Transport and Diffusion in the Transition Layer and Planetary Boundary Layer for Drainage Flows in Anderson and Putah Creeks, California, during ASCOT 1980

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.