# UKCA top boundary **URL:** \ **Category:** Unified Model\ **Created:** [11 March 2026 16:08 UTC](https://cms-helpdesk.ncas.ac.uk/t/ukca-top-boundary/2031 "2026-03-11T16:08:24Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![hannahbryant](https://avatars.discourse-cdn.com/v4/letter/h/b5a626/32.png) [@hannahbryant](https://cms-helpdesk.ncas.ac.uk/u/hannahbryant)\ **Post date:** [11 March 2026 16:08 UTC](https://cms-helpdesk.ncas.ac.uk/t/ukca-top-boundary/2031/1 "2026-03-11T16:08:24Z")
Hi! 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](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? Thanks, Hannah ---
**Author:** ![Luke](https://dub1.discourse-cdn.com/flex013/user_avatar/cms-helpdesk.ncas.ac.uk/luke/32/45_2.png) [@Luke](https://cms-helpdesk.ncas.ac.uk/u/Luke)\ **Post date:** [11 March 2026 16:57 UTC](https://cms-helpdesk.ncas.ac.uk/t/ukca-top-boundary/2031/2 "2026-03-11T16:57:02Z")
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](https://code.metoffice.gov.uk/trac/UKESM/ticket/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.’. ---
**Author:** ![system](https://europe1.discourse-cdn.com/flex013/uploads/cms_support/original/1X/1fd2411499ffcbc299fe756cd5cdf26e44956558.png) [@system](https://cms-helpdesk.ncas.ac.uk/u/system)\ **Post date:** [10 April 2026 16:57 UTC](https://cms-helpdesk.ncas.ac.uk/t/ukca-top-boundary/2031/3 "2026-04-10T16:57:54Z")
This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.