A fundamental question in atmospheric fluid dynamics is precisely how synoptic-scale eddy activity is maintained in the regional wintertime storm tracks. This understanding is also of practical importance to both climate simulations and extended range weather forecasting. It is well known that barotropic deformation plays an important role in the life cycles of transient eddies within the storm tracks. It is also well known that many general circulation models have a climatological bias in which the Northern Hemisphere wintertime circulation is anomalously zonal. Associated with this bias are regional anomalies in the horizontal deformation fields. We assess the dynamical role that such biases play in the model representation of midlatitude storm tracks.
Applying modern dynamical diagnoses to observational analyses and the output of general circulation models, we contrast the impact of barotropic deformation on regional synoptic eddy activity. We study winter-mean deformation and storm track characteristics in reanalysis datasets and extended integrations of general circulation models (including CCM2, CCM3, and GEOS). Specifically, we diagnose the envelope function, major axes, and E-vectors of synoptic eddies. These are contrasted to the local dilitation axes and deformation magnitudes of the winter-mean flow and used to calculate local barotropic energy conversions. These calculations are performed in the mid to upper troposphere, where barotropic effects are most pronounced, and are contrasted with mid-tropospheric analyses of the baroclinic growth parameter. The diagnoses are performed for Northern Hemisphere winter average conditions as well as for interannual and intraseasonal anomalous weather regimes. The differences between observations and the model simulations are quantified and the implications of the results for climate simulations and extended-range forecasting are discussed. Our results indicate that the longitudinal extent of storm tracks in atmospheric models is strongly controlled by the regional representation of barotropic deformation.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics