Hadley Global Environment Model 2 - Earth System
The HadGEM2-ES model was a two stage development from HadGEM1, representing improvements in the physical model (leading to HadGEM2-AO) and the addition of earth system components and (leading to HadGEM2-ES). [1] The HadGEM2-AO project targeted two key features of performance: ENSO and northern continent land-surface temperature biases. The latter had a particularly high priority in order for the model to be able to adequately model continental vegetation. Through focussed working groups a number of mechanisms that improved the performance were identified. Some known systematic errors in HadGEM1, such as the Indian monsoon, were not targeted for attention in HadGEM2-AO. HadGEM2-AO substantially improved mean SSTs and wind stress and improved tropical SST variability compared to HadGEM1. The northern continental warm bias in HadGEM1 has been significantly reduced. The power spectrum of El Nino is made worse, but other aspects of ENSO are improved. Overall there is a noticeable improvement from HadGEM1 to HadGEM2-AO when comparing global climate indices. [2] In HadGEM2-ES the vegetation cover is better than in the previous HadCM3LC model especially for trees, and the productivity is better than in the non-interactive HadGEM2-AO model. The presence of too much bare soil in Australia though may cause problems for the dust emissions scheme. The simulation of global soil and biomass carbon stores are good and agree well with observed estimates except in regions of errors in the vegetation cover. HadGEM2-ES compares well with the C4MIP ensemble of models. The distribution of NPP is much improved relative to HadCM3LC. At a site level the component carbon fluxes validate better against observations and in particular the timing of the growth season is significantly improved. The ocean biology (HadOCC) allows the completion of the carbon cycle and the provision of di-methyl sulphide (DMS) emissions from phytoplankton. DMS is a significant source of sulphate aerosol over the oceans. The diat-HadOCC scheme is an improvement over the standard HadOCC scheme as it differentiates between diatom and non-diatom plankton. These have different processes for removing carbon from the surface to the deep ocean, and respond differently to iron nutrients. The HadOCC scheme performs well with very reasonable plankton distributions, rates of productivity and emissions of DMS. The diat-HadOCC scheme has slightly too low levels of productivity which requires further tuning to overcome. The additions of a tropospheric chemistry scheme, new aerosol species (organic carbon and dust) and coupling between the chemistry and sulphate aerosols have significantly enhanced the earth system capabilities of the model. This has improved the tropospheric ozone distribution and the distributions of aerosol species compared to observations, both of which are important for climate forcing. Including interactive earth system components has not significantly affected the physical performance of the model.
Met Office Hadley Centre, Fitzroy Road, Exeter, Devon, UK, EX1 3PB
Bellouin N., O. Boucher, J. Haywood, C. Johnson, A. Jones, J. Rae, and S. Woodward. (2007) Improved representation of aerosols for HadGEM2.. Meteorological Office Hadley Centre, Technical Note 73, March 2007
Collins W.J. , N. Bellouin, M. Doutriaux-Boucher, N. Gedney, T. Hinton, C.D. Jones, S. Liddicoat, G. Martin, F. O'Connor, J. Rae, C. Senior, I. Totterdell, and S. Woodward (2008) Evaluation of the HadGEM2 model. Meteorological Office Hadley Centre, Technical Note 74
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Reference to a modelComponent called Atmosphere
Reference to a fileVariable called Total Solar Irradiance in a dataObject called file_solar
Reference to a componentProperty called solar_forcing in a modelComponent called Atmosphere
Coupling details for component HadGEM2-ES
The model includes interactive schemes for sulphate, sea salt, black carbon from fossil-fuel emissions, organic carbon from fossil-fuel emissions, mineral dust, and biomass-burning aerosols. The model also includes a fixed monthly climatology of mass-mixing ratios of secondary organic aerosols from terpene emissions (biogenic aerosols).
Emissions for the sulphur cycle include emissions of ammonium sulphate aerosol precursors: sulphur dioxide and dimethylsulphide. For sulphur dioxide, anthropogenic emissions are split into surface and chimney-level emissions. Natural emissions from volcanoes are provided as a 3D field. DMS emissions are provided by the ocean biogeochemistry scheme over the ocean, and provided through a dataset over land surfaces. All sulphur-cycle emissions are expressed in term of mass of sulphur within HadGEM2-ES. For black-carbon and organic-carbon aerosols from fossil-fuel and biofuel emissions, emissions are provided at near-surface level. These emissions are expressed in term of mass of carbon. For biomass-burning aerosol, emissions are provided as two fields: surface emissions, and high-level emissions. The latter field is homogeneously distributed across the boundary layer. Biomass emissions are expressed in term of mass of carbon. Emissions for sea-salt and mineral dust aerosols are computed interactively from the modelled meteorology and soil properties by their respective schemes. The climatology of monthly-averaged mass-mixing ratios of secondary organic aerosol from terpene emissions was derived from the chemistry-transport model STOCHEM (Derwent et al., 2003). Emissions based on datasets: All emission fields are provided as monthly means, which are time interpolated by the model every 5 days. Sulphur dioxide emissions are derived from sector-based emissions datasets for IPCC AR5 (Lamarque et al., 2009). Emissions for all sectors are considered surface emissions, except for energy emissions and half of industrial emissions which are considered chimney-level emissions. Emissions for land-based dimethylsulphide are taken from Spiro et al. (1992). Anthropogenic emissions for black carbon and organic carbon are from the sum of all sector-based emissions in the IPCC AR5 dataset. They are emitted in the second level of HadGEM2-ES (near-surface emissions). Anthropogenic emissions for biomass-burning aerosol are the sum of IPCC AR5 emissions for black and organic carbon from biomass-burning emissions. Grass fire emissions are assumed to be surface emissions, while forest fire emissions are distributed homogeneously across the boundary layer. Emissions based on interactive schemes: Sea-salt and mineral dust aerosol emissions are computed interactively depending on modelled near-surface wind speeds and soil properties (Jones et al., 2001; Woodward, 2001). The only 3D aerosol-related emissions used in HadGEM2-ES are natural emissions of sulphur dioxide from background volcanoes (Andres and Kasgnoc, 1998). Aircraft emissions of aerosol precursors or primary aerosols are not included in HadGEM2-ES.
Andres, R.J. and Kasgnoc, A.D. (1998) A time-averaged inventory of subaerial volcanic sulfur emissions. J. Geophys. Res., 103, 25251-25261, 1998.
Derwent, R.G., Collins, W.J., Jenkin, M.E., and Johnson, C.E. (2003) The global distribution of secondary particulate matter in a 3-D Lagrangian chemistry transport model. J. Atmos. Chem., 44, 57-95, 2003.
Jones A., D.L. Roberts, M.J. Woodage and C.E. Johnson (2001) Indirect sulphate aerosol forcing in a climate model with an interactive sulpher cycle.. J. Geophys Res., 106, 20293-20310.
Lamarque, J.F. et al. (2009) Gridded emissions in support of IPCC AR5.. IGACtivities 41, 12-18, May 2009.
Spiro, P.A., Jacob, D.J., and Logan, J.A. (1992) Global inventory of sulfur emissions with 1x1 resolution. J. Geophys. Res., 97, 6023-6036, 1992.
Woodward S., (2001) Modelling the atmospheric life cycle and radiative impact of mineral dust in the Hadley Centre climate model. J. Geophys. Res., 106, D16, 18,155-18,166, 2001.
3D mass/volume mixing ratio for aerosols
Modal scheme, mass as a tracer, number inferred from prescribed size distributions
The model includes interactive schemes for sulphate, sea salt, black carbon from fossil-fuel emissions, organic carbon from fossil-fuel emissions, mineral dust, and biomass-burning aerosols. The model also includes a fixed monthly climatology of mass-mixing ratios of secondary organic aerosols from terpene emissions (biogenic aerosols). All aerosol species, except sea-salt and biogenic aerosols, are advected using the tracer advection scheme and undergo wet and dry deposition. Wet deposition accounts for re-evaporation of precipitation (Bellouin et al., 2007). All aerosol species exert a direct effect (scattering and absorption of shortwave and longwave radiation) and thereby also a semi-direct effect (impact on atmospheric temperature and cloud profiles of aerosol absorption). All aerosol species, except black carbon and mineral dust, also contribute to both the first and second indirect effects on clouds, modifying cloud albedo and precipitation efficiency, respectively. The version of the sulphur cycle included in HadGEM2-ES is described in Jones et al. [2001] and Roberts and Jones [2004] with further improvements described hereafter. The sulphate scheme is a modal scheme where the free aerosol is assumed to have a log-normal size distribution in the Aitken and accumulation size ranges, which include particles with a radius less than 0.05 and 0.5 m, respectively. Sulphate aerosol may also be in a dissolved mode where layer clouds are present, thus affecting the cloud droplet size and exerting first and second indirect effects (modification of cloud albedo and precipitation efficiency, respectively). The sulphur cycle in HadGEM2-ES also includes improvements described by Bellouin et al. [2007]: condensation of sulphuric acid from dry oxidation of sulphur dioxide and dimethysulphide, and conversion from Aitken to accumulation mode particles by condensation. In addition, the oxidation of SO2 and DMS is now using oxidant concentrations (OH, HO2, H2O2, O3) provided by the interactive tropospheric chemistry scheme and depleted oxidants are fed back to the chemistry (alternatively, oxidant concentrations can be prescribed). Emissions of dimethylsulphide from the ocean is provided by the ocean biogeochemistry scheme. The sea salt scheme is a simple diagnostic scheme depending on wind speed and height above the surface to determine the number concentration of sea salt particles in two size modes (Jones et al., 2001). Schemes for black carbon from fossil-fuel emissions (Roberts and Jones, 2004), biomass-burning, and organic carbon from fossil-fuel emissions (Collins et al, 2008) include modes for freshly emitted particles that gradually age into another, more hygroscopic mode; there is also a mode for aerosols that have become incorporated into cloud droplets. Black carbon is considered to be slightly hygroscopic, only becoming incorporated into cloud droplets by diffusion, whereas organic carbon and biomass-burning aerosols are considered to act as cloud condensation nuclei. The mineral dust scheme is based on Woodward (2001) with further improvements made to the dust emission scheme (described in Collins et al, 2008): changes to values of the impact threshold friction velocity and parameterisation of the effect of soil moisture. The modelled horizontal flux includes a wide size range of particles from .06 to 2000 microns in 9 bins. Vertical flux is calculated for particles up to 60 microns in 6 bins, the size distribution following that of the horizontal flux in this range.
Roberts D.L., and A. Jones (2004) Climate sensitivity to black carbon aerosol from fossil fuel combustion. J. Geophys. Resr., 109,
Aerosol transport is done by HadGEM2-ES tracer advection scheme, which includes large-scale advection (Davies et al. 2005), convective transport (Gregory and Rowntree, 1990), and boundary layer mixing (Lock et al., 2000).
Davies T., M. J. P. Cullen, A. J. Malcolm, M. H. Mawson, A. Staniforth, A. A. White, and N. Wood (2005) A new dynamical core for the Met Office's global and regional modelling of the atmosphere.. Quarterly Journal Royal Meteorology Society, 131, 1759-1782.
Gregory J., and P.R. Rowntree (1990) A mass flux convection scheme with representation of cloud ensemble characteristics and stability - dependent closure.. Monthly Weather Review, 118, 1483-1506.
Lock A.P., A.R. Brown, M.R. Bush, G.M. Martin, R.N.B. Smith et al. (2000) A new boundary layer mixing scheme. Part I: scheme description and single column model tests.. Monthly Weather Review, American Meteorological Society, 128, 3187-3199.
The HadGEM2-ES model incorporates the Met Office's New Dynamics framework that provides a non-hydrostatic, fully compressible, deep atmosphere formulation with fewer approximations to the basic equations; semi-Lagrangian advection of all prognostic variables except density, permitting relatively long timesteps to be used at higher resolution; a conservative and monotone treatment of tracer transport; and improved geostrophic adjustment properties bringing better balance to the coupling. HadGEM2-ES includes interactive modelling of atmospheric aerosols, driven by surface and elevated emissions and including tropospheric chemical processes as well as physical removal processes such as washout. The aerosol species represented in the model are sulphate, black carbon, biomass smoke, sea salt, organic carbon, mineral dust and a biogenic climatology. The atmospheric component has a horizontal resolution of 1.25 degrees of latitude by 1.875 degrees of longitude with 38 layers in the vertical extending to over 39 km in height. The model uses the Arakawa C-grid horizontally and the Charney-Phillips grid vertically. The atmospheric timestep period is 30 minutes (48 timesteps per day).
The convection scheme is based on the mass flux scheme of Gregory and Rowntree (1990) but with major modifications. The scheme is explicitly coupled to the boundary layer scheme, and cumulus convection is similarly diagnosed using the mean humidity profile. If cumulus convection is diagnosed, then the boundary layer scheme is capped at the convective cloud base. The convection scheme is then triggered from the lifting condensation level in order to parameterize transports from cloud base upward. Deep and shallow convection are diagnosed separately and different thermodynamic closures are applied. In shallow convection the closure is based on Grant (2001); for deep convection a CAPE closure is used, based on Fritsch and Chappell (1980). A convective momentum transport (CMT) parameterization is used for both deep and shallow convection, based on a flux-gradient relationship obtained from the stress budget. A cloud-base closure for CMT is used, based on the assumption that large-scale horizontal pressure gradients should be continuous across cloud base. Entrainment and detrainment rates for shallow convection are parameterized as in Grant and Brown (1999). The radiative effects of convective anvils are represented by specifying a vertically varying convective cloud amount (Gregory 1999). The model includes adaptive detrainment parametrization to produce smoother mass-flux profiles and more realistic diabatic heating profiles. In addition the depth criterion for shallow convection has been removed to allow shallower clouds to rain provided their water content is sufficiently high.
Derbyshire S.H., A. V. Maidens, S. F. Milton, R. A. Stratton and M. R. Willett (2010) Adaptive detrainment in a convective parametrization. Submitted to Q.J. Royal Meteorol. Soc.
Fritsch J. M., and C. F. Chappell (1980) Numerical prediction of convectively driven mesoscale pressure systems - Part I: Convective parameterization.. Journal of Atmospheric Sciences, 37, 1722-1733.
Grant A.L.M., and A. R. Brown, (1999) A similarity hypothesis for shallow cumulus transports.. Quarterly Journal of Royal Meteorological Society, 125, 1913-1936.
Grant A.L.M., (2001) Cloud base fluxes in the cumulus-capped boundary layer.. Quarterly Journal of Royal Meteorological Society, 127: 407-421.
Gregory J., (1999) Representation of the radiative effects of convective anvils.. Hadley Centre Technical Note 7., Met. Office, Exeter
The large-scale cloud scheme for liquid cloud is that of Smith (1990), in which cloud water and cloud amount are diagnosed from total moisture and liquid water potential temperature using a triangular probability distribution function. The width of this distribution is diagnosed from the variability of the moisture and temperature of the surrounding grid points. A representation of the difference between cloud area fraction and cloud volume fraction is made by subdividing a single model layer into three. HadGEM1 and later models introduced an updated version of the Wilson and Ballard (1999) microphysics scheme. Transfers between water categories (ice, liquid water, vapor, and rain) are calculated based on physical process equations using particle size information.
Smith R. N. B., (1990) A scheme for predicting layer clouds and their water content in a general circulation model. Quarterly Journal of Royal Meteorological Society, 116, 435-460.
Wilson D. R., and S. P. Ballard (1999) A microphysically based precipitation scheme for the Met Office Unified Model.. Quarterly Journal of Royal Meteorological Society, 125, 1607-1636.
The dynamical core for HadGEM2-ES (Davies et al., 2005) was designed to avoid unnecessary approximations and includes the following features: - non-hydrostatic, fully compressible, deep atmosphere formulation using a terrain following, height-based vertical coordinate. - discretization using a horizontally staggered Arakawa C-grid and vertically staggered Charney-Phillips grid; - semi-Lagrangian advection for all prognostic variables, except density, with conservative and monotone treatment of tracers, - Eulerian treatment of the continuity equation for mass conservation; - predictor-corrector implementation of a two time level, semi-implicit, time integration scheme, - three dimensional iterative solution of a variable-coefficient elliptic equation for the pressure increment at each time step.
A semi-lagrangian advection scheme is used. The advection of potential temperature, moisture, density and winds are treated separately. Moisture is conserved using a non-hydrostatic scheme.
The gravity wave drag (GWD) scheme is that of Webster et al. (2003) and includes low-level flow blocking. The actual gravity wave drag is that due to air flowing over orography; it is deposited where wave breaking is diagnosed (typically in the lower stratosphere). The remaining drag (about 80 per cent of the total) is attributed to flow around the orography and is deposited uniformly between the surface and the subgridscale orographic height.
Webster S., A.R. Brown, D.R. Cameron and C.P. Jones (2003) Improvements to the Representation of Orography in the Met Office Unified Model. Quarterly Journal of Royal Meteorological Society, 129 (591), 1989-2010 Part B.
The radiation code is the solution to the two-stream equations by Edwards and Slingo (1996), with some developments. It includes absorption by CO2, H2O, O3, O2, N2O, CH4 and CFCs. The longwave band from 1200 to 1500 cm-1 has been split at 1330 cm-1 in order to better represent the overlap between CH4 and N2O. Gaseous absorption is based on the updated High-Resolution Transmission (HITRAN) 2000 database (Rothman et al., 2003). The water vapour continuum is version 2.4 of the Clough-Kneizys-Davies (CKD) formulation (Clough et al., 1992) and has been included in the shortwave region. Treatment of the effects of non-spherical ice cloud particles are determined using the parameterization by KristjA?nsson et al. (2000). The sea surface albedo is based on the functional form of Barker and Li (1995), modified in the light of aircraft data, and the land-surface albedo is described by Essery et al. (2003). The direct (scattering and absorption of radiation) and indirect radiative effects of aerosols is included.
Barker, H.W. and Z. Li (1995) Improved simulation of clear-sky shortwave radiative transfer in the CCC-GCM J. Climate, 8, 2213-2223
Clough, S.A, M.J. Iacono, and J. L. Moncet (1992) Line-by-line calculations of atmospheric fluxes and cooling rates: application to water vapour. J. Geophys. Res., 97(D14), 15671-15785
Edwards, J.M. and A. Slingo (1996) Studies with a flexible new radiation code. I: choosing a configuration for a large-scale model Q. J. R. Meteorol. Soc., 122, 689-720
Essery, R. L. H., Best, M. J., Betts, R. A., Cox, P. M., and Taylor, C. M. (2003) Explicit representation of subgrid heterogeneity in a GCM land-surface scheme J. Hydrometeorol., 43, 530-543.
Kristjansson, J.E., J.M. Edwards, and D.L. Mitchell (2000) Impact of a new scheme for optical properties of ice crystals on climates of two GCM's J. Geophys. Res., 105(D8), 10063-10079
Rothman, L. S., et al. (2003) The HITRAN molecular spectroscopic database: edition of 2000 including updates through 2001. J. Quant. Spectrosc. Radiat. Transfer, 82, 5-44
The atmospheric chemistry component of HadGEM2-ES is a configuration of the United Kingdom Chemistry and Aerosols (UKCA) model (O'Connor et al., 2009; 2010; www.ukca.ac.uk) running a relatively thorough description of inorganic odd oxygen (Ox), nitrogen (NOy), hydrogen (HOx), and carbon monoxide (CO) chemistry with near-explicit treatment of methane (CH4), ethane (C2H6), propane (C3H8), and acetone (Me2CO) degradation (including formaldehyde (HCHO), acetaldehyde (MeCHO), peroxy acetyl nitrate (PAN), and peroxy propionyl nitrate (PPAN)). It makes use of 25 tracers and represents 41 species. Large-scale transport, convective transport, and boundary layer mixing are treated. The chemistry scheme accounts for 25 photolytic reactions, 83 bimolecular reactions, and 13 uni- and termolecular reactions.
O'Connor, F.M., C.E. Johnson, O. Morgenstern, and W.J. Collins (2009) Interactions between tropospheric chemistry and climate model temperature and humidity biases. Geophys. Res. Lett., 36, L16801,
O'Connor, F. M., C.E. Johnson, O. Morgenstern, M.G. Sanderson, P. Young, G. Zeng, W.J. Collins, and J.A. Pyle (2010) Evaluation of the new UKCA climate-composition model. Part II. The troposphere Geosci. Model Dev. Disc., In preparation, 2010.
Atmospheric Chemistry Emission and Concentration
The tropospheric chemistry can be run fully interactively, taking account of surface emissions of CH4, CO, NOx, HCHO, C2H6, C3H8, Me2CO, and MeCHO. These are added in the surface layer of the model and then mixed within the boundary layer by mixing (Lock et al., 2000). In the case of CH4, surface emissions from all anthropogenic and natural sectors can be prescribed. Alternatively, interactive wetland emissions (Gedney et al., 2004) can be used in place of prescribed wetland emissions. Prescribed emissions for all emitted speces are provided on the model's horizontal grid as monthly mean emissions rates. The model then reads in the monthly mean emission rates and calculates an instantaneous emission rate based on a time interpolation of the monthly mean fields. Typically, this time-interpolated rate is recalculated every 5 days. The corresponding tracers are incremented in the surface layer using this time-interpolated emission rate and the dynamical timestep, followed by boundary layer mixing. The interactive chemistry in the model also includes aircraft emissions of NOx. These are provided on the 3D model grid as monthly mean emissions rates and time interpolated as for the surface emissions. The model then reads in the monthly mean emission rates and calculates an instantaneous emission rate based on a time interpolation of the monthly mean fields. Typically, this time-interpolated rate is recalculated every 5 days. The NO tracer in the model is then incremented, using this time-interpolated emission rate and the timestep. Interactive lightning emissions of NOx are also considered (Price and Rind, 1994), which have been scaled to give approximatly 5 TgN/year at the present day. These are diagnosed every dynamical timestep using convective cloud top and base from the climate model's convection scheme and added to the NO tracer every dynamical timestep. The chemistry also has the option of fixing the surface methane concentration based on concentrations provided (either fixed or varying in time). The interactive chemistry does not consider heterogeneous chemistry in the stratosphere. For this reason, stratospheric ozone is prescribed 3-5 km above the tropopause using a time-varying or time-invariant field.
Gedney, N, Cox, P. M., and Huntingford, C. (2004) Climate feedback from wetland methane emissions Geophys. Res. Lett., 31 (20), L20503, 2004.
Price, C. and Rind, D. (1994) Modelling global lightning distributions in a general circulation model. Mon. Weath. Rev., 122, 1930-1939, 1994.
Atmospheric Chemistry Gas Phase Chemistry
see description for "Atmospheric Chemistry" component
The atmospheric chemistry component (UKCA) of HadGEM2-ES takes account of 25 photolytic reactions. The corresponding photolysis rates are calculated offline in the Cambridge 2D model (Law and Pyle, 1993) using the Hough (1988) scheme. These are then read in by UKCA on the first time step of the model integration and interpolated in time and space at each model grid box.
Hough, A. M. (1998) The calculation of photolysis rates for use in global tropospheric modelling studies. AERE Report, 13259, At. Energy Res. Estab., Harwell, U.K., 1988
Law, K. S. and Pyle, J. A. (1993) Modelling trace gas budgets in the troposphere. 1. ozone and odd nitrogen J. Geophys. Res., 98, 18377-18400, 1993.
The tracers in the UKCA atmospheric chemistry in HadGEM2-ES are subject to large-scale advection (Davies et al. 2005), convective transport (Gregory and Rowntree, 1990), and boundary layer mixing (Lock et al. 2000).
The major ice sheets (Greenland and Antarctica), and minor ice caps (Ellesmere, Devon and Baffin islands, Iceland, Svalbard, Novaya and Severnaya Zemlya, and Stikine) are depicted as static ice. They are initialised with a snow depth of 50,000 mm of water equivalent. Further ablation or accumulation has an impact on sea level. Runoff follows the river routing scheme and enters the oceans at predefined river outflow points. Calving at the coastal boundaries is simulated through a fresh water flux to the ocean evenly distributed over the area of observed icebergs in both hemispheres. The value of the freshwater flux is calculated to exactly balance the time mean ice sheet surface mass balance over the preindustrial control simulation.
The second version of the U.K. Met Office Surface Exchange Scheme (MOSES-II) (Cox et al. 1999; Essery et al. 2003) is used. This allows tiling of land surface heterogeneity using nine different surface types. A separate surface energy balance is calculated for each tile and area-weighted grid box mean fluxes are computed, which are thus much more realistic than when a single surface type is assumed. In addition, vegetation leaf area is allowed to vary seasonally, providing a more realistic representation of seasonal changes in surface fluxes. Tiling of coastal grid points allows separate treatment of land and sea fractions. This in combination with the increased ocean model resolution greatly improves the coastline, particularly in island regions.
Cox P. M., R. A. Betts, C. B. Bunton, R. L. H. Essery, P. R. Rowntree, and J. Smith (1999) The impact of new land surface physics on the GCM simulation of climate and climate sensitivity.. Climate Dynamics., 15, 183-203.
A bulk albedo is used for each of the surface types within the tile scheme. Non vegetation surfaces have a fixed specified snow free albedo. For vegetation the albedo is a combination of the bare soil albedo and the vegetation albedo, using Bear's law to determine the fraction of bare soil. The overall albedo then depends upon the amount of snow, varying between the snow free albedo and a specified deep snow albedo according to the depth of snow (Essery et al. 2001).
Essery R., M. Best, and P. Cox (2001) MOSES 2.2 technical documentation.. Hadley Centre Technical Note 30, 30 pp.
The land surface scheme simulates the storage of carbon in the terrestrial biosphere and its exchange with the atmosphere. Vegetation takes up carbon through photosynthesis and releases it through autotrophic respiration. The net balance (NPP) is allocated to root, wood and leaf carbon. Turnover of vegetation carbon falls as litter to the soil, which is representedf by the 4-pool soil carbon model RothC (Coleman and Jenkinson, 1999). Decomposition of soil organic material depends on a Q10 function of temperature (q10=2) and a function of soil moisture (Cox, 2001).
Coleman K, Jenkinson D. S (1999) RothC-26.3, A Model for the Turnover of Carbon in Soil: Model Description and User's Guide Lawes Agricultural Trust, Harpenden, UK
Cox, P.M. (2001) Description of the TRIFFID dynamic global vegetation model.. Hadley Centre Technical Note 24, Hadley Centre, Met Office, UK.
(described under Land Surface Carbon Cycle)
The surface energy balance is derived following the Penman-Montheith approach. This is extended to include thermal inertia for the surface and weakening of the coupling to the underlying soil through radiative processes (Essery et al. 2001). Surface heterogeneity is modelled using the tile approach, with a separate surface energy balance calculation for each tile. The temperature change associated with the outgoing longwave radiation is taken into account in the surface energy balance, but the net change to the radiative balance is not applied throughout the atmosphere. Instead it is assumed that the small changes to the longwave radiation pass through the atmosphere and are added to the top of atmosphere balance through an additional diagnostic. The surface is implicitly coupled to the atmosphere using the method of Best et al. (2004).
Best, M.J., Beljaars, A., Polcher, J., Viterbo, P., (2004) A proposed structure for coupling tiled land-surfaces with the planetary boundary layer Journal of Hydrometeorology, 5, 1271-1278
Lakes are represented as a fixed, specified fraction of each gridbox. They provide a source of water to evaporate to the atmosphere but do not prognostically simulate water depth, storage or areal extent. Evaporated water from lakes is removed from soil moisture in unstressed regions. The model calculates the accumulated lake evaporation globally over a whole day and then applies an equal correction to all the 4 soil moisture levels for each grid box where the soil moisture is greater than the wilting point.
Snow at the land surface is represented with a single layer. This layer is merged with the first soil layer in such a way that if the total depth of snow is less than the thickness of the first soil layer, then the thermal properties become a linear weighting of those from the snow and soil. If the snow depth exceeds the first soil layer thickness, then this layer can increase in thickness and adopts only the snow thermal properties (Essery et al. 2001).
The soil has four layers of increasing thickness up to a depth of 3m each with the same soil properties. In addition there is a deeper layer (extending down to 6m below the surface) which has a saturated hydraulic conductivity which decreases exponentially with depth (Gedney et al, 2003). Soil properties are from <REFERENCE REQUIRED>
Gedney, N., and Cox, P. M. (2003) The sensitivity of global climate model simulations to the representation of soil moisture heterogeneity J. Hydromet., 4, 6, 1265-1275
The large-scale hydrology scheme (Gedney et al, 2003 and Clark and Gedney, 2008) is incorporated into the land surface soil hydrology. This allows for the deeper layers to become saturated and a water table to form. Deep run-off is lost through lateral flow below the water table. Excess (supersaturated) soil water (e.g. through snowmelt) is now drained into the soil layer below instead of simply run-off from the top layer. This increases soil moisture in the lower soil layers and so helps to reduce the water stress on vegetation following snowmelt in Northern Hemispheric continents.
Clark, D. B., and Gedney, N. (2008) Representing the effects of subgrid variability of soil moisture on runoff generation in a land surface model J. Geophys. Res., 113, D10, D10111.
The land surface scheme represents 5 vegetation functional types (PFTs): broadleaf and needleleaf tree, C3 and C4 grass and shrub, the fractions of which can be specified or simulated by the model (Cox 2001). When simulated, fractional coverage of each PFT evolves due to competition between types based on carbon balance and available land area and is represented by Lotka-Voltera competition equations. As it does not explicitly simulate crop growth, crop areas are treated as natural grass. The impact of land-use is implemented by removing trees and shrubs in regions specified as agriculture. The representation of photosynthesis is that of Collatz et al. (1991) for C3-type photosynthesis and Collatz et al. (1992) for C4-type photosynthesis, as described in Cox et al. (1999). Photosynthesis is also directly sensitive to the leaf nitrogen concentrations (specified) and the leaf temperature. The phenological status alters the leaf area index (LAI) of the canopy and is soley a function of leaf temperature (with pre-specified tolerances to low temperatures) and includes the effects of leaf dropping and budburst in a way corresponding to a chill-day parametrization. It also includes a representation of the vertical profiles of light and nitrogen through the canopy (Mercardo et al., 2007).
Collatz, G. J., Ball, J. T., Grivet, C., and Berry, J. A. (1991) Physiological and environmental-regulation of stomatal conductance, photosynthesis and transpiration - a model that includes a laminar boundary-layer Agr. Forest Meteorol., 54, 107-136.
Collatz, G. J., Ribas-Carbo, M., and Berry, J. A. (1992) Coupled photosynthesis-stomatal conductance model for leaves of C4 plants Aust. J. Plant Physiol., 19, 519-538.
Mercado, L. M., Huntingford, C., Gash, J. H. C., Cox, P. M., and Jogireddy, V. (2007) Improving the representation of radiation interception and photosynthesis for climate model applications. Tellus, 59B, 553-565.
Realistic river flow is important for the freshwater contribution to the thermohaline circulation. The new Total Runoff Integrating Pathways (TRIP) dynamic river routing scheme (Oki and Sud 1998) advects runoff along prescribed channels using an embedded 1degree x 1degree river transport submodel. All rivers flow with an effective velocity of 0.4 m/s and a meander ratio of 1.4. River outflow to the ocean is included. Ancillary files specify 'inland basins' in which the river flows into an inland delta and the moisture enters the soil column. The model also allows for inland basins to be defined seperately, and spreads water proportionately over the river outflow points.
Oki T., and Y.C. Sud (1998) Design of the Total Runoff Integrating Pathways [TRIP] - A global river channel network.. Earth Interactions, 2.
The ocean component is based on the Bryan-Cox code (Bryan 1969; Cox 1984) and follows the ocean component of HadGEM1 (Johns et al., 2006) very closely. The model uses a latitude-longitude grid with a zonal resolution of 1 degree, and a meridional resolution of 1 degree between the poles and 30 degrees, from which it increases smoothly to 0.333 degrees at the equator - giving a grid of 360 x 216 points. It has 40 unevenly spaced levels in the vertical, with enhanced resolution near the surface better to resolve the mixed layer and thermocline. The forward timestep period is 1 hour, with a mixing timestep of once per day. HadGEM2 uses a bathymetry derived from the Smith and Sandwell (1997) 1/30 degrees depth dataset merged with ETOPO5 (1988) 1/12 degrees data at high latitudes, interpolated to the model grid and smoothed using a five-point (1:4:1) two-dimensional filter. Where this procedure obstructs important narrow pathways (e.g., Denmark Strait, Faroes-Shetland Channel, Vema Channel, and around the Indonesian archipelago), the bathymetry is adjusted to allow some flow at realistic depths (with reference to Thompson 1995). The land masks for the ocean grid differs from that used for the atmosphere model, due to the differences in model resolutions. To enable daily coupling between the models a tiling scheme has been introduced. For each atmosphere grid box, fractions of the fluxes can be coupled to land, sea and sea ice models so that the total flux is conserved - though locally the flux may not be conserved so diagnosis can be difficult. The only ancillary flux used by the ocean model is to enable a balance in the freshwater flux to be maintained, since the accumulation of frozen water on land is not returned into the freshwater cycle, i.e there is no representation of icebergs calving off ice shelves. The ancillary flux is used to add freshwater back into the model, calibrated from a HadGEM2 reference integration to give a balanced freshwater budget.
Bryan K., (1969) A numerical method for the study of the circulation of the world ocean.. Journal of Computational Physics, 4, 347-376.
Cox M.D., (1984) A primitive equation, three dimensional model of the ocean.. Ocean Group Technical Report 1, GFDL, Princeton.
Johns T.C., C.F. Durman, H.T. Banks, M.J. Roberts, A.J. McLaren, J.K. Ridley, C.A. Senior, K.D. Williams, A. Jones, G.J. Rickard, S. Cusack, W.J. Ingram, M. Crucifix, D.M.H. Sexton, M.M. Joshi, B-W. Dong, H. Spencer, R.S.R. Hill, J.M. Gregory, A.B. Keen, A.K. Pardaens, J.A. Lowe, A. Bodas-Salcedo, S (2006). "The new Hadley Centre climate model HadGEM1: Evaluation of coupled simulations." Journal of Climate, American Meteorological Society, Vol. 19, No. 7, pages 1327-1353.
A pseudo fourth-order advection scheme (Pacanowski and Griffies 1998) is used, which is more accurate and generates less grid-scale noise than a second-order scheme, and includes upstream mixing at the ocean bottom to improve model stability there. A simple semi-implicit representation of linear bottom friction is included which removes momentum at the ocean bottom depending on the magnitude of the velocities found there.
Pacanowski R. C., and S. M. Griffies (1998) MOM 3.0 manual.. NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, 692 pp.
Isopycnal diffusivity takes a constant value of 500 m2 s-1 using the Griffies et al. (1998) scheme; the Gent and McWilliams (1990) adiabatic mixing scheme in the skew flux form (Griffies 1998) is used with a spatially and temporally varying coefficient (Visbeck et al. 1997; Roberts 2004), a minimum value of 150 m2s-1 and maximum of 2000 m2s-1, and spatial distribution with higher values in the western boundary currents and Antarctic Circumpolar Current. The biharmonic adiabatic scheme of Roberts and Marshall (1998) is used with a coefficient of 1.0e+12 cos3(latitude) m4 s-1. The cos(latitude) factor is required for numerical reasons owing to the convergence of meridians at high latitude. Horizontally aligned biharmonic tracer mixing in the top 20m (top two model layers), with coefficient 2.5e+12 cos3(latitude) m4 s-1 is included to partially represent enhanced mixing at the ocean surface. Momentum diffusion uses both a modified Laplacian scheme with a coefficient of 750(1-cos(latitude)) m2s-1 (modified to reduce the westward currents on the equator, giving better agreement with observations) and a biharmonic scheme with coefficient 1.0e+13 cos3(lat) m4 s-1.
Griffies S. M., (1998) The Gent-McWilliams skew flux.. Journal of Physical Oceanography, 28, 831-841.
Griffies S. M., A. Gnanadesikan, R. C. Pacanowski, V. D. Larichev, J. K. Dukowicz, and R. D. Smith (1998) Isoneutral diffusion in a z-coordinate ocean model.. Journal of Physical Oceanography, 28, 805-830.
Roberts M. J., and D. Marshall (1998) Do we require adiabatic dissipation schemes in eddy-resolving ocean models?. Journal of Physical Oceanography, 28, 2050-2063.
Roberts M. J., (2004) The Gent and McWilliams parameterisation scheme, including Visbeck and biharmonic GM schemes.. Tech. Rep., Unified Model Documentation Paper UMDP54, Met Office, 23 pp.
Visbeck M., J. Marshall, T. Haine, and M. Spall (1997) Specification of eddy transfer coefficients in coarse-resolution ocean circulation models. Journal of Physical Oceanography, 27, 381-402.
Dukowicz J. K., and R. D. Smith (1994) Implicit free surface method for the Bryan-Cox-Semtner ocean model.. Journal of Geophysical Research, C4, 7991-8014.
The active tracers (temperature and salinity) are advected using a fourth-order scheme.
There is a parameterisation of flow into three marginal seas where the passages are not resolved by the model grid. Mediterranean water is partially mixed with Atlantic water across the Strait of Gibraltar (constant flux of 0.4Sv over the top 80m and out at 600m). The Red Sea has 0.2Sv fluxed in over the top 20m and out at 40-60m. The Persian Gulf has 0.1Sv fluxed in over the top 20m and out at 40-60m. The flux volumes used are based on observed values, although the Mediterranean value was reduced because the model does not represent many of the mixing processes that would modulate the water as it flows over the sill.
There is no reference salinity, instead salinity limits are applied: 5-45 psu. An enhancement of the vertical and horizontal diffusion in the ocean wherever a river outflow occurs has been introduced to correct a known systematic salty bias close to the Amazon at a depth of 150m.
Convective mixing in the model uses the Rahmstorf (1993) full convection scheme. The equation of state remains the UNESCO 1981 polynomial approximation. There are limits on the model surface salinity which is not allowed to go outside the range 5 - 45 psu. The vertical tracer diffusivity has been lowered in the upper 1000m of the ocean to the edge of the uncertainty range. This has the effect of reducing mixing with the cooler subsurface water, increasing the SSTs and alleviating the subsurface cooling in the tropics.
Moum J.N., and T.R. Osborn (1986) Mixing in the main thermocline. Journal of Physical Oceanography, Vol 16, Issue 7, pp1250-1259.
Rahmstorf S., (1993) A fast and complete convection scheme for ocean models. Ocean Modelling, 101, 9-11.
Roether W., V. M. Roussenov and R. Well (1994) A trace study of the thermohaline circulation of the eastern Mediterranean.. In: Malanotte-Rizzoli P, Robinson AR (eds) Ocean Processes in climate dynamics, global and Mediterranean example, Kluwer Academic Press, Dordrecht, pp 371-394.
The mixed layer is represented by the Kraus and Turner (1967) bulk mixed layer for tracers, together with a quadratic representation of the Large et al. (1994) scheme for momentum mixing in the mixed layer. Vertical mixing beneath the mixed layer is performed by the Peters et al. (1988) scheme; this is a Richardson-number dependent scheme, and its parameters have been altered to better fit the observed data in their paper by Cusack (2004). This change reduces the excessive noise near the ocean surface. There is also a modification to the standard vertical mixing scheme to enhance the mixing at the base of the mixed layer, to increase the communication between the ocean surface and the deeper layers and make the profile more similar to that observed.
Kraus E. B., and J. S. Turner (1967) A one-dimensional model of the seasonal thermocline, Part II.. Tellus, 19, 98-105.
Large W. G., J. C. McWilliams and S.C. Doney (1994) Ocean vertical mixing: A review and a non-local boundary layer parameterization. Reviews of Geophysics, 32, 363-403.
Peters H., M. C. Gregg, and J. M. Toole (1988) On the parameterization of equatorial turbulence. Journal of Geophysical Research, 93, 1199-1218.
A two band scheme (one more penetrative) from Paulson and Simpson (1977), assuming pure water type 1B with coefficients adjusted.
There is a simple prognostic ecosystem model (the Diat-HadOCC model), with three nutrients (combined nitrate and ammonium, dissolved silicate and dissolved iron), two phytoplankton (diatoms and other phytoplankton), one zooplankton and three detrital compartments (detrital nitrogen, detrital carbon and detrital silicate). There are also two prognostic tracers that are only used in a diagnostic capacity (i.e. their concentrations are affected by, but do not affect, the other compartments): these are dissolved ammonium and dissolved oxygen. Nitrogen is used as the model currency. The diatoms have a variable silicate:nitrate ratio, so diatom silicate is another compartment. All three of the detrital compartments sink with a constant velocity, and are remineralised at a rate that is depth-dependent The zooplankton and both phytoplankton compartments have fixed elemental carbon:nitrogen ratios which allow the flows of carbon through the ecosystem to be linked to the corresponding nitrogen flows. As well as the ecosystem compartments listed above dissolved inorganic carbon (DIC) and total alkalinity (TAlk) are included as prognostic tracers. Those two compartments, along with the model temperature and salinity, are used to calculate the ocean surface pCO2, the air-sea flux of CO2 and the ocean pH. The air-sea fluxes of CO2 and DMS are each calculated every ocean time-step and their daily means are passed through the coupler to the atmosphere each day.
There are no exchanges of biogeochemical variables with the land (including rivers), with sea-ice or with the sea-floor sediments (which are not included in the model). Only the air-sea interface has any forcing for the ocean biogeochemistry directly. Dust from the atmospheric model is deposited on the ocean and absorbed in the top layer; its soluble iron content is added to the dissolved iron in the water and is available for biological uptake and can be transported like any other ocean tracer. In the atmospheric model the rate of dust deposition is calculated in six size classes and by four deposition processes, but on being passed to the ocean all these combinations are summed to get the total deposition. DMS is passed from the ocean to the atmosphere, but the atmospheric concentration is assumed to be zero in the calculation so there is no air-to-sea flux. In the case of CO2 the flux is calculated in both directions, with the atmospheric pCO2 required for the calculation being prescribed as specified in the CMIP5 protocols. For both DMS and CO2 the piston velocity uses the 10m wind-speed, passed from the atmosphere. In the case of dissolved oxygen, which is a prognostic tracer but whose concentration does not affect any other biogeochemical process, the surface ocean layer is assumed to be 3% over-saturated, the saturation being determined according to Equation 8 on p1310 of Garcia and Gordon (1992), modified by removing the incorrect term on the first line as indicated in the o2sato.f function in the OCMIP-2 Biotic-HOWTO document (www.ipsl.jussieu.fr/OCMIP/). The flux is determined by the change in the top-layer ocean concentration required to achieve that oversaturation, and is applied to the ocean concentration but not the atmosphere, and no forcing from the atmosphere is required. More details of the gas exchange formulations is given in the Ocean Bio Gas Exchange component. Note that all information passed from the atmosphere is passed through the coupler, which is called every 24 hours.
Garcia, H.E. & Gordon, L.I. (1992) Oxygen solubility in seawater: Better fitting equations. Limnol. Oceanogr., 37, 1307-1312
The gasses exchanged between the ocean and atmosphere are DMS (only ocean to air) and CO2 (both directions). Oxygen is not passed between ocean and atmosphere although it exists as a tracer in both models and the ocean tracer has a surface boundary condition as described in the preceding section Ocean Bio Boundary Forcing. For both DMS and CO2 the air-sea flux is calculated each (ocean) time-step in the ocean model and applied to the appropriate ocean tracers; the daily mean flux is passed through the coupler (once a day) to the atmospheric model. In the case of DMS there is no attempt to model the sulphur cycle in the ocean, a simple parameterisation being used to calculate the surface ocean concentration of DMS and thence the flux out of the ocean. The parameterisation used is that of Simo and Dachs (2002), which sets the DMS concentration as a function of chlorophyll divided by the mixed layer depth while that ratio is greater than 0.02 mg-Chla/m^2, and as a function of the log of the mixed layer depth otherwise. However, here in the latter case if the mixed layer is more than 182.536 m the scheme of Aranami & Tsunogai (2004), which make the DMS concentration inversely proportional to the mixed layer depth, is used to avoid negative concentrations. In all cases the chlorophyll is calculated from the non-diatom phytoplankton concentration only, and the mixed layer depth is calculated according to the 0.5C temperature criterion. The Schmidt number for DMS is calculated according to Saltzman et al. (1993). In the case of CO2 the atmospheric pCO2 (time-varying and prescribed according to CMIP5 protocols) is converted from partial pressure (in ppmv) to Moles-C/kg-seawater using the solubility parameterisation of Weiss (1974). The concentration of carbonic acid ([H2CO3], taken to include dissolved CO2 gas) is calculated from the model DIC and TAlk concentrations and the model temperature and salinity. The definition of TAlk used is [HCO3(-)] + 2[CO3(2-)] + [B(OH)4(-)] + [OH(-)] [H(+)], and so the calculation of [H(+)] and then [H2CO3] requires the constants K1, K2, Kb and Kw. The formulae for all these constants can be found in DOE (1994), with the K1 and K2 constants coming originally from Roy et al. (1993) and the boric acid dissociation constant Kb from Dickson (1990) The total boron concentration is linked to salinity as in Peng et al. (1987). [H(+)] is found using the iterative secant method of similar triangles following Bacastow (1981). The Schmidt number for CO2 is calculated following Table A1 in Wanninkhof (1992), which itself uses experimental results from Jahne et al. (1987). The wind-speed dependence of the piston velocity follows Wanninkhof (1992) and uses the 0.31 coefficient value.
Aranami, K. & Tsunogai, S. (2004) Seasonal and regional comparison of oceanic and atmospheric dimethylsulfide in the northern North Pacific: Dilution effects on its concentration during winter J. Geophys. Res., 109, D12303,
Bacastow, R. (1981) Numerical evaluation of the evasion factor, pp95-101 in: Scope 16: Carbon Cycle Modelling Bolin, B., editor, John Wiley, New York
Dickson, A.G. (1990) Thermodynamics of the dissociation of boric acid in synthetic seawater from 273.15 to 318.15 K, Deep-Sea Res., 37, 755-766
US Dept. of Energy. (1994) Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2. Dickson, A.G. & Goyet, C., editors, ORNL/CDIAC-74
Jahne, B., Heinz, G. & Dietrich, W. (1987) Measurement of the diffusion coefficients of sparingly soluble gases in water with a modified Barrer method, J. Geophys. Res., 92, 10,767-10,776
Peng, T.-H., Takahashi, T., Broecker, W.S. & Olafsson J. (1987) Seasonal variability of carbon dioxide, nutrients and oxygen in the northern North Atlantic surface water: observations and a model, Tellus, 39B, 439-458
Roy, R.N., Roy, L.N., Vogel, K.M., Porter-Moore, C.,Pearson, T., Good, C.E., Millero, F.J. & Campbell, D.M. (1993) The dissociation constants of carbonic acid in seawater at salinities 5 to 45 and temperatures 0 to 45C, Mar. Chem., 44, 249-267
Saltzman, E.S., King, D.B., Holmen, K. & Leck, C. (1993) Experimental Determination of the Diffusion Coefficient of Dimethylsulfide in Water J. Geophys. Res., 98C, 16481-16486
Simo, R. & Dachs, J. (2002) Global ocean emission of dimethylsulfide predicted from biogeophysical data, Global Biogeochem. Cycles, 16(4), 1018
Wanninkhof, R. (1992) Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res., 97C, 7373-7382
Weiss, R.F. (1974) Carbon dioxide in water and seawater: The solubility of a non-ideal gas. Mar. Chem., 2, 203-215
The fourteen ocean biogeochemical tracers are: dissolved inorganic carbon (DIC); total alkalinity (TAlk); dissolved oxygen; and the eleven ocean ecosystem tracers, as described in the Ocean Bio Tracers Ecosystem component including dissolved inorganic nitrogen (DIN), dissolved silicate, total dissolved iron, diatom nitrogen, diatom silicate, other non-diatom phytoplankton nitrogen, zooplankton nitrogen, detrital nitrogen, detrital carbon, detrital silicate and dissolved ammonium. Dissolved inorganic carbon (DIC): equal to [H2CO3] + [HCO3(-)] + [CO3(2-)],here [H2CO3] is taken to include both carbonic acid and dissolved CO2 gas. It is taken up, in a fixed ratio to nitrogen, by diatoms and other non-diatom phytoplankton during growth (but does not limit their rates of growth). It is recycled through the natural mortalities of diatoms, other non-diatom phytoplankton and zooplankton, zooplankton respiration and messy feeding and by remineralisation of detrital carbon. It has an exchange with the atmosphere, as defined in the Ocean Bio Gas Exchange component Units are mMoles-C/m3; Total alkalinity (TAlk): defined as [HCO3(-)] + 2[CO3(2-)] + [B(OH)4(-)] + [OH(-)] [H(+)]. Biological processes affect TAlk in two ways: when one unit of DIN is taken up (or released) one unit of TAlk is released (taken up), following Goldman and Brewer (1980), and when one unit of calcium carbonate is formed (or dissolved) two units of TAlk are taken up (released). Units are mEquivalents/m^3; Dissolved oxygen: when one unit of DIC is taken up (or released) by biology (except for calcium carbonate formation or dissolution), one unit of dissolved oxygen is released (taken up). The surface ocean boundary condition for dissolved oxygen is described in Ocean Bio Boundary Forcing component. Units are nMoles-O2/m^3.
Goldman, J.C. & Brewer, P.G. (1980) Effect of nitrogen source and growth rate on phytoplankton-mediated changes in alkalinity Limnol. Oceanogr., 25, 352-357
The eleven ocean ecosystem tracers are: a) Nutrients: - dissolved inorganic nitrogen (DIN): the sum of dissolved nitrate (and nitrite) and ammonium; units mMoles-N/m^3. The initial field at the start of the spinup was from the World Ocean Atlas (2005). Taken up by diatoms and other non-diatom phytoplankton, recycled from those compartments and from zooplankton and detrital nitrogen. - dissolved ammonium: optional prognostic tracer, but the rate of diatom and other non-diatom phytoplankton growth rates do not depend on its concentration because they depend on DIN, which includes dissolved ammonia. The cycle of dissolved ammonia is calculated in parallel with that of DIN, so that the cycle of dissolved nitrate (which is not a tracer, but the difference between DIN and ammonium) can be inferred. Units are mMoles-N/m^3. - dissolved silicate: taken up by diatoms for shell formation, recycled from detrital silicate (by dissolution). Units are mMoles-Si/m^3. - dissolved iron: taken up by diatoms and other non-diatom phytoplankton, recycled from those compartments and from zooplankton (no iron is present in detritus). Atmospheric dust deposited on the ocean is a source of dissolved iron. Although all the dissolved iron is available for biological uptake only the free (non-complexed) iron can be adsorbed onto (implicit) mineral particles and removed permanently from the model ocean. To calculate the partition between free and complexed iron a constant density of total (free and complexed) ligand and a partition constant are assumed. Units are uMoles-Fe/m^3. b) Living organisms: - diatoms: described in terms of their nitrogen content (units: mMoles-N/m^3). Their growth is limited by [DIN], dissolved silicate and light. They are subject to predation by the zooplankton and to natural mortality, and they sink at 1 m/day. The rate at which they take up silicate is a factor times the DIN uptake rate, the factor being a function of the dissolved iron concentration with a high Si:N ratio where iron is a limiting nutrient. - diatom silicate: because the Si:N uptake ratio varies, the Si:N ratio in diatoms also varies so this has to be a separate prognostic variable. Its source is the uptake of dissolved silicate during diatom growth, and it become detrital silicate when the diatom dies by predation or natural mortality. It sinks at 1 m/day, like the diatoms. Units are mMoles-Si/m^3. - other non-diatom phytoplankton: also described in terms of their nitrogen content (units: mMoles-N/m^3). They represent all phytoplankton except those that are diatoms. Their growth is limited by [DIN] and light. They are subject to predation by zooplankton and to natural mortality. They do not sink. They have a lower maximum growth rate than diatoms, but it is reduced less by iron limitation. Calcium carbonate is formed in proportion to the growth rate of this compartment (it is recognised that not all non-diatom phytoplankton calcify, and this is accounted for in the proportionality). - zooplankton: are described in terms of their nitrogen content (units: mMoles-N/m^3). They feed on diatoms, other non-diatom phytoplankton and detritus (detrital nitrogen and detrital carbon). They have dynamic feeding preferences which depend on the relative concentrations of the prey following Fasham et al. (1990). The base preference for diatoms varies with the degree of iron limitation, to represent the observed effect that iron-limited diatoms are more heavily silicified and so less appetising to predators. The rate of feeding depends on the absolute prey concentration, weighted by preference. The zooplankton respire (with a corresponding excretion of nitrogen to balance their carbon:nitrogen ratio) and suffer natural mortality, the latter term implicitly including the effects of higher trophic levels. c) Other particles: - detrital carbon, detrital nitrogen: are described in terms of their carbon and nitrogen contents respectively (units: mMoles-C/m^3, mMoles-N/m^3), and represent the elemental fractions of detrital particles. Their sources are (fractions of) the natural mortalities of diatoms, other non-diatom phytoplankton and zooplankton, and messy feeding and egestion of un-assimilated food by zooplankton. They sink at a constant speed, 10 m/day. They are remineralised to DIC or DIN at a specific rate that is inversely proportional to depth (up to an absolute maximum specific rate). If they sink to the sea-floor they are instantly remineralised and the products evenly spread over the bottom three levels of the water-column. - detrital silicate: its source is the natural mortality of diatoms and their predation by zooplankton. It sinks at a constant speed of 10 m/day, and returns to dissolved silicate at a constant specific rate (which is depth-independent). Any that reaches the sea-floor is instantly remineralised and the resulting dissolved silicate spread over the bottom three levels of the water-column. Units are mMoles-Si/m^3.
Fasham, M.J.R., Ducklow, H.W. & McKelvie, S.M. (1990) A nitrogen-based model of plankton dynamics in the oceanic mixed layer J. Mar. Res., 48, 591-639
The sea-ice model is split between the atmosphere and ocean model components. The atmosphere part calculates the atmosphere-ice heat fluxes, the diffusive heat flux through the ice, the ice surface temperature and surface melting. This allows the diurnal cycle of the surface temperature to be modelled whilst only coupling betwen the atmosphere and ocean components once a day. The ocean part deals with the remaining thermodynamics, calculates basal melting by heat supplied from the mixed layer, keeps account of snow and ice thickness changes including conversion of submerged snow to ice, and deals with ice dynamics (including ridging) using daily mean fields of the atmospheric forcings supplied via coupling routines. A multiple ice thickness category model which solves the basic equation of Thorndike et al. (1975), is used to capture the sub-gridscale ice thickness distribution. HadGEM2-ES uses 5 ice categories plus open water (leads). Ice thickness affects many sea ice properties and processes, including the growth rate, ice strength and surface energy fluxes. As ice thickness can vary greatly within the grid box length scale, a multiple ice category model should lead to an improved representation of sea ice processes.
Thorndike, A. S., D. A. Rothrock, G. A. Maykut and R. Colony (1975) The thickness distribution of sea ice. Journal of Geophysical Research, 80, 4501-4513.
The ice albedo scheme is based on the scheme of Ebert and Curry (1993) with a parameterisation based on SHEBA observations. The bare ice albedo is modified (Semtner 1987) to simulate internal scattering processes in the zero layer scheme, which act to delay the onset of melt. In addition, the snow parameterisation scheme used in the land surface scheme is included, which allows for partial and semi-transparent snow cover. The snow albedo is modified by surface temperature, representing the increase in liquid water content. As a consequence the surface albedo, in addition to surface air temperature, is highly responsive to modelled snow fall events.
Ebert E. E., and J. A. Curry (1993) An intermediate one-dimensional thermodynamic sea ice model for investigating ice-atmosphere interactions. Journal of Geophysical Research, 98, 10085-10109.
Semtner A., (1987) A numerical study of sea ice and ocean circulation in the Arctic.. Journal of Physical Oceanography, 17, 1077-1099.
The ice velocities are computed using the Elastic-Viscous-Plastic dynamics of Hunke and Dukowitz (1997). The ice momentum and stress state equations are solved to balance the effect of wind stress, ocean currents, Coriolis term and the internal ice stresses while maintaining a viscous plastic ice rheology. Ice is allowed to flow across the North Pole despite the ocean model having a polar island. Ice is advected using a first-order upwind advection scheme. The mechanical redistribution (or ridging scheme) of the Los Alamos CICE model (Hunke and Lipscomb, 2004) is also included, which can convert thinner ice to thicker ice within a grid box.
Hunke E., and J. Dukowicz (1997) An elastic-viscous-plastic model for sea-ice dynamics. Journal of Physical Oceanography, 27, 1849-1867.
Hunke E. and Lipscomb W. (2004) CICE: The Los Alamos Sea Ice Model, documentation and software, version 3.1. LA-CC-98-16, Los Alamos Natl. Lab., U.S.A.
The zero-layer thermodynamics model of Semtner (1976) is used which is applied separately to each ice thickness category. There is a one snow layer on top of the ice in each category. Once the thermodynamic ice and snow growth rates have been calculated, the linear remapping scheme of Lipscomb (2001) is used to compute the transfer of ice between categories. Ocean to ice heat flux parameterisation uses the McPhee scheme that uses both the ocean-ice temperature difference and the friction velocity in the flux parameterisation (McPhee, 1992). The ocean-ice heat flux is proportional to the ice concentration for concentrations greater than 0.05. For lower concentrations, the grid box integral of the heat flux is independent of concentration to simulate the increased heat flux on small floes in the marginal ice zone.
Lipscomb W., (2001) Remapping the thickness distribution in sea ice models.. Journal of Geophysical Research, 106, 13989-14000
McPhee M. G., (1992) Turbulent heat-flux in the upper ocean under sea ice.. Journal of Geophysical Research-Oceans, American Geophysical Union, 97(C4), 5365-5379.
Semtner A., (1976) A model for the thermodynamic growth of sea ice in numerical investigations of climate.. Journal of Physical Oceanography, 6, 379-389
The HadGEM2-ES model was a two stage development from HadGEM1, representing improvements in the physical model (leading to HadGEM2-AO) and the addition of earth system components and coupling (leading to HadGEM2-ES).
Base Simulation of Ensemble3.1 Pre-industrial Control (1860 - 2360)
Impose non-evolving, pre-industrial conditions, which may include: Prescribed atmospheric concentrations of - all well mixed gases (including CO2) - some short-lived (reactive) species Prescribed non-evolving emissions or concentrations of - natural aerosols or their precursors - some short-lived (reactive) species Unperturbed land use.
Reference to a experiment called 3.1 piControl
Reference to a NumericalRequirement called 3.1.stc.1850_500yr in a experiment called 3.1 piControl
Reference to a NumericalRequirement called 3.1.ic in a experiment called 3.1 piControl
The resources(deployment) on which this simulation ran
Reference to a platform called IBM Power 6
Reference to a modelComponent called HadGEM2-ES
this is a parameter change introduced in the atmospheric model component
3.1 Pre-industrial Control (1860 - 2360)
Pre-Industrial coupled atmosphere/ocean control run. Imposes non-evolving pre-industrial conditions.
A description of initial condition for the control run.
Prescribed atmospheric concentrations of pre-industrial well mixed gases: excluding CO2
Prescribed atmospheric concentrations of pre-industrial well mixed gas: Carbon Dioxide
Prescribed atmospheric concentration of pre-industrial short lived (reactive) gas species
Prescribed concentrations or emissions of pre-industrial natural aerosols
Prescribed concentrations or emissions of pre-industrial natural aerosol precursors
Prescribed concentrations or emissions of pre-industrial short lived (reactive) aerosol species
Annual mean variations in total solar irradiance (TSI) are partitioned across six shortwave spectral bands (0.2-10μm) to estimate the associated spectral changes with TSI variations (Lean et al., 1995). With the changes across the spectral bands the Rayleigh scattering and ozone absorption properties are also varied. The TSI data used for the historic period were recommended by CMIP5 (Lean et al., 2009 -L09; SOLARIS 2009) and are created from reconstructions of solar cycle and background variations in TSI. The annual mean TSI was processed to force the mean of the 1700-2004 period to be the same as the model control solar constant value (1365 Wm-2).
Lean J.L, O.R White and A.Skumanich, 1995. On the solar ultraviolet spectral irradiance during the Maunder Minimum, Global Biogeochecmical Cycles, 9, 2, 171-182.
Met Office Unified Model 192-column 38-level Atmosphere Grid
The key features of the N96L38 grid configuration, which is utilised by the Met Office Hadley Centre HadGAM1 and HadGAM2 atmosphere models, are as follows. The grid is logically rectangular and dimensioned (i=192, j=145, k=38), where i = longitude, j = latitude, k = level. Horizontal grid cell size is 1.875 degrees in longitude by 1.25 degrees in latitude. Vertical levels are terrain-following for levels up to k=29 and constant thickness above that level. Grid staggering (i.e. between the sub-grids used by scalar (physics) and vector quantities) is based upon the Arakawa-C grid configuration. Array indexing in the latitude direction starts at the south pole. For an unrotated grid mesh, longitude (i=0.5) corresponds to Greenwich meridian. In the Unified Model, the north and south poles coincide with the bounding half-integer j planes, i.e. j=0.5 and j=M-0.5. P-points and u-points exist at the poles, but not v-points. At the poles all values of P variables are set equal. Likewise for scalar variables rho, theta, m, w.
Met Office Unified Model 192-column Atmosphere Grid Horizontal Coordinate System
The N96 Grid represents the 192-column horizontal coordinate system utilised within the Met Office Hadley Centre HadGAM1 and HadGAM2 atmosphere models. This grid defines the horizontal locations of the physics (P) variables computed by these atmosphere models. The locations of U variables are offset by one-half of a grid cell to the east of P grid locations. The locations of V variables are offset by one-half of a grid cell to the north of P grid locations.
Met Office Unified Model 360-column 40-level Ocean Grid
The key features of the N180L40 grid, which is utilised within the Met Office Hadley Centre HadGOM1 and HadGOM2 ocean models, are as follows: The grid is logically rectangular and dimensioned (i=360, j=216, k=40), where i = longitude, j = latitude, k = depth. The grid is characterised as regular rather than uniform since the Y dimension (latitude) is specified via a 1D vector rather than a constant scalar offset. Between the poles and 30 degrees N and S the horizontal grid cell size is 1.0 degree by 1.0 degree. In the tropical region, however, the grid cell size in the Y/latitude directon decreases (and resolution increases) smoothly from 1 degree at 30 N/S to approximately 1/3 degree at the equator. Grid staggering in the HadGOM1/HadGOM2 ocean grid is based upon the Arakawa-B grid scheme. In this scheme, u and v variables are co-located at the corners points of the grid cells. T and w variables coincide in the horizontal (at the grid cell centres) but are offset from each other by one-half a grid cell in the vertical. In physical space, T/w points are exactly halfway in latitude between u/v points, but the converse does not hold true. In the vertical, T points are exactly halfway in depth between w points, but again the converse does not hold true.
Met Office Unified Model 360-column Ocean Grid Horizontal Coordinate System
Between the poles and 30 degrees N and S the horizontal grid cell size is 1.0 degree by 1.0 degree. In the tropical region, however, the grid cell size in the Y/latitude direction decreases (and resolution increases) smoothly from 1 degree at 30 N/S to approximately 1/3 degree at the equator.