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

Abstract

The daytime evolution of the thermally forced boundary layer (BL) circulation over an isolated mountain, about 30 km in diameter and 2 km high, is examined by means of numerical simulations validated with data collected in the Cumulus Photogrammetric, In Situ, and Doppler Observations (CuPIDO) field campaign. Two cases are presented, one remains cloud free in the simulations, and the second produces orographic convection just deep enough to yield a trace of precipitation. The Weather Research and Forecasting version 3 simulations, at a resolution of 1 km, compare well with CuPIDO observations. The simulations reveal a solenoidal circulation mostly contained within the convective BL, but this circulation and especially its upper-level return flow branch are not immediately apparent since they are overwhelmed by BL thermals. A warm anomaly forms over the high terrain during the day, but it is rather shallow and does not extend over the depth of the convective BL, which bulges over the mountain. Low-level mountain-scale convergence (MSC), driven by an anabatic pressure gradient, deepens during the day. Even relatively shallow and relatively small cumulus convection can temporarily overwhelm surface MSC by cloud shading and convective downdraft dynamics. In the evening drainage flow develops near the surface before the anabatic forcing ceases, and anabatic flow is still present in the residual mixed layer, decoupled from the surface. The interaction of the boundary layer circulation with deep orographic convection is examined in Part II of this study.