Why Surface Wind Measurements in Hurricanes Don't Match Satellite Estimates

Dr. Philip Klotzbach writes via email:

Discussion of near impossibility of actually recording strongest winds in a hurricane

Brian McNoldy and I wrote a blog for Capital Weather Gang yesterday where we called out Matt Drudge about his claims that the National Hurricane Center was over-estimating the winds in Hurricane Matthew. As you know, it is nearly impossible to measure the strongest winds in a hurricane due to a variety of factors. Brian and I covered these in some detail in this blog.

Matt Drudge commented on our story last night, and since then, there have been hundreds of comments from individuals claiming that NHC is just a cog in the “global warming” machine. As you know, I’m certainly not a fan of over-hyping climate change and have published several papers showing no trends in intense hurricane activity over the modern observational period.

I feel that it would be incredibly useful for a well-respected climate skeptic website to pick up this story and give some additional credence to it. If the eyewall of Hurricane Matthew had been 50 miles further west, it would have been devastating for the east coast of Florida! We may not be so lucky next time, and I don’t want people to become armchair meteorologists and think the winds are much less than NHC reports.

Happy to help – Anthony

Hurricane intensity is not exaggerated to scare people, and here’s how we know

By Phil Klotzbach and Brian McNoldy

Hurricane Matthew brushed the East Coast as a Category 4 in early October. It scoured the Florida coast with a storm surge that washed out roads and flooded homes and businesses. It dumped over a foot of rain on the Carolinas and triggered deadly flooding. And before all of this, Matthew devastated Haiti and killed at least 600 people — it’s possible we’ll never knowthe final death toll.

But the hurricane’s deadly encounter with Haiti and multibillion-dollar brush with the East Coast wasn’t bad enough for some people — mainly, Matt Drudge, a political commentator and news aggregator, who tweeted skepticism about Matthew’s actual intensity as it barreled toward Florida. Perhaps, he suggested, the National Hurricane Center was overhyping the storm’s maximum winds.

Drudge deletes his tweets regularly, so here’s a screenshot from the morning of Oct. 7:

“The deplorables are starting to wonder if govt has been lying to them about Hurricane Matthew intensity to make exaggerated point on climate,” Drudge mused. (“The deplorables” refers to supporters of Republican presidential nominee Donald Trump.)

Then this (image saved by the BBC):

Drudge argued the point based on data from Caribbean weather stations and buoys that were not reporting winds as strong as what the National Hurricane Center used in its advisories. But the National Hurricane Center uses a lot of different methods to determine a hurricane’s actual peak intensity, and there are some serious issues with relying simply on weather stations and buoys.

The National Hurricane Center — or NHC — uses a variety of satellite-based tools to estimate a hurricane’s winds when the storm is far from land, and as the storm moves closer to land, additional devices are employed.

Hurricane Hunter aircraft are dispatched to investigate the storm in more detail. These planes measure winds both at flight-level, which is approximately 10,000 feet, as well as at the surface through the use of a device called a stepped-frequency microwave radiometer which measures radiation transmitted from the ocean surface back up to the airplane. When high winds churn up the ocean surface, the amount of microwave radiation measured is diffused compared with the amount of radiation measured when the ocean surface is calm.

NHC also utilizes devices called “dropwindsondes” that are dropped from the aircraft at about 10,000 feet and measure temperature, humidity, pressure and wind speed as they descend to the surface. When a storm nears shore, radars can also be utilized to estimate wind velocity.

All of these different measurement devices, combined with surface observations from ships and buoys, are combined to assess a storm’s intensity.

There are several reasons buoys are likely to underestimate a hurricane’s wind.

First, buoys utilize an eight-minute average maximum wind, and NHC uses one-minute average wind. The shorter the wind averaging time, the stronger the reported winds will be. The adjustment factor would be approximately 0.90, which means if a buoy measures 80-mph eight-minute average winds, the one-minute average wind would be about 89 mph.

A more significant bias in the buoy data is the insanely severe ocean conditions that it’s reporting from. Waves in a major hurricane can be as high as 50 feet, and consequently, a buoy experiences very large crests and troughs as the hurricane passes over. When a buoy is on the crest of a wave, it is exposed to the full force of the surface winds, but when it is in the trough of a wave, it is going to be sheltered from the strongest winds. These weaker winds measured in the trough of a wave get averaged into the eight-minute averaging period measured by the buoy, which therefore gives the buoy’s estimated wind speed in a hurricane a low bias.

Surface stations also have significant measurement challenges in hurricanes.

Often during strong wind speeds, the anemometers break. Storm surge can also wash away weather stations before the strongest winds come ashore.

Importantly, a hurricane’s wind field is a wide swath, and the strongest winds may only be found in a small region of the hurricane. The number of surface stations in any area struck by a hurricane is likely not dense enough to accurately measure the strongest winds. For example, one 2014 study found that on average, “a single perfect anemometer experiencing a direct hit by the right side of the eyewall will underestimate the actual peak intensity by 10-20% percent. Even an unusually large number of anemometers (e.g., 3-5) experiencing direct hits by the storm together will underestimate the peak wind speeds by 5%-10%.”

The number of surface weather stations reporting hurricane-force winds, even in notoriously strong United States landfalling hurricanes are surprisingly small. For example, in NHC’s post-storm report on Hurricane Katrina (2005), a Category 3 hurricane on the Saffir-Simpson Wind Scale, only five surface weather stations reported sustained hurricane-force winds, with the strongest sustained hurricane-force winds reported being 86 mph. Many of the surface stations reporting the strongest winds were also flagged by NHC as being incomplete, indicating failure of the station before the strongest winds likely occurred. For reference, NHC estimated Katrina’s Louisiana landfall to have had maximum winds of 125 mph and its Mississippi landfall to have had maximum winds of 120 mph.

The peak winds in a hurricane are confined to a small region of the storm called the eyewall, which is the ring of thunderstorms that surround the eye. Beyond the eyewall, the wind speed drops off relatively quickly.

In the case of Hurricane Matthew, while the southeast United States suffered significant flooding damage due to both storm surge and heavy rain, the east coast of Florida dodged a huge bullet. Had Matthew tracked 50 miles farther to the west, the eyewall of the storm would have come ashore, causing much more extensive flooding as well as significant wind damage.

In any case, Matthew’s strongest winds would likely not have been measured by a weather station. The National Hurricane Center provides the best analysis that science can offer.