| Relatively long-term fluctuations are evident in the low level circulations surrounding the Andes and the Rocky Mountains. Some of these fluctuations are commonly explained by local, thermal influences that characterize monsoons as well as by regional responses to remote, tropical forcings. This study first explores the seasonal cycle of low level circulations in the vicinity of the Andes and compares it to other mountainous regions such as the Rockies and the Tibetan plateau. Pronounced mechanical flow modifications appear to be associated with orographic deflection over both the Andes and the Rockies. Reanalysis diagnostics, and medium range and month-long integrations of the Utah Global Model (UGM) are presented. Analyses of orographically forced, stationary Rossby waves suggest that climatological, annual and interannual variations observed around the largest mountain ranges may be explained in terms of mechanical, orographic flow deflection. Over South America, this effect tends to produce an orographically bound cyclone in all seasons, with a poleward low-level jet (LLJ) east of the Andes.; The relative importance of initial state detail over subtropical South America during summer is also examined. Experiments with the rotated, variable resolution UGM assess the impact of initial state changes upon regional predictability. Preliminary results show that forecasts of lower troposphere winds can be strongly influenced by the initial state specification over South America through the first 5 days.; Medium range forecasts over North America during anomalous summer floods (1993) and droughts (1988) are investigated. Results suggest that orographic processes modulate the summer, Great Plains LLJ and related hydrology of the Mississippi River basin. If anomalously strong and broad, upper level zonal flow exists over the orography, then the associated large inertia will promote predictability in all components, including the LLJ and summer precipitation, for a relatively long period. An additional hypothesis is that predictability enhancement occurs in winter cases of highly anomalous zonal flows over western North America. Results of 15 day simulations using a variety of operational and research global model configurations are presented to diagnose the predictability of precipitation and large scale features. Model forecasts of precipitation accumulation delineate qualitatively between wet and dry events at both 5 and 10 days. Improved performance may be related to the large inertia of the flow in wet events, and to increased predictability of initial, large scale anomalies in both wet and dry events. |
