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The southern Andes create a variety of atmospheric phenomena, but these have not been well studied. In this paper, we describe our preliminary observations and modeling of a few of these phenomena including: the sharp vegetation gradient on the eastern slopes, orographic triggering of roll convection, foehn wall and short lee-wave [rains, liquid clouds capping ice clouds, "bullet" clouds, and isotopic fractionation of water vapor.

A summary is given of the scientific objectives and experiment design for the upcoming Terrain-induced Rotor Experiment (T-REX), which has been proposed for March and April 2006 in the Sierra Nevada in California.

A linear model of gravity waves generated by stratified airflow over mountains is developed. The model provides simple, closed-form formulas for the surface drag in a situation where conditions for wave resonance exist. The wind is constant near the surface and decreases linearly above. The drag normalized by its value in the absence of shear is found to depend on two parameters: the height of the interface where the shear is discontinuous and the Richardson number, Ri, in the region above. This drag attains maxima when the height of the interface induces constructive interference between the upward and downward propagating reflected waves, and minima when there is destructive interference. The amplitude of the drag modulation becomes larger for lower Ri. It is also shown that, for Ri

Seven mountain wave case studies from the Mesoscale Alpine Programme (MAP) have been used to investigate the variation in the resolved surface pressure drag due to the Alps with model horizontal resolution. The three independent modelling systems tell the same story. For cases with small southerly (or northerly) wind-speeds near mountain crest level (< 10 m s-1), most of the drag was produced by low-level flow splitting around the Alpine barrier and the drags were converging. For situations where the southerly (or northerly) wind component increased strongly with height due to the location of the jet-stream directly above the Alps, the larger wind-speeds near mountain crest level forced much stronger mountain waves in the flow aloft. For such cases, the drag increased strongly as the grid-spacing was reduced from 12km to 4km. This has important consequences for the use of drag parameterisations.

The Sierra Rotors Project took place in March and April 2004 in Owens Valley, California to study terrain-induced rotors. An intense rotor event was documented during Intensive Observing Period (IOP) 8 on 24-26 March 2004. The event was characterized by a cold frontal passage and strong westerly flow at the mountain top level that induced mountain waves and rotors over Owens Valley. This case was simulated with the Naval Research Laboratory's (NRL) Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSTM) run at 333 m resolution. In this study, we analyze the evolution and structure of the observed and simulated mountain waves and rotors during the IOP 8 event, in which horizontal circulation associated with the rotor extended to the valley floor, where it was observed as easterly flow by the DR1 mesonetwork of surface stations. The model accurately reproduced the timing and spatial structure of many of the observed phenomena, including thermally forced flow in the valley at the start of the event, an intense mountain wave during the period of observed surface easterly flow, and strong westerlies on the Sierra Nevada lee slopes.