A robust numerical method for the potential vorticity based control variable transform in variational data assimilation

Sean Buckeridge, M J P Cullen, Robert Scheichl, M Wlasak

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

The potential vorticity based control variable transformation for variational data assimilation, proposed in Cullen (2003), is a promising alternative to the currently more common vorticity based transformation. It leads to a better decorrelation of the control variables, but it involves solving a highly ill-conditioned elliptic partial differential equation (PDE), with a constraint. This PDE has so far been impossible to solve to any reasonable accuracy for realistic grid resolutions in finite difference formulations. Following on from the work in Buckeridge and Scheichl (2010) we propose a numerical method for it based on a Krylov subspace method with a multigrid preconditioner. The problem of interest includes a constraint in the form of two-dimensional elliptic solves embedded within the main three-dimensional problem. Thus the discretised problem cannot be formulated as a simple linear equation system with a sparse system matrix (as usual in elliptic PDEs). Therefore, in order to precondition the system we apply the multigrid method in Buckeridge and Scheichl (2010) to a simplified form of the three-dimensional operator (without the embedded two-dimensional problems) leading to an asymptotically optimal convergence of the preconditioned Krylov subspace method. The solvers used at the Met Office typically take over 100 iterations to converge to a residual tolerance of 0.1 and fail to converge to a tolerance of 10(-2). The method proposed in this paper, in contrast, can converge to a tolerance of 10(-2) within 15 iterations on all typical grid resolutions used at the Met Office, and is convergent to a tolerance of 10(-6). In addition, the method demonstrates almost optimal parallel scalability. Copyright
Original languageEnglish
Pages (from-to)1083-1094
Number of pages12
JournalQuarterly Journal of the Royal Meteorological Society
Volume137
Issue number657
DOIs
Publication statusPublished - Apr 2011

Keywords

  • preconditioner
  • solver
  • multigrid
  • parallelisation
  • finite volume
  • potential vorticity

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