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GEORGE W. PLATZMAN

Abstract

The use of central differences on a rectangular net, in solving the primitive or vorticity equations, produces solutions on each of two lattices. By exploring this lattice structure, a formal equivalence is established between the central-difference vorticity and primitive equations. A demonstration is given also that exponential instability previously found to result from certain types of boundary conditions is suppressed by applying these conditions in such a way as to avoid coupling the lattices.

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THE PREDICTION OF SURGES IN THE SOUTHERN BASIN OF LAKE MICHIGAN

Part I. The Dynamical Basis for Prediction

GEORGE W. PLATZMAN

Abstract

A summary of numerical computations is presented, in a form designed to aid in operational prediction of surges in the Southern Basin of Lake Michigan. The computations are based upon a dynamical model in which the surge is generated by pressure gradient and wind stress in a squall line which moves across the Basin with constant speed and direction. For each of 25 combinations of squall-line propagation speed and direction, the arrival time of the surge is determined, and the amplitude estimated, at various locations along the shore. At some locations there are a well-defined peak of surge amplitude and corresponding critical values of squall-line propagation speed and direction, associated with resonant coupling between the squall line and Lagrangian body waves. Energy computations indicate the presence of another resonant peak associated with Stokesian edge waves.

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SHIRLEY M. IRISH
and
GEORGE W. PLATZMAN

Abstract

The dates of incidence of extreme wind tide on Lake Erie have been determined for the 20-year period 1940 through 1959, for all cases in which the difference in lake level between Buffalo and Toledo exceeded 6 feet. A frequency-intensity analysis shows that a set-up in excess of 10 feet may be expected once every 2 years. Extreme wind tides occur mainly in the 6-month period October through March; more than 70 percent of the cases fall in the three months November, December, and January. November is the month of most frequent incidence, having more than one-third of the total number of cases in the period studied.

The observed seasonal variation of extreme set-up frequency is interpreted as a reflection of the seasonal variation of storm frequency and of storm-track location. Secondary, but important factors are: seasonal variation of storm intensity and seasonal variation of thermal stability of the atmospheric boundary layer. The tendency for marked temperature stratification to be present in the Lake during the summer probably inhibits set-up to a significant degree during that season.

Storms which produce extreme set-up follow very similar paths during the 24 hours preceding the time of maximum set-up. However, no clear-cut relation was found between source region and set-up intensity, apart from a tendency for Alberta Lows to yield slightly smaller set-up. Brief descriptions are given of the storms of March 22, 1955, and November 17, 1955.

Finally, the writers report that no relation was found between frontal speed and set-up intensity, and infer that resonant coupling with the Lake does not contribute significantly to set-up magnitude.

It should be noted that storms accompanied by strong easterly winds, which produce high water on the western end of the Lake, have not been studied in this investigation.

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SHIRLEY M. IRISH
and
GEORGE W. PLATZMAN

Abstract

No Abstract Available.

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