Dynamics and Physics
The GISS Earth System Models are well-documented over a development timeframe, which spans more than three decades (original model paper, Hansen et al., 1983). The current physics and dynamics of the GISS ModelE2 are predominantly based on those of GISS ModelE (Schmidt et al., 2006) and GISS Model II’ before it (Hansen et al. 2002, and references therein). Principal prognostic variables in the atmosphere are potential temperature, the water vapor mixing ratio, as well as the horizontal wind components. Virtual potential temperature is used for all density/buoyancy-related calculations.
The model has a Cartesian gridpoint formulation for with a standard horizontal resolution of 2° X 2.5° latitude by longitude (although 4° X 5° and 8° X 10° resolutions are also available for historical comparison to previous model experiments and for educational uses). The effective nominal resolution for tracer transports is significantly greater, however, because nine higher-order moments are carried along with the mean tracer values in each grid cell. Velocity points in the atmosphere are on the Arakawa-B grid and sigma coordinates are used in the vertical up to 150 hPa, with constant pressure layers above. The standard vertical resolution has 40 layers and a model top at 0.1 hPa.
Four surface types are defined including, open water (lakes and oceans), ice-covered water, ground (bare soil and vegetation), and glaciers. The model uses a 30 minute time step for all physics calculations and the radiation code is called every five physics time steps. The radiation code accounts for the radiatively important trace gases, including CO2, CH4, N2O, CFCs, and ozone, all of which are prescribed, but which can be updated as the model runs to incorporate trends. The model also includes treatments for both tropospheric and stratospheric aerosols and can be run with a climate-influenced gravity wave drag scheme that separately calculates the effects of gravity waves arising from mountain drag, penetrating convection, shear, and deformation.