurrence of unusually low temperatures at high levels. The movement of the surface center is found to be in accord with the stream flow at the top of the warm core between the 700- and 500-mb. levels. A possible influence of the ocean surface temperature distribution is suggested on the basis of a striking coincidence between the Gulf Stream axis and the storm track.

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Oliver R. Wulf and Leverett Davis Jr

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It is suggested that worthwhile results may be obtained by the use of a suitable form of the second law of thermodynamics for the calculation of the efficiency of the earth's surface and atmosphere, considered as the heat engine that is driving the large-scale atmospheric circulation. A relation is obtained connecting the rate at which the kinetic energy of the general circulation is dissipated by turbulence with the integral over the atmosphere of the net rate of absorption of radiation divided by the temperature at which it is absorbed. This is done on the apparently plausible assumption that any other terms in the atmosphere's entropy budget are relatively small. The method used is described, and its application is indicated with the aid of a sample calculation using illustrative data. It appears that the engine driving the atmospheric circulation is, from one point of view, more efficient than might be supposed.

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D. R. DAVIS and W. C. BRIDGES

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A weak tropical depression moved out of the Gulf of Mexico on Sept. 19–20, 1969. With the blocking action of a surface High and in the absence of steering currents aloft, the Low became stationary on the Florida coast for approximately 48 hr. Torrential rains occurred in a small area 60–65 mi to the east and 50 mi inland from the point where the Low made landfall. Record-breaking floods resulted. The 23-in. maximum point rainfall was about 9 in. greater than the previous maximum rainfall of record produced by a 1924 tropical storm in the same area. The location of the area of maximum rainfall with respect to the point of landfall of the Low's center closely follows the pattern previously reported for the more intense hurricanes and tropical storms.

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A. M. J. Davis and R. T. McNider

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The influence of katabatic winds on the Antarctic coastal waters is examined by using simple models of the ocean and atmosphere. A katabatic flow model incorporating Coriolis dynamics is solved analytically and another with nonlinear friction is solved numerically to provide wind stress to a two-layer coastal ocean model. The resulting solutions are evidently the first to incorporate Coriolis terms with a thermodynamic equation that includes compressional warming effects. The emphasis in this paper is on delineating the parameters that control the relative adjustment of the katabatic wind into alongshore and offshore components. By including nonlinear friction, it is shown that steeper slopes and weaker stratification tend to direct the wind more toward the ocean. It is further demonstrated that the katabatic forcing supports the strong polar easterlies (winds from the east) along the periphery of the continent and that the offshore extent should be dependent on the atmospheric Rossby deformation radius. The ocean model shows that significant downwelling occurs at the coast, while upwelling is predicted at a distance of the order of the ocean Rossby radius. An alongshore coastal jet from the east is found in the model and is evidently the manifestation of the east wind drift. The upwelling offshore may be a significant aspect of polynya formation and maintenance of the Antarctic divergence zone and contribute to the biological productivity of the region.

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David R. Walker and Robert E. Davis

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A climatology of the once-daily (0000 UTC) 1000-hPa error fields of the National Meteorological Center's 80-wave Medium-Range Forecast (MRF) model is studied. An analysis of the error field has been conducted over the contiguous United States and over the Northern Hemisphere from 20° to 80°N for three warm and four cool seasons (9 September 1987 to 6 March 1991). Temporal and spatial mean error fields over various integration lengths are presented.

The skill, as measured by the anomaly correlation, has not significantly changed over the lifetime of the 80-wave MRF model. Anomaly correlation values at 1000 hPa and 500 hPa show that the model is retaining useful information about the anomalies in the height field out to about one week. A reduction in the model biases may reflect an improvement in model physics (longwave radiational calculations, etc). The cool and warm seasons have distinctly different spatial error patterns. The 1000-hPa warm season shows spurious height falls over the southwestern United States that grow with increasing integration length. The 1000-hPa cool season underestimates the intensity of low pressure systems over and east of Hudson Bay and overestimates their strength over the Pacific Northwest.

Principal components analysis of the 429-variable error covariance matrices for the cool and warm seasons identifies 6 orthogonal variables that explain over 60% of the original error variance. MRF model problems appear to be related to problems the model has with simulating the atmosphere's interaction with orographic features (Alberta and Colorado Rockies), storm tracks and baroclinic zones (Gulf Stream region and United States-Canadian border), and persistent atmospheric features (Hudson Bay low, eastern Pacific subtropical high, and desert Southwest heat low).

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P. A. Davis and W. R. Peltier

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The model used by Davis and Peltier (1976) to study the linear stability of a compressible, stratified parallel shear flow underlain by a rigid boundary is extended to include the effects of turbulent dissipation. It is shown that the incorporation of both eddy viscosity and thermal diffusivity removes the critical level singularity that occurs in the inviscid compressible model. Both Kelvin-Helmholtz and resonant instabilities continue to exist in the presence of dissipation. The stability characteristics of both modal types' are investigated as functions of the parameters of the background flow, including Reynolds number. Dissipation is found to reduce the range of horizontal wavenumbers for which Kelvin-Helmholtz instability is possible, primarily by stabilizing the short-wavelength disturbances. The dissipative resonant modes are also found over a reduced range of parameter space, but the entire region of resonant instability is shifted to shorter horizontal wavelengths. This behavior is explained in terms of three interrelated factors: the effect of dissipation on the overreflection properties of the critical level, the effect of dissipation throughout the fluid as a whole, and the fact that the vertical wavelength of the instability in the region between the bottom of the shear layer and the ground is quantized. The long-wavelength propagating disturbances generated by a nearly isentropic shear layer in the inviscid model appear to be stabilized by dissipation. The growth rates of both Kelvin-Helmholtz and resonant modes are reduced by dissipation, but the reduction is more severe in the case of the resonant modes. The evolution of instabilities in a real shear layer is discussed in the light of this result.

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P. A. Davis and W. R. Peltier

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A stably stratified parallel shear flow near the ground is able to support two distinct modes of instability, these being the well known Kelvin-Helmholtz disturbance and a secondary wave whose growth in time derives from the process of critical level overreflection. Here we first consider the two modes of instability in circumstances in which they exist in isolation from one another and present some new results concerning their finite-amplitude characteristics. In particular, we comment on the likely mechanism of secondary instability which marks the transition to turbulence in a Kelvin-Helmholtz billow. Further, simulations of the finite-amplitude evolution of an isolated resonant mode shows that nonlinearity, of itself, is not able to compensate for a small linear growth rate in making the resonant mode a prominent feature of the flow. We suggest that such may be accomplished, however, by the interaction between the two modes through the process of vortex pairing since the first subharmonic of the fastest growing Kelvin-Helmholtz mode may be a resonant mode. We provide a simple, weakly nonlinear calculation which supports this notion.

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