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An annual cycle of vegetation in a GCM. Part I: implementation and impact on evaporation

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Abstract

The sensitivity of evaporation to a prescribed vegetation annual cycle is examined globally in the Met Office Hadley Centre Unified Model (HadAM3) which incorporates the Met Office Surface Exchange Scheme (MOSES2) as the land surface scheme. A vegetation annual cycle for each plant functional type in each grid box is derived based on satellite estimates of Leaf Area Index (LAI) obtained from the nine-year International Satellite Land Surface Climatology Project II dataset. The prescribed model vegetation seasonality consists of annual cycles of a number of structural vegetation characteristics including LAI as well as canopy height, surface roughness, canopy water capacity, and canopy heat capacity, which themselves are based on empirical relationships with LAI. An annual cycle of surface albedo, which in the model is a function of soil albedo, surface soil moisture, and LAI, is also modelled and agrees reasonably with observed estimates of the surface albedo annual cycle. Two 25-year numerical experiments are completed and compared: the first with vegetation characteristics held at annual mean values, the second with prescribed realistic seasonally varying vegetation. Initial analysis uncovered an unrealistically weak relationship between evaporation and vegetation state that is primarily due to the insensitivity of evapotranspiration to LAI. This weak relationship is strengthened by the adjustment of two MOSES2 parameters that together improve the model’s LAI-surface conductance relationship by comparison with observed and theoretical estimates. The extinction coefficient for photosynthetically active radiation, k par , is adjusted downwards from 0.5 to 0.3, thereby enhancing the LAI-canopy conductance relationship. A canopy shading extinction coefficient, k sh , that controls what fraction of the soil surface beneath a canopy is directly exposed to the overlying atmosphere is increased from 0.5 to 1.0, which effectively reduces soil evaporation under a dense canopy. When the experiments are repeated with the adjusted parameters, the relationship between evaporation and vegetation state is strengthened and is more spatially consistent. At nearly all locations, the annual cycle of evaporation is enhanced in the seasonally varying vegetation experiment. Evaporation is stronger during the peak of the growing season and, in the tropics, reduced transpiration during the dry season when LAI is small leads to significantly lower total evaporation.

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Acknowledgements.

This work was supported through the EU PROMISE project (EVK2-CT-1999–00022). The LAI data was provided as part of the ISLSCP II initiative through NASA Goddard DAAC [Hall FG, Meeson B, Los S, Steyaert L, Brown de Colstoun E, Landis D (eds) ISLSCP Initiative II. NASA. DVD/CD-ROM. NASA, 2003]. The surface albedo climatology was provided by the Canada Centre for Remote Sensing (http://www.ccrs.nrcan.gc.ca ). We thank Dr. P. Cox, Dr. R. Betts, Dr. C. Taylor and T. Osborne for valuable discussions regarding this work. The authors would also like to thank the anonymous reviewers for their constructive comments, which improved the quality of the work.

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  1. CGAM, PO Box 243, Earley Gate, University of Reading, Reading, Berkshire RG6 6BB, UK

    D. M. Lawrence & J. M. Slingo

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  1. D. M. Lawrence
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  2. J. M. Slingo
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Correspondence to D. M. Lawrence.

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Lawrence, D.M., Slingo, J.M. An annual cycle of vegetation in a GCM. Part I: implementation and impact on evaporation. Climate Dynamics 22, 87–105 (2004). https://doi.org/10.1007/s00382-003-0366-9

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  • DOI: https://doi.org/10.1007/s00382-003-0366-9

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