365 Days of Climate Awareness 320 – General Circulation Models 1: Introduction

A fluid is any state of matter where the atoms or molecules do not stay in fixed spatial relationships with their neighbors. This includes liquid, gaseous and plasma states. We’re not worried about plasma because that occurs only at extremely high temperatures, like in the sun. On earth we deal with liquid and gas. Fluid dynamics deals with the motion and energy states of any fluid, and computational fluid dynamics is the computerized version of this. Geophysical fluid dynamics is a specific sub-branch of fluid dynamics.

Hydrodynamic model of a ship's hull moving through the water.

There are many different types of fluid dynamics models, including within earth sciences! Testing and computer modeling are used in conjunction to analyze the performance of machines and of natural systems, including rivers, ship hulls airplane wings, and many, many more. But to be a properly geophysical in nature, a flow of air or water must meet three conditions:1)       

Aerodynamic model of a plane wing.

Fluid dyamic model (but not geophysical) of a channel and lagoon.

 1) It occurs on a sphere (or, more properly, an oblate spheroid, since the earth bulges a little around the equator);

2)         2) The sphere is rotating, which affects the fluid’s motion;

3)        3) The aspect ratio of the fluid is extremely thin. For example, the Atlantic ocean is roughly 4 km deep and 4,000 km across, for a very approximate aspect ratio of 1:1000, not unlike that of the edge of a piece of paper.

The planet's rotation, with different velocities at different latitudes.

Approximate aspect ratio of the Atlantic Ocean (with earth's curvature included).

These three conditions impose severe limitations on the types of motion the fluid is likely to undergo, with resulting effects on the equations (not touching those here! And it’s been years since I’ve tried to knock any rust off my math skills). The main effects of these conditions is to limit the ocean and atmosphere—we’re not going to worry about the mantle or core, which are separate cases—to mostly stratified, horizontal motions, with limited convection and a strong Coriolis (rotational) component. This branch is the domain of climate modelers.

Tomorrow: General circulation models 2: basic components of flow

Be brave, be steadfast, and be well.