Model Coupling ESMF

EARTH SYSTEM MODEL FRAMEWORK (ESMF)

Overview

Significant progress has been made over the past decade in the standardization of coupling tools without reducing model diversity through the Earth System Modeling Framework (ESMF; Collins et al., 2005) and the National Unified Operational Prediction Capability (NUOPC) consortia. The NUOPC layer is a simplified infrastructure on top of the ESMF library (version 7 or higher) that provides conventions and templates to facilitate easy coupling between Earth System Models (ESM). It consists of four components: (i) a driver, (ii) the models, (iii) mediators, and (iv) connectors. The driver controls the models, mediators, connectors, and coordinates tasks such as initialization and time stepping. The mediators are custom codes that facilitate the coupling of the models, and control tasks such as flux calculations, rescaling, and averaging. The connectors join ESM components and perform operations such as regridding between the source and destination fields when needed. While the NUOPC layer is available in ESMF version 7 of higher, ROMS coupling requires version 8 or higher because it uses the RunSequence input configuration script and uses version 8 features for nesting.

The critical Fortran module that sits on top of each ESM is the so-called NUOPC cap file. There is a separate cap file for each coupled component, which, from the ROMS perspective, corresponds to the atmosphere, sea-ice, wave model, etc. Therefore, it is an abstract block that allows ROMS to communicate and exchange data seamlessly within the ESMF/NUOPC framework. The ROMS and other ESM grids may (and usually do) have a different geographical extent and horizontal resolution. So if, for example, the atmosphere component domain is larger than the ROMS domain, it is necessary to provide data from another source to the atmosphere model at all of the surface grid-points that lie outside the ROMS domain. Therefore, a DATA component and its cap file is also required for most applications. In this case, SST is exported from ROMS and DATA to the atmosphere component which imports at melded SST field.

The ROMS coupling interface with the ESMF/NUOPC library allows both driver and component methods of operation. In the driver method, it provides all the interfaces needed to couple to other ESM components including the executable driver, NUOPC-based generic ESM component services, model gridded components or NUOPC Model cap files, connectors between components for re-gridding source and destination fields, input scripts, and coupling metadata management. A NUOPC Model cap is a Fortran code layer that sits on top of the ESM component, making calls to the numerical kernel via the initialize, run, and finalize phases. Alternatively, in the component method, the NUOPC ROMS cap module is provided, and it can be adapted and incorporated into other NUOPC-based coupling systems, like the NOAA Environmental Modeling System (NEMS).

Figure 1: ROMS driver mode ESMF/NUOPC coupling framework

A prototype of the ROMS coupling framework based on the ESMF/NUOPC library is illustrated in Figure 2 showing the Driver, Models, and Connectors. The Driver controls all the aspects of the coupling between the ESM components and their connections: configuration, initialization, time-stepping sequence, data exchanges, and termination. The Models are the gridded data and gridded geophysical numerical kernels wrapped into a NUOPC cap file interface. The Connectors between ESM components execute the remapping and regridding between the source and destination fields. The interpolation is usually linear with extrapolation support in the vicinity of masked grid cells. In this project, the coupling framework will include the Atmosphere Model (WRF), the Ocean Model (ROMS), and the Data Model since the grids are incongruent. The Data Model is needed because WRF requires sea surface temperature (SST) on those grid points not covered by ROMS (see Figure X).

Sequential Vs. Concurrent Mode

In sequential mode, all of the coupled model components are executed on all of the processors one after the other sequentially. In concurrent mode, each coupled model component executes on its own set of non-overlapping processors.

Regridding and Extrapolation Methods

Regridding interpolation methods between source and destination fields can be bilinear or conservative. Unmapped cells during regridding are usually located at land/sea boundaries. These unmapped cells can be mapped using the following methods: nearest source to destination, nearest inverse distance average, creep fill at specified level, or two steps extrapolation (Turuncoglu).

Indian Ocean Land/Sea Masks Mismatch??

! Cinfo(11) Field regridding method from source to destination:
! 'bilinear' => bilinear interpolation
! 'patch' => high-order patch recovery
! 'conservative1' => first-order conservative
! 'conservative2' => second-order conservative
! 'nearest' => nearest neighbor interpolation
! Cinfo(12) Field extrapolation method for unmapped points:
! 'none' => no extrapolation
! 'nearest' => nearest source to destination
! 'idavg' => nearest inverse distance average
! 'creep' => creep fill at specified level
! '2steps' => Turuncoglu's 2 steps extrapolation

'SST'
'sea_surface_temperature' ! standard name
'sea surface temperature'
't(:,:,N,itemp)', 'ROMS' ! source variable, ROMS
'C' ! source units
'Center' ! source grid-cell
'sst', 'WRF' ! destination variable, WRF
'K' ! destination units
'Center' ! destination grid-cell
'SST' ! DATA Model file variable
'bilinear' ! regridding method
'nearest' ! unmapped extrapolation method
.TRUE. ! connected to coupler
.FALSE. ! debug write into a NetCDF file
273.15d0 ! importing add offset
1.0d0 ! importing scale

'dSST'
'sea_surface_temperature_data' ! standard name
'sea surface temperature data'
'temperature', 'HyCOM' ! source variable, DATA
'C' ! source units
'Center' ! source grid-cell
'sst', 'WRF' ! destination variable, WRF
'K' ! destination units
'Center' ! destination grid-cell
'temperature' ! DATA field and time variables
'bilinear' ! regridding method
'nearest' ! unmapped extrapolation method
.TRUE. ! connected to coupler
.FALSE. ! debug write into a NetCDF file
273.15d0 ! importing add offset
1.0d0 ! importing scale

Explicit Vs. Semi-Implicit Run Sequence

Explicit		Semi-Implicit, ATM Average
# Hurricane Irene Application (single time loop) runSeq:: @240 # timeStep = 4 min interval DATA -> WRF # DATA to WRF connector DATA WRF -> ROMS # WRF to ROMS connector ROMS -> WRF # ROMS to WRF connector WRF ROMS @ ::		# Hurricane Irene Application (single time loop) runSeq:: @* # timeStep = wildcard (*) DATA -> WRF # DATA to WRF connector DATA ROMS -> WRF # ROMS to WRF connector WRF WRF -> ROMS # WRF to ROMS connector ROMS @ ::
Atmosphere exports instantaneous fields		Atmosphere exports time-averaged fields (Recommended for Conservation)

Semi-Implicit Algorithm Details

The following diagram shows the sequence of exchanges in a DATA-ATM-ROMS coupling system. The connector ROMS-TO-ATM is explicit, whereas the connector ATM-TO-ROMS is semi-implicit. Usually, the timestep of the atmosphere kernel is smaller than that of the ocean. Thus, ATM export fields are time-averaged over the coupling interval, which is the same as the ROMS timestep. Currently, in our coupling system, we have NUOPC cap files for COAMPS and WRF replacing the generic ATM label.

That is, the detailed sequence of routines called is as follows:

ESM_SetServices

ESM_SetModelServices

ROMS_SetServices

ATM_SetServices

DATA_SetServices

Coupler_SetServices set interpolation RouteHandle ROMS-TO-ATM
Coupler_SetServices set interpolation RouteHandle ATM-TO-ROMS
Coupler_SetServices set interpolation RouteHandle DATA-TO-ATM

ESM_SetRunSequence

ROMS_SetInitializeP1

ATM_SetInitializeP1

DATA_SetInitializeP1

ROMS_SetInitializeP2
ROMS_Initialize
ROMS_SetGridArrays
ROMS_SetStates

ATM_SetInitializeP2
ATM_Initialize
ATM_SetGridArrays
ATM_SetStates

DATA_SetInitializeP2
DATA_Initialize
DATA_multifile
DATA_checkfile
DATA_inquiry
DATA_ncvarcoords
DATA_ncread
DATA_SetGridArrays
DATA_SetStates

Coupler_ComputeRH compute interpolation RouteHandle ROMS-TO-ATM
Coupler_ComputeRH compute interpolation RouteHandle ATM-TO-ROMS
Coupler_ComputeRH compute interpolation RouteHandle DATA-TO-ATM

ROMS_SetClock

ATM_SetClock

DATA_SetClock

ROMS_DataInit
ROMS_Export

ATM_DataInit

Coupler_ExecuteRH apply interpolation RouteHandle DATA-TO-ATM

DATA_ModelAdvance
DATA_ncread
DATA_Export
DATA_TimeInterp

Coupler_ExecuteRH apply interpolation RouteHandle ROMS-TO-ATM

ATM_SetRunClock
ATM_CheckImport
ATM_ModelAdvance
ATM_Import
ATM_ProcessImport
ATM_Export

Coupler_ExecuteRH apply interpolation RouteHandle ATM-TO-ROMS

ROMS_SetRunClock
ROMS_CheckImport
ROMS_ModelAdvance
ROMS_Import
ROMS_Export

Coupler_ExecuteRH apply interpolation RouteHandle DATA-TO-ATM

Coupling with Nested Grids

Component	Grid	CoupledSet	ImportState	ExportState	ConnectedTo
-------------------------------------------------------------------------------------------------------------------------------------------------
ROMS	1	ESM_02	Import_ESM_02	Export_ESM_02	COAMPS
COAMPS	1	ESM_01	Import_ESM_01	Export_ESM_01	DATA
COAMPS	2	ESM_02	Import_ESM_02	Export_ESM_02	ROMS, DATA
DATA	1	ESM_01	NONE	Export_ESM_01	COAMPS
DATA	1	ESM_02	NONE	Export_ESM_02	COAMPS

• Active coupled Earth System Models (ESM) switches. If a component is nested, each grid is considered an independent component. Specify which nested grids are coupled. The first grid of active ESM components must always be coupled.
  IsActive(roms) = T  ! ROMS (NgridsR)
  IsActive(atmos) = T  ! atmosphere model (NgridsA)
  IsActive(seaice) = F  ! seaice model (NgridsI)
  IsActive(waves) = F  ! wave model (NgridsW)
  IsActive(data) = T  ! DATA model

• Set which ESM components are connected to the ocean component (ROMS) during coupling. If ROMS has nested grids, indicate which grids are connected to the other ESM components, [1:NgridsR] expected.
  Coupled(ATM2OCN) == T  ! atmosphere -> ROMS connected
  Coupled(ICE2OCN) == F  ! seaice -> ROMS connected
  Coupled(WAV2OCN) == F  ! wave -> ROMS connected
  Coupled(DAT2OCN) == F  ! DATA -> ROMS connected

• Set which ESM components are connected to the atmosphere component during coupling. If the atmosphere component has nested grids, indicate which grids are connected to the other ESM components, [1:NgridsA] expected.
  Coupled(OCN2ATM) == F T  ! ROMS -> atmosphere connected
  Coupled(ICE2ATM) == F  ! seaice -> atmosphere connected
  Coupled(WAV2ATM) == F  ! wave -> atmosphere connected
  Coupled(DAT2ATM) == T T  ! DATA -> atmosphere connected