Extended Abstract (1.2M)P2.26
Influence of ambient flows and topography on the interannual signal and medium-range predictability over the western U.S. during winter
Lee A. Byerle, Univ. of Utah, Salt Lake City, UT; and J. Paegle
We summarize evidence for an orographic modulation of large-scale ambient flows by the western Cordillera that is characterized by cyclonic acceleration of the orographic vortex in the presence of anomalously strong zonal flow, and anticyclonic accelerations for weaker zonal flow. The orography provides a scale-transfer mechanism that focusses global scale flow anomalies into regional-scale responses in both the atmospheric circulation as well as the precipitation. We follow a classical predictability theory that suggests that large-scale features ought to have relatively long predictability, and hypothesize that the interaction of large-scale atmospheric anomalies with the Rocky mountains should then enhance the predictability of regionally induced atmospheric anomalies, including regional precipitation anomalies. This hypothesis is tested in a series of two-week global simulations including the periods of the western winter drought of 1977, and floods during contrasting periods of winter 1986. We found that the forecast model is able to distinguish substantial precipitation differences through week two of the forecast period, although predicted anomalies are less pronounced than observed anomalies.
Poster Session 2, Tuesday Posters
Tuesday, 13 January 2004, 9:45 AM-11:00 AM, Room 4AB
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