# Vertical Coordinates

Climate models use a variety of vertical coordinate systems, here are the values for the models commonly in use in the Centre

## UM Vertical Coordinates

The UM uses a hybrid height coordinate system, it is terrain following up to a specified height, above which it is regular height levels.

The Iris library can automatically work out altitude values from a UM file, provided it has access to orography.

The full definition of UM height levels can be found in Appendix A of UMDP F03

Z(i, j, k) = Zsea(k) + C(k) * orography(i, j) Zsea(k) = eta(k) * top_level_height C(k) = (1 - eta(k) / eta(first_constant_r_rho_level))**2 # for k <= first_constant_r_rho_level = 0 # for k > first_constant_r_rho_level

The values of `eta(k)`

, `top_level_height`

and `first_constant_rho_level`

can be found in the UM vertical coordinate namelist, standard namelists are found on Raijin under `/projects/access/umdir/vn${VN}/ctldata/vert/`

. There are separate `eta`

values for variables defined on theta or rho levels.

The values of Zsea and C are also available in the LOOKUP header of UM output files, as values `blev`

and `bhlev`

respectively. These header values can be accessed using Mule

## WRF Vertical Coordinates

WRF uses a hybrid pressure coordinate system

eta = (Ph - Pht)/(Phs - Pht)

with `Ph`

the hydrostatic pressure, and `Pht`

and `Phs`

the values at the topmost and surface levels respectively