Research project is (co) funded by the Slovenian Research Agency.
UL Member: Faculty of Mathematics and Physics
Project: Quantification of the contribution of Rossby and inertia-gravity waves to the vertical velocity and momentum fluxes in the atmosphere
Period: 1.7.2018 - 30.6.2020
Range per year: 1,50 FTE, category C
Head: Nedjeljka Žagar
Research activity: Natural sciences and mathematics
Research Organisations: link on SICRIS
Researchers: link on SICRIS
Citations for bibliographic records: link on SICRIS
Motions in the atmosphere are three-dimensional. In average conditions, horizontal velocity components (u,v) are much greater than the vertical velocity (w), w<<u,v. Therefore, the majority of global weather and climate models applies the hydrostatic approximation which removes w as prognostic quantity. Global fields of vertical velocity w are instead diagnosed using the conservation of energy or mass. In practice, vertical motions are usually estimated at constant pressure levels in Pascals per second (so-called omega variable).
On one hand, omega is not an observable variable while on the other hand it is a crucial ingredient of weather and climate, especially the global water and energy cycle. The computation of vertical velocity using the thermodynamic equation or the mass continuity equation has become a part of every hydrostatic weather and climate model. However, there are significant differences among the models in their estimates of the omega field. There are also significant uncertainties in the estimates of vertical momentum fluxes associated with different wave oscillations in climate models, especially in the tropics.
Two kinds of wave motions used to understand basic atmospheric dynamics are the Rossby waves and inertia-gravity waves. The former describe large-scale, quasi-horizontal, quasi-geostrophic motions whereas the latter are characterized by much greater divergence and a significant vertical propagation. In the extra-tropics, the two kinds of wave motions are well separated while in the tropics the inertia-gravity (IG) waves are important on all scales.
The project proposes a new approach for the computation of vertical velocity in global models that promises to separate vertical motions associated with the Rossby and IG waves across many scales. The approach is based on the normal-mode function decomposition for the separation of the Rossby and IG dynamics in the terrain-following coordinate that was established in the MODES project (http://modes.fmf.uni-lj.si). In the computation of vertical velocity, the continuity equation will be solved by replacing the horizontal wind divergence by analytical functions derived from the normal-mode function representation. The approach will provide 3D structure of vertical motions in the global atmosphere split into contributions from the Rossby and IG waves as a function of the zonal wavenumber, meridional mode and vertical structure function.
The new method will be applied for the quantification of the two omega components in reanalysis data and climate models. Their relative role in driving the middle atmosphere variability, such as the quasi-biennial oscillation, will be estimated. The final goal is to develop a new metric for the validation of the momentum fluxes and vertical transport in climate models.