Variables over Orography in CAM4

[e_middlemas@rsmas_miami_edu]

I am wondering how surface through lower-level variables are computed in CAM4 runs with prescribed SST (with a F_2000_CN compset).  I am specifically looking at a CAM4 run in which the SSTs are the climatology from a pi-ctrl fully-coupled CCSM4 run.  In the CCSM4 run, the 850-mb zonal winds are undefined over mountains, which makes sense when the mountains are tall enough to exceed the 850 mb pressure level.  In the CAM4 run, though, winds are defined everywhere down to the surface.  How and why is this?Thanks for any help or ideas!EleanorAttached: 850mb Zonal winds averaged over course of run both in CAM4-fixed SST run and in pictrl CCSM4 

 

[eaton]

This appears to be a post processing issue.  The CAM4 model uses
hybrid vertical coordinates which are terrain following near the
surface, pure pressure at the model top, and merged in
between (as described in the scientific description document).
This is completely independent of whether CAM4 is running with
prescribed SSTs (as an F compset) or with an active ocean
model (as a B compset).  The model output for the 3D fields is on
the hybrid vertical coordinates.  If the 3D field is interpolated
to a fixed pressure surface by software that puts fill values in
locations that are below the surface then the plot will show
where the mountains are (e.g. the 'CCSM4' plot).  On the other
hand, there is an output field produced by CAM called 'U850'
which is interpolated internally to 850mb before being output as
a spatially 2D field.  The interpolation code is just using the
surface value at locations where the surface pressure exceeds
850mb (as in the 'CAM4' plot).  This is very old code which
should be updated to make use of fill values.