Accuracy of past hurricane counts good

November 26, 2007

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

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2 comments

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mikiwud
not rated yet Nov 27, 2007
50% variations plus 50% random = complete guesstimation.Oh,and pick the count that suits your playstation climate game.
Quantum_Conundrum
not rated yet Jan 12, 2008
"We cannot explain the warming trend in the tropics without considering human impacts on climate. This is not a natural variation," says Mann.
........
Garbage. It is verifyable fact that we still have not reached Medieval warm period highs. How can you blame modern warming on humans, or say with any validity that this is not a natural cycle?

....

"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.
....

Garbage. The strongest landfalling hurricane on record is still the labor day hurricane of 1935, which is still the third strongest storm on record period.

There have been at least 4 or 5 "Katrinas" in the past 100 years for Louisiana alone: Betsy, Audry, Andrew, The Ruddock Hurricane (Historians have found literally whole houses and other artifacts buried under the mud from the water level rise), Camille. These are just the ones I can think of off the top of my head, and most of them are 40 years ago.

All you have to do is go ask an elderly person about it. For some reason people forget, and younger generations don't believe it, but it has happened before, and every time people are like "Oh this has never happened before, water's never been here before, etc." Well, they didn't ask the right person, I can promise you. Because the water in Katrina still did not get as high as a Betsy survivor told me he had seen it, and I have a great-uncle who is in his mid 80's who remembers other storms with worse water level rise in this same area. So just go ask an old person old enough to remember what Steve Lyons and Max Mayfield don't remember.

Also, in terms rainfall, Katrina was a relatively dry hurricane. The inland flooding was virtually non-existent compared to Audry and other Louisiana storms. I kept a rain gauge myself, and we got roughly 7 inches. I've seen far worse rainfall totals from other hurricanes and just frontal systems in my life, and I'm only 27 years old.
...

Finally, I agree with the poster above as well. Arbitrary input yields arbitrary results. A computer can only tell you what you (or someone) already know, or perhaps organize it in a different way, but it can't just pop out some number out of thin air, regardless of how good the programming is.

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