Mesoscale Perturbations on Airflow in the Urban Mixing Layer

Urban perturbations on the mixing layer airflow over St. Louis were studied as a part of METROMEX. Case studies were conducted on summer days under generally undisturbed synoptic weather conditions. Based on simultaneous airborne wind and temperature measurements, the relative effects of the urban heat island, surface friction and local terrain features were estimated. When the elevated heat island was strong and the winds were light, the thermally induced pressure perturbation was the dominant force and accelerated the airflow as it converged into the elevated heat island. With strong winds and a weak thermal field, frictional drag appeared to become a significant force and decreased the wind speeds over the city. The higher terrain south of the city produced only a minor deflection of the airflow.