Vertical Resolutions (Levels)

In a similar way to the horizontal grid, the vertical profile of the atmosphere is divided into a number of different levels. The model used in has 19 vertical levels in the atmosphere (and 20 in the ocean). Unlike the horizontal grid, the vertical grid is not evenly spaced. They’re not even equally spaced in pressure, which could make sense as, for example, the 950 hPa (near the surface) and 900hPa levels (a bit further up) have the same mass of air between them as the 100hPa and 50hPa levels, even though the physical distance between them is much less. This is because the density of air decreases exponentially with distance from the Earth’s surface: the difference in pressure between the top of Everest (about 10km up) and about 9km up is much smaller than the difference in pressure between sea level and 1km up.

The horizontal and vertical structure of the HadCM3 climate model.
Source: Met Office

The levels are unevenly spaced in terms of pressure so that they can be concentrated in the areas, i.e. near the surface, where we are more interested in knowing what is going on than at other levels. The model levels take into account what the surface is doing; so a level doesn’t suddenly vanish as it intersects a mountain! The top level is at about 30km; in the middle of the stratosphere.