Post on 25-Feb-2016
description
UBC/UW 2011 Hydrology and Water Resources
SymposiumFriday, September 30, 2011
DIAGNOSIS OF CHANGING COOL SEASON PRECIPITATION STATISTICS IN THE WESTERN
U.S. FROM 1916-2003
Joe Hamman, Research AssistantDept. of Civil and Environmental EngineeringUniversity of Washington
Alan F. Hamlet, Research Assistant Professor JISAO/CSES Climate Impacts GroupDept. of Civil and Environmental Engineering, University of Washington
Motivation Why we’re studying cool season precipitation statistics.
This Study (Part 1): Telling the Statistical Story Trying to answer questions about trends and changes in variance. We’ll throw around cool season statistics and a few colorful
graphs.This Study (Part 2): EOF Analysis
What can we learn from the leading modes? Interested in the reduced dipole signal.
Conclusions Implications of changing precipitation regimes. What does the paleo-record tell us? Can we distinguish between green house gas forced climate
change and natural variability?
OUTLINE
THE DATA
Mean (mm)
Standard Deviation
Coefficient of Variation
Source: Hamlet, A. F., & Lettenmaier, D. P. (2005). Production of
Temporally Consistent Gridded Precipitation and Temperature Fields for the Continental United States. Journal of Hydrometeorology, 330-336.
1/16 t h degree grid cel ls Cool Season (Oct – Mar) totals for each grid cel l Period of Record: 1916 - 2003
Mean Annual Streamflow Strong interannual
variability Mountainous west
is heavily dependent on winter precip for summer flows
Implications for hydropower, agricultural and in stream uses
MOTIVATION
1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 20200
1000
2000
3000
4000
5000
6000
7000
Skykomish River, WA Mean Annual Flow
Dis
char
ge, c
ubic
fee
t pe
r se
cond
MOTIVATION
Peak Annual Streamflow Observed
expanded variance
Implications for flood control
Is this increase in variance part of natural variability?
MOTIVATION
-4
-3
-2
-1
0
1
2
3
4
Sta
ndar
dize
d A
nom
alie
s
Cool Season Precipitation
PNWGBRCACRB
1920 1930 1940 1950 1960 1970 1980 1990 2000-4
-2
0
2
4
ENSO / PDO Indecies
ENSOPDO
warm
warm
cool
Largest positive trends are along coast
All 4 regions were wetter after 1977 Regional trends are only
significant in the Colorado River Basin post 1977
Other regions do not show significant changes post 1977
TRENDS IN COOL SEASON PRECIPITATION
13.5
-3.0Cool season precipitation
trends (1916-2003)
Statistically significant change in variance (p < 0.05) in all four regions after 1977.
OBSERVED CHANGES IN VARIANCE
-4
-3
-2
-1
0
1
2
3
4
Std
. Ano
mal
ies
Cool Season Precipitation
1920 1930 1940 1950 1960 1970 1980 1990 20000.050.1
0.150.2
0.250.3
0.35
CV
21 Year Running Coefficient of VariationPNWGBRCACRB
1920 1930 1940 1950 1960 1970 1980 1990 2000-4
-2
0
2
4PC 1
1920 1930 1940 1950 1960 1970 1980 1990 20000
0.01
0.02
0.03
0.0421 Year Running Standard Deviation
Represents west wide synchronicity. Variance increases earl ier than in observed record. The expansion of variance in PC1 explains both increased variance and
increased regional coherence in the regional average t ime series. PC1 shows more red noise characterist ics post 1975 indicat ing increasing
persistence and ampl itude of cohesion throughout the west.
EOF ANALYSISMODE 1
1 -11 -1
EOF 1
Dipole structure between southwest and northwest Steady increase in variance• PC2 seems to be functioning more on interannual time scales
and exhibits less long-term persistence.
EOF ANALYSISMODE 2
1 -11 -1
1920 1930 1940 1950 1960 1970 1980 1990 2000-4
-2
0
2
4PC 2
1920 1930 1940 1950 1960 1970 1980 1990 20000
0.02
0.04
0.06
0.08 21 Year Running Standard Deviation
EOF 2
IMPLICATIONS AND FUTURE RESEARCH
Implications on Hydropower Production Expanded variance Increased inter-regional correlation
-2
-1
0
1
2
3
1917
1919
1921
1923
1925
1927
1929
1931
1933
1935
1937
1939
1941
1943
1945
1947
1949
1951
1953
1955
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
Sys
tem
Wid
e H
ydro
pow
er
Pro
duct
ion
(Std
Ano
mal
ies)
CRB
SSJ
PNW
Correlation:CRB-SSJ = 0.07CRB-PNW = 0.08SSJ-PNW = 0.36
Correlation:CRB-SSJ = 0.14CRB-PNW = -0.14SSJ-PNW = 0.06
Correlation:CRB-SSJ = 0.73CRB-PNW = 0.51SSJ-PNW = 0.65
Figure: Alan Hamlet
IMPLICATIONS AND FUTURE RESEARCH
Paleo Records Paleological records support the argument that natural
variability is sufficient to explain the late 20th century patterns in the CORB and SSJ.
Currently, paleological records are being developed for the other basins
CONCLUSIONS
Strong spatial and temporal correlations between the leading EOFs and the ENSO/PDO patterns further confirm the role of these patterns in Western U.S precipitation.
Marked changes in statistics coincide with statistically significant changes in the variance associated ENSO and PDO.
It remains unclear whether global climate change is contributing to the effects. GCMs do not reliably simulate the changes in precipitation
variability seen in the observed record, thus conventional detection and attribution studies typically fail in the initial step. However, this could also be related to other factors such as GCM deficiencies in simulating cool season precipitation variability at the regional scale.
There is currently little evidence to support the hypothesis that recent changes in cool season precipitation variability are an expression of global greenhouse forcing.
Questions?
QUESTIONS?
SNOWPACK
Snowpack Produced by cool season precipitation Important in terms of annual flows and
flooding
Figure: Effects o f prec ip i ta t ion t rends on Apr i l 1 SWE exc lud ing temperature changes , 1916-2003. (Hamlet e t a l . , 2005)
Fig. 2 Decadal departures in reconstructed 1 April SWE for watersheds predominately within the U.S. portion of the North American cordillera.
G T Pederson et al. Science 2011;333:332-335