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Regarding the Eta-12 cloud fraction tending toward clear or 100% overcast, please note that this issue has come up on the new COMET newserver for discussion on the Eta and AVN. The link is at
http://meted.ucar.edu/nwp/newsgroups/
with Bill Bua and I attempting to provide answers like Stan and company do here. The format is similar to this RUC forum except through newserver so that people can post gif and jpg images in their messages.
In the Eta, the cloud fraction algorithm was not adjusted or retuned when the microphysics was overhauled, a change which occurred together with the resolution change to 12 km. The new microphysics has less extensive areas of condensate - no condensate = clear. But the model climatology with the new scheme is toward higher RH than with the old scheme. Since the cloud fraction formula was (and thus still is) a function of RH, the diagnosed cloud fractions are higher where the model has clouds.
Stephen Jascourt
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