Best Track Re-Analysis Activities at the National Hurricane Center NATIONAL HURRICANE CENTER JACK BEVEN WHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGIN

Outline Overview of re-analysis activities Recent work in the early-recon period in the Atlantic The test bed Eastern Pacific re-analysis Overview of re-analysis activities Recent work in the early-recon period in the Atlantic The test bed Eastern Pacific re-analysis

Re-analysis overview Chris Landsea’s Atlantic re-analysis project has ‘completed’ the period. The period has been partly examined. While great progress has been made, the project has moved slower than originally planned. The progress is expected to slow further as the project reaches periods with more data. No period has been truly ‘completed’. New data and new potential storms are still being found for the period. Chris Landsea’s Atlantic re-analysis project has ‘completed’ the period. The period has been partly examined. While great progress has been made, the project has moved slower than originally planned. The progress is expected to slow further as the project reaches periods with more data. No period has been truly ‘completed’. New data and new potential storms are still being found for the period.

Some re-analysis issues Many data gaps for older systems could potentially be filled. The search for old data/anecdotal accounts is not complete, especially in the Caribbean countries and Central America. This requires people willing to perform data mining in libraries and archives. Changes in and problems with instrumentation must be watched for and documented. For example, pre-1928 four- cup anemometers in the United States had a high speed bias in hurricane-force winds. Re-analysis of the aircraft reconnaissance period will deal with changes in flight procedures, instrumentation, and data interpretation. Standards for this need to be worked out. Standards also need to be set on how satellite data (e. g. Dvorak estimates) should be used. For example, how many Dvorak analyses per best track time is optimum? Many data gaps for older systems could potentially be filled. The search for old data/anecdotal accounts is not complete, especially in the Caribbean countries and Central America. This requires people willing to perform data mining in libraries and archives. Changes in and problems with instrumentation must be watched for and documented. For example, pre-1928 four- cup anemometers in the United States had a high speed bias in hurricane-force winds. Re-analysis of the aircraft reconnaissance period will deal with changes in flight procedures, instrumentation, and data interpretation. Standards for this need to be worked out. Standards also need to be set on how satellite data (e. g. Dvorak estimates) should be used. For example, how many Dvorak analyses per best track time is optimum?

The NHC Best Track Change Committee Excerpt from the committee charter: “The final “best-tracks” are prepared by the National Hurricane Center (NHC) hurricane specialists at the conclusion of every Atlantic and eastern north Pacific tropical cyclone. These tracks represent the best estimate of tropical cyclone position, intensity and status every six hours during its life cycle. They are based on a comprehensive analysis of all available observational data. Once completed, these tracks are added to the historical tropical cyclone database for each basin. These files (1851 to the present for the Atlantic, 1949 to the present for the eastern north Pacific) are used at NHC for verification of forecasts and other internal applications. They are also used extensively by the research, academic and insurance communities, as well as the media and the public. The best-track files are not static, but rather represent the best current estimates for tropical cyclone track and intensity, given the observations and science available at the time. Newly acquired observations, re-interpretation, or re-analysis may indicate changes to these tracks. Such changes must be dealt with in an organized and well-documented manner.” Excerpt from the committee charter: “The final “best-tracks” are prepared by the National Hurricane Center (NHC) hurricane specialists at the conclusion of every Atlantic and eastern north Pacific tropical cyclone. These tracks represent the best estimate of tropical cyclone position, intensity and status every six hours during its life cycle. They are based on a comprehensive analysis of all available observational data. Once completed, these tracks are added to the historical tropical cyclone database for each basin. These files (1851 to the present for the Atlantic, 1949 to the present for the eastern north Pacific) are used at NHC for verification of forecasts and other internal applications. They are also used extensively by the research, academic and insurance communities, as well as the media and the public. The best-track files are not static, but rather represent the best current estimates for tropical cyclone track and intensity, given the observations and science available at the time. Newly acquired observations, re-interpretation, or re-analysis may indicate changes to these tracks. Such changes must be dealt with in an organized and well-documented manner.”

The NHC Best Track Change Committee The committee is the ‘keeper’ of the Atlantic and Eastern North Pacific best track records. It is the committee‘s job to analytically examine all proposed changes to the records and decide whether they should be incorporated or rejected. This includes modifications to existing storms, addition of new storms, and deletion of existing storms that may not have actually met tropical storm/ hurricane criteria.

Reanalysis Steps Obtain all raw data (and center fixes) into a single database. Plot all surface obs onto synoptic maps at least once daily. Choose track/intensity from synoptic maps and center fixes. Explain track and intensity determinations in a metadata file, highlighting reasons for large changes to track and intensity Look for “missing storms” and follow similar procedure When complete, the following files should be stored in the folder for that storm (or year): –A spreadsheet - database containing all raw obs –A metadata file (described above) and your 6-hourly best track data –All synoptic maps utilized –All info from all sources/references utilized

Rules for Determining Intensity If you have a central pressure, use p-w relationship –If you know RMW, Penv, size, and/or speed of the cyclone, add or subtract 5 or 10 kt to the p-w relationship as appropriate In the absence of a central pressure, assign a wind speed 5 kt higher than the highest wind ob available If you have a peripheral pressure ob, you at least know a lower bound for the intensity from p-w relationship Continuity if lack of data (to a point). But no changes to HURDAT if no data, because interpolating through data gaps is problematic.

Reanalyzing Intensity: 1953 Hurricane Carol Key observations: 9/1 18Z: Ship kt NE (x nmi) from center. –Should choose at least kt. –Chose 90 kt (55 kt) at 18Z based on 944 mb 18 hours later. 9/2 13Z: AC – 944 mb central pressure = 119 kt. –Chose 120 kt (75 kt) at 12Z. 9/3 18Z: AC – 929 mb central pressure = 133 kt. –Chose 140 kt (130 kt) at 18Z due to tiny RMW (3 nmi) **(eye diameter)** and fast forward speed (16 kt). Carol Portion of Original HURDAT track for Hurricane Carol (1953). Red numbers are the intensities listed in the original HURDAT Pressure- wind relationships used are southern- intensifying subset of Brown et al.

Chose Intensity: New Storm with no AC New to HURDAT: /11/1944 M= 7 12 SNBR= 707 NOT NAMED XING= /11* * * * * /12* * * * * /13* * * * * Oct 12, 12Z map Key observations: Determine intensity at 10/12 12Z: -Highest wind: 70 kt at 14Z -Lowest p: 996 mb at 11Z cp wind>53 kt -Would choose 75 kt if adhere to methodology of choosing 5 kt above highest observed wind Chosen 70 kt (subject to revision) b/c lowest p was 996 mb with dense data. (65 & 70 kt ship far from center)

Track analysis with recon fixes All center fixes compiledAll center fixes compiled Fixes plotted and interpolated to 6-hourly positionsFixes plotted and interpolated to 6-hourly positions Ship and station data plotted against aircraft dataShip and station data plotted against aircraft data Final revised positions are a subjective consensus of all dataFinal revised positions are a subjective consensus of all data When lack of data, significant changes are not implementedWhen lack of data, significant changes are not implemented Radar fixPenetration fix 34N 32N 30N 76W78W74W 1948 Storm # Barbara

Early Aircraft Recon Data Position: Center fixes and navigation accurate to within 30 nmi on average Surface wind speeds (estimated visually) and flight-level wind speed measurements were both highly inaccurate and displayed a high bias Aircraft central pressures (when available) are used to determine intensity via the Brown et al. (2006) pressure-wind relationships Major hurricanes were rarely penetrated in – central pressures below 950 mb were extremely rare

Aircraft data can cause problems! Post flight summary 9/2/1952 Storm Baker: “Entered eye…min pressure 993 mb…max wind encountered 130 kt”

Eastern Pacific Re-analysis Motivation There are known problems with the eastern Pacific HURDAT. –Severe discontinuities in the data (around the late 1960’s and around the time NHC assumed responsibility in 1988) Need to ensure a consistent application of analysis across the data set. We want to get it right! It’s our job to do so as caretakers of HURDAT. Utilized heavily in climate studies

Scope and Data of the EPAC Re-analysis Re-analysis generally covers the pre-1988 period (before NHC assumed operational responsibilities for the basin) Primary data sources include NOAA CLASS satellite imagery, ICOADS, and Mexican surface observations (in NCDC’s EDADS data base) NAVY “best tracking” discovered and will be utilized Tracks in the 1950’s and 1960’s are too smoothed Boxes in NCDC basement contain archived data useful for the re-analysis Data have not been organized nor digitized Thorough review of the literature (e.g., Monthly Weather Review, Mariners Weather Log) and Historical Weather Maps to discover any missing systems Searching Mexican periodicals for evidence of missing Mexican landfalls or data regarding known landfalls

#1 #2 Issue - Oversimplification of Tracks?

An Early Look at Potential HURDAT Errors Socorro Is.. Position in HURDAT 0.5° to the east HURDAT Position T4.5/5.5 in “re-analysis”. HURDAT: 115 kt Dreaded Pinhole Eye. HURDAT: 65 kt Genesis at this time?

08/ / D D D / / D / D / /0545 D / /1145 D LG /1745 B /2345 D / / D / D D >1.5 11/ D D B B D B D /1745 B D /2345 B S /0545 W D /1145 LG D /1745 MG W /2345 LG Vis CDO = 5.5 W /0545DG W /1145 DG 4.0 W /1745MG 4.0 W /2345 MG 4.0 W Missed RI and subsequent arrested development Current HURDAT Missed Rapid Weakening Hurricane Max, 1987

15/ /1145 <3/4 W W / W / W /2345 MISSING / / TWTC Hurricane Max, 1987

Available Satellite Imagery and Issues NOAA CLASS data: McIdas AREA files are 8-bit, half hourly, and has a “4 km” resolution Calibration issues: N-AWIPS can only display 7-bit data and thus only half of the 256 possible colors Other software may display more but smoothing algorithms overly refine data Resolution: some of the imagery is of uneven resolution (e.g., 8km in latitude but 4 km in longitude) All imagery remapped to 4 km Missing or no data: occasional gaps, some of poor quality; imagery becomes less frequently available earlier 1977 has no digitized imagery Other issues? TBD Polar Orbiter data to be used to supplement imagery or fill in gaps