Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 10 7 2010 10.5194/acpd-10-16797-2010 http://www.atmos-chem-phys-discuss.net/10/16797/2010/ http://www.atmos-chem-phys-discuss.net/10/16797/2010/acpd-10-16797-2010.html http://www.atmos-chem-phys-discuss.net/10/16797/2010/acpd-10-16797-2010.pdf 16797 16835 2010-07-07 Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx S. J. Abel steven.abel@metoffice.gov.uk D. N. Walters G. Allen Met Office, Exeter, UK University of Manchester, Manchester, UK Observations in the subtropical south east Pacific obtained during the VOCALS-REx field experiment are used to evaluate the representation of stratocumulus cloud in the Met Office forecast model and to identify key areas where model biases exist. Marked variations in the large scale structure of the cloud field were observed during the experiment on both day to day and on diurnal timescales. In the remote maritime region the model is shown to have a good representation of synoptically induced variability in both cloud cover and marine boundary layer depth. Satellite observations show a strong diurnal cycle in cloud fraction and liquid water path in the stratocumulus with enhanced clearances of the cloud deck along the Chilean and Peruvian coasts on certain days. The model accurately simulates the phase of the diurnal cycle but is unable to capture the coastal clearing of cloud. Observations along the 20° S latitude line show a gradual increase in the depth of the boundary layer away from the coast. This trend is well captured by the model (typical low bias of 200 m) although significant errors exist at the coast where the model marine boundary layer is too shallow and moist. 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