New Vegetation Radiative Transfer Schemes for Land Surface Models

The land surface components of climate and Earth system models tend to utilise relatively simple representations of vegetation radiative transfer processes to determine key land surface properties such as albedo, land surface temperature and the absorption of sunlight for photosynthesis. This simplicity is driven, in large part, by a need for computational efficiency. However, a growing number of studies have pointed to the need for more complex radiative transfer in these models.

An almost ubiquitous assumption in such radiative transfer schemes is that a vegetation canopy can be represented by a plane-parallel, turbid medium – a perfectly flat box in which scattering elements (i.e. leaves, branches, trunks, etc.) are randomly distributed. Real canopies typically exhibit quite complex, non-random structures often involving the clumping of leaves and branches at multiple scales. Furthermore, the optical properties of canopies are typically assumed to be vertically and horizontally homogeneous which does not allow for realistic representation of, for example, forest stands with mixed species or understory vegetation.

This presentation examines recent developments that have the potential to overcome these and other deficiencies in land surface model radiative transfer schemes, whilst maintaining sufficient computational efficiency to make them viable for inclusion in climate and Earth system models. This is achieved by using the same solutions to the transfer problem as currently employed in climate models as the building blocks to construct canopies that can vary both vertically and horizontally.