Group seminar on 26. April, 14:15 CET

Modal decomposition of the vertical momentum fluxes

Valentino Neduhal

The transfer of momentum by atmospheric waves is an important process in the global momentum budget of the atmosphere. As the waves propagate upward and dissipate, they provide forcing for the large-scale atmospheric flows such as the quasi-biennial oscillation in the tropical stratosphere. Due to models' inability to represent all effects of the vertically-propagating waves, especially the inertia-gravity (IG) waves, the effects of vertically propagating waves still need to be parameterized. This is especially important in climate models that are still largely run at low horizontal and vertical resolutions. The parameterization involves assumptions about the spectra of momentum fluxes and IG waves. A typical assumption is that the horizontal wind perturbations beyond a certain scale are associated purely with the IG waves. Such an assumption can not be applied in the tropics since the variance of the Kelvin and mixed Rossby-gravity waves is largely on large scales.

In this seminar, I will present a new approach to the regime-dependent computation of vertical momentum fluxes that also provides momentum flux spectra as a function of the pressure altitude, latitude, and the balanced (or Rossby) and unbalanced or IG modes including the Kelvin and mixed Rossby-gravity waves. The approach relies on the normal-mode function decomposition and MODES software that is extended to include the computation of the pressure vertical velocity and the vertical momentum fluxes. I will present the first results of the new method applied to the operational high-resolution ECMWF analyses and reanalyses data and compare them with previous works.