GRACE/TWS Data Assimilation

GRACE/TWS Data Assimilation#

This page describes the assimilation of Terrestrial Water Storage (TWS) anomaly observations derived from GRACE/GRACE-FO satellite data into the eCLM land-surface model using the PDAF-OMI framework.

Configuration#

TWS DA is enabled by setting CLM:update_tws = 1 in enkfpf.par. The following additional parameters control its behaviour:

Parameter	Section	Purpose
`CLM:update_tws`	`[CLM]`	Enable/disable TWS DA
`DA:state_setup`	`[DA]`	State vector composition
`DA:TWS_smoother`	`[DA]`	Instantaneous vs. monthly-mean water storage
`DA:max_inc`	`[DA]`	Maximum increment as fraction of state value
`DA:exclude_greenland`	`[DA]`	Exclude Greenland from state vector
`DA:set_zero_start`	`[DA]`	Reset running average before first GRACE obs

State Vector#

Only hydrologically active gridcells (columns where col%hydrologically_active is .TRUE.) contribute to the TWS state vector. Greenland can optionally be excluded via DA:exclude_greenland.

The composition of the state vector is controlled by DA:state_setup:

`state_setup`	Contents per gridcell
`0`	Liquid + ice summed per soil layer (all layers down to bedrock), snow water equivalent, surface water
`1`	Single pre-aggregated TWS value
`2`	Surface soil moisture (layer 1), root-zone moisture (layer 4), deep moisture (layer 13), snow water equivalent

Temporal Averaging#

GRACE observations represent monthly averages. The parameter DA:TWS_smoother selects which CLM fields are used to populate the state vector:

TWS_smoother = 0: Instantaneous CLM fields are used. This is appropriate when the observation file already contains monthly-mean GRACE anomalies referenced to a model climatology.
TWS_smoother = 1: Monthly running-mean CLM fields (computed internally by eCLM) are used. The reset of the running average is controlled by a flag inside the observation file; the first reset can be advanced by DA:set_zero_start.

Observation Operator#

The observation operator (implemented in obs_GRACE_pdafomi.F90) maps the state vector to an observed TWS value by summing all water-storage components for each gridcell and subtracting the temporal mean read from a reference file (mean_filename). This converts absolute water storage to a TWS anomaly comparable to GRACE observations.

The PDAF-OMI localization radius for GRACE observations is specified in units of grid cells (not km, unlike the SM localization radius).

Increment Capping#

To prevent unphysically large updates, DA:max_inc caps the analysis increment for each water-storage component:

if |Δx| > max_inc × x  →  Δx = sign(max_inc × x, Δx)

The default value of 1.0 limits the increment to 100 % of the current state value (i.e. the updated value cannot become negative through this mechanism).

Safety Checks#

Columns that are not hydrologically active are never included in the state vector.
The running temporal mean is only reset when the observation file requests it (or once at start-up via DA:set_zero_start).
PDAF-OMI deallocates observation arrays after each analysis step to avoid memory leaks in multi-step runs.

Configuration Examples#

Minimal GRACE DA setup#

[CLM]
update_tws = 1

[DA]
state_setup    = 0
TWS_smoother   = 0
max_inc        = 1.0
set_zero_start = 0

Monthly-mean TWS with Greenland excluded#

[CLM]
update_tws = 1

[DA]
state_setup        = 0
TWS_smoother       = 1
max_inc            = 0.5
exclude_greenland  = 1
set_zero_start     = 720

set_zero_start = 720 resets the running monthly mean 720 CLM time steps before the first GRACE observation (e.g. one month at 30-minute time steps).