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On the role of tuning in parametrizations of unresolved diffusive processes in atmospheric models



This article addresses the role of the tuning of the parameters of the subgrid parametrizations of diffusive processes in numerical models. To this end, it examines the stability and accuracy of a set of fractional time-split integration schemes for the nonlinear diffusion equation. The best values of the time-decentring parameters are established. The dependency of the diffusion on the exchange coefficient is tuned to get the most accurate simulation of the steady-state solution and this is then tested in a non-steady-state model integration. The following questions are then addressed: (i) what is the relative role of parameter ‘tuning’ compared to the choice of the numerical scheme? and (ii) what is the effect of using a parametrization in one model that has been tuned in another model?

The presented tests suggest that the best results are obtained by controlling the details of the numerics while taking the physical value of the exchange coefficient, instead of tuning the physics parametrization. This suggests that it is beneficial to consider ‘tuning’ the physics parametrization schemes via the freedom in the numerics as an alternative to tuning the parameters of the physical processes. Copyright © 2012 Royal Meteorological Society

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