Combining ERBE and ISCCP data to assess clouds in the Hadley Centre, ECMWF and LMD atmospheric climate models |
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Authors: | M Webb C Senior S Bony J-J Morcrette |
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Institution: | (1) Hadley Centre for Climate Prediction and Research, The Met Office, London Road, Bracknell, Berkshire RG12 2SY, UK E-mail: mjwebb@meto.gov.uk, GB;(2) MIT & LMD/CNRS, Department of Earth, Atmospheric and Planetary Sciences, Room 54-1721, 77 Massachusetts Avenue, Cambridge, MA 02139, USA, US;(3) European Centre for Medium-range Weather Forecasts, Reading, Berkshire RG2 9AX, UK, GB |
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Abstract: | This study compares radiative fluxes and cloudiness fields from three general circulation models (the HadAM4 version of the
Hadley Centre Unified model, cycle 16r2 of the ECMWF model and version LMDZ 2.0 of the LMD GCM), using a combination of satellite
observations from the Earth Radiation Budget Experiment (ERBE) and the International Satellite Cloud Climatology Project (ISCCP).
To facilitate a meaningful comparison with the ISCCP C1 data, values of column cloud optical thickness and cloud top pressure
are diagnosed from the models in a manner consistent with the satellite view from space. Decomposing the cloud radiative effect
into contributions from low-medium- and high-level clouds reveals a tendency for the models' low-level clouds to compensate
for underestimates in the shortwave cloud radiative effect caused by a lack of high-level or mid-level clouds. The low clouds
fail to compensate for the associated errors in the longwave. Consequently, disproportionate errors in the longwave and shortwave
cloud radiative effect in models may be taken as an indication that compensating errors are likely to be present. Mid-level
cloud errors in the mid-latitudes appear to depend as much on the choice of the convection scheme as on the cloud scheme.
Convective and boundary layer mixing schemes require as much consideration as cloud and precipitation schemes when it comes
to assessing the simulation of clouds by models. Two distinct types of cloud feedback are discussed. While there is reason
to doubt that current models are able to simulate potential `cloud regime' type feedbacks with skill, there is hope that a
model capable of simulating potential `cloud amount' type feedbacks will be achievable once the reasons for the remaining
differences between the models are understood.
Received: 23 January 2000 / Accepted: 24 January 2001 |
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