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Alison F. C. Bridger
and
Duane E. Stevens

Abstract

A version of Holton's numerical model of the stratospheric sudden warming is found to be very sensitive to the assumed initial mean zonal wind distribution. In tests with three initial wind profiles, in only one case can we simulate a wavenumber 1 warming involving flow reversal over a deep layer of the polar atmosphere and substantial warming (i.e., a major warming). In the other two cases, flow reversal is noted in restricted areas and warming is less intense. Examination of Eliassen-Palm cross sections and of Matsuno's refractive index squared throughout each integration reveals how the different warnings develop, and why they differ. Examination of refractive index squared in the atmosphere at the time of enhanced wave propagation out of the troposphere may be valuable aid in predicting the likelihood of a warming. Some characteristics of an “ideal” mean zonal wind profile (with which a major warming can develop) are discussed.

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Alison F. C. Bridger
and
Duane E. Stevens

Abstract

The spherical geometry of the earth is replaced by polar cylindrical geometry, with a plane tangential to the earth at the pole. The resulting frequency and structure of free motions in an isothermal, adiabatic atmosphere with a resting basic state is studied in both geometries. The solutions for ν (meridional wind) may be written as a single Bessel function if certain approximations are made. For positive equivalent depths, the geometrical approximation is best when the Lamb parameter ε≳ 10, so that Rossby waves are well modeled, while fast moving gravity waves are not well approximated. The impact of setting f to a constant value when undifferentiated, as in the usual midlatitude beta-plane approximation, is examined. It is found that the value of f is as important in determining how well the model behaves as are the geometrical and other approximations.

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Alison F. C. Bridger
,
William C. Brick
, and
Peter F. Lester

Abstract

Data collected on board an instrumented aircraft during the Marine Atmospheric Boundary Layer Experiments, West Coast (MABLES WC) field study are examined. The data were collected at several levels in the marine and inversion layers approximately 200 km west of the central California coast during August 1978. Composite sounding and cross sections of wind and temperature variables are presented to provide an overview of the mesoscale structure of the offshore marine and inversion layers at the time of MABLES WC.

Under typical summertime synoptic conditions, which prevailed early in the study period (three flights examined), a strong inversion is found. Base heights and temperature increases across the inversion base are comparable to values observed in the San Francisco Bay area. The topography of the inversion base shows a slope downward toward the east during all three of these flights and also suggests the presence of convective activity, gravity-wave activity, or both, during two of the three flights. The inversion weakened later in the study period when atypical synoptic conditions prevailed.

During typical conditions, wind-speed minima were found near the base of the inversion, and jets with speeds as high as 17–18 m−1 were found within the inversion layer. These observations of the vertical wind profile agree with those made in the San Francisco Bay area. Further, in two of the three cases, a well-organized horizontal structure of the jet was apparent. Data gathered during a flight later in the study period (“atypical” synoptic conditions) are also presented and show a particularly good example of the jet in the offshore region.

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Alison F. C. Bridger
,
Allen J. Becker
,
Francis L. Ludwig
, and
Roy M. Endlich

Abstract

Applications of the Winds on Critical Streamline Surfaces (WOCSS) model in the San Francisco Bay Area are described. Three case studies, chosen to represent important classes of airflow in the region, were conducted. Two cases involved a prevailing northeasterly flow with or without an inversion, and the third case involved northeasterly flow at the time of the Oakland hills firestorm of 20 October 1991. The dependence of model results (surface winds) on input winds and on the specification of inversion topography is discussed. Dependable results are produced with relatively few well-placed surface observations and with a single sounding. The results suggest that the model is quite suitable for routine, real-time analyses and other practical applications.

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