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Page 1© Crown copyright 2007 Initial tendencies of cloud regimes in the Met Office Unified Model Keith Williams and Malcolm Brooks Met Office, Hadley Centre for Climate Change Submitted to J. Climate ENSEMBLES/CFMIP workshop, Paris, 12/04/07
Page 2© Crown copyright 2007 Why look at cloud regimes in short range forecasts? Differences in the simulation of present-day cloud regimes amongst GCMs has been shown to contribute to a significant proportion of the spread in climate sensitivity (Williams and Tselioudis 2007). However, it may not be easy to identify the cause of errors in a particular regime from the model climatology. The Met Office has the unique asset of using the same physical model for its operational data assimilation, NWP forecasts and climate change projection (HadGEM1). Evaluation of cloud regimes in short range forecasts provides a framework in which the initial meteorological conditions are constrained by observations. Thus the evolution of the errors may provide information on the cause of systematic model bias.
Page 3© Crown copyright 2007 Principle questions to be addressed Are the properties of the simulated cloud regimes (e.g. frequency of occurrence; radiative effect) similar in a short simulation (a few days) as in the model climatology? Does the increased resolution in the NWP model improve the simulation of the cloud regimes? Are the cloud regime properties any closer to observations immediately after the model is initialised from operational analyses? Can initial tendencies in the state variables be associated with particular cloud regimes?
Page 4© Crown copyright 2007 Principal tropical cloud regimes
Page 5© Crown copyright 2007 Principal extra-tropical cloud regimes
Page 6© Crown copyright 2007 Initial tendencies in cloud regime properties
Page 7© Crown copyright 2007 Initial temperature tendency in cloud regimes
Page 8© Crown copyright 2007 Conclusions The simulated cloud regimes are essentially the same in a short run as for the model climatology, hence improvements (which will be relevant to both NWP and climate) can be tested in short runs (although it would be good to address a few initialisation issues). Increased resolution generally has little effect on the cloud regimes, although the simulation of tropical shallow cumulus is improved, whereas tropical deep convection is too infrequent when compared with ISCCP. The errors in the simulated cloud regimes are generally no smaller at T+0, which suggests weaknesses in the local processes (boundary layer/cloud/convection). Some of the initial tendencies in the state variables appear to be associated with particular regimes, which may help with identifying a cause.
Page 9© Crown copyright 2007
Met Office Hadley Centre, FitzRoy Road, Exeter, Devon, EX1 3PB United Kingdom Tel: +44 (0) Fax: +44 (0)
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