[RegCNET] error in drag coefficient calculation

[Jie Song]

Dear RegCM Users:

In RegCM version 4.1.1, there is a line in mod_vecbats.F90: 

 zlgdis(n,i) = dlog(z1d(n,i)-displa(lveg(n,i)) / &
                               rough(lveg(n,i)))

As it currently stands, the roughness length is divided by the displacement height only, which is not correct, there should be an extra set of parentheses in the numerator.  The correct code should be the following:

 zlgdis(n,i) = dlog((z1d(n,i)-displa(lveg(n,i))) / &
                               rough(lveg(n,i)))

This error was not present in version 3, but it appeared during the transition to version 4, including RegCM-4.4-rc22 and RegCM-4.3.5.6. 

We found that this bug influences our simulation results significantly because it is directly used in calculating drag coefficient for the vegetation.

We wanted to bring attention to this so other people are aware of this issue.  We hope that this bug is fixed in the new version.

Thanks
-Jie Song

Professor
Department of Geography
Northern Illinois University
DeKalb, IL 60115
jsong at niu.edu
815-753-6837(office)
815-753-6872(fax)