Weather and Climate Extremes in a Changing Climate. Regions of ...

The U.S. Climate Change Science Program Chapter 2
Figure 2.7 Area of drought in the western United States as reconstructed from
tree rings (Cook et al., 2004).
One of the clearest
trends in the U.S.
observational record
is an increasing
frequency and
intensity of heavy
precipitation events.
46
episodes of extremely high temperatures. The
month of July 1936 in the central United States
is a notable example. To illustrate, Lincoln, NE
received only 0.05” of precipitation that month
(after receiving less than 1 inch the previous
month) while experiencing temperatures reaching
or exceeding 110°F on 10 days, including
117°F on July 24. Although no studies of trends
in such “compound” extreme events have been
performed, they represent a significant societal
risk.
2.2.2.2 shOrt dur atiOn heav y
PreCiPitatiOn
2.2.2.2.1 Data Considerations and Terms
Intense precipitation often exhibits higher
geographic variability than many other extreme
phenomena. This poses challenges for the analysis
of observed data since the heaviest area of
precipitation in many events may fall between
stations. This adds uncertainty to estimates of
regional trends based on the climate network.
The uncertainty issue is explicitly addressed in
some recent studies.
Precipitation extremes are typically defined
based on the frequency of occurrence (by
percentile [e.g., upper 5%, 1%, 0.1%, etc.] or
by return period [e.g., an average occurrence of
once every 5 years, once every 20 years, etc.]),
and/or their absolute values (e.g., above 50 mm,
100 mm, 150 mm, or more). Values of percentile
or return period thresholds vary considerably
across North America. For example, in the
United States, regional average values of the
99.9 percentile threshold for daily precipitation
are lowest in the Northwest and Southwest
(average of 55 mm) and highest in the South
(average of 130mm) 19 .
As noted above, spatial patterns of precipitation
have smaller spatial correlation scales (for
example, compared to temperature and atmospheric
pressure) which means that a denser
network is required in order to achieve a given
uncertainty level. While monthly precipitation
time series for flat terrain have typical radii
of correlation 20 (ρ) of approximately 300 km
or even more, daily precipitation may have
ρ less than 100 km with typical values for
convective rainfall in isolated thunderstorms
of approximately 15 to 30 km (Gandin and
Kagan, 1976). Values of ρ can be very small
for extreme rainfall events, and sparse networks
may not be adequate to detect a desired minimum
magnitude of change that can result in
societally-important impacts and can indicate
important changes in the climate system.
2.2.2.2.2 United States
One of the clearest trends in the United States
observational record is an increasing frequency
and intensity of heavy precipitation events (Karl
and Knight, 1998; Groisman et al., 1999, 2001,
2004, 2005; Kunkel et al., 1999b; Easterling et
al., 2000; IPCC, 2001; Semenov and Bengtsson,
2002; Kunkel, 2003). One measure of this
is how much of the annual precipitation at a
location comes from days with precipitation
exceeding 50.8 mm (2 inches) (Karl and Knight,
1998). The area of the United States affected
by a much above normal contribution from
these heavy precipitation days increased by
a statistically significant amount, from about
9% in the 1910s to about 11% in the 1980s and
1990s (Karl and Knight, 1998). Total precipitation
also increased during this time, due in
large part to increases in the intensity of heavy
precipitation events (Karl and Knight, 1998). In
19 The large magnitude of these differences is a major
motivation for the use of regionally-varying thresholds
based on percentiles.
20 Spatial correlation decay with distance, r, for many
meteorological variables, X, can be approximated by
an exponential function of distance: Corr (X(A), X(B))
~ e - r/ρ where r is a distance between point A and B and
ρ is a radius of correlation, which is a distance where
the correlation between the points is reduced to 1/e
compared to an initial “zero” distance.