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Sensitivity of general
circulation model performance to convective
parametrization
March 1997
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1 . Introduction
The paper attempts to describe the sensitivity of aspects of global circulation model (GCM) performance to convective parametrization. This is a difficult task for a number of reasons. Firstly we have to define what is meant by "performance". From a numerical weather forecasting perspective a model must produces a good estimate of the three-dimensional structure and evolution of synoptic-scale weather systems. On climate time scales the model should capture a realistic mean distribution of thermodynamic variables (including cloud cover), wind and surface precipitation. For coupled ocean-atmosphere modelling the surface fluxes of heat and water vapour need to be correctly simulated. Also the simulated variability of the atmosphere (from diurnal through to the intraseasonal and interannual time scale) needs to be well captured. Convective parametrization has a large influence upon all these areas and it is difficult to make definitive statements covering each area in such a short paper.
Secondly, however, it is difficult to make definitive statements concerning the impact of convection schemes alone as other parametrizations (for example the boundary-layer scheme), together with the methods used to simulated resolved motions, also have a impact upon the ability of atmospheric models to simulated the features of the general circulation referred to above. Even if identical convection schemes were used in two GCMs in which other processes were represented in a substantially different manner, it would not be certain that the distribution of precipitation simulated, together with other features of the mean climate, would be similar. However, as will be illustrated below, some features of GCM simulations appear well correlated with certain convective processes.
This paper will be limited to illustrating areas where simulations of the global circulation are known to be sensitive to the representation of convection, emphasing the mean climate and variability of the tropics. Reference will be mainly made to recent studies using the climate version of the UK Meterological Office (UKMO) Unified Model (UM) (described briefly by Gregory and Morris (1996) and having a horizontal resolution of 2.5 x 3.75o latitude-longitude and 19 levels) and the T63 31-level configuration of a recent version of the ECMWF model (CY14R3). Other studies will also be referred to but only a summary of relevant results is provided; the reader is referred to the original material for greater details.
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12.06.2002 |
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