VERY IMPORTANT: Update WRF ESMF/NUOPC cap module

[arango]

We have carefully examined the DATA-WRF-ROMS coupling system with the ESMF/NUOPC library. We fixed esmf_atm_wrf.h NUOPC cap module after a detailed analysis of the heat flux balance equation. The documentation of WRF fluxes in the code is confusing about the direction (downward/upward) and the sign (positive/negative) of the latent and sensible heat fluxes. The net heat flux previously exported from WRF to ROMS was incorrect due to the confusing information.

• There are several variables in WRF for the heat component fluxes:

-- There is downwelling shortwave flux (swdnb, downward) and upwelling shortwave flux (swupb, upward) at the atmosphere bottom. Both are positive and with W/m² units. Therefore, the net shortwave radiation flux is swdnb minus swupb, stored in variable gsw, is positive and downward since swupb is around 8 percent of the magnitude of swdnb. Notice that the variable swdown is equivalent to swdnb. Both are positive and with W/m² units.

-- Since the ocean needs net shortwave radiation flux, we decided to use the downward (swdnb) and upward (swupb) fluxes for clarity. Also, the time-averaged values swdnb_mean and swupb_mean are available when the RAMS diagnostics are activated for semi-implicit coupling.

-- Similarly, there is downwelling longtwave flux (lwdnb, downward) and upwelling longwave flux (lwupb, upward) at the atmosphere bottom. The variable lwupb is basically StefBo * emiss * Tsur⁴. Tsur is the land and ocean temperature at the atmosphere bottom (Kelvin). Likewise, notice that the variable glw is equivalent to lwdnb. We will use glw because its equivalent time-averaged variable is glw_mean when the RAMS diagnostics are activated for semi-implicit coupling. The net longwave radiation flux, glw minus lwupb, can be positive or negative.

-- The variable hfx is the sensible heat flux (W/m²), and lh is the latent heat flux (W/m²). Both fluxes are positive and upward from the bottom of the atmosphere. Please ignore the conflicting documentation in the WRF source code for these fluxes. The time-averaged values are hfx_mean and lh_mean, respectively. Therefore, we need to flip the sign and use minus when computing the ocean surface net heat flux.

-- Therefore, if BULK_FLUXES is off, the surface net heat flux (W/m²) exported to the ocean in subroutine WRF_Export of cap module esmf_atm_wrf.h needs to be:

            CASE ('nflx', 'shflux')
              MyFmin(1)= MISSING_dp
              MyFmax(1)=-MISSING_dp
              DO j=Jstr,Jend
                DO i=Istr,Iend
# ifdef WRF_TIMEAVG
                  Fval=(grid%swdnb_mean(i,j)-grid%swupb_mean(i,j))+     &
     &                 (grid%glw_mean(i,j)-grid%lwupb_mean(i,j))-       &
     &                 grid%lh_mean (i,j)-                              &
     &                 grid%hfx_mean(i,j)
# else
                  Fval=(grid%swdnb(i,j)-grid%swupb(i,j))+               &
     &                 (grid%glw(i,j)-grid%lwupb(i,j))-                 &
     &                 grid%lh (i,j)-                                   &
     &                 grid%hfx(i,j)
# endif
                  MyFmin(1)=MIN(MyFmin(1),Fval)
                  MyFmax(1)=MAX(MyFmax(1),Fval)
                  ptr2d(i,j)=Fval
                END DO
              END DO

Here, the CPP option WRF_TIMEAVG is used to compute the flux with the RAMS time-averaged diagnostics for semi-implicit coupling (see ​RunSequence in wikiROMS).

• Several routines in ROMS were updated to allow importing the components of the heat flux balance for debugging and output purposes when BULK_FLUXES is off.

• The test case for Hurricane Irene has been revised accordingly in the test repository. Please check this application when configuring your coupling system.

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Snapshots of WRF land and ocean surface temperature and heat flux components are shown below for the hurricane Irene application:

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Then, the atmospheric field imported by ROMS are:

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Many thanks to my colleagues at Rutgers, John Wilkin, Julia Levin, and Travis Miles, for the discussions about WRF-ROMS coupling.