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