Accuracy of past hurricane counts good

Counting tropical storms that occurred before the advent of aircraft and satellites relies on ships logs and hurricane landfalls, making many believe that the numbers of historic tropical storms in the Atlantic are seriously undercounted. However, a statistical model based on the climate factors that influence Atlantic tropical storm activity shows that the estimates currently used are only slightly below modeled numbers and indicate that the numbers of tropical storms in the recent past are increasing, according to researchers.

"We are not the first to come up with an estimate of the number of undercounted storms," says Michael E. Mann, associate professor of meteorology, Penn State, and director of the Earth System Science Center.

In the past, some researchers assumed that a constant percentage of all the storms made landfall and so they compared the number of tropical storms making landfall with the total number of reported storms for that year. Other researchers looked at ship logs and ship tracks to determine how likely a tropical storm would have been missed. In the early 1900s and before, there were probably not sufficient ships crossing the Atlantic to ensure full coverage.

The researchers report in the current issue of Geophysical Review Letters "that the long-term record of historical Atlantic tropical cyclone counts is likely largely reliable, with an average undercount bias at most of approximately one tropical storm per year back to 1870."

The previously estimated undercounts of three or more storms are inaccurate.

"We have a very accurate count of Atlantic tropical cyclones beginning in 1944 when aircraft became common," says Mann. "In the 1970s, satellites were added to that mix."

With more than 60 years of accurate hurricane counts, the researchers, who included Thomas Sabbatelli, an undergraduate in meteorology and the Schreyer Honors College at Penn State, and Urs Neu, a research scientist at ProClim, Swiss Academy of Sciences, looked at other, independent ways to determine the number of hurricanes before 1944.

They looked at how the cycle of El Nino/La Nina, the pattern of the northern hemisphere jet stream and tropical Atlantic sea surface temperatures influence tropical storm generation by creating a model that includes these three climate variables. The information is available back to 1870.

The statistical model proved successful in various tests of accuracy. The model also predicted 15 total Atlantic tropical storms with an error margin of 4 before the current season began. So far, 14 storms have formed, with a little more than one week left in the season.

The model, trained on the tropical storm occurrence information from 1944 to 2006 showed an undercount before 1944 of 1.2 storms per year. When the researchers considered a possible undercount of three storms per year, their model predicted too few storms total. The model only works in the range of around 1.2 undercounted storms per year with the climate data available. The model was statistically significant in its findings.

"Fifty percent of the variation in storm numbers from one year to the next appears to be predictable in terms of the three key climate variables we used," says Mann. "The other 50 percent appears to be pure random variation. The model ties the increase in storm numbers over the past decade to increasing tropical ocean surface temperatures.

"We cannot explain the warming trend in the tropics without considering human impacts on climate. This is not a natural variation," says Mann.

"This . . . supports other work suggesting that increases in frequency, as well as powerfulness, of Atlantic tropical cyclones are potentially related to long-term trends in tropical Atlantic sea surface temperatures, trends that have in turn been connected to anthropogenic influences on climate," the researchers report.

Source: Penn State

Related stories:

Future risk of hurricanes: The role of climate change
Researchers are homing in on the hurricane-prone Gulf of Mexico and Caribbean Sea to assess the likely changes, between now and the middle of the century, in the frequency, intensity, and tracks of these powerful storms. Initial results are expected early next year.
Warmer seas linked to strengthening hurricanes
The theory that global warming may be contributing to stronger hurricanes in the Atlantic over the past 30 years is bolstered by a new study led by a Florida State University researcher. The study will be published in the Sept. 4 edition of the journal Nature.
Boston Hurricane Frequency Over Last Millennium Linked To Ocean Surface Temperatures
(PhysOrg.com) -- The frequency of hurricanes striking the Boston area has varied widely over the last millennium, with periods of lowest activity corresponding to cooler surface temperatures in the tropical North Atlantic Ocean, according to newly published research from the University of Massachusetts Amherst.
Greenland ice core analysis shows drastic climate change near end of last ice age
Temperatures spiked 22 degrees F in just 50 years, researchers say
Information gleaned from a Greenland ice core by an international science team shows that two huge Northern Hemisphere temperature spikes prior to the close of the last ice age some 11,500 years ago were tied to fundamental shifts in atmospheric circulation.
For Hurricanes, Storms, Raindrop Size Makes All the Difference
When Tropical Storm Gaston hit Richmond, Va., in August 2004, its notable abundance of small and mid-sized raindrops created torrential rains that led to unexpected flash flooding throughout the city and its suburbs. New research from NASA has concluded that tropical cyclones like Gaston produce rain differently than another class of storms called "extra-tropical" cyclones.
African dust forecast may help hurricane season predictions
As the official June 1 start of the Atlantic hurricane season approaches, forecasters are developing predictions about the severity of this year's season. For the first time this year, African dust may provide a piece of this puzzle.
New study validates hurricane prediction
Hurricanes in some areas, including the North Atlantic, are likely to become more intense as a result of global warming even though the number of such storms worldwide may decline, according to a new study by MIT researchers.
Forecasters Implement New Hurricane-Tracking Technique
A new technique that helps forecasters continuously monitor landfalling hurricanes, giving them frequent and detailed images of a storm's location, will be implemented this summer.

News discussion:

Space & Earth science news