An experimental study of the atmospheric boundary layer modified by a change in surface roughness and surface temperature

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Virginia Tech

Three-dimensional wind measurements and temperature measurements were obtained on a 250-foot meteorological tower located near the Atlantic Ocean at Wallops Island, Virginia. The type of flow measured approached the tower from the ocean resulting in a complex three-dimensional type of flow as it sees a change in roughness and a possible change in surface temperature when passing the shoreline. During warm summer afternoons, the stable air is heated from below, and an internal boundary layer (IBL) with an unstable stratification develops within the stable layer which originated over the ocean. As this flow moves inland the IBL grows vertically depending on changes in surface roughness and surface temperature. Eventually, far enough inland, the IBL replaces the original stable layer. The vertical heat flux is positive in the IBL and negative in the overlying inversion. The point where the heat flux changes sign corresponds to the height of the IBL.

Measurements of the mean and turbulent flow quantities were made with a special computer-controlled data-acquisition system for the aforementioned type of flow. Data analysis includes the following statistical parameters: mean values, variances, covariances (heat flux and Reynold's stresses), spectra and cospectra. Nine, one-hour runs were analyzed and the results agreed closely with the suggested model.

In addition, the spectra and cospectra measured in the IBL, as well as those from the overlying inversion layer, reduce to a family of curves when expressed in appropriate similarity coordinates. These results for moderately, thermally stratified flows compare quite well with the Kansas data which were obtained in the surface layer. The results for very stable flow (z/L > 2.0) do not follow the same trend as was established in the moderately stable range.