Supplement Material for Paper
Multiformity of tropical cyclone wind-pressure relationship
in western North Pacific: discrepancies among four
best-track archives
Mien-Tze Kueh
Department of Atmospheric Sciences,
National Central University, Taiwan
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Introduction
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This supplement material contains information on the
construction of the dataset and three figures.
The information include:
Data sources, Data selection, tropical cyclone wind-pressure
relationships, wind speed conversions, a list of auxiliary
figures, and references.
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Data Sources
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Historical tropical cyclone (TC) position and intensity
information was obtained from the version v03r03 of the
International Best Track Archive for Climate Stewardship
[IBTrACS; Knapp et al., 2010], which is a centralized
collection of many global sources of TC data. This most
recent version includes best track TC information through
the end of 2010. The original maximum sustained wind (MSW)
and minimum sea level pressure (MSLP) estimated by four
operational agencies in the western North Pacific (WNP)
were utilized for this study.
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Data Selection
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This study utilizes best-track archives from four operational
agencies in the western North Pacific:
1) U.S. Department of Defense Joint Typhoon Warning Center (JTWC),
2) Japan Meteorological Agency (JMA; RSMC Tokyo),
3) China Meteorological Administration's Shanghai Typhoon Institute (CMA)
4) Hong Kong Observatory (HKO)
Only the concurrent MSW and MSLP records carried by
co-tracking TCs in the period 1991-2010 are included
in the study. The co-tracking TCs are TCs that were
tracked by all the four agencies. For all of these
co-tracking TCs, their track (positions) have been
visually examined to ensure the position differences
are virtually small such that a set of identical TC
will be included for the study. During this period,
652 individual TCs were identified in the IBTrACS,
of which 463 were co-tracking by all the agencies.
The remaining 189 TCs were weak and short-lived storms,
of which the MSWs often less than 25 knot.
The JTWC only reported MSW in the first decade. In the
second decade, both MSW and MSLP were reported in the
JTWC but the number of MSW records was slightly more
than that of MSLP records. In contrast, JMA reported
more MSLP records than the MSW records. The numbers
for both MSW and MSLP records in CMA and HKO were even.
To perform the interagency comparison, both the MSW and
MSLP records for these co-tracking TCs were sorted such
that only the records that were concurrently reported
by all the agencies were sampled.
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Tropical cyclone wind-pressure relationships (WPRs)
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While the MSW speed is the official intensity estimate
and determines, the value of MSW is almost never measured
directly. It is generally inferred from satellite using
the Dvorak technique [Dvorak, 1975, 1984].
The MSW estimate is supplemented by flight-level winds
from aircraft reconnaissance when available. However,
routine aircraft reconnaissance has been terminated since
1987 summer for the WNP. In some cases, the MSW could also
be converted from estimates of MSLP (while dropsonde-derived
or in-situ measured MSLP are available) using a wind-pressure
relationship.
The Dvorak technique [Dvorak, 1975, 1984] has been used at
TC operational agencies worldwide for decades and is the
primary tool for determining TC intensity when aircraft
reconnaissance or other in-situ measurements are not available
(see Velden et al. [2006] for further information).
The Dvorak technique yields intensity estimates in terms of
T numbers. The T numbers are then related to current intensity
(CI) numbers that are then directly related to MSLP and Vmax
using a simple look-up table. A unique (one-to-one)
CI-wind-pressure relationship is tabulated on the look-up table.
Besides, a variety of wind-pressure relationships (WPRs) have
alse beed derived over time, for instance, the WPR given by
Atkinson and Holliday [1977]. The Atkinson and Holliday [1977]
wind-pressure relationship [hereafter AH77 WPR] was adopted for
operational application in the western North Pacific in the
revised Dvorak technique [Dvorak, 1984].
For intensity estimation of TCs in the WNP,the JTWC applied
the AH77 WPR during the period from 1985 to 2006, and replaced
it by a modified version [Knaff and Zehr, 2007] since 2007.
The modified version of AH77 is derived after binning the
original AH77 dataset [also see Appendix A in Knaff and Zehr,2007].
The modified AH77 will be termed as AHb hereafter.
On the other hand, the Japanese Meteorological Agency (JMA)
adopting an alternative Dvorak scale to relate the satellite-based
TC intensity to the MSW and MSLP, based upon the WPR derived by
Koba et al. [1990, 1991]. Also see supplement to Kamahori et al.
[2006] for the chronology of satellite-based TC intensity
estimates at the JMA.
Below gives a table for the look-up tables used operationally
for agencies in the WNP. While there are differences among
these operational WPRs, all of them yield kind of monotonic
connection between winds and pressures. That is, an one-to-one
relationship from which a unique value of the wind is assigned
to a particular pressure,
Table A1. Current intensity (CI)-number and corresponding
minimum sea level pressure (MSLP) and maximum sustained
wind (MSW) speed of tropical cyclone in the western North
Pacific. Note that the MSW are one-minute averaged and
ten-minute averaged wind speeds for Dvorak and Koba,
respectively.
========================================================
MSW MSLP
_____________ ______________________
CI Dvorak Koba Dvorak Dvorak Koba
[1975] [1984] [1990]
========================================================
1.0 25 22 1005
1.5 25 29 1002
2.0 30 36 1003 1000 998
2.5 35 43 999 997 993
3.0 45 50 994 991 987
3.5 55 57 988 984 981
4.0 65 64 981 976 973
4.5 77 71 973 966 965
5.0 90 78 964 954 956
5.5 102 85 954 941 947
6.0 115 93 942 927 937
6.5 127 100 929 914 926
7.0 140 107 915 898 914
7.5 155 115 900 879 901
8.0 170 122 884 858 888
========================================================
Agencies report TC intensity differently from each other and may
have changed due to changes in their operating procedures.
For instance, the JMA appears to prefer convert CI to a MSLP first
rather than MSW, and it tends to omit to assign a MSW for a values
of MSLP exceed 1000 hPa. As a result, the number of reported MSLP
is apparently larger than that of MSW in JMA archive.
According to the Annual Tropical Cyclone Report available from the
JTWC, the values of MSLP are converted from estimated maximum
surface winds using WPR. The JTWC drops the MSLP for CI less than
two based upon the Dvorak table.As a results, the number of
reported MSLP is slightly less than that of MSW.
A plot for these WPRs, as well as the wind-pressure scatter plots
derived from un-binned archive of each agency are given in Figure A2.
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Wind Speed Conversions
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Intensity for a TC is reported as the maximum sustained wind speed
over a specified time period. However, operational agencies report
wind speeds using different averaging periods, which becomes a
significant issue when comparing wind speed estimates from different
agencies. Averaging periods include one, two, and the WMO ten-minute
standard.
In the WNP, a one-minute averaging period is used by the JTWC,
a two-minute averaging period is used at CMA/STI, and the WMO
ten-minute standard is used both at JMA and HKO.
For converting the one-minute MSW to ten-minute MSW, a factor of
0.88 is used operationally at JTWC (Sampson et al. 1995):
MSW_10 = 0.88 x MSW_1
For converting non-ten-minute MSW to one-minute MSW, the wind
speed conversions listed in Table 1 of Knapp and Kurk [2010]
were adopted:
MSW_10 = 0.60 x MSW_1 + 23.3 for JMA
MSW_10 = 0.871 x MSW_1 for CMA
MSW_10 = 0.90 x MSW_1 for HKO
The aforesaid wind speed conversions were utilized for
Figure A1 and Figure A3.
There are considerable scatters found in the archived MSW
and MSLP data, and the scatter patterns differ among agencies.
Apparent interagency discrepancies are still found while
converting the JTWC one-minute MSW into ten-minute MSW using
a factor of 0.88 (Figure A1, Left). The large discrepancies
in the stronger intensity range can be diminished to some
extent while converting all non-ten-minute MSWs into one-minute
MSW (Figure A1, Right). However, the apparent interagency
discrepancies in the scatter patterns remain.
In other word, the differences in MSW averaging period are
inadequate to fully account for the interagency discrepancies
in the distribution of MSW versus MSLP.
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List of Auxiliary Figures
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1. Figure_A1.tiff
Figure A1. Scatter plots of the MSW versus MSLP taken from
the four best-track datasets as shown in Figure 1.
(Left) The one-minute MSW from JTWC were converted to
ten-minute wind using a factor of 0.88 and superimposed on
the plot (black crosses). (Right) The ten-minute MSW from
JMA and HKO, and the two-minute MSW from CMA were converted
to one-minute MSW using factors as listed by Knapp and Kurk [2010].
2. Figure_A2.tiff
Figure A2.(Left) Wind-pressure relationships used in different
tropical cyclone warning agencies: Atkinson and Holliday [1977],
binned AH77 as introduced by Knaff and Zehr [2007],
Koba et al., [1991], and Dvorak [1984]. A modified Koba curve,
for which the MSW were converted to 1-minute averaged values,
is also plotted for reference.
(Right) Scatter plots of the MSW versus MSLP taken from the four
best-track datasets. The maximum MSW and minimum MSLP in each
TC lifecycle are superimposed and denoted as black hollow triangles.
As for JTWC, the values at TC peak strength in 1991-2000 were
taken from the ATCR in this period. Also shown are the reference
WPRs derived by Atkinson and Holliday [1977], Dvorak [1984],
and Koba [1990, 1991].
3. Figure_A3.tiff
Figure A3. Distributions of MSW plotted against that of MSLP
from the four agencies in the first (left) and second (right)
decades during the period from 1991 to 2010. The plot shows the
median values (symbols) as well as the inter-quartile ranges
(lines) both for MSLP and MSW. Vertical and horizontal lines
represent the inter-quartile ranges of MSLP and MSW, respectively.
The upper panels show the distribution of all the pairing records
that reach the typhoon strength whereas the lower panels show the
distribution of the peak strength of each individual typhoon event
in the 20-year period. This figure is the same as in Figure 3
except that the one-minute MSW from JTWC were converted to
ten-minute wind using a factor of 0.88 before the data sorting
and matching among the four agencies.
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References
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