Postcards II: hurricane database issues, and the Bill Gray show

I'm in Orlando this week for the 28th Conference on Hurricanes and Tropical Meteorology, sponsored by the American Meteorological Society. The conference, held once every two years, brings together the world's experts on hurricane science. A few snapshots from the past 24 hours:

Northern Hemisphere tropical cyclone inactivity in 2007
Ryan Maue of Florida State University showed that tropical cyclone activity in 2007 the Northern Hemisphere (Atlantic, Eastern Pacific, Western Pacific, and North Indian Oceans) was at its lowest level since 1977. The Accumulated Cyclone Energy (ACE) is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher), at six-hour intervals. The numbers are usually divided by 10,000 to make them more manageable.

For the Atlantic in 2007, the season's ACE index was 68, 31% below the average of 96. For comparison, the Hurricane Season of 2005 had an ACE of 248. A single storm of the 2004 hurricane season--Hurricane Ivan--had an ACE of 70, more than the ACE index for the entire 2007 hurricane season. For the Northern Hemisphere as a whole, 2007 had the lowest ACE value since 1977. The North Indian Ocean was the only Ocean basin in the Northern Hemisphere that had an above-average ACE in 2007:

Ocean Basin....2007 ACE..Average ACE..%change
-------------------------------------------------------------------------
Atlantic .............68.........96.........-31%
E. Pacific............52........132.........-60%
W. Pacific...........209........303.........-31%
N. Indian.............44.........16........+275%
N. Hemisphere..376........551.........-31%

Uncertainties in the hurricane data base
HURDAT, the official Atlantic hurricane database, has many significant errors that are slowly being corrected, thanks to a major re-analysis effort being led by Dr. Chris Landsea of the National Hurricane Center. For example, HURDAT lists the Great New England Hurricane of 1938 as a Category 2 extratropical storm (85 knots) at landfall on Long Island, when it was really a Category 3 hurricane with 105 knot winds. Dr. Landsea presented the results of the re-analysis of 12 major hurricanes that hit the U.S. Nine of these 12 storms, including the 1938 hurricane, were re-analyzed to have higher winds at landfall. Dr. Landsea cautioned that our knowledge of past storms in the 1950s and 1960s is quite poor, compared to current capabilities. This occurs despite the fact the hurricane hunters were flying. For example, Hurricane Wilma of 2005 had 280 measurements of its maximum intensity, while Hurricane Carol of 1954, a Category 3 storm that hit North Carolina, had only seven. There were hurricane hunter flights into Carol, but they usually did not fly into the eyewall. It was common for the hurricane hunters to only get close enough to the center to estimate the position using radar back in those days. Carol stalled off the coast of North Carolina for four days, during which time no hurricane hunter flights penetrated into the eye. Could Carol have intensified into a Category 5 storm during that time? We'll never know. Because of such uncertainties, making estimations of trends in Atlantic hurricanes based on HURDAT is difficult to do, Dr. Landsea cautioned.

The Bill Gray show
Dr. Bill Gray of Colorado State University, as usual, generated the most laughter. He commented that he had been to all 28 of these AMS hurricane conferences, with the exception of the first (he was in grad school) and the fourth, when he was in Tokyo. Dr. Gray presented an educational talk, emphasizing the role of natural decades-long cycles in the salinity changes in the Atlantic as being the primary driver of observed increases in Atlantic hurricane activity in recent years. He showed that during 1945-1969 (25 years), during a period the globe was cooling slightly, there were three times as many intense hurricanes in the Atlantic compared to the 25 year period 1970-1994--a period the globe warmed significantly. His tongue-in-cheek conclusion: "CO2 gets into these storms and squashes them!" Extending this result to landfalling U.S. hurricanes, one could claim that we should expect zero landfalling U.S. hurricanes by 2050. Dr. Gray cautioned that this ridiculous result showed that one can manipulate statistics to show virtually any result you want.


Figure 1. Tounge-in-cheek misuse of statistics by Bill Gray to show that the historical record of U.S. landfalling hurricanes predicts zero landfalling hurricanes in the U.S. by 2050 as a result of increasing CO2 in the atmosphere.

Contribution of increases in SST to Atlantic hurricane activity
Adam Lea of University College London presented results showing that the 0.27°C increase in Sea Surface Temperature(SST) between 1996 and 2005 in the Main Development Region (MDR) for hurricanes in the tropical Atlantic was responsible for a 40% increase in hurricanes and intense hurricanes in the Atlantic.

The benefits of hurricanes: rainfall in the Southeast U.S.
David Knight of the University of Virginia showed that hurricanes and tropical storms form an important part of the water budget in the Southeastern U.S. For example, up to 15% of the total rainfall in eastern South Carolina and North Carolina during the six months of hurricane season (June-November) was due to tropical storms or hurricanes between 1980-2004. These numbers are 10-14% for Florida, and 8-10% for Atlanta.

More postcards tomorrow!

Jeff Masters