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Atlantic Hurricane Database Uncertainty and Presentation of a New Database Format 3576 MONTHLY WEATHER REVIEW VOLUME 141 Atlantic Hurricane Database Uncertainty and Presentation of a New Database Format CHRISTOPHER W. LANDSEA AND JAMES L. FRANKLIN NOAA/NWS/NCEP/National Hurricane Center, Miami, Florida (Manuscript received 4 September 2012, in final form 31 December 2012) ABSTRACT ‘‘Best tracks’’ are National Hurricane Center (NHC) poststorm analyses of the intensity, central pressure, position, and size of Atlantic and eastern North Pacific basin tropical and subtropical cyclones. This paper estimates the uncertainty (average error) for Atlantic basin best track parameters through a survey of the NHC Hurricane Specialists who maintain and update the Atlantic hurricane database. A comparison is then made with a survey conducted over a decade ago to qualitatively assess changes in the uncertainties. Finally, the implications of the uncertainty estimates for NHC analysis and forecast products as well as for the pre- diction goals of the Hurricane Forecast Improvement Program are discussed. 1. Introduction and compares the survey results to independently de- rived estimates from Torn and Snyder (2012). A similar ‘‘Best tracks’’ are National Hurricane Center (NHC) survey conducted in 1999 provides some insight into poststorm analyses of the intensity, central pressure, changes in dataset quality during the last decade. Fi- position, and size of tropical and subtropical cyclones nally, we discuss implications of the uncertainty esti- (Jarvinen et al. 1984), and represent the official histori- mates for NHC analysis/forecast products, as well as cal record for each storm. These analyses (apart from for the predictability goals of the Hurricane Forecast those for size) make up the database known as the hur- Improvement Program (Gall et al. 2013). ricane database (HURDAT) and have been used for a wide variety of applications: verification of official and model predictions of track and intensity (McAdie 2. Best tracks—Definition, content, and procedures and Lawrence 2000), development of intensity fore- The NHC develops best tracks for intensity,1 central casting techniques (DeMaria 2009), seasonal forecasting pressure, position, and size2 with a precision of 5 kt (Klotzbach 2007), setting of appropriate building codes 2 (1kt 5 0.5144 m s 1), 1 mb, 0.18 latitude/longitude [;6nmi for coastal zones (American Society of Civil Engineers (1 n mi 5 1.852 km)], 5 n mi, 5 n mi, and 5 n mi, respec- 1999), risk assessment for emergency managers (Jarrell tively. Best track intensity and position estimates have et al. 1992), analysis of potential losses for insurance and been provided for every synoptic time (0000, 0600, 1200, business interests (Malmquist and Michaels 2000), and and 1800 UTC) for all tropical storms, hurricanes, and climatic change studies (Knutson et al. 2010). subtropical storms since 1956 (Jarvinen et al. 1984). Prior Given the widespread use of HURDAT for meteo- to 1956, best track information was analyzed only once rological, engineering, and financial decision making, it or twice a day; interpolation was used to obtain best- is surprising that very little has been published regard- track estimates for the remaining synoptic times when ing the uncertainties inherent in the database; Torn the HURDAT database was constructed in the early and Snyder (2012) is a notable exception. This current 1980s (Jarvinen et al. 1984). work estimates the uncertainties through a survey of the best track authors, the NHC Hurricane Specialists, 1 Maximum 1-min-average wind associated with the tropical cyclone at an elevation of 10 m with an unobstructed exposure Corresponding author address: Christopher W. Landsea, NOAA/ (Office of the Federal Coordinator for Meteorological Services and NWS/NCEP/National Hurricane Center, 11691 SW 17th St., Miami, Supporting Research 2012). FL 33165. 2 Cyclone size is described by the maximum extent of winds of 34, E-mail: [email protected] 50, and 64 kt in each of four quadrants about the center. DOI: 10.1175/MWR-D-12-00254.1 OCTOBER 2013 L A N D S E A A N D F R A N K L I N 3577 Originally, central pressure best track values were discrepancies between the actual (and well known) only included into HURDAT if there was a specific locations of a tropical cyclone and its corresponding observation that could be used explicitly as the best best track value. The smoothing places greater weight track value. Beginning in 1979, central pressures have to data for which confidence is relatively high (e.g., day- been estimated for every synoptic time. Size informa- light positions are considered more reliable than night- tion has been included in the best track data since 2004. time positions). An exception to this smoothing paradigm Finally, asynpotic points (primarily to denote times of is made for landfall. Because landfall is defined as the landfall as well as peak intensities that occurred at times intersection of the tropical cyclone center and the coast- other than the synoptic hours) have been incorporated line, these points cannot logically be smoothed in time into the best tracks for the years 1851–1945 and 1991 or space; landfall data in the HURDAT2 therefore onward. Because the HURDAT format could not ac- represent NHC’s best estimates of the precise location, commodate either the size of the data or asynoptic re- intensity, and timing of landfall. cords, a new format, the second-generation hurricane At the conclusion of each storm, one of the NHC database (HURDAT2), has been developed (see the Hurricane Specialists is assigned to conduct the post- appendix). storm analysis on a rotating basis. The specialist creates A best track is defined as a subjectively smoothed a draft best track, which is reviewed at NHC by the other representation of a tropical cyclone’s history over its specialists, the Hurricane Specialists Unit (HSU) branch lifetime, based on a poststorm assessment of all avail- chief, the science and operations officer, the deputy di- able data. It is important to recognize that the best rector, and the director. The review process ensures track is not simply a reissuance of the operational values. a measure of continuity across the various best track Many types of meteorological data arrive with some la- authors. tency [e.g., microwave imagery, scatterometer data, and Advanced Microwave Sounding Unit (AMSU) data], and some data do not become available until well after 3. Observations available for best track a storm is over. Furthermore, knowing what happened assessments subsequent to a given point in time can be instrumental a. Intensity in the correct assessment of what was occurring at that point in time. NHC Hurricane Specialists review the One would expect that the quality of the best tracks entire track with all the available information and put would vary depending on the amount and reliability of together, from often contradictory data, a history that observations that are available for the poststorm as- makes sense with respect to known tropical cyclone sessments. Figure 1 illustrates how the available data can dynamics.3 vary from cyclone to cyclone. Hurricane Gordon in 2006 Because the best tracks are subjectively smoothed, was a cyclone almost exclusively monitored remotely they will not precisely recreate a storm’s history, even by satellite measurements (Blake 2006), with the ma- when that history is known to great accuracy. Aliasing jority of data provided by the Satellite Analysis Branch considerations suggest that variations with periods shorter (SAB) and Tropical Analysis and Forecast Branch than about 24 h (4 times the 6-h resolution of the best (TAFB) Dvorak analyses (Dvorak 1975, 1984). In ad- tracks) cannot be represented by HURDAT. So as the dition, observations available in recent years for tropical best tracks are constructed, apparent variations, whether cyclones well away from land include the Advanced in intensity, central pressure, location, or size, with pe- Dvorak Technique (ADT; Olander and Velden 2007), riods shorter than 24 h are typically not captured. This AMSU (Brueske and Velden 2003; Demuth et al. 2006), helps ensure that the best track values are represen- and scatterometer data from the Quick Scatterometer tative of the 6-h interval surrounding the best track (QuikSCAT) and Advanced Scatterometer (ASCAT) time. On the other hand, the smoothing (particularly satellites (Brennan et al. 2009). with track) means that there will routinely be small Hurricane Dean had much more aircraft reconnaiss- ance data available for most of its lifetime. Aircraft re- connaissance missions (Franklin 2008), from both the 3 There are some objective methodologies available for weight- U.S. Air Force Reserve’s 53rd Weather Reconnaissance ing various observations to assist in providing best tracks. (e.g., the Squadron C-130s and the National Oceanic and Atmo- Automated Tropical Cyclone Forecast system; Sampson and spheric Administration (NOAA) Aircraft Operations Schrader 2000). The current procedure at NHC is for the NHC Hurricane Specialists to use their knowledge and experience to Center Orion P-3s, provide flight-level winds that can subjectively weight the various observations available and de- be adjusted to the surface (Franklin et al. 2003), termine the best tracks manually. Stepped Frequency Microwave Radiometer (SFMR) 3578 MONTHLY WEATHER REVIEW VOLUME 141 FIG. 1. Examples of tropical cyclone best track intensities based upon (a) mainly satellite data and (b) upon a mixture of satellite and aircraft reconnaissance data. The figures indicate the best track intensities as the green line with blue symbols indicating Dvorak classifications, red symbols indicating aircraft reconnaissance observations, purple symbols indicating NASA QuikSCAT measurements, and gold symbols indicating dropsonde observations. winds (Uhlhorn et al. 2007), and global positioning system many of the data plotted for Dean will not be represen- (GPS) dropwindsonde winds (Franklin et al. 2003). tative of the cyclone’s intensity (e.g., flight-level-adjusted Figure 1 gives the appearance of less spread in the winds from the left-rear quadrant of the cyclone).
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