Parametrization of area cloud fraction

Abstract

Comparisons of radiosonde data to model profiles show that the relative coarseness of the vertical grid can lead to poorly resolved changes in thermodynamic properties. This can lead to the model underestimating cloud cover. A parametrization for area cloud fraction, compatible with a prognostic cloud scheme, is developed and tested. The parametrization is then implemented in a general circulation model and used to produce a weather forecast and a climate simulation. The simulated cloud cover is improved in both models, whilst feedbacks within the climate model lead to an improvement in the surface temperature, particularly in the Northern-Hemisphere winter. © Crown Copyright 2010. Published with the permission of the Controller of HMSO and the Queen's Printer for Scotland

References Bony S, Dufresne J-L. 2005. Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models. Geophysical Research Letters 32: L20806. Brooks ME, Hogan RJ, Illingworth AJ. 2005. Parameterizing the difference in cloud fraction defined by area and volume as observed with radar and lidar. Journal of Atmospheric Sciences 62: 2248–2260. Collins W, Bellouin N, Doutriaux-Boucher M, Gedney N, Hinton T, Jones CD, Liddicoat S, Martin G, O'Connor F, Rae J, Senior C, Totterdell I, Woodward S. 2008. Evaluation of the HadGEM2 Model. Hadley Centre Technical Note 74. Available online at: http://www.metoffice.gov.uk/publications/HCTN/HCTN_74.pdf. Del Genio AD, Yao M-S, Kovari W, Lo KK-W. 1996. A prognostic cloud water parameterization for global climate models. Journal of Climate 9: 270–304. Edwards JM, Slingo A. 1996. Studies with a flexible new radiation code. I: choosing a configuration for a large-scale model. Quarterly Journal of the Royal Meteorological Society 122: 689–719. Grenier H, Bretherton CS. 2001. A moist PBL parametrization for large-scale models and its application to subtropical cloud-topped marine boundary layers. Monthly Weather Review 129: 357–377. Hogan RJ, Jakob C, Illingworth AJ. 2002. Comparison of ECMWF winter-season cloud fraction with radar-derived values. Journal of Applied Meteorology 40: 513–525. Legates DR, Willmott CJ. 1990. Mean seasonal and spatial variability in global surface air temperature. Theoretical and Applied Climatology 41: 11–21. Rossow WB, Schiffer RA. 1999. Advances in understanding clouds from ISCCP. Bulletin of the American Meteorological Society 80: 2261–2287. Slingo A. 1990. Sensitivity of the earth's radiation budget to changes in low clouds. Nature 343: 49–51. Smith RNB. 1990. A scheme for predicting layer clouds and their water content in a general circulation model. Quarterly Journal of the Royal Meteorological Society 116: 435–460. Tiedtke M. 1993. Representation of clouds in large-scale models. Monthly Weather Review 121: 3040–3061. Tompkins AM. 2002. A prognostic parameterization for the subgrid-scale variability of water vapour and clouds in large-scale models and its use to diagnose cloud cover. Journal of the Atmospheric Sciences 59: 1917–1942. Tselioudis G, Jakob C. 2002. Evaluation of midlatitude cloud properties in a weather and climate model: dependence on dynamic regime and spatial resolution. Journal of Geophysics Research 107(D24): 4781. Wilson DR, Ballard SP. 1999. A microphysically based precipitation scheme for the meteorological office unified model. Quarterly Journal of the Royal Meteorological Society 125: 1607–1636. Wilson DR, Bushell AC, Kerr-Munslow AM, Price JD, Morcrette CJ. 2008. PC2: a prognostic cloud fraction and condensation scheme. I: scheme description. Quarterly Journal of the Royal Meteorological Society 134: 2093–2107. Wilson DR, Gregory D. 2003. The behaviour of large-scale model cloud schemes under idealized forcing scenarios. Quarterly Journal of the Royal Meteorological Society 129: 967–986. Wilson DR, Smith RNB, Gregory D, Wilson CA, Bushell AC, Cusack S. 2007. The large-scale cloud scheme and saturated specific humidity. Unified Model Documentation Paper 29: 14–16. Wood R, Bretherton C, Fairall C, Gallardo Klenner L, Esbensen S, Feingold G, Garreaud R, Huebert B, Leon D, Roberto Mechoso C, McWilliams J, Miller A, Pizarro O, Rutllant J, Snider J, Takahashi K, Weller R, Wijesekera H, Yuter S, Doherty S. 2006. VOCALS Regional Experiment (REx): Scientific Program Over-view. Available online at: http://www.atmos.washington.edu/∼robwood/VOCALS/VOCALS_SPO_Complete.pdf.