Model Vertical Coordinates and Levels and Nesting

1. Model Vertical Coordinates and Levels and Nesting

2. 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)

4. 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.

5. ECMWF Levels

6. 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.

9. Next Generation Models May Not Have Separate Nests

10. Problem: What kind of physics parameterization can handle multiple resolutions at SAME TIME?