RESUME / ABSTRACT

The trend to higher lids of weather forecast models continues with the
recent rise of ECMWF's model top to 0.01 hPa (80 km), highlighting the
need for an improved understanding of vertical coupling in data
assimilation systems.  This trend is fuelled by the desire to better
assimilate nadir satellite soundings which sense deep layers of the
atmosphere.  Because tropospheric channels also sense the stratosphere,
an improved representation of the middle atmosphere can improve weather
forecasts.  However, vertical coupling of the various regions can occur
through both the analysis and forecast steps of the assimilation cycle.
Because waves propagate upward, they can carry information from the
model troposphere upward through background error covariances, resolved
waves and parameterized gravity waves.  A nice example of vertical
coupling is seen in the phenomenon of stratospheric sudden warming, a
not uncommon occurrence in Arctic winters.  Understanding and predicting
such polar phenomena will be a focus of stratospheric projects for the
coming International Polar Year (2007-8).