HAFS Coupled HYCOM Report v0_1_0

HAFS Coupled HYCOM Report v0.1.0

Directly Coupled FV3-HYCOM

Tag

To Be Tagged

Milestones

• HAFS Coupling (1a-3-1c)
• Reworking Caps (1a-3-2a)

Status

Standalone regional ocean coupling within HAFS is now available in a directly coupled FV3-HYCOM configuration, see architecture. This configuration includes two models Finite Volume Cubed-Sphere (FV3) Model and Hybrid Coordinate Ocean Model (HyCOM) each wrapped in National Unified Operational Prediction Capability (NUOPC) interfaces and coupled through standard NUOPC connectors. Once wrapped with NUOPC interfaces these components are referred to as caps. The configuration is currently undergoing scientific verification as it is being prepared for a HAFS Real-Time Experiments release.

The directly coupled FV3-HYCOM configuration includes the FV3 cap utilized by other Unified Forecast System (UFS) applications. The existing UFS-FV3 atmosphere cap supports running over a regional domain and coupling to ocean models. The FV3 cap exports several atmospheric forcing variables and imports sea surface temperature, see fields exchanges. HAFS experiments are simulated over the Atlantic Basin. The domains used by FV3 and HYCOM differ in size, resolution, and spatial location, see regional domains, causing large areas of missing import data. In this release a feature has been added to FV3 that incorporates initial sea surface data into the imported data. This results in better coverage of the domain discrepancies, see field merging. Diagnostic output has also been added to validate the FV3 ESMF grid used during data exchanges and the sea surface data after its initial values are merged into the imported data.

This configuration also includes a HYCOM cap based on the cap provided in the Earth System Prediction Capability and extended for use in UFS applications. The HYCOM cap converts imported data into the correct units or rotation each time coupled data is received. FV3 does not provide each field needed by HYCOM and these fields are calculated within the cap. The total radiation flux is computed based on shortwave and longwave fluxes. The total wind speed is computed based on zonal and meridional wind speeds. HYCOM also handles missing imported data through incorporating offline data. GFS data is loaded into memory during initialization and merged into the unmapped points, see field merging. The previous quarter also includes HYCOM cap clean-up. The code has been cleaned up for readability, maintainability, and optimization. These changes include: added comments, trimmed white-space, subroutine argument organization, and elimination of redundant loops.

Component coupling is handled by standard NUOPC Connectors. These connectors have built in ESMF field regridding, which can be configured at run-time. The FV3-HYCOM coupling was first tested using nearest neighbor remapping. This algorithm maps the closest available source point to a destination point and unrealistically extrapolates data over a large area, see field remapping nearest neighbor. Next, the coupling was tested using bilinear interpolation as a remapping algorithm. By default, the NUOPC connector zeros all unmapped points. This results in large areas of no data, see field remapping bilinear. In order to accurately trace unmapped cells the zero select option was added to the NUOPC Connector in ESMF v8.1.0 beta snapshot 17. The zero select option allows for a fill value, in this case 9.99 x 10^20, to indicate an unmapped cell. The fill values are later replaced by offline data.

The preliminary results using bilinear coupling and merging offline data into imported data were presented at the discussion on HAFS Real-Time Experiments on June 29th, 2020 (D. Rosen, B. Liu, H.S. Kim). The presentation included technical validation, workflow developments, and preliminary scientific analysis. The initial analysis included a bug related to newly generated forcing files, which has since been resolved. During the next few weeks an in depth scientific analysis of different configuration options will be conducted to determine the best settings for HAFS Real-Time experiments. In parallel to this testing the CMEPS mediator is being integrated into the coupled system. This mediator will handle multi-component data merging, flux calculations, time interpolation, and other multi-component tasks.

Architecture

Regional Domains

Field Exchanges From FV3 to HYCOM

Standard Name	Units	HYCOM Transformation	HYCOM Hookup
inst_zonal_wind_height10m	m / s	rotated	wndspx
inst_merid_wind_height10m	m / s	rotated	wndspy
mean_zonal_moment_flx_atm	N / m2	rotated	taux
mean_merid_moment_flx_atm	N / m2	rotated	tau
inst_temp_height2m	K	converted to Celsius	airtmp
inst_spec_humid_height2m	%	converted to mixing ratio	vapmix
mean_prec_rate	Kg / m2 / s	converted to m / s	prcp
mean_net_sw_flx	W / m2	(none)	swflx
mean_net_lw_flx	W / m2	(none)	lwflx
inst_pres_height_surface	Pa	(none)	mslprs

Field Exchanges From HYCOM to FV3

Standard Name	Units	HYCOM Transformation	HYCOM Hookup
sea_surface_temperature	K	converted to Kelvin	temp

Field Remapping Nearest Neighbor

Exported Data	Imported Data
FV3 air temperature at 2 meters on FV3 domain.	FV3 air temperature at 2 meters nearest neighbor remapped onto HYCOM domain.
HYCOM sea surface temperature on HYCOM domain.	HYCOM sea surface temperature nearest neighbor remapped onto FV3 domain.

Field Remapping Bilinear

Exported Data	Imported Data
FV3 air temperature at 2 meters on FV3 domain.	FV3 air temperature at 2 meters bilinear remapped onto HYCOM domain.
HYCOM sea surface temperature on HYCOM domain.	HYCOM sea surface temperature bilinear remapped onto FV3 domain.

Field Merging

Imported Data	Merged Data
FV3 air temperature at 2 meters bilinear remapped onto HYCOM domain with unmapped cells.	FV3 air temperature at 2 meters merged with HYCOM GFS air temperature.
HYCOM sea surface temperature bilinear remapped onto FV3 domain with unmapped cells.	HYCOM sea surface temperature merged with FV3 initial sea surface temperature.