It’s Not the Wind, It’s the Rain

By Vladimir Brezina

So, Hurricane Irene has come and gone.

In Manhattan—certainly in our corner of the Upper East Side—she hardly happened. Yes, there was quite a lot of rain (in Central Park, almost 7 inches), but not all that much wind. Walking round the Central Park Reservoir yesterday afternoon, I didn’t see a single tree down. (As compared, for instance, to the hundreds brought down almost exactly two years ago by a single violent thunderstorm, the scars of which are still visible.) There were torn leaves scattered everywhere, and the Reservoir was much fuller than usual. But that’s it.

In the rest of Manhattan, too, reporters were hard pushed to show anything really dramatic. The water lapping over the seawall at the Battery was shown over and over.

As a result, New Yorkers feel let down, and worse. The readers’ comments in the New York Times are running heavily against Mayor Bloomberg for ordering major precautions—massive evacuations, a shut-down of the entire transportation system—that, according to the Monday morning quarterbacks, were an absurd overreaction that was obviously motivated by the Mayor’s desire to compensate for his underreaction to last winter’s snowstorm. And where do we get a refund for the two lost days on our Metrocard?

Be that as it may, that’s taking a very narrow view of how catastrophic Irene in fact was.

Irene cut a broad swath straight up the densely populated US East Coast from North Carolina all the way up to Vermont. And she was a very large, slow-moving system—just right to drop a lot of rain.  And Irene has. It’s been not so much the wind, or even the dreaded coastal storm surge, but rather inland floods that have been the major cause of devastation.

Here is Margaretville, in the Catskill Mountains of New York, being wiped out by the overflowing East Branch of the Delaware River yesterday.

Such scenes are being repeated in hundreds of towns all the way up the East Coast. Philadelphia is particularly hard hit, and the entire state of Vermont faces “catastrophic” flooding. Here’s the historic covered bridge in Bartonsville, VT, built in 1870, being swept away by the flood.

“Unbelievable” and “worst I’ve ever seen” are frequently heard comments. And it’s not over yet. As they channel the runoff, some of the creeks and rivers are only just now building to flood stage.  In some places, the worst may be yet to come.

In the New York Times readers’ comments, much was made of the fact that by the time Irene reached New York the system had been degraded from a hurricane to “just” a tropical storm.  But tropical storms have plenty of rain in them.  In 2008, for example, Tropical Storm Fay, which remained “just” a tropical storm and never grew into a hurricane, nevertheless caused some of the most devastating floods in Florida.

In less well developed countries, it’s typically the intense rainfall and flooding, rather than the wind, that cause the most deaths. Take 1998’s Hurricane Mitch:

Due to its slow motion from October 29 to November 3, Hurricane Mitch dropped historic amounts of rainfall in Honduras, Guatemala, and Nicaragua, with unofficial reports of up to 75 in (1900 mm). Deaths due to catastrophic flooding made it the second deadliest Atlantic hurricane in history; nearly 11,000 people were killed with over 11,000 left missing by the end of 1998. As of 2008, the official death toll from Mitch was placed at 19,325, with thousands more unaccounted for. Additionally, roughly 2.7 million were left homeless as a result of the hurricane. The flooding caused extreme damage, estimated at over $5 billion (1998 USD, $6.74 billion 2011 USD).

These tropical systems generate an amazing amount of rainwater. According to the Atlantic Oceanographic and Meteorological Laboratory,

An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 nautical miles). (More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands.) Converting this to a volume of rain gives 2.1 . 1016 cm3 /day.

… which is 1.2 . 1013 liters or about 2.6 trillion gallons per day.

It would be interesting to know how this volume, assuming that a large proportion of it appears as runoff, compares to the typical volume of water already contained in the area’s creeks and streams—proportionately how much more water is being added to them?

Further,

A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives 5.2 . 1019 Joules/day or 6.0 . 1014 Watts. This is equivalent to 200 times the world-wide electrical generating capacity—an incredible amount of energy produced!

So the hurricane has plenty of energy for destruction…

3 responses to “It’s Not the Wind, It’s the Rain

  1. A very interesting perspective on the matter, Vlad. And 6.0 E14 W! Wow. Who knew?

    Like

    • Another way to calculate the energy of a hurricane is to focus not on thermodynamics but instead on the kinetic energy of the rotating mass of air.

      The total kinetic energy of the spiralling horizontal winds of a mature hurricane has been calculated to be ~7.1017 Joules. Using a different kinetic argument, the dissipation of this energy has been calculated to proceed at ~1.1017 Joules/day. Although arrived at by different arguments from different starting assumptions, these two numbers are roughly consistent with each other, and imply that the entire kinetic energy of a hurricane is dissipated through turbulence and atmospheric heating—but simultaneously replenished if the hurricane is to maintain its rotation—once every few days.

      Making the obligatory conversion of these energies to the equivalent number of Hiroshima atomic bombs, we get numbers of the order of 10,000 Hiroshima bombs.

      But these numbers are still two orders of magnitude below the ~5.1019 Joules/day (1 million Hiroshima bombs!) generated by the condensation of water vapor into rain. So only a very small part of the energy produced by a hurricane actually goes into maintaining its spiralling horizontal winds. The rest goes into the vertical updrafts of the hurricane.

      Given these vast energies, it’s no wonder that various human attempts to weaken or channel hurricanes have all failed…

      Like

  2. Pingback: Sandy Saga, Part 4 | Wind Against Current

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