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Michael A. Fosberg

Abstract

Observations of airflow over the Santa Ana Mountains were analyzed by numerical techniques. The Boussinesq equations and their energy properties were used to define the terms computed from the observed data. The numerical results were based on a 2-km horizontal and 200-m vertical mesh.

Flow over the ridge can be divided into three distinct stages. The first stage is that of the valley wind regime with ridge-top convection. The second and third stages are associated with flow across the ridge. After a short transition period, the flow streams across the ridge and oscillates in the vertical plane. This wave-like motion is apparently produced by the response to the thermal field.

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Michael A. Fosberg

Abstract

Approximately 100 constant-volume, superpressured balloons (tetroons) were tracked in the Anderson and Putah Creek drainages of the Geysers geothermal area of northern California as part of the ASCOT (Atmospheric Studies in Complex Terrain) program. These tetroons were tracked by radar. Each tetroon was equipped with a transponder which provided a 100 MHz shift in frequency, providing a means of discriminating the tetroon-transponder broadcast signal from radar signals reflected from the hard terrain.

Tetroons were used to provide direct measurements of trajectories of individual air parcels, to provide transport speeds of individual air parcels and to provide direct measurements of the turbulence associated with an individual parcel of air. In addition, clusters of tetroons were released, providing direct measurement of the very low frequency turbulence associated with air flow meander within the two valleys.

All tetroon flights were in the transition layer between the katabatic drainage flow and the larger scale prevailing winds. Resultant analysis of the eddy diffusivities, in essence, K theory (first-order closure of the turbulence), and the very low frequency dispersion characteristics as indicated by the variance about the centroid of a balloon cluster, showed that the transition layer was well mixed on all experimental nights. In particular, the lateral dispersion was equivalent to Pasquill-Gifford Class B.

The K theory was used to estimate the dispersion coefficients. These estimated variances showed characteristics of dispersion similar to those obtained from the tetroon clusters. Dispersion estimates both by tetroon clusters and by transformed eddy diffusivities showed large variability.

Dispersion was greater in Anderson Creek than in Putah Creek. Also, a suppression of dispersion rate was found in Putah Creek between ∼1 and 2 km downwind of Larry's Cabin where Ford Flats is separated by a small ridge from the Anderson Creek drainage and from Anderson Springs.

The ultimate fate of the tetroons, once they were beyond the Collayomi Valley, was determined from the locations where they were recovered. Most tetroons were found in the Napa Valley or along the east side of the Sacramento Valley from Yuba City to Chico. The tetroons were found at large distances, one near Mount Hood, Oregon and the other on Vancouver Island, British Columbia.

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Michael A. Fosberg

Abstract

The convection associated with a valley wind regime was analyzed by numerical techniques. The Boussinesq equations and their energy properties were used to define the terms computed from observed data. The numerical results are based on a 1-km horizontal and 100-m vertical mesh. Results from the numerical analysis of the observed data were compared with numerical simulations of valley winds and convection published in the meteorological literature.

In general, agreement was found between the data and the numerical simulations. The simulations have reproduced most of the features and processes of the valley wind system. The perturbation motion field was partially recovered from the total wind field. The perturbation stream functions, the solenoid fields, and to a certain extent, the energy properties provide the best evidence of the agreement.

The valley wind reaches a quasi-steady state in the afternoon. This steady state results from an apparent maximum rate of conversion of potential to kinetic energy.

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Michael A. Fosberg and Mark J. Schroeder

Abstract

Three-dimensional analysis shows the sea-breeze front to be primarily a wind shift line in which the thermal discontinuity lags behind the shear line and becomes quasi-stationary in the afternoon. Two synoptic patterns control the temperature regime. On cool days, when a high level trough is present the sea-breeze front is weak, moves rapidly and penetrates far inland; the marine layer is deep and the sea breeze spills over the low mountain ridges. Warm days occur, when the Pacific high penetrates into Oregon and Washington; the marine layer is shallow and the sea-breeze front is intense and slow moving.

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Mark J. Schroeder, Michael A. Fosberg, Owen P. Cramer, and Clyde A. O'Dell

This paper reviews the literature on the main aspects of marine air invasion on the Pacific coast. In particular, it considers the sea breeze, Pacific coast monsoon, and airflow over coastal mountains. The major problems associated with marine air invasion are: 1) the relationship of microscale convection, waves, and banded convection- waves to the sea-breeze energetics and sea-breeze front, 2) the interaction of energy forming and dissipating processes from the general circulation down to the micrometeorological scale, 3) the climatology and dynamics of the monsoon, and 4) the mechanism of the lee waves observed in the lee of the heated coastal mountains. Lee waves often form in a statically unstable atmosphere.

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