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Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Trends and patterns in extreme precipitation usingextreme value analysis
Chris Paciorek(Department of Statistics; University of California, Berkeley)
Michael Wehner (LBNL)Prabhat (LBNL)
www.stat.berkeley.edu/˜paciorek
Research supported by DOE DE-AC02-05CH11231
August 2013
Chris Paciorek Extreme precipitation 1
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Statistical extreme value theory
The Generalized Extreme Value (GEV) distribution:
F (x) = exp
(−[
1 + ξ
(x − µσ
)]−1/ξ)Location parameter µ, scale parameter σ, shape parameter ξ,generally fit via maximum likelihood.Unites the bounded Weibull distribution (ξ < 0 ),exponential-tailed Gumbell distribution (ξ = 0), andheavy-tailed Frechet distribution (ξ > 0)Asymptotic theory says that the distribution of block maxima(or minima) converges to the GEV distribution as the blocksize goes to infinity.By the quantiles of the GEV distribution, the MLE for the
1/p-year return level is: ẑp = µ̂− σ̂ξ̂(
1− (− log(1− p))−ξ̂)
Chris Paciorek Extreme precipitation 2
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Example: Berkeley winter precipitation
1950 1970 1990 2010
510
15
max. winter daily precip.
year
prec
ipita
tion
(cm
)
precipitation (cm)
Den
sity
5 10 15
0.00
0.05
0.10
0.15
0.20
estimated 100−yrreturn value
Chris Paciorek Extreme precipitation 3
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Nonstationary extreme value analysis
Extremes may also vary by season, by time, and with covariates (inparticular teleconnections such as ENSO).
A basic strategy:
Fit separate models by seasonFit nonstationary models with respect to time and ENSO:
F (xt) = exp
(−[
1 + ξt
(xt − µtσt
)]−1/ξt)One might have all three parameters vary with time andENSO (linearly, polynomially, or based on splines).
Analyses often find little evidence (based on likelihood ratiotests) that ξ (and even σ) are varying with time, though ξ inparticular is hard to estimate even in a stationary model.
A basic model is linear in time (and possibly ENSO) in µ only,as a first-order estimate of the trend over time.
Chris Paciorek Extreme precipitation 4
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Peaks over threshold (POT) analysis
An alternative is to model all the exceedances over a highthreshold, c (e.g., the 95%ile or 99%ile of all rainy days in thedata). Why?
Don’t ’waste’ extreme observations that are not the blockmaximumReadily allow for missing data when data are missing forreasons unrelated to weatherFor precipitation, not clear that block maxima are appropriatein dry regions/seasons when there are few wet days(asymptotic conditions may not be satisfied)
A disadvantage is requiring the raw daily data, whereas blockmaxima can use data summaries/indices (e.g., HadEX2available only as indices)
Chris Paciorek Extreme precipitation 5
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Point processing modeling
The point process model implements the peaks-over-thresholdapproach by specifying the probability of the number ofexceedances (the intensity measure) and the likelihood of theactual exceedances (the intensity function). The stationaryversion is:
L(µ, σ, ξ; x1, . . . xn) ∝ exp
(−ny
[1 + ξ
(c − µσ
)]−1/ξ)·
N(A)∏i=1
1
σ
[1 + ξ
(xi − µσ
)]−1/ξ−1The parameters are equivalent to the GEV parameters andcan be used to compute return levels.
Asymptotics are with respect to the threshold getting larger.
Chris Paciorek Extreme precipitation 6
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
Example: Berkeley winter precipitation
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1950 1970 1990 2010
46
810
1214
1618
daily precip. (cm)
year
prec
ipita
tion
(cm
)
precipitation (cm)
Den
sity
4 6 8 10 12 14 16 18
0.0
0.1
0.2
0.3
0.4
estimated 100−yrreturn value
Chris Paciorek Extreme precipitation 7
Statistical Extreme Value AnalysisMethodological improvements
ResultsComments and software development
US extreme precipitation
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