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Warren J. Tennant, Glenn J. Shutts, Alberto Arribas, and Simon A. Thompson

Abstract

An improved stochastic kinetic energy backscatter scheme, version 2 (SKEB2) has been developed for the Met Office Global and Regional Ensemble Prediction System (MOGREPS). Wind increments at each model time step are derived from a streamfunction forcing pattern that is modulated by a locally diagnosed field of likely energy loss due to numerical smoothing and unrepresented convective sources of kinetic energy near the grid scale. The scheme has a positive impact on the root-mean-square error of the ensemble mean and spread of the ensemble. An improved growth rate of spread results in a better match with ensemble-mean forecast error at all forecast lead times, with a corresponding improvement in probabilistic forecast skill from a more realistic representation of model error. Other examples of positive impact include improved forecast blocking frequency and reduced forecast jumpiness. The paper describes the formulation of the SKEB2 and its assessment in various experiments.

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Andrew C. Bushell, Neal Butchart, Stephen H. Derbyshire, David R. Jackson, Glenn J. Shutts, Simon B. Vosper, and Stuart Webster

Abstract

Analysis of a high-resolution, convection-permitting simulation of the tropical Indian Ocean has revealed empirical relationships between precipitation and gravity wave vertical momentum flux on grid scales typical of earth system models. Hence, the authors take a rough functional form, whereby the wave flux source spectrum has an amplitude proportional to the square root of total precipitation, to represent gravity wave source strengths in the Met Office global model’s spectral nonorographic scheme. Key advantages of the new source are simplicity and responsiveness to changes in convection processes without dependence upon model-specific details of their representation. Thus, the new source scheme is potentially a straightforward adaptation for a class of spectral gravity wave schemes widely used for current state-of-the-art earth system models. Against an invariant source, the new parameterized source generates launch-level flux amplitudes with greater spatial and temporal variability, producing probability density functions for absolute momentum flux over the ocean that have extended tails of large-amplitude, low-occurrence events. Such distributions appear more realistic in comparison with reported balloon observations. Source intermittency at the launch level affects mean fluxes at higher levels in two ways: directly, as a result of upward propagation of the new source variation, and indirectly, through changes in filtering characteristics that arise from intermittency. Initial assessment of the new scheme in the Met Office global model indicates an improved representation of the quasi-biennial oscillation and sensitivity that offers potential for further impact in the future.

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