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Robert L. Smith

On 15 October 1957, an RHI radar observation was made of a reported funnel cloud with the CPS-9 radar at Texas A. & M. College, College Station, Texas. The funnel was reported by the Weather Station at Bryan Air Force Base at the time the radar observation was made. The synoptic situation and radar observations are discussed. PPI scope pictures as well as RHI illustrations are presented.

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Adriana Huyer and Robert L. Smith

Abstract

Observations of the hydrographic regime over the continental shelf off Oregon from the R/V Yaquina during the summer of 1972 showed the presence of an alongshore, subsurface ribbon of relatively cool water. Its properties and its evolution during the 1972 season are described. Examination of earlier observations showed that evidence of the ribbon was found during the upwelling season in almost every year between 1961 and 1971. It is usually observed when the upwelling index (the monthly mean Ekman transport directed offshore) is high. The ribbon can be accounted for by southward advection of sub-arctic water due to the coastal jet associated with upwelling. A warm temperature anomaly, occurring at somewhat higher salinity, is frequently observed inshore of the ribbon.

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David L. Cutchin and Robert L. Smith

Abstract

Sea level variations and currents on the Oregon continental shelf exhibit wavelike characteristics in a frequency band from approximately 0.15 to 0.45 cycle per day (cpd). Shelf wave dispersion curves and eigenfunctions for the Oregon continental shelf profile computed using a numerical technique are compared with a low-frequency (∼0.03–0.75 cpd) spectral analysis of the current, sea level, and atmospheric pressure records. In a narrow band around 0.22 cpd the current, sea level relationship is consistent with the predicted values for free barotropic continental shelf waves.

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Richard D. Romea and Robert L. Smith

Abstract

Time series of coastal sea level during 1976–77, from 2°12′S to 17°S along the west coast of South America, show that low-frequency, ω < 0.25 cycles per day (cpd), fluctuations propagate poleward with the phase speed of baroclinic Kelvin waves (2–3 m s−1). The alongshore coherence is highest in the frequency band 0.1–0.2 cpd. Computing the frequency-domain empirical orthogonal functions (EOF) for alongshore current, from an army of current meters extending from 5°S to 15°S during March-May 1977, gives 70% of the variance in the 0.1–0.2 cpd frequency band to an EOF mode with poleward phase propagation at 2.75 m s−1. The vertical structure of the alongshore current fluctuations (0.1–0.2 cpd) over the continental slope at 5°S and 15°S is consistent with a first-mode baroclinic Kelvin wave. The current and sea-level fluctuations are coherent and propagate poleward through latitudes where their frequency equals the local inertial frequency. The fluctuations are not significantly coherent with coastal winds from 4°S to 15°S and am therefore presumed to have an equatorial origin. Intermittent sea-level data at the Galapagos Islands during the period provide tenuous evidence that these fluctuations, propagating poleward as coastally trapped waves, previously traveled in the equatorial wave guide.

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Robert L. M. Rausch and Phillip J. Smith

Abstract

This paper examines a simulation of the explosive development phase of the ERICA IOP 4 extratropical cyclone case obtained from the 70-km Limited Area Mesoscale Prediction System. A detailed diagnosis of the simulated cyclone is performed using the Zwack–Okossi equation to examine the forcing mechanisms influencing the near-surface synoptic-scale pressure tendency. The diagnosis includes two special features, the inclusion of forcing terms representing synoptic-subsynoptic exchange processes and a unique approach to evaluating the contribution of individual levels to the vertically integrated forcing.

Explosive cyclogenesis was initiated by the combined effects of warm-air advection, cyclonic vorticity advection, and latent heat release, the first two maximizing above 400 mb and the latter below 750 mb. At the cyclone center the temperature advection profiles also featured a secondary warm-air advection maximum below 800 mb whose impact on surface pressure tendency was often equal to or greater than the upper-level maximum. The end of explosive development was signaled by a marked decrease in development rate that corresponded to a reversal in sign of the temperature advection contribution. Finally, subsynoptic processes contributed significantly through the synoptic-subsynoptic exchange of temperature.

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Walter P. Smith and Robert L. Gall

Abstract

Squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern Arizona and northwestern Mexico. Initial thunderstorm formation is over the Continental Divide in the late afternoon and the systems become organized within a few hours. Satellite imagery, cloud-to-ground lightning strike data, and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than the environmental flow at all levels below 20 kPa so that most of the anvil clouds lag behind.

The synoptic-scale circulation is anomalous with a strong ridge located over the western United States and a deep trough located over the eastern United States. West to northwest winds are found in the boundary layer over southern Arizona and northwest Mexico while a deep layer of east winds are observed above. As a result most of the environmental wind shear is confined to the lowest 2.5 km above the ground with very little shear at higher altitudes. The low-level wind shear seems to be required for the westward propagation of thunderstorms and the formation of the squall lines. Extremely dry midtropospheric air develops in the easterly flow through some combination of advection and subsidence and also appears to be an important factor in the development of the squall lines and in the creation of severe thunderstorms.

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ROBERT L. SMITH and DAVID W. HOLMES

Abstract

The U.S. Weather Bureau has been experimenting with a radar operating on the Doppler principle to determine whether apparatus of this type would detect and uniquely identify tornadoes. The principles of Doppler radar as applied to meteorology and results of recent experiments with equipment of this type are discussed. Calculations of anomalous wind speeds of 206 m.p.h. in a funnel cloud and 94 m.p.h. in a dust devil are presented in detail. In addition, data have been gathered from squall lines and isolated thunderstorms. Recommendations are made for an optimum Doppler radar system for the detection of tornadoes.

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Robert Cox, Bruce L. Bauer, and Thomas Smith

An intercomparison of four mesoscale numerical prediction models that could lead to the selection of a model for use in the theater of operations by United States Air Force (USAF) meteorological personnel is described. Mesoscale numerical prediction models have matured, and recent advances in computer hardware make this a realizable objective.

Two studies were launched to determine if a mesoscale model could be used operationally in theater and to select the model that produced the best forecast under simulated operational conditions. Of prime concern was not whether the model could produce reliable forecasts in data-rich areas, but how well the models operated and thus produced forecasts in data-sparse areas. The first study did an overall review of the available mesoscale numerical weather prediction models resulting in a general ranking of the models by expected forecast ability and operational maturity. At the conclusion of this study it became apparent that a more in-depth analysis was needed to distinguish among the higher-ranking models. Thus, this study was initiated.

This study compared four models for quality of forecasts in different climate regions in the world. Two are considered state-of-the-art models that could easily be made operational. These are the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model 5 (MM5) and the Colorado State University Regional Atmospheric Modeling System (RAMS). The third model was the Navy Operational Regional Prediction System Version 6 (NORAPS6), the navy's operational regional forecast model. The fourth model is the current USAF mesoscale model, the Relocatable Window Model (RWM), that was used to provide a baseline of the current USAF capability.

The models were scored by comparing the forecast values with observations. The relative ranking of the models varied with parameter, but overall, the rank order was RAMS, MM5, NORAPS6, and RWTVL The score disparity between the models was not large.

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Benoit Cushman-Roisin, James J. O'Brien, and Robert L. Smith

Abstract

This note studies the response of a simple linear baroclinic coastal-upwelling model to fluctuating longshore winds. Correlations between wind stress and velocities are computed explicitly. It is shown that these correlations depend primarily upon the wind-stress spectrum and that, for a realistic spectrum, the wind stress leads the longshore velocity by approximately one day. The computed correlations agree remarkably well with observations and dismiss the belief that time lags ought to be directly related to the local inertial period, i.e., some fraction of the local pendulum day.

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Kenneth H. Brink, J. S. Allen, and Robert L. Smith

Abstract

An analysis is presented of low-frequency (<0.4 cpd) fluctuations in currents, temperature and tide gage data collected during the March-September 1976 segment of the CUEA JOINT-II experiment off the coast of Peru. The observations were made near 15°S, a region of particularly strong and persistent coastal upwelling. Conclusions about the dynamics of motions over the continental shelf and slope are reached by means of correlations, empirical orthogonal functions and other indicators. It is found that flow over the shelf, where stratification was weak, was generally dominated by vertical turbulent frictional effects and was strongly coupled to the effectively inviscid, baroclinic flow over the slope. The momentum balance was three dimensional, with the alongshore pressure gradient playing an important role. In contrast to behavior in other coastal upwelling regions, the alongshore velocity field over the shelf and slope was evidently not strongly driven by the local alongshore component of the wind stress. The mean wind stress throughout the period was equatorward (upwelling favorable), whereas the mean alongshore currents over the shelf were poleward. The alongshore current fluctuations, which propagated poleward along the coast, were initially poorly correlated with the local wind stress, but during the course of the experiment, the wind stress increased in magnitude and gained in importance as a driving mechanism. The temperature and onshore-offshore current fluctuations over the shelf and, therefore, presumably the upwelling circulation were, however, correlated with the local wind stress throughout the experiment.

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