Intra-regional variability of CBL mixing depth as a function of wind speed near Nashville during SOS

Significant differences in the height of the daytime convective mixed layer Zi over distances of a few 10's of km were evident in both airborne lidar cross sections and radar wind profiler backscatter data during SOS-95 near Nashville, Tennessee (Banta et al., 1998: JGR, 103, 22519-22544). These intra-regional differences, amounting to 500 m or more, were observed during 3 days of a stagnation episode during July 1995. Although the region is relatively flat, it was hypothesized that these Zi differences were due to a difference in land use between 2 subregions, forested vs. mixed cropland/coniferous forest. If this hypothesis is correct, we expected to see a decrease in the Zi difference as the homogenizing effect of boundary-layer wind speeds increased. This study is an analysis of dry days in which peak boundary-layer growth was not affected by deep moist convection. It shows that this expectation was correct: Zi differences between the regions decreased steadily with increasing wind speed until the differences were negligible at 6 m/s. As a further indication of the veracity of the hypothesis, 2 days with light winds but with the surface saturated by widespread rainfall on the previous day also showed negligible differences in Zi between the subregions.