[RegCNET] Max cloud fraction

[Travis O'Brien]

Dear RegCNET Colleagues,

Alex and I followed up on this thread offline, and we realized that both fcmax
and cftotmax must be set to 1 in the namelist in order for the maximum
cloud fraction to reach 1.

The fcmax variable is the maximum volume of a grid cell occupied by cloud
(i.e., it is 3D), whereas cftotmax is the maximum area of a column occupied
by cloud (as far as radiation is concerned; it is 2D).  Both of these
variables need to be set because of the cloud overlap assumption that RegCM
4.3 uses:

totcf(n) =  d_half * ( totcf(n) + maxval(cld(n,:)) )  [line 920 of
mod_rad_radiation.F90 in RegCM 4.3.5.6]

The RegCM 4.3/4.4 reference manuals note that this assumption is used.
Users of previous RegCM versions should keep this change in mind, if they
aren't aware of it already, since it differs from the overlap assumption
used in previous version of RegCM.

With Kind Regards,
-Travis-


On Tue, Jun 3, 2014 at 2:13 PM, Alexander Bryan <ambrya at umich.edu> wrote:

> Hi RegCNet,
>
> Could someone point me to a reference that explains where the default
> value for maximum cloud fraction (cftotmax = 0.75) comes from? Does this
> cap mean that the cloud fraction output variable (totcf) does not allow for
> overcast skies? Can this variable be reliably used as input for a lake
> circulation model?
>
> Alex Bryan
> Ph.D. Candidate
> Department of Atmospheric, Oceanic, and Space Sciences
> University of Michigan, Ann Arbor, MI
> Email: ambrya at umich.edu
> URL: http://aoss.engin.umich.edu/people/ambrya
>
>
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> RegCNET at lists.ictp.it
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>



-- 
Travis O'Brien
Research Scientist, Earth Sciences Division
Lawrence Berkeley National Lab
http://esd.lbl.gov/about/staff/travisobrien/
<http://esd.lbl.gov/about/staff/travisobrien/index.html>
(510) 495-8047
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