Tuesday, 13 January 2004
Influence of ambient flows and topography on the interannual signal and medium-range predictability over the western U.S. during winter
Room 4AB
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.
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