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J. E. Geisler and F. P. Bretherton

Abstract

The response of a stably stratified atmosphere to differential heating across a coastline is studied. A linear, inviscid, Boussinesq theory is used to analyze the growth caused by switching on a small temperature contrast at an initial time. The propagation of the disturbance is interpreted in terms of internal gravity waves. The outer part of the disturbance, where the linearization is always valid, arrives ahead of the main nonlinear overturning, the sea breeze proper, and can therefore be described as the sea-breeze forerunner. Solutions are obtained and discussed for three different assumed model atmospheres, illustrating the influence of thermal stratification upon the structure of the forerunner. Some modifications are introduced to enable the effects of viscosity to be described. Reasonable values of internal viscosity and of surface drag are inserted into the modified theory, and these effects, respectivly, are found to limit the amplitude of the forerunner at moderate distances inland to a few meters per second and to limit the inland penetration distance to about 60 km.

Implications of the results of this study for a more general nonlinear theory are emphasized. In particular, the role of the upper boundary condition in a model of the sea-breeze circulation is clarified. The momentum balance argument advanced here should also be valid in the nonlinear case.

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Maurice L. Blackmon, John E. Geisler, and Eric J. Pitcher

Abstract

A general circulation model has been run in the perpetual January mode to produce several long-term simulations, each distinguished by a different imposed equatorial Pacific sea surface temperature. From each of them simulations we have extracted an eight-member ensemble of 90-day averaged fields. Ensemble-mean difference maps are presented in this paper, together with an estimate of the statistical significance of features which appear in thee maps. These results are compared with observational studies in the literature that present difference maps of Northern Hemisphere winter fields composited according to some index related to the two extremes of equatorial Pacific sea-surface temperature variation.

The results show many anomaly patterns of high statistical significance that are also in good agreement with those observed. In the tropics, them include 990 mb wind, sea level pressure and rainfall anomalies constituting the Southern Oscillation, as well as a 200 mb height anomaly at all longitudes. In extratropical latitudes there is a 500 mb height anomaly which agrees closely with the observed Pacific/North American (PNA) pattern. Other manifestations of the simulated PNA pattern which are in good agreement with observations are anomalies of height, zonal wind and temperature all at 700 mb. The model also reproduces large anomalies that are observed in the 10 mb height and zonal wind fields in the polar stratosphere.

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