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Microscale pressure fluctuations measured within the lower atmospheric boundary layer Elliott, James Arthur

Abstract

An instrument was developed to measure the static pressure fluctuations within the turbulent flow of the atmospheric boundary layer. This instrument was used to measure some of the properties of pressure fluctuations over a flat boundary and over water waves and has provided the first reliable pressure data within a turbulent boundary layer. For all observations over a flat boundary the root-mean-square pressure produced by the boundary layer turbulence was about 2.6 times the mean stress. The spectra had a power law behaviour with a mean slope of -1.7 for scales above the peak of the vertical velocity spectrum. Pressure fluctuations were approximately spherical in shape, and propagated downstream at a rate equal to the 'local' mean wind. Above the boundary, the large scale pressure fluctuations were approximately in phase with the downstream velocity fluctuations; at small scales there was a large phase difference (≃135°). These phase differences were interpreted to be the result of the large pressure producing scales interacting with the earth's surface, while the small scales were 'free' of the surface. Pressure forces resulted in an energy flux out of the downstream velocity fluctuations of about 0.45 of the total energy source for the turbulence within the band of 0.05 < kz < 20. The pressure term in the net energy budget was found to be about 1/10 of the energy feeding term. Pressure measurements near wind generated waves showed a large spectral hump at the wave frequencies. The amplitude of this hump increased, and its vertical rate of decay decreased, as the mean wind speed increased. The phase difference between pressure and waves during active generation was found to be about 135°, pressure lagging waves. This did not change vertically.

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