A climatology of synoptic forcing for vertical motions over the western United States

The climatology and interannual variability of synoptic-scale ascending motion is examined over the Western United States. Using ERA-Interim reanalysis grids every 6 hours from 1 October- 30 April within the 20-year period from 1989-2008, several statistical measures are used to summarize the occurrence and variability of positive convergence of the quasi-geostrophic Q-vector at multiple pressure levels, which is assumed to represent forcing for synoptic scale upward vertical motion. Case studies are employed to present a more detailed examination of the temporal and spatial evolution of the convergent Q-vector field, revealing an inclusive range of structures and scales as well as the interaction of synoptic systems with the underlying terrain of the western United States. Year-to-year variability in the intensity of synoptic scale weather systems is represented by the standardized anomaly of accumulated positive Q-vector convergence, at each grid point, with respect to its long-term mean. This approach is in contrast to the more traditional use of the seasonal departure in mid-tropospheric heights from the long-term average. Subsequently, the linkage between interannual variations in precipitation and synoptic forcing is explored on the regional scale as well as over narrow mountain barriers in Northern Utah.