GENIE:ENTS

Tim Lenton (t.lenton@uea.ac.uk)

Summary

 * Name of model: ENTS (Efficient Numerical Terrestrial Scheme)


 * Authors/Constructors: Mark Williamson (Tyndall) & Tim Lenton (UEA)


 * Reference and description: Williamson et al. (2006)


 * Original version: n/a


 * Units: SI


 * Sign convention: positive into atmosphere (fluxes of heat, moisture and carbon)


 * Grid: As ocean (only currently available on 36x36 equal area grid)


 * Timestep: default 5 per ocean timestep (500/year, as for EMBM)


 * Checks to ensure continuity:


 * Dynamical variables: Vegetation carbon, soil carbon, soil water (equilibrium land temperature is calculated analytically at each timestep)

Processes modelled

 * Vertical exchange of heat, moisture and carbon dioxide with atmosphere
 * Uptake of carbon dioxide in photosynthesis, release in vegetation respiration and soil respiration, exchange from vegetation to soil in litter fall
 * Accumulation (from precipitation) and loss (in runoff) of soil water using a simple bucket model but with bucket capacity a function of soil carbon content (sponge effect)
 * Precipitation as snow and maintenance of snow cover if land temperature less than 0 deg.C
 * Changes in land surface albedo due to changes in vegetation, soil and snow cover

Model tuning
Long term monthly mean NCEP reanalysis air temperature, precipitation and relative humidity re-gridded data and pre-industrial atmospheric CO2 were used to tune certain model parameters. The targets were global annual average carbon fluxes of net photosynthesis (120 PgC yr-1), vegetation respiration (60 PgC yr-1), leaf litter (60 PgC yr-1) and soil respiration (60 PgC yr-1), and global annual average values of vegetation (451 PgC) and soil (1306 PgC) carbon. The model was tuned by linearising the equations, and allowing the values of the rate constants k18, k24, k26, and k29 to vary. They were then fine tuned in an iterative trial and error method to correct for small non-linearities. The resulting values are given in Table 1 below.

Values for key parameters
From Table 1 of Williamson et al. (2006).


 * {| border="1" cellpadding="3"

!Parameter !Label !Value !Units
 * Controls dependence of surface albedo on vegetation carbon
 * k7
 * 0.461
 * m2kgC-1
 * Maximum soil field capacity
 * k8
 * 0.45
 * m
 * Minimum soil field capacity
 * k9
 * 0.15
 * m
 * Effect of soil carbon on soil field capacity
 * k10
 * 0.02
 * m3kgC-1
 * Photosynthesis cut-off control for tropical vegetation
 * k11a
 * 309.15
 * K
 * Photosynthesis cut-off control for boreal vegetation
 * k11b
 * 283.15
 * K
 * Another photosynthesis temperature control
 * k12
 * 268.25
 * K
 * CO2 compensation point
 * k13
 * 29
 * ppmv
 * Michaelis-Menten half-saturation constant for CO2
 * k14
 * 145
 * ppmv
 * Controls self-shading effect of vegetation
 * k16
 * 11.5
 * m2kgC-1
 * Control of dependence of vegetation fraction on carbon content
 * k17
 * 0.35
 * kgCm-2
 * Maximum photosynthesis rate
 * k18
 * 4.35
 * kgCm-2yr-1
 * Activation energy for vegetation respiration
 * k20
 * 54830
 * Jmol-1
 * Rate constant for vegetation respiration
 * k24
 * 0.215
 * yr-1
 * Rate constant for leaf litter turnover
 * k26
 * 0.131
 * yr-1
 * Rate constant for soil respiration
 * k29
 * 0.125
 * yr-1
 * Temperature control for soil respiration
 * k31
 * 308.56
 * K
 * Another temperature control for soil respiration
 * k32
 * 227.13
 * K
 * Dependence of roughness length on vegetation carbon
 * kz
 * 0.0452
 * m3kgC-1
 * }
 * k29
 * 0.125
 * yr-1
 * Temperature control for soil respiration
 * k31
 * 308.56
 * K
 * Another temperature control for soil respiration
 * k32
 * 227.13
 * K
 * Dependence of roughness length on vegetation carbon
 * kz
 * 0.0452
 * m3kgC-1
 * }
 * 0.0452
 * m3kgC-1
 * }
 * }

Couplings to other modules

 * Land-Atmosphere (EMBM)
 * Land-Ocean (GOLDSTEIN)
 * Land-Atmospheric Chemistry (ATCHEM)

Land-Atmosphere I/O

 * Inputs:
 * Air temperature (K)
 * Wind speed (m2s-1)


 * Outputs:
 * Land surface temperature (K)
 * Soil Moisture (m water equivalent)

Land-Ocean I/O

 * Outputs:
 * Runoff (m s-1 equivalent at coastal gridboxes, according to simple catchment areas)

Land-Atmospheric Chemistry I/O

 * Inputs:
 * CO2 (ppm)


 * Outputs:
 * net CO2 exchange flux (PgC/yr)

Prescribed inputs
None

Empirical relationships
There are a couple of new empirical relationships used in the model:
 * Roughess length (z0) is found to depend linearly on vegetation carbon (Cv) by regressing vegetation carbon from Olson et al. (1985) against roughness length derived from NCEP reanalysis to give parameter kz (value in Table 1, above).
 * z0 = min{0.001m, kz*Cv}
 * Vegetation fractional cover (fv) has a saturating dependence on vegetation carbon (Olson et al. 1985)
 * fv = 1 - exp(-k17*Cv)