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

to calibrate against existing techniques, and
with poor documentation where such calibrations
were done. Thus the available tropical
cyclone data contain an inhomogeneous mix of
changes in quality of observing systems, reporting
policies, and the methods utilized to analyze
the data. As one example, the Dvorak technique
– a subjective method that is routinely applied
in all ocean basins to estimate tropical cyclone
intensity using satellite imagery – was not
introduced until the early 1970s and has evolved
markedly since then. The technique originally
utilized visible satellite imagery and was based
on subjective pattern recognition. At that time,
intensity estimates could only be made during
daylight hours. In the early- to mid 1980s, the
Dvorak technique was significantly modified to
include digital infrared satellite imagery (which
is available 24 hours per day), and has become
the de facto method for estimating intensity in
the absence of aircraft reconnaissance. (See
Chapter 4 for suggested measures to improve
consistency).
Insufficient efforts in re-examining and quality
controlling the tropical cyclone record on a year
to year basis, particularly outside the Atlantic
and eastern North Pacific regions, have resulted
in substantial uncertainties when using best
track data to calculate changes over time. Efforts
are ongoing to reanalyze the historic best
track data, but such a posteriori reanalyses are
less than optimal because not all of the original
data that the best track was based on are readily
available.
Compared to earlier periods, tropical cyclonecounts
are acceptable for application to
long-term trend studies back to about 1945 in
the Atlantic and 1970 in the Eastern Pacific
(e.g., Holland and Webster, 2007 and references
therein), and back to about 1975 for the Western
and Southern Pacific basins, thanks to earthorbiting
satellites (e.g., Holland, 1981). Until
the launch of MeteoSat-7 in 1998, the Indian
Oceans were seen only obliquely, but storm
counts may still be expected to be accurate
after 1977. In earlier periods, it is more likely
that storms could be missed entirely, especially
if they did not pass near ships at sea or land
masses. For the North Atlantic, it is likely that
up to 3 storms per year were missing before
1900, dropping to zero by the early 1960s
Weather and Climate Extremes in a Changing Climate
Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands
(Holland and Webster, 2007; Chang and Guo,
2007). Estimates of the duration of storms are
considered to be less reliable prior to the 1970s
due particularly to a lack of good information
on their time of genesis. Since the 1970s, storms
were more accurately tracked throughout their
lifetimes by geostationary satellites.
Estimates of storm intensity are far less reliable,
and this remains true for large portions of the
globe even today. Airborne hurricane reconnaissance
flight became increasingly routine
in the North Atlantic and western North Pacific
regions after 1945, but was discontinued in the
western North Pacific region in 1987. Some
missions are today being conducted under the
auspices of the government of Taiwan. However,
airborne reconnaissance only samples a small
fraction of storms, and then only over a fraction
of their lifetimes; moreover, good, quantitative
estimates of wind speeds from aircraft did not
become available until the late 1950s. Beginning
in the mid-1970s, tropical cyclone intensity
has been estimated from satellite imagery. Until
relatively recently, techniques for doing so
were largely subjective, and the known lack of
homogeneity in both the data and techniques
applied in the post-analyses has resulted in
significant skepticism regarding the consistency
of the intensity estimates in the data set. This
lack of temporal consistency renders the data
suspect for identifying trends, particularly in
metrics related to intensity.
Recent studies have addressed these known
data issues. Kossin et al. (2007a) constructed a
more homogeneous record of hurricane activity,
and found remarkably good agreement in
Compared to
earlier periods,
tropical cyclone
counts are
acceptable for
application to
long-term trend
studies back to
about 1945 in the
Atlantic and 1970 in
the Eastern Pacific.
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