Orographic Modulation of Lee-Side Precipitation

We present evidence of a correlation of cross-mountain zonal flows and ambient vorticity fields. When the zonal flow across the Rockies increases, a cyclonic vortex tends to spin up during the summer season, promoting stronger low-level-jets to the east, and enhancing dynamic support for precipitation over the Mississippi River Basin. We present statistics for monthly averages of the NCEP Reanalysis that support these connections, and numerical simulations with a global model that illustrate the relevance of this dynamical process relative to regional surface evaporation anomalies. The results suggest that dynamical atmospheric effects related to the orographically bound vortex may be at least as important as anomalies of surface evaporation on time scales of a week or more. Furthermore, the predictability of related precipitation anomalies may be relatively long in cases where the initial atmospheric anomaly possesses large scale and carries a longer "memory". In such cases, the orography of western North America focusses the large-scale signal into smaller-scale synoptic responses that determine regional rainfall evolution. Sample 2 week simulations will be presented and discussed.