Transcript Discharge Comparing NARR, ERA-40, and Observed Hudson Bay River Stephen J. Déry

Comparing NARR, ERA-40, and Observed Hudson Bay River Discharge
Stephen J. Déry1, Eric F. Wood2, and Christopher Kerr3
1Atmospheric
and Oceanic Sciences Program, Princeton University, Princeton, NJ
2Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ
3Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, NJ
Introduction
Rivers provide a natural pathway for freshwater on the land surface and constitute a vital link between the
atmosphere, the land surface, and the oceans. Streams and rivers integrate spatially and temporally
atmospheric and land surface processes at the catchment-scale, providing a mechanism by which climate
change may be detected. One area where significant climate change is ongoing is the Arctic (Serreze et al.
2000). Rising surface air temperatures are altering the hydrologic cycle of pan-Arctic drainage basins,
including freshwater discharge (Peterson et al. 2002; Déry et al. 2005).
One major pan-Arctic river basin that has undergone recent changes is the Hudson Bay Basin (which
includes the James Bay and Ungava Bay basins, hereafter referred to as HBB). The HBB covers an area of
~3.7 × 106 km2, or more than one third of Canada (Fig. 1). Its freshwater discharge of ~900 km3 yr-1 equates
one fifth of the total annual runoff to the Arctic Ocean and affects high-latitude oceanographic,
atmospheric, cryospheric, and biologic processes (Aagaard and Carmack 1989).
Given the importance of the HBB drainage network in the global freshwater budget and the recent decline
in the number of monitoring flow gauges in Canada (Shiklomanov et al. 2002), there is an urgent need to
report historical discharge rates of Canadian rivers with outlets into Hudson Bay. It is also critical to
validate comprehensive hydrometeorological datasets such as the North American Regional Reanalysis
(NARR) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis data
(ERA-40) in the polar environment. The goal of this preliminary study is to compare HBB river discharge
measurements with the modeled streamflow in the NARR and the ERA-40.
Results (cont.)
Figure 3 presents the annual cycle of daily river
discharge into HBB. The measured flow rates
are relatively low during winter and early
spring, achieving a mean minimum of 0.21 mm
day-1 on 5 April. As spring advances, snowmelt
accelerates the rate of discharge and the mean
annual maximum flow rate of 1.37 mm day-1,
as a whole, is reached on average each 25 June.
During summer and early fall, river discharge
remains relatively high, with a secondary
plateau attained on average in late September
and early October. Observed discharge rates
then gradually decrease to the low flow regime
of the winter season.
Compared to the observed state, the
reanalysis products exhibit an amplified
seasonal cycle, with higher daily discharge
rates during the snowmelt period (Fig. 3).
The modeled freshets occur nearly two
months prior to the observed spring peaks.
The NARR discharge rates decrease during
the summer and approach zero during
winter. Discrepancies in the annual cycle of
daily discharge arise primarily owing to the
lack of river routing schemes in the
reanalysis products.
Figure 3: Mean annual cycle of observed, ERA-40,
and NARR daily river discharge in HBB, 1982-1987.
Figure 4 illustrates the mean annual observed,
ERA-40, and NARR river discharge per
contributing area in the HBB over the period
1982-1987. The measured discharge records
show higher (lower) discharge rates per
contributing area in eastern (western) Canada
where precipitation rates are relatively high
(low) and evaporation rates are relatively low
(high).
Figure 1: River basins of Canada (Source: Atlas of Canada). The Hudson Bay Basin is highlighted in
yellow/orange and covers ~3.7 × 106 km2 in central Canada.
Data
1) OBSERVATIONS: Measured freshwater discharge rates for rivers that drain into HBB are compiled
in Environment Canada's Hydrometric Database (HYDAT; http://www.msc.ec.gc.ca/wsc/hydat/H2O/
index_e.cfm, 2004). HYDAT is a comprehensive observational database that provides daily discharge
rates for 42 of the main rivers with outlets into Hudson Bay (see Fig. 1).
2) ERA-40: The ERA-40 dataset provides instantaneous discharge rates over the global land surface
generated by a numerical weather prediction model using a constant analysis framework (ECMWF;
http://data.ecmwf.int/data/d/era40_daily/, 2004). The ERA-40 data used in this study are 6-hourly values
of river runoff on a 1.1o reduced Gaussian grid. A subset of 266 model grid cells covering 3.5 × 106 km2
represents the HBB.
3) NARR: Instantaneous discharge rates from the NARR are also used in the analysis (NCEP;
http://wwwt.emc.ncep.noaa.gov/mmb/rreanl/, 2004). The NARR provides 3-hourly surface and
subsurface runoff as generated by the Eta numerical weather prediction model using a fixed analysis
scheme. A subset of ~3400 grid points from the original NARR mesh (32-km horizontal resolution) are
used to represent HBB river discharge. The surface and subsurface components are summed to provide
the 1982-1987 daily and annual HBB river discharge rates that are compared to the other two datasets.
Overall, the ERA-40 and NARR discharge rates
exhibit similar spatial patterns to the
observations. For instance, discharge rates are
greater (lesser) on the eastern shores of Hudson
Bay. The NARR captures the observed
streamflow minimum in the Canadian Prairies
and the observed streamflow maximum in
central Québec. The NARR grid point discharge
data tend to be more intense than observed or in
the ERA-40 dataset owing to its finescale
resolution.
Figure 4: Mean annual observed (top), ERA-40
(middle), and NARR (bottom) river discharge
(mm) in HBB (bold outline), 1982-1987.
Conclusion and Future Work
A comparison of the observed, ERA-40, and NARR river discharge rates in HBB has been conducted.
Our preliminary results show that, for the period 1982-1987, the NARR annual discharge rates compare
favorably with the observed data with a mean absolute difference of 31 mm yr-1. The presented annual
cycle of daily discharge rates from the two reanalysis products exhibits an amplified seasonal cycle
compared to the observations owing to the lack of a river routing for the ERA-40 and NARR grid-level
runoff. The NARR discharge rates per contributing area show similar patterns to the observations, with
high discharge rates occurring in central Québec and low streamflow rates in the Canadian Prairies.
Further analysis of the NARR terrestrial hydrologic cycle, using the complete period (1979-2003), is being
undertaken by the authors and should allow a more complete assessment of the NARR dataset for
hydrologic studies. Further investigations of large-scale atmospheric teleconnections and their role in the
HBB hydrologic cycle such as the recent study by Déry and Wood (2004) will also be conducted.
Results
References
Figure 2 shows that the mean annual discharge
rate observed in 42 HBB rivers is 235 mm yr-1.
The mean annual discharge rates from the
ERA-40 and NARR are 302 and 204 mm yr-1,
respectively (Table 1). The mean absolute
errors between for the ERA-40 and NARR
compared to the observations are 67 and 31
mm yr-1, with the NARR showing a larger
standard deviation than the ERA-40.
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Déry, S. J., M. Stieglitz, E. C. McKenna, and E. F. Wood, 2005: Characteristics and trends of river
discharge into Hudson, James, and Ungava Bays, 1964-2000. Submitted to J. Climate.
Peterson, B. J., R. M. Holmes, J. W. McClelland, C. J. Vörösmarty, R. B. Lammers, A. I. Shiklomanov,
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Table 1: Statistics on the comparisons
between the annual observed and the ERA40 and NARR river discharge rates in HBB,
1982-1987. All units are mm yr-1. SD,
standard deviation; MAE, mean absolute
error.
Statistic
Observed ERA-40
NARR
Mean
235
302
204
SD
8.4
8.4
24.7
MAE
…
67.0
31.3
Acknowledgments
Figure 2: Annual observed, ERA-40, and NARR
river discharge in HBB, 1982-1987.
This research is supported by NSF grant OPP02-30211 and NOAA grant NA17RJ2612. The authors
thank Ken Mitchell (NCEP), Marco Carrera (NCEP), and Kirsten Findell (GFDL) for technical support
and their useful comments.