Data Assimilation of Terrestrial Water Storage Observations to Estimate Precipitation Fluxes: A Synthetic Experiment

Manuela Girotto,Rolf Reichle,Matthew Rodell,Viviana Maggioni

doi:10.3390/rs13061223

Abstract

The Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On (GRACE-FO) mission provide unprecedented observations of terrestrial water storage (TWS) dynamics at basin to continental scales. Established GRACE data assimilation techniques directly adjust the simulated water storage components to improve the estimation of groundwater, streamflow, and snow water equivalent. Such techniques artificially add/subtract water to/from prognostic variables, thus upsetting the simulated water balance. To overcome this limitation, we propose and test an alternative assimilation scheme in which precipitation fluxes are adjusted to achieve the desired changes in simulated TWS. Using a synthetic data assimilation experiment, we show that the scheme improves performance skill in precipitation estimates in general, but that it is more robust for snowfall than for rainfall, and it fails in certain regions with strong horizontal gradients in precipitation. The results demonstrate that assimilation of TWS observations can help correct (adjust) the model’s precipitation forcing and, in turn, enhance model estimates of TWS, snow mass, soil moisture, runoff, and evaporation. A key limitation of the approach is the assumption that all errors in TWS originate from errors in precipitation. Nevertheless, the proposed approach produces more consistent improvements in simulated runoff than the established GRACE data assimilation techniques.

Highlights

Introduction and Research QuestionsThe Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On (GRACE-FO) mission have been providing observations of changes in the Earth’s water storage since2002 ([1,2])
This study focuses on terrestrial water storage (TWS) data assimilation, broadly similar to previous experiments but with some technical differences that highlight another way of leveraging GRACE observations to further improve land surface model output
The GRACE and GRACE-FO satellite missions have been measuring TWS globally for nearly two decades at spatial and temporal resolutions that are often considered too coarse for practical hydrological applications

Summary

Introduction

The Gravity Recovery and Climate Experiment (GRACE) mission and its Follow-On (GRACE-FO) mission (hereinafter “GRACE” will be used to refer to both GRACE and GRACE-FO) have been providing observations of changes in the Earth’s water storage since. 2002 ([1,2]) These mass change observations are temporally averaged, typically monthly, measurements of anomalies (departures from the long term mean at a given location) of the total mass of water stored on and beneath the land surface. The measured quantity is often referred to as terrestrial water storage (TWS), which encompasses snow, groundwater, soil moisture, and surface water in lakes, wetlands, and rivers. The assimilation of TWS data into a land surface model has been used to downscale the coarse resolution of GRACE observations and to separate the TWS components (e.g., [4,5,6]).

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