Mesoscale Anticyclonic Circulations in the Lee of the Central Rocky Mountains

Composite analyses of terrain-forced, mesoscale anticyclonic circulations over southern Wyoming and northern Colorado are constructed. These suggest two different types of circulations, based on upstream flow direction. The air within type 1 circulations originates to the west of the Continental Divide and contains little moisture. Type 2 circulations form in more moist, northerly flow and are sometimes associated with snowbands. Both types tend to form during the afternoon and dissipate after sunset, although type 2 events may follow frontal passages and occur at night. Case studies of one event of each type suggest that anticyclonic vorticity generation occurs within the lowest kilometer above ground level when that layer is nearly vertically mixed in both potential temperature and velocity. An analogy with vorticity generation in mixed-layer models is considered, and it is shown that the conditions for generating negative vorticity in those models are satisfied in each observed case. The mixed-layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations and may therefore explain the observed, late-day snowfall maximum along the Front Range of Colorado.