Concurrent Airplane and Balloon Measurements of Atmospheric Boundary-Layer Structure over a Forest

Abstract

During three observational periods in the spring and summer of 1964, an experimental program was conducted over a forested area in northeastern Wisconsin in which airplane measurements in the planetary boundary layer were obtained in conjunction with simultaneous pilot-balloon observations of mean wind profiles up to a height of 2000 m and supporting surface-layer measurements. Sufficient data were obtained to permit the investigation of several features of boundary-layer structure. A strong dependence of horizontal and vertical velocity variances upon stability is found, and velocity spectra for stable and unstable conditions are quite different. A clear distinction between the eddy sizes responsible for momentum transport in near-neutral and unstable situations is shown by the co-spectral densities of horizontal and vertical velocities. Heat-flux profiles derived from the combined data are qualitatively similar to those found by previous investigators and to Ball's theory, in that the heat flux decreases steadily with height and tends to become negative in the upper portion of the boundary layer. However, the eddy-energy budget fails to show the strong transport of eddy energy to the inversion level that is required by Ball's model, indicating that if such a transport occurs, it must be at wavelengths longer than those measured. An approximate balance of the eddy-energy budget is possible when allowance is made for the bandwidth restrictions on the vertical flux measurements.