Abstract
The zonal mountain pressure torque across a portion of the Rocky Mountains is estimated for three lee cyclones simulated with the National Centers for Environmental Prediction’s Eta model. The relative importance of the mountain torque to the regional balance of atmospheric angular momentum is examined through the use of a momentum budget.
The magnitude of the regional mountain torque is generally greater than that of the tendency of atmospheric angular momentum, and the torque’s negative sign at all times in all cases examined indicates the transfer of angular momentum from the atmosphere to the earth. The average mountain torque associated with the strongest lee cyclone is found to be the same magnitude and sign as the tendency of global atmospheric angular momentum during the time frame of the simulation, and the cyclone is concurrent with the maximum negative tendency observed during the month.
Estimates of the friction torque suggest that this force also transfers momentum to the earth over the regional domain, but that it is generally much smaller in magnitude than the mountain torque associated with lee cyclones. Flux processes and the torque exerted by the surrounding atmosphere are the dominant processes in the regional momentum budget, whereas each would vanish over a global domain.
Meridional time sections of the mountain torque illustrate its dependence upon the zonal pressure gradient, the height of the orography, and the lever arm to the earth’s axis of rotation. The pressure trough associated with each cyclone, a characteristic of Rocky Mountain lee cyclones in general, plays an important role in the torque distribution by increasing the pressure difference to the south of each storm, where the lever arm is greater, over that which would occur with a more symmetric lee cyclone.
Corresponding author address: Prof. Alan C. Czarnetzki, Department of Earth Science, University of Northern Iowa, Cedar Falls, IA 50614-0335.
Email: alan.czarnetzki@uni.edu
