id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
786	Reading forcing data with DT < 1 second	jcwarner		"I am using a NetCDF forcing data file with a baroclinic '''DT''' of 0.2 sec to drive a lab test case. But ROMS does not interpolate the data correctly because in '''set_ngfld''' (same for '''set_2dlfd''' and '''set_3dlfd''') we have:
 {{{
      fac1=ANINT(Tintrp(it2,ifield,ng)-time(ng),r8)
      fac2=ANINT(time(ng)-Tintrp(it1,ifield,ng),r8)
}}}
which truncates the time interpolation weights to a whole number for the nearest second towards zero.

We got it to work for smaller than a second baroclinic timestep by using:
{{{
      fac1=ANINT((Tintrp(it2,ifield,ng)-time(ng))*SecScale,r8)
      fac2=ANINT((time(ng)-Tintrp(it1,ifield,ng))*SecScale,r8)
}}}
where '''!SecScale=1000'''. That is, the time interpolation weights are rounded to the nearest millisecond instead.

The following statements at the full precision did not work:
 {{{
      fac1=Tintrp(it2,ifield,ng)-time(ng)
      fac2=time(ng)-Tintrp(it1,ifield,ng)
}}}

because there can be a small value of '''fac1''' that is negative because of roundoff, and then the interpolation is stopped by
 {{{
      ELSE IF (((fac1*fac2).ge.0.0_r8).and.(fac1+fac2).gt.0.0_r8) THEN
...
}}}

indicating unbounded interpolants.

----

== WARNING: ==

Notice that we no longer will get identical solutions with previous versions due to very small differences in the time interpolated fields. It does not matter much because the differences are in the order of roundoff. However, users need to be aware of such fact.


 "	defect	closed	major	Release ROMS/TOMS 3.7	Nonlinear	3.7	Fixed