Atmospheric Model

Atmospheric Model

Illustration of all processes and physical model elements that are parameterized in numerical weather prediction models. Includes 20 different items, such as topography, deep convection, longwave radiation absorption and emission, microphysical processes, land surface processes and land use types, soil and vegetation processes, snow/water/ice at the earth surface, atmospheric radiation transfer, etc.

The atmospheric component of the climate model is very similar to a numerical weather prediction model. The model solves for both resolved processes, like motion, and parameterized processes, like cloud physics.

The model numerically solves the equations of motion at each grid point. In other words, the model directly simulates motion in the atmosphere between grid layers by balancing the pressure gradient force, Coriolis effect, and forces resulting from curvature in the flow.

There are also parameterized dynamics for aspects of atmospheric flow too small to resolve, such as gravity wave drag. We know from theory and observations that mountains generate gravity waves that propagate up into the atmosphere and transport momentum from the surface up into the atmosphere. This affects elements of the larger-scale flow such as the placement of the jet stream and storm tracks and the location and strength of planetary scale atmospheric waves.

Most physical processes must be parameterized. These include the following:

  • Radiative transfer, or how sunlight and long-wave radiation propagate up and down through the atmosphere.
  • Processes related to cloud formation resulting from both large-scale (resolved) lifting and from sub-grid scale convective processes. Traditionally, these processes have been hardest to parameterize.
  • Boundary layer and surface exchange processes that occur on scales much smaller than a typical grid box.
  • Dissipation of kinetic energy or momentum away from the boundary layer.

None of these processes are unique to climate models. Every weather forecast model includes them, too. If you look "under the hood" of any weather model, this is what you're going to find.