Using GRACE to estimate changes in land water storage: present limitations

and future potential

John Wahr, Sean Swenson, Isabella VelicognaUniversity of Colorado

GRACELaunched March, 2002. A NASA/DLR mission. Managed by U Texas,

JPL, and GFZ. Anticipated lifetime: 8-9 years.

Objective: map out the gravity field to high accuracy every month

Time variable gravity: the general idea

•Use GRACE to map the Earth’s gravity field at ~monthly time intervals. These fields are provided to users.•Remove the mean, to obtain time-variations in gravity. •Use the results to solve for changes in mass at the Earth’s surface: in the oceans, the polar ice sheets, and the water stored on land.

40 fields ~monthly fields, between April, 2002 and Feb, 2006, are now available.

A fundamental limitation of time-variable gravity:

The mass results have no vertical resolution. Implications: (1) can’t distinguish between water on the surface or in

the ground. (2) can’t distinguish between land water storage, and

mass variability in the atmosphere or in the underlying solid Earth.

The atmosphere: ECMWF meteorological fields are used to remove atmospheric contributions before constructing gravity fields.

The solid Earth: post-glacial-rebound causes secular gravity signals that must be modeled and removed by the user.

A mission-dependent limitation

The mass results must be averaged over several hundred km or more,

to be accurate.

Annual Mass Cycle From GRACESmoothed using 750-km averaging radius.

Cosine is max on Jan 1; Sine is max on April 1.

GRACE Annual Cycle: 750 km Averaging Radius

Seasonal mass signal. 400 km smoothing radius

Sean Swenson has found a way to filter the Stokes coefficients to reduce noise but not signal.

The error depends on smoothing radius.

Examples of water storage in specific regions

Besides using finding smoothed estimates of water storage, you can also find water storage in specific geographic regions (river basins, for example).

Averaging kernels for the Mississippi and Amazon basins

The Mass Balance Equation

• Rate of water storage change = precip –evapotranspiration – runoff• dS/dt = P – ET – R

Possible Applications:

1. P - ET = dS/dt (S from GRACE) + R (from river discharge)

2. R = P - ET - dS/dt

Examples of estimating P - ET = dS/dt + R

GRACE water storage (S)

dS/dt

Red: dS/dt + R

Black: P-ET from ECMWF & from NCEP

Red: dS/dt + RBlack: P-ET from ECMWF & from NCEP

dS/dt

GRACE water storage (S)

Examples of estimating R = P - ET - dS/dt

GRACE water storage (S)

P-ET.

Green: ECMWF Yellow:NCEP

dS/dt

P-ET-dS/dt(runoff)

GRACE water storage (S)

P-ET.

Green: ECMWF Yellow:NCEP

dS/dt

P-ET-dS/dt

(runoff)

GRACE water storage (S)

P-ET. Green: ECMWF Yellow:NCEP

dS/dt

P-ET-dS/dt(runoff)

Linear Trend between April, 2002 and Feb, 2006

Secular Mass Solution.750-km Smoothing Radius. PGR Removed.

Antarctic Mass Variation From GRACE

__ GRACE - PGR

Trend -152+/-80 km3/yr ~0.4+/-0.2 mm/yr sea level rise

WAIS and EAIS Mass Variation From GRACE

WAIS:-148+/-21 km3/yr EAIS:0+/-56 km3/yr

Greenland Mass Variation From GRACE

Greenland Mass Variation From GRACE

The Future

• GRACE lifetime expected to be 8-9 years.

• GOCE (an ESA satellite gradiometer) will launch in 2007.

Will provide excellent results at short wavelengths. Not designed to rival GRACE for time-variable studies.

• NASA is considering a GRACE follow-on mission. Has the potential of obtaining mass variability down to scales

of ~100 km.