Two-Time-Step Oscillations in Numerical Weather Prediction Models

Ulla Hammarstrand Department of Meteorology, Stockholm University, Stockholm, Sweden

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Abstract

Spurious, nonamplifying, two-time-step oscillations are present in several numerical models of the atmosphere where the vertical diffusion is parameterized using a nonlinear diffusion equation. The problems become particularly pronounced when the convective activity is studied, since convection is directly influenced by the vertical fluxes and occurs when these fluxes are large. Different numerical formulations for the solution of the diffusion equation in the High Resolution Limited Area Model have been tested in this paper. The results show that a perfect solution has not been found but that there are solutions that should be recommended to replace the traditional formulation with an explicit formulation of the diffusion coefficient.

Corresponding author address: Ulla Hammarstand, Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden.

Email: ullah@misu.su.se

Abstract

Spurious, nonamplifying, two-time-step oscillations are present in several numerical models of the atmosphere where the vertical diffusion is parameterized using a nonlinear diffusion equation. The problems become particularly pronounced when the convective activity is studied, since convection is directly influenced by the vertical fluxes and occurs when these fluxes are large. Different numerical formulations for the solution of the diffusion equation in the High Resolution Limited Area Model have been tested in this paper. The results show that a perfect solution has not been found but that there are solutions that should be recommended to replace the traditional formulation with an explicit formulation of the diffusion coefficient.

Corresponding author address: Ulla Hammarstand, Department of Meteorology, Stockholm University, S-106 91 Stockholm, Sweden.

Email: ullah@misu.su.se

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