Landscape variability and surface flux parameterization in climate models

The Earth's surface shows variability at the landscape scale (1-10 km); the influence of surface variability at this scale has been analysed to provide a parameterization for use in large-scale atmospheric models with a grid size unable to solve the landscape scale explicitly. Landscape variations are found to add drag to the atmosphere, owing to sudden changes in vegetation height. The drag increases momentum flux and indirectly influences the transfer of heat and gases between the landscape and the atmosphere. Consequently, the exchange between a variable landscape and the atmosphere deviates from a simple sum of the exchanges between landscape elements and the contiguous air layer. Strong influences are found for tree lines and forest edges. Most of the existing aggregation schemes for grid-averaged fluxes in large-scale models strongly underestimate the consequences of landscape variability owing to the neglect of drag at surface transitions. The supplementary drag can easily be incorporated in an aggregation scheme of surface fluxes in a large-scale model. New experiments on the landscape scale are recommended to improve the accuracy of the method.