Upslope Flow and Cumulus Development Over an Isolated Heated Mountain: Observations and Simulations, isolated mountain

Surface and upper-air data, collected as part of the Cumulus Photogrammetric, In situ and Doppler Observations (CuPIDO) experiment during the 2006 monsoon season around the Santa Catalina Mountains in southeast Arizona, are used to study the diurnal variation of the mountain-scale surface convergence, its thermal forcing, and its relation to cumulus convection. The thermal forcing is examined both in terms of a radial potential temperature gradient within the convective boundary layer, and in terms of a horizontal pressure gradient force, which is derived assuming hydrostatic balance. The evolution of cumulus convection is traced using digital stereo-cameras (Zehnder et al. 2007). The mountain is ~30 km in diameter, ~2 km high, and relatively isolated. The environment is characterized by weak winds, a deep convective boundary layer in the afternoon, and sufficient low-level moisture for orographic cumulus convection on most days.

The katabatic, divergent surface flow at night and anabatic, convergent flow during the day are in phase with the diurnal variation of the horizontal pressure gradient force, which points towards the mountain during the day and away from the mountain at night. The daytime pressure deficit over the mountain of 0.5-1.0 mb is hydrostatically consistent with the observed 1-2 K virtual potential temperature excess over the mountain. The interplay between surface convergence and orographic thunderstorms is examined. It turns out that the consequence of deep convection (outflow spreading) is more apparent in the surface flow pattern than its possible trigger (enhanced convergence).