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Atmospheric Power Law BehaviorA Look at Southeastern US Temperatures
James Duncan
Motivation & Introduction
Extreme climatic events are weather phenomena
that occupy the tails of a dataset‟s probability density
function (PDF).
Advanced stochastic theory asserts that power law
distributions should exist in the tail ends of our data.
Questions to Answer:
Show That Power Law Distributions Are Evident within
Temperature Data.
Analyze how power law distributions change with varying
weather and climatic patterns (seasons, ENSO, etc.).
What is a Power Law
Distribution?Mathematically, a power law probability distribution of
quantity x may be written as:
Where α is the exponent or scaling parameter and C is
the normalization constant.
p(x) =a -1
xmin
x
xmin
æ
èç
ö
ø÷
-a
p x( ) =Cx-a
[Neelin et al. 2011]
Data & Methods
Data
Methods
Daily observed maximum and minimum temperatures across the
southeastern United States (AL, FL, GA, NC, SC) spanning 1960-
2009.
Measures of quality control have been put in place resulting in the
omission of 20 stations.
Trends have been removed from the data. If data is to follow a power law distribution, it does so above some
lower bound xmin.
To find our lower bound, we employ the Kolmogorov-Smirnov or KS
Statistic which calculates the maximum difference between the CDF of the
observed data and estimated power law distribution.
To calculate our scaling parameter α, we employ the “method of
maximum likelihood”.
ˆ a =1+ n lnx i
xmini=1
n
åé
ë ê
ù
û ú
-1
D = maxx³xmin
|F(x) -P(x) |
Significance Testing
Employ the use of a goodness of fit test which will
measure and analyze the KS distance of our power
law distribution with that of other synthetically
derived power law distributions.
From this goodness of fit test, we are able to derive
a „p-value‟ which expresses the probability that the
estimated power law distribution is a good fit to the
observed data.
Presence of Power Law
Distributions
Power Law Fit &
Significance Skewness
KurtosisP-Value Tests
Criteria Is Power Law Fit Significant?
Ppower>0.10 and Pgauss<0.10 YES
Ppower<0.10 and Pgauss>0.10 NO
Ppower>0.10 and Pgauss>0.10 but Ppower>Pgauss Both Fits Are Significant, But Can Say Power Law is Better Fit (YES)
Ppower<0.10 and Pgauss<0.10 NO
Xmin & Alpha
More analysis is needed to adequately note whether patterns exist in the spatial
distributions of Xmin and Alpha.
Distinguished Power Laws
Tamiami, FL
Distinguished Criteria (ppower>0.90 & pgauss~0)
Hialeah, FL
Asheville, NC
Henderson, NC
Maximum
Temperatures
Minimum
Temperature
s
Seasonal Shifts in
Power Law Distributions
Now that we have established that power law
distributions are existent, how are they
modulated by changes in the seasonal cycle?
Fall Power Law Fit &
Significance Skewness
KurtosisP-Value Tests
Criteria Is Power Law Fit Significant?
Ppower>0.10 and Pgauss<0.10 YES
Ppower<0.10 and Pgauss>0.10 NO
Ppower>0.10 and Pgauss>0.10 but Ppower>Pgauss Both Fits Are Significant, But Can Say Power Law is Better Fit (YES)
Ppower<0.10 and Pgauss<0.10 NO
Future Work & Conclusions
There does appear to be a dynamic link between
areas of significant power law fit and areas of distinct
skewness and kurtosis.
Further examine how power law distributions change
with respect to season, ENSO, and other climatic
cycles.
Look to see if these modulations in the power law
distribution may be explained by any specific physical
processes.
Look into more ways to objectively characterize
changes in the power law parameters (Xmin and
Alpha) and distribution.
ReferencesClauset, A., C. R. Shalizi, and M. E. J. Newman, 2009: Power-law distributions in empirical data, SIAM Rev., 51, 661-
703.
Neelin, D., and T. W. Ruff, 2011: Long tails in regional surface temperature probability distributions with implications for
extremes under global warming. Geophys. Res. Lett., 39, l04704, doi: 10.1029/2011GL05061.
Newman, M. E. J., 2005: Power laws, Pareto distributions and Zipf‟s law, Contemp. Phys., 46, 323-351.
Sura, P., 2011: A general perspective of extreme events in weather and cliamte. Atmos. Res., 101, 1-21.
Stefanova, L., P. Sura, and M. Griffin, 2012: Quantifying the non-Gaussianity of wintertime daily maximum and
minimum temperatures in the Southeast United States. J. Climate, in press.
Winter Power Law Fit &
SignificanceSkewness KurtosisP-Value Tests
Criteria Is Power Law Fit Significant?
Ppower>0.10 and Pgauss<0.10 YES
Ppower<0.10 and Pgauss>0.10 NO
Ppower>0.10 and Pgauss>0.10 but Ppower>Pgauss Both Fits Are Significant, But Can Say Power Law is Better Fit (YES)
Ppower<0.10 and Pgauss<0.10 NO
Values of Xmin
Appears to be several more distinct regions of behavior than Annual Behavior;
however, more analysis and comparison is need to adequately depict the
potential patterns developing spatially.