Reszka M.K.,Environment Canada |
Reszka M.K.,The Meteorological Research Division |
Polavarapu S.M.,Environment Canada |
Polavarapu S.M.,The Meteorological Research Division
Quarterly Journal of the Royal Meteorological Society | Year: 2012
Mass-wind and vorticity-divergence balance constraints based on the linearized Charney and quasi-geostrophic omega equations, respectively, are assessed in a developmental version of the global, three-dimensional variational data-assimilation system at Environment Canada. Unlike traditional balance constraints, which are averaged in time, the new constraints are flow-dependent and reflect a more complete set of dynamics. Single observation experiments demonstrate that the new covariance model leads to asymmetrical increments that are qualitatively aligned with the instantaneous background wind field. Data-assimilation experiments using real observations are performed for a period of five weeks during two different seasons, employing the control and experimental constraints. Subsequent forecast verification against radiosondes shows a definite benefit of the new covariances in the Tropics; however, the impact in the Extratropics is neutral or slightly negative. Verifications against analysis show virtually no change in the troposphere; however, a significant improvement is observed in the stratosphere at all lead times. Compared with the Charney mass-wind balance, the contribution of the quasi-geostrophic omega constraint is rather minimal, at least in its current adiabatic form. The new balance scheme requires a considerable amount of computational time in the context of our 3D-Var system, although the relative cost in a 4D-Var setting may be far less significant. Moreover, the present experiments are useful in elucidating several important aspects of covariance modelling, particularly the dependence of balance dynamics on spatial scale. © 2012 Crown in the right of Canada.