Seasonal Hydroclimate Variability over North America: Global and Regional Reanalyses Faulty Evapotra

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Seasonal Hydroclimate Variability over North
AmericaGlobal and Regional Reanalyses Faulty
Evapotranspiration

• Alfredo Ruiz-Barradas
• Sumant Nigam

Department of Atmospheric and Oceanic
Science University of Maryland
CPPA 2008 PIs Meeting
Silver Spring, MD September 29-October 1
September 30, 2008
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Data Sets

• Global Reanalyses NCEP/NCAR
• ERA-40
• JRA-25
• Regional Reanalysis NARR
• Other data sets GSWP-2 (multi-model mean,
  global)
• VIC (US optimized)
• UNH/GRDC (Fekete
  et al. 2002)
• US-MEX
• Period 1979-1999

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The Beginning An interannual precipitation
variability analysis NARR
J, J, A GPP Index Regressions
Precipitation JJA Standard Deviation
MFC gt ET in reanalyses. High expectations on NARR
Box (100º-90ºW,35º-45ºN) defines Great
Plains Precipitation (GPP) Index
Region is at the center of the Discussion of
local vs remote forcing of precipitation
variability.
CI0.3 mm/day
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Atmospheric Water Balance (1979-1999)
Area-average over the Great Plains (100ºW-90ºW,35
º-45ºN)
Is ET large in NARR?
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Atmospheric Water Balance (1979-1999)
Is ET large in NARR? It is comparable
to NCEP/NCAR Larger than ERA-40 JRA-25 VIC
Residual NARR
Yes, ET seems to be large From the AWB point of
view.
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Comparison with Rasmusson (1968 MWR, 96,
720-734) over Central Plains and Eastern US
regions
(110º-80ºW,30º-50ºN)
From Rasmusson, E. (1968 MWR, 96, 720-734)
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Terrestrial Water Balance NARR (1979-1999)
Annual mean Background Annual cycle Dials
P-E Runoff
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River Discharge
Composite Runoff constrained by spatially
distributed river discharge
NARR
Annual mean runoff in NARR is smaller than the
observationally constrained product
This implies that annual mean P E is
underestimated or
E is overestimated because
P is
very well assimilated in NARR
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The Terrestrial Water Balance in Global Reanalyses
P-E Runoff
Driest
Wettest
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The Terrestrial Water Balance in Offline
Land-Sfc. Models
P-E Runoff
Runoff in VIC and UNH/GRDC products have a
similar structure in the annual mean
This supports the idea that Runoff (E) is
underestimated (overestimated) in NARR
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P-E
Winter
Runoff
In NARR P-E ?ws/?t
P-E
Summer
Runoff
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Area-average over the Great Plains (100ºW-90ºW,35
º-45ºN)

• Seasonal imbalances in NARR from spring to fall
  are due to the highlighted errors in runoff and
  evaporation.
• JRA-25 has the largest seasonal imbalances,
  although its annual
• imbalance is comparable to that
• in ERA-40 and 1/2 of that in NCEP/NCAR.
• Seasonal imbalances in JRA-25
• are due to overestimation in
• P - E, the change in water storage, and runoff as
  well as the almost nonexistent annual cycle in P
  - E.
• ERA-40 has larger imbalances
• from spring to fall due to the
• small and almost uniform change
• in water storage during those
• seasons.

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It is known that
Noah, the LSM in NARR, had a large positive bias
in summer evaporation over regions of non-sparse
vegetation cover, such as the eastern US, and
that the bias was related to canopy resistance
parameters in the model (Mitchell et al
2005 http//www.emc.ncep.noaa.gov/gc_wmb/Document
ation/TPBoct05/T382.TPB.FINAL.htm). Upgrades to
the Noah model, including the correction of the
evaporation bias, were implemented in middle
2005, almost two years after the completion of
NARR at the end of 2003.
Conclusions

• It is clear that reanalyses have some problems in
  reproducing the terrestrial water cycle over
  North America, particularly the global products.
  The most recent global reanalysis, the Japanese
  reanalysis, does not improve the representation
  of the climatological features of the terrestrial
  water cycle over North America.
• The regional reanalysis NARR severely
  overestimates evaporation (and the change in
  water storage) that leads to the underestimation
  of runoff. Is the problem fix in its LSM?
• In any case, these results make clear the need
  for a correction in the assimilation process in
  NARR in which some observational constraints on
  the land-surface part are needed to generate
  realistic hydroclimate fields.