GENIE module intros

The IGCM Atmosphere model
IGCM stands for Intermediate General Circulation Model. It was developed mainly in the Meteorology Department at the University of Reading. It is a spectral primitive equation model - the 'intermediate' refers to the physics, rather than the dynamics, which are similar to those found in a full GCM, such as the Hadley centre model. It can technically be run at any resolution, but the default is for a triangular truncation of 42 spectral coefficients - T42. Physical parameterisations include the Betts-Miller or Tiedtke convection schemes; a detailed description of the model can be found in Forster et al, 1998.

The IGCM communicates to genie.F via a number of subroutines. Firstly there is an initialisation step (initialise_atmos). Then there is the main timestep loop of default 1 hour, which starts with the adiabatic step (igcm3_adiab). Then follows a mini-loop of default 10 minutes, which includes calls to the land surface scheme (igcm_land_surflux), the ocean surface scheme (igcm_ocean_surflux), and boundary layer schemes (igcm_land_blayer and igcm_ocean_blayer). Then, outside of the mini-loop is a call to the diabatic physical parameterisations (igcm_diab).

When run with the Goldstein seaice, the ocean surface scheme is replaced by a call to surf_ocn_sic.

When run with MOSES land, the land surface scheme is replaced by a call to genie_land.

When run with the goldstein ocean, there are also calls to a spatial interpolation routine - interp_ocn_atm, if goldstein is running at a different resolution to the IGCM.

The EMBM Atmosphere model
EMBM stands for Energy-Moisture Balance Model. The EMBM represents the atmosphere as a single 2-D layer with an advective-diffusive transport scheme for heat and moisture. The prognostic variables are surface air temperature and surface specific humidity. Planetary albedo and the wind fields for advective transports are prescribed, while transport parameters have been calibrated using data assimilation techniques. Continental precipitation is returned instantaneously to coastal ocean points via a runoff map. Outgoing longwave radiation is parameterised by an empirical polynomial function, with the addition of a greenhouse warming term, for anthropogenically forced experiments, which is proportional to the log of increase in carbon dioxide concentration (relative to a reference level). An implicit numerical scheme is used to allow long EMBM timesteps.

The GOLDSTEIN Ocean model
GOLDSTEIN stands for Global Ocean Linear Drag Salt and Temperature Equation Integrator. GOLDSTEIN comprises a reduced physics (frictional geostrophic) 3-D ocean model configured in realistic global geometry, including topography and multiple islands. The prognostic variables are temperature and salinity. GOLDSTEIN includes an isoneutral and eddy-induced mixing scheme, an efficient convection scheme, and spatially variable drag. Unlike primitive-equation ocean models, momentum advection and acceleration terms are neglected in the equation of motion, allowing the use of timesteps which are long relative to those generally used in 3-D ocean models.

The Slab Ocean model
This module is a very simple representation of the ocean. It evolves the surface temperature based on the flux balance at the surface. It has no interaction with the hydrological cycle. It can be run with pre-calculated implied ocean heat fluxes.

The Fixed Ocean model
This simply outputs surface ocean temperature based on values read in from a file.

The Dynamic-Thermodynamic Sea Ice model
The model represents sea ice as a single 2-D layer. The prognostic variables are ice thickness and ice areal fraction. Sea-ice temperature is a diagnostic variable. Sea ice grows in extent and thickness, or melts, according to net heat flux into the ice from ocean and atmosphere. A net freshwater flux due to freezing and melting is also included. Thickness and extent are further modified by a combination of advection by ocean currents at the uppermost level and lateral diffusion. An implicit numerical scheme is used to allow long sea-ice model timesteps.

For further details of these three GENIE components see: Edwards, N. R. and R. Marsh (2005). Uncertainties due to transport-parameter sensitivity in an efficient 3-D ocean-climate model. Climate Dynamics, 24, 415-433.

The Slab Sea Ice model
This module is a very simple representation of seaice. It evolves the surface temperature based on the flux balance at the surface. It has no interaction with the hydrological cycle. It can be run with pre-calculated implied ocean heat fluxes.

The Fixed Sea Ice model
This simply outputs surface seaice temperature based on values read in from a file.

The Efficient Numerical Terrestrial Scheme (ENTS)
ENTS is a dynamic vegetation model with a single class of vegetation. Dynamic variables are vegetation and soil carbon, which are linked via leaf litter, and depend functionally on atmospheric light, carbon dioxide, and temperature levels, and on carbon content and available soil water, specified via a 'leaky bucket' scheme. Vegetation fractional cover is a function of vegetation carbon. Land albedo depends on vegetation fraction, soil carbon and snow cover. Spatial discretization is matched to the atmosphere model.

Land and vegetation model: GENIE-land
The influence of the land-surface in the Earth system is significant over a wide range of temporal and spatial scales. Land interacts closely with the atmosphere through its control of their exchange of energy (radiation and heat) and mass (principally water and carbon). Energy gained by the land-surface from absorbed radiation is partitioned into fluxes of heat and moisture to the overlying atmosphere. The processes governing this partitioning locally are complex functions of surface structure (albedo, surface roughness, vegetation type and coverage) and environment (solar radiation, air temperature and humidity, sub-surface water availability and atmospheric CO2), and the interaction with the atmosphere is complicated further by surface heterogeneity.

Over timescales of one year and longer land-atmosphere exchange of carbon dioxide becomes important in the Earth system through its effects on radiative forcing and surface structure. Carbon uptake by land occurs primarily through vegetation photosynthesis, which is linked closely with surface moisture exchange through plant transpiration. Changes in the land carbon reservoir occur through plant growth and spread, turnover of plant mass to the soil, and respiration of soil carbon to the atmosphere, each of which are functions of climate.

Interaction of land with the oceans is less intimate, but significant nonetheless. Principally, the routing of runoff by river basins aggregates the effects of local changes in land-atmosphere and ice sheet moisture budgets and concentrates them at particular points along land-ocean boundaries. The magnitude and location of these freshwater fluxes are thought to have potentially profound effects on global ocean circulation, notably fluxes into the Atlantic and Arctic oceans associated with Heinrich events during the last deglaciation.

GENIE-land is a simplified version of the Met Office land surface scheme (MOSES2) including the full dynamic vegetation model TRIFFID. Computational expense is reduced by omitting the complex boundary-layer description of MOSES2 - necessary for fine-resolution weather forecasts - and using only a single soil layer. For each land gridbox the surface energy partition is calculated for seven tile types (five vegetation, bare soil and land-ice) under the same gridbox mean atmospheric forcing. Surface heterogeneity within each gridbox is accounted for by aggregating the fluxes from each tile type weighted by the tile fractional coverages. For vegetation tiles, a `big-leaf' approach is used, with leaf-level fluxes scaled to canopy-level as a simple function of leaf area index assuming Beer's law of light attenuation through the canopy. GENIE-land calculates the net photosynthesis of each vegetation type (broad- and needle-leaf trees, C3 and C4 grasses, and shrubs) on the same short timestep used for the surface energy balance. The time aggregated carbon fluxes are used by TRIFFID to update vegetation mass and fractional coverage over a longer timestep of typically one year, with Lotka-Voltera based competition between species for fractional coverage. Soil carbon comprises a single pool for each gridbox which is updated by TRIFFID on the long (e.g. annual) timestep. Runoff routing is applied from a pre-calculated mask, with the default mask based on present-day observed river destinations.

Land ice model: GLIMMER
Ice sheets play an important role in the of the climate system, especially over the timescale of a glacial cycle. Numerous feedbacks operate between the ice and the rest of the climate system, notably though albedo changes, ice elevation evolution and the fresh water budget. Ice sheet modeling poses a horizontal resolution challenge for Earth system modeling. Important physical processes that determine the evolution of the ice sheet tend to occur in small regions --- examples of this are the melting of ice in a narrow margin at the edge of an ice sheet, and the formation of mountain glaciers, which can potentially lead to large-scale glacial inception. In both cases, the scales involved are in the 10s of km.

Traditionally, ice sheet modeling in an earth-system context has been done on a global latitude-longitude grid. However, with this approach, it is hard to generate the required resolution, as well as being potentially inefficient (since only limited regions are generally ice-covered), especially if a uniform resolution is used. To address this issue, we have coupled a regional ice model (GLIMMER) into the GENIE framework.

GLIMMER is based on a stand-alone ice sheet model, written by Tony Payne (see Payne (1999)). It uses a projected grid scalable over 20-100+ km grid cells. The time steps (years to decades) are longer than other components. Different levels of physics are available; mass balance calculations using different day-degree calculation schemes; coupled ice flow, thermodynamics and ice-thickness evolution; and isostasy. Isostasy along with a simple parameterization of sliding is necessary for Ice Age simulations and, in particular, work on Heinrich events. The code uses quasi-object-oriented techniques in Fortran 95 to allow the model to be run as multiple instances over different regions of the globe. Thus the individual ice sheets of the world (Greenland, Antarctica, Laurentide, Scandinavian etc) can be modeled on separate, local grids and the need for high-resolution global coverage is avoided.

The GLIMMER homepage is http://glimmer.forge.nesc.ac.uk/

More detailed information on running GLIMMER can be found on the project wiki: http://wiki.nesc.ac.uk/read/glimmer-project

Fixedicesheet model
This provides the orography and icesheet fraction to genie. The values are read in from a file.

Atmospheric chemistry model: GENIE-atchem
The atmospheric reservoirs of trace gases such as carbon and nitrogen compounds (notably CO2, CH4 and N2O) are small relative to the ocean and land reservoirs, but their effect on radiative forcing in the Earth system cannot be neglected. The direct exchange of such compounds between the land and ocean is extremely weak making the atmosphere and important chemical intermediary between them. The atmospheric reservoir is not passive, however, as chemical and photochemical processes within the atmosphere governing the lifetime of each species. It is desirable for an Earth system model to represent as many of these processes as possible.

GENIE-atchem provides an embryonic model of atmospheric chemistry for GENIE. At present it is extremely simple, providing simply a method for other model components to update atmospheric CO2, CH4 and N2O and conserve these compounds over the Earth system. The model reservoirs are defined on the atmospheric latitude-longitude grid with a single vertical level. There is no advection, but there is the option for the compound to be well-mixed (globally uniform concentrations). The intention is that this component is written in a way that will make it relatively simple to add processes and chemical species as GENIE develops.

Fixedchem
This module simply outputs concentrations of various gases to genie.f, based on values read in from a file or a namelist.