Wednesday, November 13, 2013

Assessing Super Typhoon Haiyan’s Winds - Alaskan waters deserve extreme caution by NWP vessels

In the absence of direct wind speed measurements, one of the common methods used to estimate the intensity of tropical cyclones is the Dvorak technique. Developed four decades ago by American meteorologist Vernon Dvorak, the technique estimates maximum wind speeds by analyzing subtle differences in visible and infrared satellite imagery. However, the Dvorak method does not directly measure a storm’s winds, and some meteorologists think it overestimates maximum wind speed in some circumstances.

Since meteorological organizations do not send hurricane hunter aircraft to monitor typhoons and cyclones in the Pacific—and few ground instruments survived the storm—the Joint Typhoon Warning Center and other groups had to rely heavily on the Dvorak method to estimate wind speeds for Super Typhoon Haiyan. As meteorologist Eric Holthaus pointed out, the storm even maxed out the Dvorak scale, scoring an 8.0 on an 8.0 scale as Haiyan approached the Philippines on November 7.

Scatterometers, a type of microwave radar, can also measure the strength of a storm’s winds. The dual-beam rotating scatterometer on the Indian Space Research Organization’s Oceansat-2 satellite, for instance, can be used to measure the strength of the winds at the ocean surface. On November 7, 2013, Oceansat-2 measured Haiyan’s surface winds at 9:30 a.m. local time (5:30 p.m. PST), as shown in the image above. Arrows indicate wind direction and colors indicate wind speed, with darker shades of purple indicating stronger winds. (The strongest are red.) As is typical of cyclones in the northern hemisphere, the area of strongest winds was northeast of the storm center.

According to the Oceansat-2 data, which was processed by scientists at NASA’s Jet Propulsion Laboratory (JPL) using an experimental technique, the storm’s winds peaked at 206 kilometers (128 miles) per hour at the time of measurement—strong enough to devastate the landscape.

However, it is important to note that the maximum winds were likely stronger than what Oceansat-2 measured, explained Bryan Stiles of JPL. His group’s algorithm averages Oceansat-2 data over a 24 by 24 kilometer (15 by 15 mile) area, which yields a value somewhat lower than the storm’s absolute maximum winds. Stiles estimated that the maximum wind speeds were probably about 20 percent faster—about 240 kilometers (150 miles) per hour—when Oceansat-2 acquired the data, but his team has not yet had time to perform a rigorous analysis.

“The resolution of scatterometers is usually around 15 to 30 miles (25 to 50 kilometers), so they are not capable of resolving a storm’s maximum winds,” explained University of Miami meteorologist Brian McNoldy. “They work on the principle of wind roughening the ocean’s surface. So, in a sense, they don’t really measure the wind in a tropical cyclone. What they do is detect differences in how radiation is scattered off the ocean surface, and then a complex model is used to back out what wind speed would be responsible for that amount of roughness.”

“The bottom line is that meteorologists are going to be debating what Haiyan’s top wind speeds were for some time,” said Jeffrey Halverson, a meteorologist at the University of Maryland at Baltimore County. “The best we can do is point to the strengths and shortcomings of each piece of technology or technique that we use to estimate winds—be it Dvorak, a scatterometer, or a barometer. Since we lack reliable in situ measurement for Haiyan, we have to use wide error bars.”

Further Reading:

NASA Earth Observatory (2012, October 28) Comparing the Winds of Sandy and Katrina. 

NASA Jet Propulsion Laboratory (2013, November 8) NASA Peers Into One of Earth’s Strongest Storms Ever. 

New Republic (2013, November 11) How Strong is Super Typhoon Haiyan? 

Data courtesy of the Jet Propulsion Laboratory and the Indian Space Research Organization’s Oceansat-2mission. Caption by Adam Voiland, with information from Alexander Fore, Brian McNoldy, Jeffrey Halverson, andBryan Stiles.

Instrument: OceanSat-2 - OSCAT

After months underway on an east to west Northwest Passage you cross the Bering Sea Arctic Circle finish line and go into Nome Alaska exhausted... you think it is over... but it really it is just beginning... Nome does not have good facilities for haul-out nor storing your yacht in the Alaskan subzero weather... you must continue on south to a warmer climate as fast as possible... winter is approaching... you face another 1,000 nautical miles through the Bering Sea to turn the corner at Unimak Pass for King Cover on the Alaska Peninsula in the North Pacific Ocean with over 1500 miles to Washington State... but the good summer weather is now over... it is Fall and the STORMS and TYPHOONS are rolling east across the North Pacific Ocean... its time to really respect the weather and make even better decisions on planning a route - be it: a Gulf of Alaska coastwise route then the S.E. Alaska Inside Passage with many protected harbors (recommended) or across open waters to Dixon Entrance or the great circle direct route to the Straits of San Juan de Fuca into Victoria B.C. Canada or to Seattle Washington USA. Judging weather windows will be key to making a good decision or what could turn out to be an extremely poor decision.  

The Northwest Passage was a monumental challenge but the last two thousand nautical miles might prove to be the biggest trial of your sailing history... or you could turn them into the most rewarding passage of your life... Alaska has it all... how you choose to experience it will determine if it becomes a rewarding memory or the darkest night filled with terror... 

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