Author: Eric Blake
“If you want to understand today, you have to search yesterday.” ~ Pearl S. Buck
One of the lesser-known but important functions of the NHC is to maintain a historical hurricane database that supports a wide variety of uses in the research community, private sector, and the general public. This database, known as HURDAT (short for HURricane DATabase), documents the life cycle of each known tropical or subtropical cyclone. In the Atlantic basin, this dataset extends back to 1851; in the eastern North Pacific, the records start in 1949. The HURDAT includes 6-hourly estimates of position, intensity, cyclone type (i.e,, whether the system was tropical, subtropical, or extratropical), and in recent years also includes estimates of cyclone size. Currently, after each hurricane season ends, a post-analysis of the season’s cyclones is conducted by NHC, and the results are added to the database. The Atlantic dataset was created in the mid-1960s, originally in support of the space program to study the climatological impacts of tropical cyclones at Kennedy Space Center. It became obvious a couple of decades ago, however, that the HURDAT needed to be revised because it was incomplete, contained significant errors, or did not reflect the latest scientific understanding regarding the interpretation of past data. Charlie Neumann, a former NHC employee, documented many of these problems and obtained a grant to address them under a program eventually called the Atlantic Hurricane Database Re-analysis Project. Chris Landsea, then employed by the NOAA Hurricane Research Division (HRD) and now currently the Science and Operations Officer at the NHC, has served as the lead scientist and program manager of the Re-analysis Project since the late 1990s.
In response to the re-analysis effort, NHC established the Best Track Change Committee (BTCC) in 1999 to review proposed changes to the HURDAT (whether originating from the Re-analysis Project or elsewhere) to ensure a scientifically sound tropical cyclone database. The committee currently consists of six NOAA scientists, four of whom work for the NHC and two who do not (currently, one is from HRD and the other is from the Weather Prediction Center).
Over the past two decades, Landsea, researchers Andrew Hagen and Sandy Delgado, and some local meteorology students have systematically searched for and compiled any available data related to each known storm in past hurricane seasons. This compilation also includes systems not in the HURDAT that could potentially be classified as tropical cyclones. The data are carefully examined using standardized analysis techniques, and a best track is developed for each system, many of which would be different from the existing tracks in the original dataset. Typically, a season’s worth of proposed revised or new tracks is submitted for review by the BTCC. Fig. 1 provides an example set of data that helped the BTCC identify a previously unknown tropical storm in 1955.
The BTCC members review the suggested changes submitted by the Re-analysis Project, noting areas of agreement and proposed changes requiring additional data or clarification. The committee chairman, Dr. Jack Beven, then assembles the comments into a formal reply from the BTCC to the Re-analysis Project. Occasionally, the committee’s analysis is presented along with any relevant documentation that would help Landsea and his group of re-analyzers account for the differing interpretation. The vast majority of the suggested changes to HURDAT are accepted by the BTCC. In cases where the proposed changes are not accepted, the BTCC and members of the Re-Analysis Project attempt to resolve any disagreements, with the BTCC having final say.
In the early days of the Re-analysis Project, the amount of data available for any given tropical cyclone or even a single season was quite small, and so were the number of suggested changes. This allowed the re-analysis of HURDAT to progress relatively quickly. However, since the project has reached the aircraft reconnaissance era (post 1944), the amount of data and the corresponding complexity of the analyses have rapidly increased, which has slowed the project’s progress during the last couple of years.
The BTCC’s approved changes have been significant. On average, the BTCC has approved the addition of one to two new storms per season. One of the most highly visible changes was made 14 years ago, when the committee approved Hurricane Andrew’s upgrade from a category 4 to a category 5 hurricane. This decision was made on the basis of (then) new research regarding the relationship between flight-level and surface winds from data gathered by reconnaissance aircraft using dropsondes.
The figures below show the revisions made to the best tracks of the 1936 hurricane season, and give a flavor of the type, significance, and number of changes being made as part of the re-analysis. More recent results from the BTCC include the re-analysis of the New England 1938 hurricane, which reaffirmed its major hurricane status in New England from a careful analysis of surface observations. Hurricane Diane in 1955, which brought tremendous destruction to parts of the Mid-Atlantic states due to its flooding rains, was judged to be a tropical storm at landfall after re-analysis. Also of note is the re-analysis of Hurricane Camille in 1969, one of three category 5 hurricanes to have struck the United States in the historical record. The re-analysis confirmed that Camille was indeed a category 5 hurricane, but revealed fluctuations in its intensity prior to its landfall in Mississippi that were not previously documented.
The most recent activity of the BTCC was an examination of the landfall of the Great Manzanillo Hurricane of 1959. It was originally designated as a category 5 hurricane landfall in HURDAT and was the strongest landfalling hurricane on record for the Pacific coast of Mexico. A re-analysis of ship and previously undiscovered land data, however, revealed that the landfall intensity was significantly lower (140 mph). Thus, 2015’s Hurricane Patricia is now the strongest landfalling hurricane on record for the Pacific coast of Mexico, with an intensity of 150 mph.
The BTCC is currently examining data from the late 1950s and hopes to have the 1956-1960 re-analysis released before next hurricane season. This analysis will include fresh looks at Hurricane Audrey in 1957 and Hurricane Donna in 1960, both of which were classified as category 4 hurricane landfalls in the United States. As the re-analysis progresses into the 1960s, the committee will be tackling the tricky issue of how to incorporate satellite images into the re-analysis, including satellite imagery’s irregular frequency and quality during that decade. The long-term plan is to extend the re-analysis until about the year 2000, when current operational practices for estimating tropical cyclone intensity became established using GPS dropsonde data and flight-level wind reduction techniques.
For more reading on this topic:
- Atlantic hurricane database reanalysis website – http://www.aoml.noaa.gov/hrd/data_sub/re_anal.html
- Documentation about the HURDAT database format – http://www.aoml.noaa.gov/hrd/Landsea/landsea-franklin-mwr2013.pdf
— Todd Kimberlain and Eric Blake
A previous blog entry described the new NHC five-day tropical cyclone formation (or genesis) products. In this blog entry, we discuss the factors that go into these predictions.
The primary tool used at NHC for five-day tropical cyclone genesis forecasts is global numerical modeling. Global models can predict many of the environmental factors that influence tropical cyclone formation, and the skill of these models has been improving with time. More tropical cyclone formations are being forecast with longer lead times, and weather prediction models show fewer “false alarms” than in the past. Recent studies suggest, and forecaster experience seems to confirm, that a consensus of the available model guidance usually outperforms any single model. This “two heads are better than one” approach works as long as the models (or heads) are somewhat independent of one another. In addition, NHC is currently evaluating a few statistical techniques that use the global model output to produce objective guidance designed to assist hurricane specialists in developing the probabilities of formation issued in the Tropical Weather Outlook.
Kelvin Waves and the Madden-Julian Oscillation
Global model guidance is not the only tool available to NHC forecasters, however. Researchers have learned that a majority of lower latitude tropical cyclone formations are associated with waves in the atmosphere moving through the global Tropics from west to east. Two particularly important wave types are the Convectively Coupled Kelvin Wave (CCKW), which circumnavigates the equator in about 15 to 20 days, and the Madden-Julian Oscillation (MJO), which transits the globe in 30 to 60 days. These waves are normally initiated by large areas of thunderstorm activity over tropical regions, especially near India and southeastern Asia. These waves are different in both frequency and direction of motion from the more well-known tropical waves that originate over Africa and often spawn tropical cyclones as they move westward across the Atlantic and eastern North Pacific basins.
Tropical cyclone formation often accompanies the passage of the “active phase” of either the faster-moving CCKWs or the slower-moving MJO. Figure 1 shows tropical cyclone tracks over a 37-year period in active and inactive phases of the MJO as the wave moves around the globe, along with increased or decreased rainfall anomalies associated with the two phases of the MJO (Zhang 2013). In the figure, the active phase of the MJO for the Atlantic occurs in panel (a), while for the eastern Pacific the active phase occurs in panel (d). The less active phases for these two basins fall in panels (c) and (b), respectively.
This concentration of tropical cyclone activity occurs because each type of wave temporarily makes large-scale environmental conditions, such as vertical wind shear or atmospheric moisture, more conducive for tropical cyclone formation. Although not every wave causes a tropical cyclone to form, pre-existing disturbances have a greater likelihood of developing into tropical cyclones after the passage of a CCKW or the MJO. High-activity periods can last as long as a week or more with the MJO, but are generally followed by days to possibly weeks of little to no activity during the inactive phases of these waves, when large-scale conditions become unfavorable for tropical cyclone formation. Forecasters use real-time atmospheric data and other tools to diagnose the location and motion of these important catalysts for tropical cyclone formation.
Here is an example from the 2014 hurricane season of how forecasters used these atmospheric signals. The graphic below, called a Hovmöeller diagram, shows where large areas of rising air (cool colors) and sinking air (warm colors) exist near the equator as a function of time. The dashed black contours depict the active phase of successive CCKWs, and the solid red contours show the inactive phases. In this particular case, forecasters noted that there was a strong CCKW moving through the eastern Pacific in the middle part of October. Extrapolating the wave forward in time, along with numerical models forecasts of the wave’s location and strength, suggested that a tropical cyclone could form within a few days over the far eastern Pacific from a disturbance that was already in the area. The green dot indicates where Tropical Storm Trudy formed, a day or two after the CCKW passed the disturbance. Although CCKW tracking is only a secondary factor in determining a Tropical Weather Outlook forecast, a basic knowledge of this atmospheric phenomenon is an important part of the process.
Forecasters consider many factors when preparing the five-day genesis probabilities for the Tropical Weather Outlook, including explicit forecasts from the global models and knowledge of any ongoing CCKWs or the MJO. In addition, the final NHC forecast also reflects the current trends of the disturbance, which are weighted much more heavily in the two-day outlook, but also can affect the five-day forecast as well. There are several ongoing research projects that will hopefully yield objective probabilities and other tools designed to help better predict tropical cyclone formation. These tools, in combination with the dynamical guidance from numerical models, should improve the quality of genesis forecasts and perhaps in the next five years extend reliable tropical cyclone formation forecasts from five days to one week.
— Eric Blake and Todd Kimberlain
Thanks to Chidong Zhang and David Zermeno, University of Miami RSMAS, for Figure 1.