Momentum transport by gravity waves in a stochastically driven jet

The important role of internal gravity waves in transporting momentum and energy to regions of the atmosphere remote from their sources is widely acknowledged and has many implications for the general circulation and thermal structure of the atmosphere. Therefore a comprehensive understanding of the physical mechanisms that maintain the statistical equilibrium wave fluxes is needed. This problem is highly complex, as it involves knowledge of source characteristics, in addition to understanding and modelling of wave-mean flow and wave-wave interactions that influence and filter the emitted wave spectrum. An advantageous conceptual and mathematical framework for studying the wave mean flow interactions is the generalized stability theory (GST), as it separates on the one hand the non-linear mechanisms and other processes that enter as forcing and the growth and dissipation processes on the other. In this context the complex non-linear interactions, as well as the various sources of gravity waves, are parameterised as stochastic forcing white in space and time to account for the broad band of generated wave frequencies and wave numbers and for the complex morphology and localized intermittent nature of the various sources. Applying the methods of GST on a linear two-dimensional gaussian jet, revealed both a tendency for radiation of momentum away from the jet, as well as deposition inside the jet due to critical level filtering. The statistical steady state flux divergence resulted in a significant net drag on the jet, a result that proved to be robust regardless of changes in the temporal and spatial correlation of the forcing and in the static stability of the background flow across the tropopause.