Atlantic Hurricane Database Uncertainty and Presentation of a New Database Format
By Franklin, James L | |
Proquest LLC |
ABSTRACT
"Best tracks" are
1. Introduction
''Best tracks'' are
Given the widespread use of HURDAT for meteorological, engineering, and financial decision making, it is surprising that very little has been published regarding the uncertainties inherent in the database; Torn and Snyder (2012) is a notable exception. This current work estimates the uncertainties through a survey of the best track authors, the
2. Best tracks-Definition, content, and procedures
The NHC develops best tracks for intensity,1 central pressure, position, and size2 with a precision of 5 kt (1kt5 0.5144ms21), 1mb, 0.18 latitude/longitude [;6nmi (1 nmi 5 1.852 km)], 5 n mi, 5 nmi, and 5 nmi, respectively. Best track intensity and position estimates have been provided for every synoptic time (0000, 0600, 1200, and
Originally, central pressure best track values were only included into HURDAT if there was a specific observation that could be used explicitly as the best track value. Beginning in 1979, central pressures have been estimated for every synoptic time. Size information has been included in the best track data since 2004. Finally, asynpotic points (primarily to denote times of landfall as well as peak intensities that occurred at times other than the synoptic hours) have been incorporated into the best tracks for the years 1851-1945 and 1991 onward. Because the HURDAT format could not accommodate either the size of the data or asynoptic records, a new format, the second-generation hurricane database (HURDAT2), has been developed (see the appendix).
A best track is defined as a subjectively smoothed representation of a tropical cyclone's history over its lifetime, based on a poststorm assessment of all available data. It is important to recognize that the best track is not simply a reissuance of the operational values. Many types of meteorological data arrive with some latency [e.g., microwave imagery, scatterometer data, and Advanced Microwave Sounding Unit (AMSU) data], and some data do not become available until well after a storm is over. Furthermore, knowing what happened subsequent to a given point in time can be instrumental in the correct assessment of what was occurring at that point in time.
Because the best tracks are subjectively smoothed, they will not precisely recreate a storm's history, even when that history is known to great accuracy. Aliasing considerations suggest that variations with periods shorter than about 24 h (4 times the 6-h resolution of the best tracks) cannot be represented by HURDAT. So as the best tracks are constructed, apparent variations, whether in intensity, central pressure, location, or size, with periods shorter than 24 h are typically not captured. This helps ensure that the best track values are representative of the 6-h interval surrounding the best track time. On the other hand, the smoothing (particularly with track) means that there will routinely be small discrepancies between the actual (and well known) locations of a tropical cyclone and its corresponding best track value. The smoothing places greater weight to data for which confidence is relatively high (e.g., daylight positions are considered more reliable than nighttime positions). An exception to this smoothing paradigm is made for landfall. Because landfall is defined as the intersection of the tropical cyclone center and the coastline, these points cannot logically be smoothed in time or space; landfall data in the HURDAT2 therefore represent NHC's best estimates of the precise location, intensity, and timing of landfall.
At the conclusion of each storm, one of the
3. Observations available for best track assessments
a. Intensity
One would expect that the quality of the best tracks would vary depending on the amount and reliability of observations that are available for the poststorm assessments. Figure 1 illustrates how the available data can vary from cyclone to cyclone. Hurricane Gordon in 2006 was a cyclone almost exclusively monitored remotely by satellite measurements (Blake 2006), with the majority of data provided by the Satellite Analysis Branch (SAB) and Tropical Analysis and Forecast Branch (TAFB) Dvorak analyses (Dvorak 1975, 1984). In addition, observations available in recent years for tropical cyclones well away from land include the Advanced Dvorak Technique (ADT; Olander and Velden 2007), AMSU (Brueske and Velden 2003; Demuth et al. 2006), and scatterometer data from the Quick Scatterometer (QuikSCAT) and Advanced Scatterometer (ASCAT) satellites (Brennan et al. 2009).
Hurricane Dean had much more aircraftreconnaissance data available for most of its lifetime. Aircraftreconnaissance missions (Franklin 2008), from both the
Figure 1 gives the appearance of less spread in the observations for Gordon relative to Dean. However, many of the data plotted for Dean will not be representative of the cyclone's intensity (e.g., flight-level-adjusted winds from the left-rear quadrant of the cyclone). Moreover, close agreement between SAB and TAFB Dvorak estimates does not necessarily indicate smaller uncertainty, because it has been shown that Dvorak intensity analyses are not overly sensitive to the individual performing the analysis (Mayfield et al. 1988; Torn and Snyder 2012).
b. Central pressure
Best track central pressures for cyclones observed primarily by satellite are determined from SAB and TAFB Dvorak analyses, the ADT, and AMSU. In addition, since 2010 the analysis has also used the Knaff-Zehr-Courtney pressure-wind relationship (Knaffand Zehr 2007;
c. Position
Figure 2 illustrates examples again from Gordon and Dean of the tropical cyclone best track positions and the available fixes. Position estimates for systems like Gordon over the open
d. Wind radii
Observations to support wind radii analyses are quite limited. Two satellite-based instruments for estimating wind radii are the ASCAT and the (now defunct) QuikSCAT scatterometers. However, scatterometer passes are infrequent (on the order of one every day or two), they often only sample a portion of the cyclone, and their winds are not well calibrated at the tropicalstorm- force wind threshold as a result of ambiguities introduced by rain contamination (Brennan et al. 2009). Data fromthe passive WindSat radiometer andOceanSat scatterometer have also been received at NHC in the last couple of years. However, WindSat cannot obtain useful data in rainy conditions and the calibrations for OceanSat are still evolving, making it currently unsuitable for estimating cyclone size. Aircraftreconnaissance observations, such as adjusted flight-level winds, SFMR winds, and GPS dropwindsonde winds, do assist in determining wind radii, but do not provide complete coverage of the surface wind field, given that flight-level and SFMR winds are only available directly along the flight track and GPS dropwindsonde winds are only spot measurements.
e. Additional considerations
Other data sources, such as ships, moored buoys, and coastal weather stations, are used. But because of their wide spacing and distance from the storm as well as the propensity for them to either actively avoid tropical cyclones (ships) or fail during tropical cyclone events (buoys and stations), these usually do not play a major role in determining tropical cyclone best tracks.
The WSR-88Ds provide center fixes within about 200 nmi of the U.S. coast and wind data from these radars have even a shorter range. Moreover, the radars only measure the wind component directly toward or away from the radar site, and not lower than a few hundred meters above the ground (necessitating amethod for adjusting the winds to 10 m). As a consequence, the use of land-based Doppler radar for best track purposes is largely restricted to those few cases near landfall when reconnaissance data are unavailable.
Overall, about 30% of the Atlantic basin best track times for tropical cyclones have the benefit of aircraftreconnaissance observations (Rappaport et al. 2009). Typically these data are obtained for any tropical cyclone within 500 nmi of landfall and west of 52.58W in the Atlantic (
4. Methodology for estimating best track uncertainties
In early 1999, an unpublished survey was conducted of the six
A very similar survey was conducted in early 2010 of the 10
Some discussion of the limitations of the survey approach is appropriate here. While the estimates are quantitative, they are subjectively determined by each NHC Hurricane Specialist. In addition, while the average of these estimates is shown here, the sample of participants is small (7 in 1999 and 11 in 2010).
5. Results of best track uncertainty estimates
The two surveys conducted a decade apart allow for an assessment of the current uncertainty for all of the best track parameters, and provide insight into how the uncertainty for position and intensity has changed over time. Tables 1 and 2 and Figs. 3-6 provide summaries of the average best track uncertainty estimates as provided by the
a. Intensity
Intensity best track uncertainty in 2010 (Fig. 3, Table 2) shows a moderate dependence upon observations available and a weak dependence upon intensity. Tropical storms have an uncertainty in the peak winds of about 12 kt when sampled primarily by satellite, which drops to about 8 kt for both satellite and aircraftdata and U.S. landfalling cyclones. This uncertainty is nearly the same for category 1 and 2 hurricanes. For major hurricanes, the average uncertainty in intensity is larger- about 14 kt for satellite-only observations, dropping to about 11 kt for satellite and aircraftmonitoring, and to about 10 kt for U.S. landfalling cyclones. While the values are only moderately sensitive to the intensity, if one puts these results into the context of the uncertainty relative to the absolute value of the intensity, then the relative uncertainty via satellite-only observations in tropical storms is about 25%, in category 1 and 2 hurricanes ;15%, and in major hurricanes ;10%. For aircraft/satellite monitoring and for U.S. landfalling cyclones, the relative uncertainty decreases to about 15% for tropical storms, ;10% for category 1 and 2 hurricanes, and ;8% for major hurricanes. The intensity uncertainty values from
b. Central pressure
For central pressure best tracks (Fig. 4, Table 2), the uncertainty in 2010 increases for stronger cyclones, but only for satellite-based measurements. In this bin, tropical storm central pressures have an uncertainty of about 6 mb, category 1 and 2 hurricanes about 8mb, and major hurricanes about 10 mb. To put these central pressure uncertainty values into perspective, one could compare them versus the average pressure-deficit of Atlantic basin tropical cyclones, which would be about 20mb for tropical storms, ;40mb for category 1 and 2 hurricanes, and ;70mb for major hurricanes (
c. Position
For position best tracks (Fig. 5, Table 2), the uncertainty in 2010 is strongly a function of intensity (more intense cyclones have less position uncertainty) and observational platform (more comprehensive observations decrease the position uncertainty). For tropical storms, satellite-only best tracks have a quite large uncertainty of about 35 n mi. This uncertainty decreases to about 22 nmi for aircraftand satellite measurements and even further decreases to about 18 nmi for U.S. landfalling tropical storms. To put these position uncertainty values into perspective, one could compare them versus the average size of Atlantic basin tropical cyclones based upon a measure of the surface circulation size, such as the outer closed isobar, which has a median of about 150 nmi for tropical storms and 200 nmi for both category 1 and 2 hurricanes and major hurricanes (Kimball and Mulekar 2004). This suggests a relative uncertainty in position for cyclones monitored primarily by satellite of about 20% for tropical storms and ;10% for both category 1 and 2 hurricanes and major hurricanes. Inclusion of aircraftreconnaissance information reduces the uncertainty of position substantially with estimated values of about 22 nmi for tropical storms (about 15% relative uncertainty), ;15 nmi for category 1 and 2 hurricanes (;7.5%), and ;11 nmi for major hurricanes (;5%). Finally, for cyclones making landfall in
d. Wind radii
The average uncertainty in 2010 of the size best tracks (maximum extent of 34-, 50-, and 64-kt wind radii) is presented in Table 2 and Fig. 6. These are fairly invariant with respect to intensity, but appear to be strongly related to the observational capabilities available. For example, the 34-kt wind radii has an average uncertainty from satellite-only measurements of about 40 nmi regardless of intensity, ;30 nmi from satellite and aircraftmonitored tropical cyclones, and ;25 nmi for those systems making landfall. These uncertainties are quite large relative to the average wind radii itself (Kimball and Mulekar 2004): about 45% for tropical storms (with median 34-kt radii of 85 nmi),;30%for category 1 and 2 hurricanes (median 34-kt radii of 130 nmi), and ;30% for major hurricanes (median 34-kt radii of 140 nmi) for those systems primarily monitored by satellite. This relative uncertainty drops some to about 35% for tropical storms,;25%for category 1 and 2 hurricanes, and;20% for major hurricanes being observed by both satellite and aircraft. The estimate is further reduced for those cyclones making a U.S. landfall to about 30% relative uncertainty for tropical storms and ;20% for both category 1 and 2 hurricanes, and ;20%for major hurricanes.
The estimated uncertainty in 2010 for the 50-kt wind radii is about 30 nmi from satellite-only monitoring, ;23 nmi from satellite and aircraftobservations, and about ;18 nmi for U.S. landfalling tropical cyclones (Table 2). Climatological median 50-kt wind radii is about 50 nmi for tropical storms, ;70 nmi for category 1 and 2 hurricanes, and about 85 nmi for major hurricanes (Kimball and Mulekar 2004). This suggests relative uncertainty from satellite-only, satellite and aircraft, and U.S. landfalling of the following: ;55%, 40%, and 35% for tropical storms; ;45%, 35%, and 30% for category 1 and 2 hurricanes; and ;40%, 30%, and 25% for major hurricanes, respectively.
The estimated uncertainty in 2010 for the 64-kt wind radii is about 24 nmi from satellite-only monitoring, ;17 nmi from satellite and aircraftobservations, and about;13 nmi for U.S. landfalling hurricanes (Table 2). Climatological median 64-kt wind radii is about 40 nmi for category 1 and 2 hurricanes and about 50 n mi for major hurricanes (Kimball and Mulekar 2004). This suggests relative uncertainty from satellite-only, satellite and aircraft, and U.S. landfalling of the following: ;55%, 40%, and 35% for category 1 and 2 hurricanes; and ;50%, 35%, and 25% formajor hurricanes, respectively.
6. Comparison of uncertainty estimates with earlier work
There has only been one previous study that has attempted to quantify the uncertainty in the Atlantic basin best tracks-Torn and Snyder (2012). Their study addressed best track uncertainty for intensity and central pressure as well as operational uncertainty for position.
Torn and Snyder (2012) were able to derive estimates of intensity and central pressure best track uncertainties (binned by intensity) for satellite-only observations by comparing the 2000-09 SAB and TAFB Dvorak classifications when there existed aircraftreconnaissance within 2 h of the best track time to provide ground truth. They suggested uncertainty values of about 10 kt for tropical storm and about 12 kt for category 1 and 2 hurricanes and for major hurricanes monitored by satellite-only measurements. Likewise, they analyzed uncertainty values of about 7mb for tropical storms, 10 mb for category 1 and 2 hurricanes, and 12mb for major hurricanes. These values are relatively close to those estimated here in Table 2 based upon a completely different methodology.
For position uncertainty, Torn and Snyder (2012) examined the operational position uncertainty estimates contained in NHC tropical cyclone products. In this case, the operational estimates are described as ''position accurate within'' x miles, which may be more of an upper bound estimate of the likely error, rather than the average error. Torn and Snyder (2012) analyzed tropical storm uncertainty in position to be about 35 n mi, category 1 and 2 hurricanes to be about 25 nmi, and major hurricanes to be about 20 nmi, using data from the 2000 to the 2009 hurricane seasons. These are somewhat larger than the uncertainty estimates provided here for best track positions of 30 nmi for tropical storms, 20 nmi for category 1 and 2 hurricanes, and 12 nmi for major hurricanes (combining the satellite-only and the satellite and aircraftbins in a 30:70 ratio; Rappaport et al. 2009). However, best track values of center locations can differ significantly from NHC operational assessments of these quantities because of additional observations becoming available as well as the opportunity to put subsequent measurements into the context of the life cycle of the tropical cyclone. However, one would expect that in general the best track position uncertainty should be smaller-at times substantially smaller-than the operational estimates. This is because, for example, at night for systems onlymonitored by infrared geostationary satellites there can be quite large ambiguity in the operational positions. It is not uncommon for the first light visual imagery from geostationary satellites to reveal a position quite far removed from that analyzed overnight. This is known colloquially at NHC as the ''sunrise surprise.'' The best tracks have the ability of hindsight to correct these overnight positions accordingly with this subsequent information and thus would have substantially smaller uncertainty than the operational estimates, which again may be thought of as an upper bound error estimate. Torn and Snyder (2012) did, in contrast to the subjective results obtained here, find a reduction in the position uncertainty during the first decade of the twenty-first century. It is possible that the disagreement in the uncertainty changes is due to the semiquantitative nature of this survey, differing members of the
One can additionally compare the uncertainty results here versus those estimated for best tracks in the presatellite and preaircraftreconnaissance (Landsea et al. 2012) era. For intensity, the uncertainty today is roughly half of what is was in the late nineteenth and early twentieth centuries. For position, the uncertainty in recent years has been reduced by about 75% in areas monitored today by satellite primarily and by about 85% for those tropical cyclones with aircraftreconnaissance available today. This is a dramatic increase in accuracy of analysis over a century time scale.
7. Implications of the results for analysis and forecasting
The results obtained with these surveys of the
As noted earlier, NHC provides wind radii information both operationally and in best track in quadrants expressed as a single value representing the largest radial extent within that quadrant. This somewhat crude depiction of the surface winds is also used to forecast tropical cyclone size, with 34- and 50-kt size forecasts going out to 72 h and 64-kt size forecasts going out to 36 h. Such very large uncertainties and very low signalto- noise ratio in the wind radii is a strong argument for not providing additional specification of the tropical cyclone wind field and for not currently extending the size forecasts out further in time.
These uncertainties also have implications for the forecasting goals of the Hurricane Forecast Improvement Program (HFIP; Gall et al. 2013). The goals for this program include reducing the average track and intensity error by 50% through 120 h by 2019. The overall position uncertainty is about 20 nmi for all tropical storms and hurricanes. Figure 8 puts this current uncertainty in position into context with regards to the days 1 through 5 NHC forecast track errors over the last two decades. It is unlikely that the uncertainty in position will have an effect on the ability to reduce track errors as hoped by HFIP, except perhaps at the day 1 forecast time, which is currently about 50 nmi.
Figure 9 compares the current estimated uncertainty in intensity-about 10 kt-with the NHC forecast errors for intensity between days 1 and 5. It is apparent that the current estimated uncertainty in intensity forecasts is of similar magnitude to the existing average intensity forecast errors at 24 h. Any sizable reductions in large forecast busts (usually associated with either rapid intensification or rapid weakening) will somewhat lower the average intensity forecast errors. However, unless there are also substantial improvements in our capability to observe the intensity of tropical cyclones, achieving the quantitative HFIP intensity forecast goals could prove very challenging, especially at the shorter forecast leads.
8. Summary and discussion
This paper provides estimates of the Atlantic basin best track uncertainties for intensity, central pressure, position, and size for today's tropical cyclones. This is accomplished by taking a survey of the
* The best track intensity uncertainty increases moderately with intensity and decreases substantially with availability of aircraftmonitoring compared with satellite-only observations.
* The best track central pressure uncertainty increases moderately with intensity and decreases to much smaller values with the availability of aircraftmonitoring.
* The best track position uncertainty decreases substantially both with increasing intensity and with the availability of aircraftmonitoring.
* The best track size (wind radii) uncertainty changes little with intensity, but decreases moderately with the availability of aircraftmonitoring.
* The only best track parameter substantially improved with additional monitoring afforded by coastal radars and stations when a cyclone makes a U.S. landfall is the best track size.
* Substantial improvement in the perceived intensity uncertainty was suggested between the 1999 and 2010 surveys. However, little change in the position uncertainty was indicated between the two surveys.
* The best track size (wind radii) has a very poor signal-tonoise ratio, which suggests that any expansion of the current NHC operational analyses of the surface wind and its forecast would be premature at this time.
* The uncertainty inherent in today's best track positions should not be a hindrance to the HFIP track forecast goals by 2019.
* The uncertainty estimated in the current best track intensities may make achieving the HFIP intensity forecast goals by 2019 problematic.
There may be opportunities in the next decade or so to improve our monitoring capabilities and reduce the uncertainties both in operations and in the best track database. For intensity, four potential improvements may be possible. The first is the use of Hurricane Imaging Radiometer instrument (HIRAD; Miller et al. 2011). This aircraftinstrument-similar in design to the SFMR-allows for a wide swath of surface winds to be measured, rather than single point values directly below the aircraft. The second is from the use of airborne Doppler radar winds adjusted to approximate surface observations (
For central pressure uncertainty improvements, this could be obtained by either deployment of small unmanned aircraftinto the center of tropical cyclones or the use of tethered blimps (Duvel et al. 2009) to provide these measurements. However, when manned aircraftis available (about 30% of the time in the Atlantic and about 5% of the time in the northeast Pacific), central pressure values already have quite small uncertainties.
The uncertainty in tropical cyclone position currently is relatively small, but still a difficult operational problem in some circumstances. A next-generation scatterometer could provide some improvements in determining the position of tropical storms and category 1 and 2 hurricanes. Of concern is the possibility of a degradation of current capabilities due to a reduction in the number of low-Earth-orbiting satellites providing microwave image fixes (Velden and Hawkins 2010; Hawkins and Velden 2011). If this degradation were to occur, it could make the position uncertainties that are currently small somewhat worse.
Tropical cyclone size (wind radii) has the largest room for improvement in the current parameters that best tracks are being provided. There are some recently available wind field techniques that have not been widely used inNHC operations that may improve our analyses of tropical cyclone size. The AMSU-based analyses (DeMuth et al. 2006) and the multisatellite-based analyses (Knaffet al. 2011) are undergoing evaluation to determine their skill and utility for improving NHC's wind radii estimates. In the next decade or so, substantially improved wind radii could be obtained from operational implementation of an aircraft-deployed HIRAD, airborne Doppler radar, or a next-generation satellite-based scatterometer. Finally, a geostationary satellite-based AMSU (
As noted earlier, the substantial uncertainties- especially with regards to intensity and wind radii-may limit the forecast improvements possible in coming years at NHC. New observational capabilities and improved utilization of existing measurements provide optimism for both reduced uncertainties in analyzing crucial tropical cyclone parameters as well as improved predictability.
Acknowledgments. The authors thank the former and current
1 Maximum 1-min-average wind associated with the tropical cyclone at an elevation of 10m with an unobstructed exposure (
2 Cyclone size is described by the maximum extent of winds of 34, 50, and 64 kt in each of four quadrants about the center.
3 There are some objective methodologies available for weighting various observations to assist in providing best tracks. (e.g., the Automated Tropical Cyclone Forecast system; Sampson and Schrader 2000). The current procedure at NHC is for the
4 Three NHC Hurricane Specialists-Avila, Beven, and Pasch- participated in both the 1999 and 2010 surveys, allowing for a more homogeneous comparison of the results based just upon their responses. These showed quite similar changes in the estimates of uncertainty compared with the whole sample that is reported here.
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CHRISTOPHER W. LANDSEA AND JAMES L. FRANKLIN
(Manuscript received
Corresponding author address:
E-mail: [email protected]
APPENDIX
The Revised Atlantic Hurricane Database (HURDAT2)
The original HURDAT format substantially limited the type of best track information that could be conveyed. The format of this new version, the secondgeneration hurricane data (HURDAT2), is based upon the ''best tracks'' available from the b-decks in the Automated Tropical Cyclone Forecast (ATCF; Sampson and Schrader 2000) system database and is described below. Reasons for the revised version include 1) inclusion of nonsynoptic (other than 0000, 0600, 1200, and
An example of the new HURDAT2 format for Hurricane Irene from 2011 follows in Table A1. There are two types of lines of data in the new format: the header line and the data lines. The format is comma delimited to maximize its ease in use. The header line has the following format: spaces 1 and 2-basin-Atlantic; spaces 3 and4-ATCF cyclone number for that year; spaces 5-8, before first comma-year; spaces 20-29, before second comma-name, if available, or else ''UNNAMED''; spaces 35-37-number of best track entries (i.e., rows) to follow.
Notes:
1) Cyclone number: In HURDAT2, the order cyclones appear in the file is determined by the date/time of the first tropical or subtropical cyclone record in the best track. This sequence may or may not correspond to the ATCF cyclone number. For example, the 2011 unnamed tropical storm AL20 that formed on 1 September is sequenced here between AL12 (Katia-formed on 29August) andAL13 (Lee- formed on 2 September). This mismatch between ATCF cyclone number and the HURDAT2 sequencing can occur if poststorm analysis alters the relative genesis times between two cyclones. In addition, in 2011 it became practice to assign operationally unnamed cyclones ATCF numbers from the end of the list, rather than insert them in sequence and alter the ATCF numbers of cyclones previously assigned.
2) Name: Tropical cyclones were not formally named before 1950 and are thus referred to as UNNAMED in the database. Systems that were added into the database after the season (such as AL20 in 2011) also are considered UNNAMED. Nondeveloping tropical depressions formally were given names (actually numbers, such as ''TEN'') that were included into the ATCF b-decks starting in 2003. Nondeveloping tropical depressions before this year are also referred to as UNNAMED. Note that the nondeveloping tropical depressions for 1988 are currently missing from the b-deck files and are therefore not available here either. (These should be included into the new HURDAT2 sometime during 2014.)
The remaining rows of data in the new format are the data lines (Table A1). These have the following format: spaces 1-4-year; spaces 5-6-month; spaces 7-8, before first comma-day; spaces 11-12-hours in coordinated universal time (UTC); spaces 13-14, before second comma-minutes; space 17-record identifier (see notes below)
L Landfall (center of system crossing a coastline)
W Peak maximum sustained wind speed
P Minimum in central pressure
I An intensity peak in terms of both pressure and wind
C Closest approach to a coast, not followed by a landfall
S Change of status of the system
G Genesis
T Provides additional detail on the track (position) of the cyclone; spaces 20-21, before third comma-status of system.
Options are as follows:
TD Tropical cyclone of tropical depression intensity (<34 kt)
TS Tropical cyclone of tropical storm intensity (34-63 kt)
HU Tropical cyclone of hurricane intensity (>64 kt)
EX Extratropical cyclone (of any intensity)
SD Subtropical cyclone of subtropical depression intensity (<34 kt)
SS Subtropical cyclone of subtropical storm intensity (>34 kt)
LO A low that is neither a tropical cyclone, a subtropical cyclone, nor an extratropical cyclone (of any intensity)
WV Tropical wave (of any intensity)
DB Disturbance (of any intensity);
spaces 24-27-latitude; space 28, before fourth comma- hemisphere (north or south); spaces 31-35-longitude; space 36, before fifth comma-hemisphere (west or east); spaces 39-41, before sixth comma-maximum sustained wind (in knots); spaces 44-47, before seventh comma- minimum pressure (in millibars); spaces 50-53, before eighth comma-34-kt wind radii maximum extent in northeastern quadrant (in nautical miles); spaces 56-59, before ninth comma-34-kt wind radii maximum extent in southeastern quadrant (in nautical miles); spaces 62-65, before tenth comma-34-kt wind radii maximum extent in southwestern quadrant (in nautical miles); spaces 68-71, before eleventh comma-34-kt wind radii maximum extent in northwestern quadrant (in nauticalmiles); spaces 74-77, before twelfth comma- 50-kt wind radii maximum extent in northeastern quadrant (in nautical miles); spaces 80-83, before thirteenth comma-50-kt wind radii maximum extent in southeastern quadrant (in nautical miles); spaces 86-89, before fourteenth comma-50-kt wind radii maximum extent in southwestern quadrant (in nautical miles); spaces 92-95, before fifteenth comma-50-kt wind radii maximum extent in northwestern quadrant (in nauticalmiles); spaces 98-101, before sixteenth comma- 64-kt wind radii maximum extent in northeastern quadrant (in nautical miles); spaces 104-07, before seventeenth comma-64-kt wind radii maximum extent in southeastern quadrant (in nautical miles); spaces 110-113, before eighteenth comma-64-kt wind radii maximumextent in southwestern quadrant (in nautical miles); spaces 116-119, before nineteenth comma-64-kt wind radii maximum extent in northwestern quadrant (in nautical miles).
Notes:
1) Record identifier: This code is used to identify records that correspond to landfalls or to indicate the reason for inclusion of a record not at the standard synoptic times (0000, 0600, 1200, and
2) Time: Nearly all HURDAT2 records correspond to the synoptic times of 0000, 0600, 1200, and
3) Status: Tropical cyclones with an ending tropical depression status (the dissipating stage) were first used in the best track beginning in 1871, primarily for systems weakening over land. Tropical cyclones with beginning tropical depression (the formation stage) were first included in the best track beginning in 1882. Subtropical depression and subtropical storm status were first used beginning in 1968 at the advent of routine satellite imagery for the Atlantic basin. The low status-first used in 1987-is for cyclones that are neither tropical cyclone or subtropical cyclones, nor extratropical cyclones. These typically are assigned at the beginning of a system's life cycle and/or at the end of a system's life cycle. The tropical wave status-first used in 1981-is almost exclusively for cyclones that degenerate into an open trough for a time, but then redevelop later in time into a tropical cyclone (e.g., AL10-DENNIS in 1981 between 13 and 15 August). The disturbance status is similar to tropical wave and was first used in 1980. It should be noted that for tropical wave and disturbance status the location given is the approximate position of the lower-tropospheric vorticity center, as the surface center no longer exists for these stages.
4) Maximum sustained surface wind: This is defined as the maximum 1-min-average wind associated with the tropical cyclone at an elevation of 10m with an unobstructed exposure. Values are given to the nearest 10 kt for the years 1851 through 1885 and to the nearest 5 kt from 1886 onward. A value is assigned for every cyclone at every best track time. Note that the nondeveloping tropical depressions of 1967 did not have intensities assigned to them in the b-decks. These are indicated as ''-99'' currently, but will be revised and assigned an intensity when the Atlantic hurricane database reanalysis project (Hagen et al. 2012) reaches that hurricane season.
5) Central pressure: These values are given to the nearest millibar. Originally, central pressure best track values were only included if there was a specific observation that could be used explicitly. Missing central pressure values are noted as ''-999''. Beginning in 1979, central pressures have been analyzed and included for every best track entry, even if there was not a specific in situ measurement available.
6) Wind radii: These values have been best tracked since 2004 and are thus available here from that year forward with a resolution to the nearest 5 n mi. Best tracks of the wind radii have not been done before 2004 and are listed as ''-999'' to denote missing data. Note that occasionally when there is a nonsynoptic time best track entry included for either landfall or peak intensity, that the wind radii best tracks were not provided. These instances are also denoted with a ''-999'' in the database.
General notes
The database goes back to 1851, but it is far from being complete and accurate for the entire century and a half. Uncertainty estimates of the best track parameters available for are available for various era in Landsea et al. (2012), Hagen et al. (2012), Torn and Snyder (2012), and within this paper. Moreover, as one goes back further in time in addition to larger uncertainties, biases become more pronounced as well with tropical cyclone frequencies being underreported and the tropical cyclone intensities being underanalyzed. That is, some storms were missed and many intensities are too low in the preaircraftreconnaissance era (before 1944 for the western half of the basin) and in the presatellite era (before 1972 for the entire basin). Even in the last decade or two, new technologies affect the best tracks in a nontrivial way because of our generally improving ability to observe the frequency, intensity, and size of tropical cyclones. SeeVecchi and Knutson (2008), Landsea et al. (2010), Vecchi and Knutson (2011), and Uhlhorn and Nolan (2012) for methods that have been determined to address some of the undersampling issues that arise in monitoring these mesoscale, oceanic phenomenon.
The only aspect of the original HURDAT database that is not contained in the new HURDAT2 is the stateby- state categorization of the Saffir-Simpson Hurricane Wind Scale for continental U.S. hurricanes. This information is not a best track quantity and thus will not be included here. However, such U.S. Saffir-Simpson Hurricane Wind Scale impact records will continue to be maintained, but within a separate database on the NHC website.
Copyright: | (c) 2013 American Meteorological Society |
Wordcount: | 9073 |
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