search

NOAA-GFDL / GFDL_atmos_cubed_sphere

The GFDL atmos_cubed_sphere dynamical core code
Other
56 stars 118 forks source link

Potential Cancellation Issue in deln_flux #323

Open oelbert opened 8 months ago

oelbert commented 8 months ago

The d2 calculation near the end of deln_flux has discrepant results between 64- and 32-bit caulculations:

https://github.com/NOAA-GFDL/GFDL_atmos_cubed_sphere/blob/9ca1468b9f247a8d747e877be270c9e7b980421f/model/tp_core.F90#L1316

The GEOS team has resolved this by enforcing the order-of-operations in the flux addition/subtractions: d2(i,j) = ((fx2(i,j)-fx2(i+1,j))+(fy2(i,j)-fy2(i,j+1)))*gridstruct%rarea(i,j)

This could be a good improvement to 32-bit runs

lharris4 commented 8 months ago

Hi, @oelbert. Thank you for identifying these changes from GMAO. Indeed this better representation of order-of-operations when computing flux divergences, which are done at a number of places in the solver, could go a long way to avoiding some of the cancellations that can occur.

They also define the Flux Capacitors mfx,mfy and accumulated Courant numbers cx and cy as double-precision in all runs, and a double-precision version of delp. I definitely think these are worth trying out.