Incorporating vegetation as a dynamic element in the Hadley Centre GCM

Vegetation and climate are tightly coupled. The global distribution of vegetation is closely correlated with climatic regimes, whilst the structure and function of vegetation are key factors in determining the land-atmosphere fluxes of heat, water, momentum and CO2. Despite these interactions, GCM land surface schemes have tended to assume fixed vegetation properties even under changes in climate and atmospheric CO2. Simulations with these models will therefore have missed modes of behaviour (and feedbacks) associated with the interactive coupling between vegetation and climate.

Recently a number of groups have attempted to quantify the missing feedbacks by coupling equilibrium vegetation models "asynchronously" to GCMs. This involves an iterative procedure in which the GCM calculates the climate implied by a given land cover, and the vegetation model calculates the land cover implied by a given climate. This process is repeated until a mutual climate-vegetation equilibrium is reached. Such techniques have yielded interesting results, but are only appropriate for simulating equilibria, and suffer from inconsistencies associated with the independent calculation of surface fluxes in the vegetation model and the GCM.

In order to fully understand the role of climate-vegetation feedbacks we need to treat the land cover as a interactive element, by incorporating dynamic global vegetation models (DGVMs) within GCMs. A model of this type, called "TRIFFID" ("Top-down Representation of Interactive Foliage and Flora Including Dynamics"), has been developed at the Hadley Centre for use in coupled climate-carbon cycle simulations.

TRIFFID defines the state of the terrestrial biosphere in terms of the soil carbon, and the structure and coverage of five vegetation types (Broadleaf tree, Needleleaf tree, C3 grass, C4 grass and shrub) within each GCM gridbox. Competition between the vegetation types is based upon a Lotka-Volterra approach. The areal coverage, leaf area index and canopy height of each type are updated using the carbon fluxes calculated within a tiled land surface scheme, called "MOSES II".

The land-atmosphere fluxes are calculated within MOSES II on every 30 minute GCM timestep and aggregated before being passed to TRIFFID (usually every 10 days). Leaf phenology (bud-burst and leaf drop) is updated on an intermediate timescale of 1 day, using accumulated temperature-dependent leaf turnover rates. After each call to TRIFFID the land surface parameters required by MOSES II (e.g. albedo, roughness length) are updated based on the new vegetation state.

A description of the model and its interface to the GCM will be presented, along wih early results from coupled climate-vegetation simulations.