Mixing-height differences between land use types: Dependence on wind speed

Mixing height and its spatial variation are important for a variety of issues and applications. In a previous study in the region around Nashville, Tennessee, we found a significant difference in the daytime mixed-layer inversion height z(i) between the area to the north and west of the city and the area to the south and east. These observations were from 3 days of very light daytime winds. The subregion to the north and west is deciduous forest, and that to the south and east is agricultural and mixed forest land. The differences in z(i), which we are referring to as Delta z(i), were attributed to differences in surface heat flux characteristics arising from the land use variation. In the present study we investigate the effects of increasing wind speed on Delta z(i) between the subregions, using a sample of 15 dry days from Southern Oxidants Study 1995. As convective boundary layer (CBL) wind speeds increased from less than or similar to1 to 6 m s(-1), Delta z(i) decreased from 600 m to negligible. We found that an increase in shear-generated turbulence in the CBL was a probable factor in this decline. Two light-wind days that followed days with widespread rainfall were also noted. The Delta z(i) on these days were small. Thus either strong winds or widespread surface moisture availability were able to nullify the effects of the land use/surface heat flux differences on Delta z(i) between the two subregions.