[MITgcm-support] Problem with OBCs (bug ?)

[Nicolas Grisouard]

Hi  everyone,

I am running experiments about internal waves forced through open 
boundary condition on the western boundary (a little bit like the 
internal_wave experiment, but with more complex forcings).

I have been getting a weird behaviour for the past months but only 
noticed it a few days ago, since this problem now induces unphysical 
features. I think I isolated one problem and think it involves the OBCS 
package.

About the first issue. I tried to implement two forcings that are about 
the same. Configuration #1 (C1), in obcs_calc.F :

        DO K=1,Nr
          DO J=1-Oly,sNy+Oly
             OBWu(J,K,bi,bj)=0. _d 0
     &            +Uinflow*ramptime2
     &            * sin(2*PI*(float(k)/float(Nr)-5.D-1))
     &            * sin(2.0 _d 0*PI*futureTime/obTimeScale)
             OBWv(J,K,bi,bj)=0. _d 0
     &            +Uinflow
     &            *f0/(2.0 _d 0*PI/obTimeScale)
     &            *cos(2. _d 0*PI*futureTime/obTimeScale )
             OBWt(J,K,bi,bj)=tRef(K)
#ifdef ALLOW_NONHYDROSTATIC     ! I do allow nonhydrosatic effects
             OBWw(J,K,bi,bj)=0.
#endif
          ENDDO
        ENDDO

That is, in human-friendly format :
u(z,t) = U.f(t) . sin[2.pi.(z/H - 1/2)] . sin(2.pi.t/T)
everything_else = 0 as f0 = 0 (nonrotating case)
f(t) = function that smoothes the activation of the forcing and equal to 
1.8% at t= 0
H = height of the domain
kz = 2*pi/H
T = period of the forcing so that frequency < N

So we have a "stationary" forcing. At time t = 0, the fields are like 
they should be : the stratification is as set in gendata and the 
velocities fields are = 0.

If we now take the exact same configuration except for the forcing that 
is now "propagative" (C2) (the one I need) :

        DO K=1,Nr
          DO J=1-Oly,sNy+Oly
             OBWu(J,K,bi,bj)=0.D0
     &            +Uinflow*ramptime2
     &            * sin(2.D0 *PI*(float(k)/float(Nr)-5. _d -1
     &            - futureTime/obTimeScale))
             OBWv(J,K,bi,bj)=0. _d 0
     &            +Uinflow
     &            *f0/(2.0 _d 0*PI/obTimeScale)
     &            *cos(2. _d 0*PI*futureTime/obTimeScale )
             OBWt(J,K,bi,bj)=tRef(K)
#ifdef ALLOW_NONHYDROSTATIC
             OBWw(J,K,bi,bj)=0.
#endif
          ENDDO
        ENDDO

Again :
u(z,t) = U.f(t). sin[2.pi.(z/H - 1/2 - t/T)]

Now at time t = 0, the U and T fields are like they should be, but the W 
field shows someting odd, as displayed in figure propagative.jpg. The 
vertical velocity is non-zero at the column that is next to the point i 
= 1. This induces parasite emission of waves in the best scenario and 
maybe the blowing of the computation in the worst scenario.


Third configuration (C3), to complete C2 :
u(z,t) = U.f(t). sin[2.pi.(z/H - t/T)]

I get a W field as shown in propagative2.jpg : same as in 
propagative.jpg but opposite values of velocities. I would have expected 
a node in the middle...



I have no idea how to fix this and am ready to give any additional 
information to who could help me of course.

Thank you,
Nicolas

-- 
Nicolas GRISOUARD
PhD Student - ERES division
Laboratoire des Ecoulements Geophysiques et Industriels
BP 53
38041 Grenoble cedex 9 France
tel : +33 (0)476 825 037 - fax : +33 (0)476 825 271

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