Marco Milan, Met Office

Control of gravity waves in the Met Office convective-scale 4DVAR system

Dingmin Li, Laura Pitcher, Gordon Inverarity, Gareth Dow, Bruce Macpherson, Robert Tubbs, Peter Weston, Graeme Kelly, Lee Hawkness-Smith


An initial demonstration of convective scale 4DVAR at the Met Office was run during summer 2012 for the London Olympics. This hourly cycling assimilation system was run with a 1.5 km resolution model over the southern UK. Building on this success, the Met Office is now developing 4DVAR for operational use over a larger domain in its UKV model. The basic features of this system will be summarised. The introduction of analysis increments in 4DVAR can break the dynamical balance present in the NWP model, generating spurious gravity waves. One source of imbalance is the vertically adaptive grid which concentrates analysis resolution in regions of high static stability. The approach chosen to control such imbalance is a weak constraint digital filter implemented via an extra term in the 4DVAR cost function – the Jc term. The Jc term is specified by a set of parameters which depend on model resolution in space and time. We present the outcome of sensitivity tests to determine suitable parameters for the UKV model and results from a week long experiment in summer 2014. Impact on gravity wave activity, precipitation spin-up, forecast performance and cost of the 4DVAR minimisation are considered. It is found that a well tuned Jc term is able to remove spurious gravity waves without deterioration of forecast skill or damage to the evolution of precipitation, and with an acceptable cost overhead.

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