I was hoping to ask a clarification of i_ukca_topboundary (at vn12.0). I am running with this set to 2 (rewrite all species at the top level with the level below, following: https://code.metoffice.gov.uk/trac/um/wiki/ticket/5700/TicketDetails. I have the h2o_feedback switched off (and methox switched on) and this combination seems to be fine, however I can see that there is a fail-if warning for ukca_topboundary == 4 and the h2o_feedback false, with the description: “fail-if=this==4 and namelist:run_ukca=l_ukca_h2o_feedback==‘.false.’ =# Cannot impose top-boundary for H2O if there is no H2O feedback”. Is there a reason that it is ok to impose the h2o top boundary without the h2o feedback for option 2 top boundary but not for option 4?
Good question! As I wrote this change I’m thinking back as to the reasons here.
H2O (q) is advected outside of the UKCA tracers, and option 1 explicitly doesn’t consider q. Option 4 won’t work if there is no feedback as you then can’t set the top bounday for q as UKCA doesn’t have any control over it if the feedback is turned off.
UKESM:#729 (which I also wrote) says that q currently has no top boundary at all.
In UKESM though this feedback is turned on by default, so option 2 would make sense.
I suspect that what is happening is that UKCA is attempting to put this top boundary condition on q, but it isn’t being fed back. In your case q won’t have a top boundary condition at all (which is how it usually works). There probably needs to be some text to say that “if l_ukca_h2o_feedback==‘.false.’ then H2O will not have a boundary condition”.
The reason that there is a fail with option 4 is that this explicitly wants to overwrite H2O, but this won’t work if l_ukca_h2o_feedback==‘.false.’.