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been achieved in so-called ''off-line'' (land-only) assimilation systems. These advances
need to be incorporated into the coupled land-atmosphere systems used in atmospheric
data assimilation and numerical weather prediction (NWP). Ground-breaking advances in
coupled land-atmosphere data assimilation are being made, for example, at ECMWF (de
Rosnay et al.
2012a
,
b
). At the same time, the coupling of the GEOS-5 LDAS to the
GEOS-5 atmospheric data assimilation system is underway at the NASA GMAO.
Moreover, much of the progress in land data assimilation has been with systems that
assimilate only one type of observation, often surface soil moisture. In future, more
emphasis will need to be placed on the assimilation of multiple types of observations
within a single assimilation system, including observations of water cycle components
such as soil moisture, SWE, snow cover fraction, TWS, and precipitation.
Future development should also address the addition or improvement of runoff routing
and surface water storage model components in the global land surface models used in
NWP. The planned NASA Surface Water and Ocean Topography (SWOT; Durand et al.
2010
) mission, for instance, will provide high-resolution observations of surface water
elevation. To improve our understanding of the global hydrological cycle, it will be crucial
to incorporate these new observations into global land data assimilation systems, building
on early studies such as those by Andreadis et al. (
2007
), Biancamaria et al. (
2011
), and
Durand et al. (
2008
).
Finally, the existing global land data assimilation systems will need to consider the
modeling of vegetation dynamics and the assimilation of current or planned satellite obser-
vations such as the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR),
the Leaf Area Index (LAI), or the multi-angular Photochemical Reflectance Index (PRI)
(Albergel et al.
2010
; Hilker et al.
2012
; Kaminski et al.
2012
; Knorr et al.
2010
; Mu˜ oz
Sabater et al.
2008
; St
¨
ckli et al.
2011
). Furthermore, current microwave sensors already
provide observations of the freeze-thaw state of the landscape at coarse scales (Kim et al.
2010
), and SMAP will provide much higher-resolution observations with continental cov-
erage (Entekhabi et al.
2010
). These vegetation and freeze-thaw observations link the
hydrological and carbon cycles and should be used in global land data assimilation systems.
Acknowledgments The authors thank the organizers of the ISSI Workshop on ''The Earth's Hydrological
Cycle'' held February 6-10, 2012 and two anonymous reviewers for their efforts. The research was sup-
ported by the NASA program on The Science of Terra and Aqua, the NASA Soil Moisture Active Passive
mission, the NASA Postdoctoral Program, and the NASA High-End Computing program.
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