Influence of mesoscale winds on the turbulent structure of the urban boundary layer over St. Louis

Two fair weather afternoons have been examined, where the urban boundary layer over St. Louis, though exhibiting similar thermal characteristics, had a markedly different kinematic structure. The turbulent nature of the boundary layer was examined through analysis of double theodolite wind profiles at an urban and at a rural site on each day. On 14 July 1975, the winds increased with height above the inversion at both sites and on the following day, the winds decreased above the boundary layer in the same region. While the mean wind speed in the lowest 0.8 km agl was similar on both days, the turbulence characteristics of the urban boundary-layer winds were distinctly different on these two afternoons. This was evidenced by the variance of the wind and is in agreement with simultaneous aircraft measurements reported by Hildebrand and Ackerman (1984). A similar difference in turbulence was not found over the rural site. It is suggested that the enhanced turbulence at the urban site on 14 July is likely associated with the wind profile immediately above the boundary layer, where the downward flux of high momentum air from above the inversion may have resulted in stronger mechanical mixing within the boundary layer.