The influence of an orographic feature on an idealized mid-latitude cyclone
(Book)

Includes bibliographical references (pages 153-162).

The interaction of a mid-latitude cyclone with an isolated north-south mountain barrier is examined using idealized numerical simulation. A prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge, producing a cold front that interacts strongly with the topography. The structure and evolution of the lee waves launched by the topography are analyzed, including their temporal and their north-south variation along the ridge. Typical mountain wave patterns are generated by a 500-m high mountain, while substantial wave breaking occurs above a mountain with 2-km height, both at low levels in the lee and in the lower stratosphere. Both local wave characteristics (like their structure and magnitude) and integrated effects of these waves (the pressure drag and momentum flux) often exhibit significant differences from the waves produced in 2D or 3D simulations with steady large-scale flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow. Stratospheric wave breaking over a sufficiently high ridge causes significant removal of the cross-mountain momentum, and strong regions of deceleration are observed above the jet core. Low-level blocking and displacement of the developing cyclone are other ways the mountain influences the synoptic system. Small-amplitude perturbations strongly influence the domain-averaged flow response to the terrain. The mountain waves are also observed to have an influence on the atmospheric KE spectra, producing a k^{-5/3} spectra over a wide range of mesoscale wave umbers in the stratosphere; this slope is not present in the absence of terrain. The spectral energy budget is calculated, and the gravity waves directly inject energy into the mesoscale, which is then cascaded upscale. These results suggest that terrain forcing is sufficient for building a k^{-5/3} slope, and that direct forcing of the mesoscale is necessary for production of the observed mesoscale slopes, invalidating inertial cascade assumptions.