Title: The synoptic-to-mesoscale transition: a new look at the data

Abstract: While synoptic-scale midlatitude perturbations are well-understood to be governed by quasi-geostrophic dynamics, the nature of mesoscale flows has attracted considerable controversy. Several theories have been put forward to explain aircraft observations that exhibits a change in the slope of the energy spectrum at the synoptic-to-mesoscale transition at about 500 km. These theories aim at explaining the relatively small spectral slope in the mesoscale range.

We here go back to the data and ask what can be inferred from them in addition to the spectral slope. We show that the observed spectra can be decomposed in two steps: (1) Using a Helmholtz decomposition, the rotational and divergent components can be separated. (2) Using the dispersion and polarization relations of inertia–gravity waves, the spectra can further be decomposed into a geostrophic component and an inertia–gravity wave component.

This decomposition suggests that the synoptic-to-mesoscale transition is due to a transition from quasi-geostrophic dynamics in the synoptic scales to flow that is dominated by inertia–gravity waves in the mesoscale. Implications, for example for atmospheric predictability, will be discussed.


Speaker's website: http://eaps-www.mit.edu/paoc/people/j%C3%B6rn-callies