Model Output Overview
- Introduction
- File Format: NetCDF
- Directory Structure & Data Organization
- Calendar Used In Experiments
- Using ts_catalog Files To Find Variables
Model Components
Control & Transient Forcing Experiments
References & Citations
If you have comments or questions regarding these GFDL R30 Coupled Climate Model webpages please e-mail Keith.Dixon @ noaa.gov
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GFDL R30 COUPLED CLIMATE MODELS:
An introduction to the R30 coupled model output available on GFDL's NOMADS Server
http://nomads.gfdl.noaa.gov/nomads/forms/climate.html
A Guide To Accessing R30 Model Output Stored On GFDL's NOMADS Server for DecCen Climate Research
GFDL R30 COUPLED MODEL COMPONENTS
Many elements of the model descriptions found on this page are taken from two manuscripts. To download copies of these manuscripts, go to our REFERENCES &CITATIONS web page.
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Atmospheric GCM
The atmospheric model component of the GFDL R30 coupled model solves the primitive equations on a sphere using a spectral transform method. Fields of horizontal variables are represented by a truncated series of spherical harmonics and grid point values, with zonal truncation at wavenumber 30 (Rhomboidal 30 truncation, abbreviated as R30). The atmospheric model data found on this server is on the transform grid that has grid point spacing of 3.75 longitude by ~2.25 latitude (a 96x80 global grid). In the vertical, a finite difference scheme is used in conjunction with a sigma coordinate system, where sigma = p/p* is the vertical coordinate, and p* is the surface pressure. There are 14 unevenly spaced levels extending from sigma=0.9965 near the surface to sigma=0.015. The model uses a filtered orography (Lindberg and Broccoli, 1996) to alleviate unrealistic small-scale features associated with using a finite number of spherical harmonics in the spectral representation of topography.
The grid information is contained in the netCDF files. Executing an "ncdump -c" command using an atmospheric model netCDF file as input will list the grid information and other data file documentation. The 96 "longitude" points (0.0 to 356.25 by 3.75 degree increments) will be listed as will the 80 "latitude" rows (88.28838S to -88.28838N, unevenly spaced according to the Gaussian grid of the R30 spectral model). The model's 14 full sigma levels are listed as "sigma_full" (0.015 to 0.99665) and the 13 half-levels are listed as "sigma_half" (0.03 to 0.9933). A netCDF file's "sigma_ts" variable documents the level of the atmospheric model data found in that particular file. The 2-D field "LSEA" gives the model's land-sea mask on the atmospheric model's transform grid [0=land, 1=sea]. The 2-D field "ZSTAR" gives the elevation of the surface [cm].
A relative humidity based cloud prediction scheme and moist convective adjustment (Manabe et al., 1965) are used in the atmospheric model. Clouds are predicted whenever the relative humidity exceeds a critical threshold which varies with height (from 100% near the surface to 90% in the upper atmosphere). Where predicted, clouds are assumed to fill the grid box. Precipitation occurs when water vapor supersaturation is simulated, falling as snow when the temperature of the lower atmosphere is below freezing.
A seasonal cycle of insolation is prescribed at the top of the atmosphere, with a solar constant of 1365 W m-2. For the sake of simplicity and computational speed, no diurnal cycle of insolation is included in the model. The solar constant does not vary during these model integrations.
The effects of clouds, water vapor, cardon dioxide and ozone are included in the calculation of solar and terrestrial radiation. Ozone is specified as a function of latitude, height, and season based on observations. The mixing ratio of carbon dioxide is assumed to be uniform throughout the atmosphere. Carbon dioxide levels in the model are use to represent the well mixed greenhouse gases CO2, CH4, N20 and halocarbons. Thus, these greenhouse gases can be thought of as being represented by an "effective CO2" level in this model.
Land Surface Model
The land model component's horizontal grid matches the atmospheric model's transform grid. At the atmosphere-land surface interface, the sum of the shortwave, longwave, latent, and land-air sensible heat fluxes are assumed to be zero (i.e., the soil has no heat storage capability, and the land surface temperature is determined diagnostically).
A simple bucket hydrology scheme is used for soil moisture calculations, with a uniform field capacity of 0.15 cm of liquid water assumed (Manabe, 1969). At each land surface grid box a budget is computed in which precipitation and snow melt are inputs of water to the 15 cm bucket. Evaporation and sublimation remove moisture from the bucket. Evaporation is calculated as the product of the potential evaporation from a surface saturated at the local surface temperature and pressure and a evapotranspiration efficiency factor. Over land, the evaporation efficiency is given by the ratio of local soil moisture to a critical value that is 75 percent of field capacity, and is set to unity if soil moisture exceeds this critical value (Milly, 1992).
When the soil moisture is at field capacity, excess water added via precipitation or snowmelt is converted to runoff. Runoff from land points is instantaneously routed to the ocean following observed river drainage basins. Similarly, when the model-predicted snow depth exceeds a specified value at a grid point (0.1 m water equivalent), the excess snow is routed to the ocean as runoff. The water thus transported to the ocean as runoff changes oceanic salinity.
Ocean GCM
The z-coordinate ocean model used in the GFDL R30 coupled model solves the primitive equation of motion with the use of the Boussinesq, rigid-lid, and hydrostatic approximations, and is based upon the GFDL Modular Ocean Model version 1 code (Pacanowski et al., 1990). The Cox (1987) implementation of sub-grid scale isopycnal tracer diffusion is used here.
The ocean model component has the same number of latitude rows as the atmospheric model to which it is coupled and twice as many grid points in the zonal direction (i.e., a 192x80 grid for the R30 ocean model component). There are 18 unevenly spaced vertical levels in the R30 ocean model.
The ocean grid information is contained in the netCDF files. Note that the ocean model used the staggered B-grid arrangement, in which the internal model velocity components are defined at points one-half of a grid cel to the northeast of the points where tracer (e.g., potential temperature, salinity) and barotropic stream function variables are defined. Vertical velocities are defined at the top and bottom of model levels, while tracers and internal model velocities are defined at the model level midpoints.
Executing an "ncdump -c" command using an atmospheric model netCDF file as input will list the grid information and other data file documentation. Two sets of 192 longitude points (each separated by 1.875 degrees) will be listed ("longitude_ts" and "longitude_uv"). Similarly, two sets of latitude rows "latitude_ts" and "latitude_uv" will be listed. The 80 "latitude_ts" values match those from the R30 atmospheric model. Tracer and internal velocites cware defined on the "depth_zt" vertical axis. Vertical velocities are defined on the "depth_zw" axis. A netCDF file's "depth_zt1" variable documents the vertical level of the ocean model data found in that particular file.
The 2-D field "FKMT" gives the model bathymetry on the tracer grid, listed as the integer number of model levels. The 2-D field "FKMU" gives the model bathymetry on the uv grid, determined as the minimum of the four surrounding tracer point depths.
The relatively narrow waterways that connect the Mediterranean Sea and Hudson Bay to the Atlantic Ocean are not resolved by the ocean model grid. To account for these unresolved connections, a scheme is used that mixes potential temperature, salinity and any other tracers between specified non-adjacent water columns in a conservative manner (momentum is not mixed). The volume rate of mixing and the depth range are specified and time invariant.
The Bering Strait is open in the model, allowing water to flow between the Pacific and Arctic Oceans. However, since the Americas are not treated as an island in the computation of the barotropic streamfunction, the net volume of water exchanged through the Bering Strait is always zero, although non-zero advective fluxes of heat and salinity do occur between the Pacific and Arctic basins.
Sea Ice Model
The GFDL R30 coupled model uses a relatively simple sea ice model that neglects the internal pressure of the sea ice. The sea ice model's lineage can be traced back to the model developed by Bryan (1969), although modifications have been made over time, several of which are noted in Manabe et al. (1990). The horizontal grid spacing of the sea ice model matches that of the underlying ocean model. At each grid point, average sea ice thickness is predicted, but not fractional coverage (i.e., the grid cell is either considered completely ice free or entirely covered by ice of some model-predicted thickness). Thus, the model considers a single ice layer, and leads are not included in the sea ice model.
Sea ice moves freely with the ocean currents, provided the ice thickness is less than four meters. Additional convergence of sea ice is not permitted at grid points where the thickness exceeds four meters.
Fluxes involving the latent heat of fusion are conserved within the coupled model system during freezing and melting. Also, surface water freshening and brine rejection occur in a conservative manner as sea ice melts or forms. Any snow that falls upon pre-existing sea ice is instantly converted into sea ice. The sea ice has no sensible heat content.
Following Broccoli and Manabe (1987), the albedo of sea ice depends on surface temperature and thickness. For thick ice (at least 1 m thick), the surface albedo is 80% if the surface temperature is below -10C and 55% at 0C, with a linear interpolation between these values for intermediate temperatures. If the ice thickness is less than 1 m, the albedo decreases with a square root function of ice thickness from the thick ice values to the albedo of the underlying water surface.
Flux Exchange Among Model Components
Fluxes of momentum, heat, and freshwater between the model's atmosphere-land component and the ocean-sea ice component are computed and exchanged once per day. The heat flux exchanged is the sum of the radiative, sensible, and latent components. The water flux consists of evaporation, sublimation, precipitation, and runoff from the continents. The runoff from a set of land points defining a river drainage basin is deposited into the ocean at the uppermost level of the ocean grid box corresponding to the "mouth" of the river. In a few cases where the river outflow is quite large (e.g., the Amazon), the "river" flow is instantaneously spread across several adjacent grid boxes in the horizontal, and the top two grid boxes in the vertical. This reduces the likelihood of numerical instabilities associated with large gradients of salinity in the ocean model component.
To minimize climate drift, seasonally and spatially varying heat and freshwater fluxes are added to the ocean surface. As described in Manabe et al. (1991), these flux adjustments are determined before the coupled model is initialized, and they do not vary interannually.
This is a documentation file for R30 coupled model output available on GFDL's NOMADS Server http://nomads.gfdl.noaa.gov/dods-data/DecCen/r30/coupled/">