id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
474	IMPORTANT: Corrected parallel bug in float's vertical random walk	arango	arango	"Finally, I figured out how to correct the shared-memory bug in the floats vertical random walk. I mentioned this problem before in src:ticket:472. Getting random number sequences in parallel is very tricky. It is very difficult to get identical ROMS solutions in serial and parallel when random numbers are involved, but it is possible.

 * Changed the strategy of generating a normally-distributed random number sequence for random vertical walk velocity.  A new global variable '''rwalk''' is created in '''mod_floats.F''' to store random number sequences for floats. This fixed the shared-memory bug when generating the random numbers.

 * Changed the initialization handling of out-of-bounds floats. Previously, we have in '''inp_par.F''':
{{{
         FLT(ng)%Tinfo(ixgrd,nc)=MIN(MAX(0.5_r8,Fx0(i,ng)),  &
     &                           REAL(Lm(ng),r8)+0.5_r8)
         FLT(ng)%Tinfo(iygrd,nc)=MIN(MAX(0.5_r8,Fy0(i,ng)),  &
     &                           REAL(Mm(ng),r8)+0.5_r8)
}}}
 This limiting is eliminated in '''inp_par.F''' and '''grid_coords.F''' and now we have:
{{{
         FLT(ng)%Tinfo(ixgrd,nc)=Fx0(i,ng)
         FLT(ng)%Tinfo(iygrd,nc)=Fy0(i,ng)
}}}

 * The checking for out-of-bounds initial locations is now done in '''step_floats.F''' when the floats are released:

{{{
          IF ((Tinfo(ixgrd,l).lt.0.5_r8).or.                            &
     &        (Tinfo(iygrd,l).lt.0.5_r8).or.                            &
     &        (Tinfo(ixgrd,l).gt.REAL(Lm(ng),r8)+0.5_r8).or.            &
     &        (Tinfo(iygrd,l).gt.REAL(Mm(ng),r8)+0.5_r8)) THEN
            bounded(l)=.FALSE.               ! outside application grid
          END IF
}}}
 This fixes a parallel bug when comparing trajectoris in serial and distributed-memory.  Now the out-of-bounds floats in MPI solutions have the correct !FillValue instead of a zero value.

 * The random numbers are now computed with the parallel friendly routine '''gasdev.F'''.  It uses the same efficient routines used to generate random numbers in the adjoint-based algorithms. The routine '''nrng.F''' (file '''utility.F''') is eliminated.

 * The random number sequence is only generated in the floats predictor step.  This fixes the problem reported by Mark Hadfield in src:ticket:472.  The '''1/SQRT(2)=0.707''' factor was due to the two independent random sequences (predictor and corrector steps). Many thanks to John Wilkin for bringing to our attention: if the variance of the random velocity on the predictor step is '''1''', and this is independent of the random velocity on the corrector step (also '''1'''), then the two random step combined have a variance of only '''1/2''' with an standard deviation of '''0.707'''."	bug	closed	major	Release ROMS/TOMS 3.4	Nonlinear	3.4	Fixed