Abstract
For the f-plane shallow-water primitive equations (PEs), hierarchies of balance conditions relating the gravity manifold (divergence δ and ageostrophic vorticity γ = fζ − g∇2h) to the Rossby manifold (linearized potential vorticity qℓ = ζ − fh/H) are introduced. These hierarchies are ∂Nδ/∂tN = ∂N+1δ/∂tN+1 = 0 (δ balance), ∂Nδ/∂tN = ∂Nγ/∂tN = 0 (δ–γ balance), and ∂Nγ/∂tN = ∂N+1γ/∂tN+1 = 0 (γ balance), for N = 0, 1, … . How well these balance conditions represent the balance accessible to a given PE flow is explored. Detailed numerical experiments are carried out on an idealized potential vorticity distribution for which the domain maximum Rossby and Froude numbers are Romax ≐ 0.73 and Frmax ≐ 0.28. The numerical results reveal that all these hierarchies are asymptotic: as N increases, imbalance first decreases and then increases, as measured for instance by a linearized available energy. The minimum imbalance, over all the balance conditions considered, is attained by γ balance at N = 2. The most accurate balance conditions (e.g., γ and δ balances at N = 2) all exhibit slightly different energy spectra for the imbalance at medium to largest scales. Further, the greatest improvement shown by these accurate balance conditions over the less accurate conditions like quasigeostrophy occurs at large scales.
* Current affiliation: School of Mathematics, University of St Andrews, North Haugh, St. Andrews, United Kingdom.
Corresponding author address: A. R. Mohebalhojeh, School of Mathematics, University of St Andrews, North Haugh, St. Andrews KY16 9SS, United Kingdom. Email: arm@mcs.st-and.ac.uk
