Colloquium, 22 March, 3:00 pm, EN6085A

The fine-scale details of cumulus evolution in the transition from shallow to deep convection, and the accompanying changes in the environment and boundary layer over mountainous terrain have been the objects of a field campaign in July-August 2006, over and around the Santa Catalina Mountains in southern Arizona. The transport of boundary-layer air into the deep troposphere by anabatic flow and orographic cumuli is still poorly understood and a challenge for NWP models. The Santa Catalina Mountains serve as natural laboratory to study convection, given the spatially and temporally regular development of cumulus driven by convergent boundary-layer flow. The pre-convective environment as well as its successive modifications by moist convection were sampled via coordinated in situ observations from the surface, soundings, and aircraft, along with airborne radar data and stereo-photogrammetry from two angles.

This talk focuses on the mountain-wide convergence of energy by the anabatic flow that develops in response to diurnal heating over the mountain, a number of hours after sunrise. Horizontal fluxes were determined from a series of circumnavigations around the mountains allowing us to calculate the mountain-wide mass, heat, and moisture budget.  15 days are examined, 3 in detail, illustrating different convective regimes. Around the time of first cumulus development, the wind was generally directed towards the mountain, i.e. it had an anabatic component, part of a thermally-direct toroidal circulation.  The transport of energy by the toroidal circulation was almost entirely driven by the mean anabatic wind, rather than by eddies in the horizontal flow. The mean anabatic flow was highly variable, but when it was present, the horizontal influx of latent and sensible heat (averaged over the mountain footprint area, and integrated over the inflow depth) was far larger than the vertical influx of the same at the surface.

These findings imply that surface heating over the mountain only indirectly triggers orographic convection, via the toroidal convergence that it drives. The forcing for this convergence will be subject of a follow-up study.