Latest Stepped-Frequency Microwave Radiometer Classifies Hurricane Earl's Wind Force

By Ashok Bindra September 01, 2010

As weather experts warn East Coast residents of Hurricane Earl approaching the coastline by late Thursday, federal agencies are employing old and new technologies to track the massive storm’s movement and the force with which it was coming. Thus, ensuring that the citizens of the area have accurate and complete information about the approaching hurricane, giving them enough time to prepare and take steps to tackle the fury of the storm.

In speaking to the reporter Jill R. Aitoro, James Franklin, branch chief of the hurricane specialist unit at the National Hurricane Center (NHC) in Miami, said “We have all kinds of tools, some of which we’ve had a long time, and others more recent.”

While traditional satellite images track a storm’s movement, "newer tools are helping the agencies to do a better job,” Franklin told Aitoro.

Speaking of new instruments, the National Hurricane Center (NHC) uses a novel instrument called the Stepped-Frequency Microwave Radiometer (SFMR) to classify a hurricane’s wind force. SFMR picks up the microwave radiation emitted from the foam the hurricane winds create on the ocean’s surface. SFMR measurements on Tuesday indicated that Earl’s top winds measured at 135 mph, making the storm a Category 4, according to NHC.

However, today the NHC Earl advisory posted wind speed of 125 mph, downgrading the storm to Category 3.

By definition, the SFMR, affectionately known as ”smurf,” is installed on the WC-130J aircraft within a pod attached to the aircraft’s wing. As the plane flies through a storm, the SFMR senses microwave radiation naturally emitted from foam created on the sea by winds at the surface. Computers then determine wind speeds based on the levels of microwave radiation detected. 

Currently, winds are extrapolated from the winds at the aircraft’s altitude or from a dropsonde released from the aircraft, however, the SFMR directly measures the surface winds and is not confined to a single point like the dropsonde. This constant measurement of surface winds gives the NHC center in Miami a more complete picture of the storm.

Hence, explaining this phenomenon to Aitoro, Franklin said, “The windier it is, the more disrupted the sea surface and the greater the amount of foam,” creating more radiation.” He added “That’s kind of a big deal because until recently we had to estimate the intensity of a storm based on the winds observed by an aircraft,” flying at 10,000 feet through the hurricane. “Of course, no one lives at 10,000 feet,” he continued. “We need to know how strong the winds are at the surface.”

As explained to the reporter, for about 10 years, the NHC has been using what is called a dropwindsondes to measure a hurricane’s winds. The instrument is loaded with a GPS receiver and a parachute, which opens when dropped from an aircraft. As a result, scientists can measure wind speed based on how fast a dropwindsondes descends. While the center still uses the device, it’s less reliable than SFMR because it measures wind only in a specific location, noted Franklin. In addition, dropwindsondes costs $750 each and can’t be reused, stated Franklin in the interview

“Having continuous readings below where the aircraft is flying allows us to have better estimates of current [storm] intensity, which is the starting point for a forecast,” Franklin said. “

Designed to operate over a tuning bandwidth of 4.6 to 7.2 GHz, SFMR offers up to 8 channels with receiver channel bandwidth of 50 MHz. It uses a corrugated horn type antenna with a beamwidth (3 dB) of 20-28 degrees and linear polarization. Measurement precision is rated at 0.17 K (1second averaging) and uses a 28 Vdc, 2 A power supply. Its power rating is 168 W and operating temperature range is -65 °C to +40 °C. Weighing 40 lbs, the unit measures (L x W x H) 24 x 11 x 13 inches.

Ashok Bindra is a veteran writer and editor with more than 25 years of editorial experience covering RF/wireless technologies, semiconductors and power electronics. To read more of his articles, please visit his columnist page.

Edited by Erin Harrison

TechZone360 Contributor

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