Observational Verification of Previously Unidentified Tropical Cyclones in the Western & Southern Pacific Basins
Sabrina Abesamis, Lily Dove, Ryan Truchelut, and Robert E. Hart
Young Scholars Program, Florida State University
Background
Due to modern enhancements to
satellite technology, the detection
of present-day tropical cyclones
(TCs) has greatly improved in
accuracy. The lack of such scientific
means preceding the 1960s has
generated concern regarding the
oversight of prior undetected TCs,
which may have created a possible
bias in the historical record before
and including the early 20th century.
As seen in Figure 1, this under
sampling is particularly significant
in the Pacific basins in the presatellite era. This study is capable of
improving the historical record,
furthering research involving longterm TC trends and the possible
increase in storms per year due to
global climate change.
Type 1
A Type 1 Event is one for which sufficient data
provides evidence that no TC was present at the
time. For this conclusion to be drawn, no closed
surface circulation – which involves winds from all
cardinal directions within the same observation
time – is present. Winds greater than 33 knots or
sea level pressures below 1002 millibars are
typically not observed. If an event is definitively
non-tropical/frontal or there is clearly no closed
surface circulation, it is automatically classified as
Type 1, regardless of whether strong winds or low
pressures are present.
Candidate Event Classification Criteria
Figure 2
This sample observation
describes the sets of four
numbers located on the
candidate event study
maps.
Type 2
Type 2 events are those which cannot be fully
placed into either the Type 1 or Type 3 category.
Events which receive this classification often have
few or limited observations, which may occur within
a candidate. Other events classified as Type 2 are
those for which a closed circulation is present
without low pressures or strong winds or vice versa.
Type 3
Type 3 status is designated towards events which
are considered possible missing tropical cyclones.
In order to receive this classification, events must
have a closed circulation around a well-defined
center as well as two or more observations of
either winds greater than 33 knots or sea level
pressures less than 1002 millibars within any six
hour
observational
time period.
Such
observations do not have to be present during
the entire life of an event. These events are
clearly non-frontal and must have thickness
anomalies which are fairly symmetrical about the
estimated center of circulation.
Discussion
In the present day, the average tropical cyclone
seasons in the Western and Southern Pacific
Basins experience 20 to 25 storms per year.
Although the reanalysis method used in this study
is useful in detecting and presenting potential presatellite storms (designated as Type 3s), the lack
of observations in multiple candidate events
caused many promising events to be classified as
ambiguous, or Type 2, particularly in the Southern
Pacific. This significant weakness of the method
used can be seen in our results displayed in Figure
7 a and b.
Figure 7 (a & b)
Results of each event type received for both the Southern Pacific
and the Western Pacific Basins.
Conclusion
Figure 3
Figure 1
This plot (Truchelut and Hart, 2013) reveals the
notable undercount of TCs present in the Pacific
basins before the mid-20th century. The years
reanalyzed by this study are enclosed in the
boxes.
Methodology
Our
reanalysis
has
been
concentrated on the Southern and
Western Pacific Basins during the
1920s-1940s
due
to
the
considerable undercount present in
the current archives (see Figure 1).
By utilizing a thickness algorithm
developed by Truchelut and Hart
2011, the technique of identifying
candidate events is completely
automated, which has greatly
expedited the process compared to
former
manual
methods.
Classification was based on
National Hurricane Center (NHC)
criteria (Landsea et al., 2008) in
order to ensure consistent and
reliable results. The center panel
displays more information about
the three classifications.
This Type 1 event shows a frontal nature due to
its asymmetrical shape. It also demonstrates
weak winds and average pressures.
Figure 5
Figure 4
This Type 2 example lacks sufficient observations.
This Type 3 example exhibits excellent symmetry
around a well-defined circulation center. It has
multiple observations of 37 knot winds and
pressures below 1000 millibars.
Results
for
the
Reanalysis
of
TCs
in
Southern
&
Western
Pacific
Basins
Southern Pacific (1920-1937)
Western Pacific (1920-1929, 1937-1944)
6
6
Both graphs display the total number of candidate events for each type in each of the respective basins.
The data collected from this research indicate
that an average of about 9.7 events per year
from the Southern Pacific and 18 from the
Western Pacific were generated by the automatic
reanalysis technique. Of these candidate events,
an average of 0.41 and 6.18 missing TCs per year
were found in the Southern and Western Pacific
basins, respectively. From this data, we can
conclude that the Western Pacific Basin’s
candidate events provided more observations
and opportunities for higher classification as a
Type 3 than those events found in the Southern
Pacific Basin. The thickness algorithm has
already identified thousands more events that
await observational verification. With further use
of this reanalysis process, the accuracy and
completeness of the historical record of tropical
cyclones can continue to grow and expand.
Landsea, C. W., et al 2008: A Reanalysis of the 1911-20 Atlantic
Hurricane Database. J.Climate.
Truchelut, R. E., and R. E. Hart (2011), Quantifying the possible
existence of undocumented Atlantic warm‐core cyclones in
NOAA/CIRES 20th Century Reanalysis data, Geophys. Res. Lett.,38
Truchelut, Ryan and Robert Hart. "Global Identification of
Previously Undetected Tropical Cyclone Candidates in
NOAA/CIRES 20th Century Reanalysis Data." 30th Conference on
Hurricanes and Tropical Meteorology, Ponte Vedra Beach, FL. 16
April 2012. Conference presentation.
Truchelut, R., R. Hart, and B. Luthman, 2013: Global identification
of previously undetected pre-satellite era tropical cyclone
candidates in NOAA/CIRES 20th Century Reanalysis data. J. Appl.
Meteor. Climatol. doi:10.1175/JAMC-D-12-0276.1, in press.