The role of terrain and pressure stresses in Rocky Mountain lee cyclones

The earth-atmosphere exchange of storm absolute dynamic circulation by mountain-induced surface pressure stress and the response of the circulation in a Rocky Mountain lee cyclone is examined. Surface pressure stresses that transfer horizontal momentum across the earth-atmosphere interface stem from the hydrostatic weight of the atmosphere resting against the inclined surfaces of orography. In a lee cyclone, mass asymmetries must be combined with terrain variability for net transfer across the interface. Within the storm structure, the acceleration of the dynamic circulation by the pressure gradient force is determined by the line integral of the azimuthally directed components of pressure stress, in effect an angular momentum pressure torque. The pressure torque is compared to the tendencies of specific relative circulation and absolute dynamic circulation in a lee cyclone simulated by the eta model. The mountain-induced pressure torque is found to be negative in the lower layers of the cyclone vortex throughout the simulation. Negative pressure torque, which indicates the transfer of absolute dynamic circulation from the cyclone to the mountain, acts, in conjunction with other processes, to force convergence of the mass transport in the lower layers of the cyclone. The import of absolute angular momentum from the storm's environment by the converging mass circulation exceeds the loss of angular momentum to the earth by pressure and viscous stresses, and thus leads to the spinup of the storm-relative circulation. The negative pressure torque diagnosed in the simulated cyclone results from an asymmetric distribution of surface pressure stress about the cyclone's circulation center in conjunction with a stronger pressure gradient to the north and northwest of the cyclone than to the south. This asymmetry is shown to be a characteristic of Rocky Mountain lee cyclones and the results illuminate its relation to the storm's life cycle.