A numerical study of the evolving convective boundary layer and orographic circulation around the Santa Catalina Mountains in Arizona. Part I: circulation without deep convection

Observational data collected during the 2006 Southwest Monsoon around the Santa Catalina Mountains approximately 30 km in diameter near Tucson, AZ using radiosonde data, UW King Air data, and surface station data shows the development of anabatic surface flow and Boundary Layer (BL) circulation around and on the mountain. Observations show the development of mountain-scale surface convergence (MSC) a few hours after sunrise, peaking near local solar noon, and decaying through the afternoon and evening. An orographic toroidal circulation with a low-level anabatic component and divergence near the BL top is sometimes but not always present. The data also confirm that the anabatic flow is driven by surface heating over the mountain, resulting in solenoidal forcing and a hydrostatic horizontal pressure gradient force towards the mountain. The present paper summarizes the results from detailed numerical simulations for several cases using the Weather, Research, and Forecasting (WRF) modeling system. The observational data make it possible, for the first time, to validate simulations of flow and convective development over a real mountain by means of measured MSC and cloud top chronology. Initial findings show WRF accurately captures the diurnal variation of MSC at the surface, which is driven by a perturbation pressure gradient forced directed towards the mountain. This daytime pressure deficit is attributed to a warm anomaly over the mountain; therefore, solenoidally driven. The level of non-divergence within the BL generally rises in conjunction with deepening BL. Even benign orographic cumulus congestus can produce localized cool anomalies indicative of dry, weak microburst which temporarily disrupts the MSC.