Model Vertical Coordinates and Levels and Nesting

Model Vertical Coordinates

• Earliest models were in z or p. – A problem near terrain (complex boundary conditions

where model level intersected mountains)• Next innovation were terrain-following

geometries…sigma coordinates. Sigma p and z. Still heavily used today.

• Also used: potential temperature (theta coordinates, eta coordinates—like p) and hybrids (such as sigma-p/theta)

Model Levels

• Modern models generally have been 35 and 100 levels.

• Generally NOT uniform in height.• Virtually all have many model levels near the

surface to define the PBL.• Many have higher density of levels near the

tropopause as well.

ECMWF Levels

Nesting

• If one has lots of computer power and a global model, no worry about horizontal boundary conditions or nesting

• But if want to run at high resolution over a limited domain, can use nesting—running a separate model domain at high resolution embedded in a lower-resolution simulation.

• Can have one way or two way nesting.

Next Generation Models May Not Have Separate Nests

Problem: What kind of physics parameterization can handle multiple

resolutions at SAME TIME?