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The structure of convection in the planetary boundary layer Davison, Douglas Stewart

Abstract

During July 1971, an intensive experimental program was undertaken in south-eastern Alberta to measure the turbulent statistics in the planetary boundary layer using a 92m tower, an array of four smaller towers and an instrumented T-33 aircraft from the National Aeronautical Establishment of the National Research Council. The height variation of the high wavenumber spectral levels of temperature and vertical velocity were found to follow the z-dependence predicted by semi-empirical theories applicable to the constant flux layer up to a height of at least 300m. However, unlike the constant flux layer, the assumption of Ri = z/L is not applicable at higher levels; thus, at large z, the spectral levels in the k[sup -5/3] region for temperature and vertical velocity have the form, Φ[sub TT] ∝z and Φ[sub ww] ∝z°. The translation velocity of convective plumes was measured with an array of towers, at a height of 3.5m. The direction of plume motion was found to be close to that of the surface wind. The speed of plume motion was found to be close to the wind speed near the top of the surface shear zone; this result is different from previous estimates using different techniques. A dynamic model of the plume was used to show that a translation speed close to the wind speed near the top of the surface shear zone (as observed) allows plumes to exist at a lower thermal instability level then any other possible translation speed. The horizontal cross-sectional shape of the temperature field of the plumes at 3.5m was observed to be very elongated in the downwind direction with a ratio of the downstream to cross-stream diameters of about 8:1. This shape was dynamically justified.

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