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 Gadi under ~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