Colloquium, Tuesday, Nov. 17, 3:00 pm, EN6085A: (ical.ics)

Aircraft and surface measurements of the boundary-layer (BL) transport of mass and moisture towards an isolated, heated mountain are presented. The data were collected as part of CuPIDO around the Santa Catalina Mountains (CM) in Arizona, 20-30 km in diameter, during the North American monsoon, on days with weak winds and cumulus congestus to cumulonimbus development over the mountain. Flights in the BL around the mountain and surface station data indicate that mountain-scale anabatic surface wind generally develops shortly after sunrise, peaking at ~1 ms-1 in strength close to solar noon. There is some evidence for a toroidal heat-island circulation, with divergence in the upper boundary layer. The aircraft data and mainly the diurnal surface temperature and pressure patterns confirm that this circulation is driven by surface heating over the mountain. Observations for three days suggest that growth spurts of orographic cumulus and cumulonimbus are not preceded by enhanced mountain-scale mass convergence near the surface, and that the decay of orographic deep convection is associated with divergence around the mountain.

The daytime evolution of the thermally-forced BL circulation over the CM, and its interaction with cumulus convection, is examined by means of numerical simulations. Three cases are presented, one remains cloud-free in the simulations, the second produces orographic convection just deep enough to yield a trace of precipitation, and the third produces numerous cumulonimbi (Cbs) over the CM. The Weather, Research and Forecasting v.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. 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. Even relatively shallow and relatively small cumulus convection can temporarily disrupt the surface MSC by cloud shading and convective downdraft dynamics. Deeper and more widespread convection can shut down the MSC, and new convection tends to develop where outflow boundaries converge with each other or with the terrain.