My lab is new to using EQUATES data. We are using EQUATES concentrations but have been confused about the vertical height over which concentrations are averaged for a given grid cell. We know that the model uses 35 layers but couldn’t find the heights for the layers.
Any insight would be helpful. Thanks!
The vertical levels in Equates are
VGLVLS = 1.f, 0.9975f, 0.995f, 0.99f, 0.985f, 0.98f, 0.97f, 0.96f, 0.95f, 0.94f, 0.93f, 0.92f, 0.91f, 0.9f, 0.88f, 0.86f, 0.84f, 0.82f, 0.8f, 0.77f, 0.74f, 0.7f, 0.65f, 0.6f, 0.55f, 0.5f, 0.45f, 0.4f, 0.35f, 0.3f, 0.25f, 0.2f, 0.15f, 0.1f, 0.05f, 0.f;
You can get this by doing ncdump on any METCRO3D file.
ncdump -h METCRO3D_20170412.nc (name of the file is just for example)
and look for VGVLS row. You will find the information.
These are sigma level or terrain following or eta levels defined in WRF.
Documentation regarding this is in the following link: -
The WRF simulations performed for EQUATES used the hybrid vertical coordinate system. Therefore, the VGLVLS attribute in the METCRO3D files (or other EQUATES 3D files) do NOT allow you to compute pressure levels at the layer interfaces using surface pressure and the pressure at the top (ptop), as one could have done if the older terrain-following “sigma” coordinate system had been used.
Moreover, in the non-hydrostatic framework used by WRF, layer heights vary in space and time and cannot easily be computed only from VGLVLS and other time-invariant attributes.
To obtain the space-and-time-varying layer heights, you need to read the ZH (heights at the layer midpoint) or ZF (heights at the top of each layer) variables from METCRO3D for each day. The approximate height at the top of the first layer for the EQUATES setup is 20 meters, but this can vary by several meters over the course of a year and across the 12US1 domain.
I was looking for an image of the hybrid hybrid sigma-pressure vertical levels, and found this. I need to update the existing CMAQ training to reflect the use of ZH (heights at the layer midpoint) or ZF (heights at the top of each layer) variables from METCRO3D.
This thread provides some additional discussion on the hybrid vertical coordinate system that is the default setting in WRF since version 4.0 and its implications for the use of the VGLVLS attribute in downstream programs.