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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

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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.

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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?

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Principal tropical cloud regimes

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Principal extra-tropical cloud regimes

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Initial tendencies in cloud regime properties

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Initial temperature tendency in cloud regimes

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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.

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