Spatial and temporal resolution

The BETHY scheme can be run on any spatial grid, with several vegetation types present at each grid point. Here, the resolution chosen is 111 km by 111 km (on an equal area grid with 1° by 1° resolution at the equator) and up to three vegetation types per grid cell. The model is run for 11,069 land grid points excluding Antarctica.

While the BETHY scheme is driven by monthly climate data, the strongly non-linear behaviour of the water balance makes it necessary to compute this process internally on a daily time step, distributing monthly precipitation according to the stochastic model of Geng et al. [1986]. This model requires daily probabilities of wet days which are, together with temperature and incoming PAR, interpolated linearly from monthly to daily mean values.

To account for diurnal variations, radiation, energy balance and photosynthesis are calculated every 10 days on an hourly time step, such that those diurnal processes are recomputed every time the LAI is updated. Computation of the diurnal temperature cycle assumes maximum temperature as mean plus half the diurnal amplitude, reached at 14:00 hours, with a sinusoidal time course between dawn and dusk and a linear decrease during the night [Rosenberg, 1974, p. 91]. Specific humidity is assumed to be close to saturated humidity at dawn, and to grow during the day by an amount depending on the ratio of actual to potential evapotranspiration [Müller, 1982; Rosenberg, 1974].

On the output side of the BETHY scheme, those internal calculations at higher temporal resolution are averaged for each grid point over a month and over vegetation types to provide carbon and water fluxes as well as LAI and fAPAR at the same time resolution as the input data. To ensure that results are independent of initial conditions, the model is run for six years with mean-climate input data, of which the first three are considered as spinup and the last three are again averaged for each month to compute a mean response of vegetation and land surfaces to the climatic forcing. A detailed description of the BETHY scheme can be found in [Knorr, 1997].

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