Seasonal Hydroclimate Variability over North America:

Global and Regional Reanalyses Faulty Evapotranspiration

Alfredo Ruiz-Barradas

Sumant NigamDepartment of

Atmospheric and Oceanic ScienceUniversity of Maryland

September 30, 2008

CPPA: 2008 PIs MeetingSilver Spring, MD

September 29-October 1

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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PrecipitationJJA Standard Deviation

J, J, A GPP Index

Regressions

Box (100º-90ºW,35º-45ºN) defines Great Plains

Precipitation (GPP) Index

Region is at the center of theDiscussion of local vs remote forcing of

precipitation variability. CI=0.3 mm/day

The Beginning: An interannual

precipitation variability analysis: NARR

MFC > ETin reanalyses.

High expectations on NARR

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Atmospheric Water Balance

(1979-1999)

Area-average over the Great Plains

(100ºW-90ºW,35º-45ºN)

Is ET large in NARR?

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Atmospheric Water Balance

(1979-1999)

Yes, ET seems to be large From the AWB point of view.

Is ET large in NARR?

It is comparable to:NCEP/NCAR

Larger than:ERA-40JRA-25VICResidual NARR

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From Rasmusson, E. (1968; MWR, 96, 720-734)

(110º-80ºW,30º-50ºN)

Comparison with Rasmusson (1968; MWR, 96, 720-734) over Central Plains and Eastern US regions

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Terrestrial Water Balance

NARR(1979-1999)

Annual mean: BackgroundAnnual cycle: Dials

P-E ≈ Runoff

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NA

RR

Annual mean runoff in NARR is smaller than the observationally constrained product

River Discharge Composite

Runoff constrained byspatially distributed river discharge

This implies that annual mean P – E is underestimated or E is overestimated because P is very well assimilated in NARR 8

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 NARR10

Winter

Summer

P-E

Run

off

P-E

Run

off

In NARR:P-E ≈ ∂ws/∂t

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Area-average over the Great Plains

(100ºW-90ºW,35º-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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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/Documentation/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.

It is known that:

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. 13