Good news for the 2007 hurricane season

There's plenty of hurricane-related news to report from last week's annual meeting of the American Meteorological Society (AMS) in San Antonio, Texas. The best news is that the Air Force Hurricane Hunter C-130 aircraft are expected to get a major upgrade of their instrumentation for the 2007 season, thanks to $10.5 million in supplemental funding approved by Congress in December 2004. At least four Air Force C-130 aircraft will receive new Stepped Frequency Microwave Radiometer (SFMR) instruments. The first SFMR-equipped Air Force C-130 is scheduled to be on-line in June. The SFMR (or "Smurf") is able to directly measure the wind speed at the ocean surface. The instrument has been flown operationally by NOAA's two P-3 hurricane hunter aircraft since 1999, and has repeatedly provided crucial estimates of the landfall intensity of numerous hurricanes, particularly during the ferocious hurricane seasons of 2004 and 2005. The SFMR works by studying the brightness of the surface at microwave wavelengths. Strong winds kick up sea spray that makes the ocean white with large foam patches. The amount of microwave energy seen by the instrument is proportional to the whiteness of the ocean surface, and therefore the wind speed. This measurement of the surface winds is far more accurate than trying to infer the surface winds from winds measured at flight level. For example, as Hurricane Katrina approached landfall in 2005, the winds measured at flight level (10,000 feet) stayed roughly constant, while the surface winds fell from Category 5 to Category 3 speeds as the hurricane weakened. The SFMR correctly diagnosed Katrina's sudden weakening at landfall, enabling NHC to issue more accurate advisories.

NOAA's two P-3 hurricane hunter aircraft have flown the SFMR instrument since the late 1980's, so we have a lot of calibration data that has greatly increased our confidence in the reliability of these wind measurements. Comparison of SFMR winds with dropsonde and buoy measurements in hurricanes have shown that the SFMR winds are in error by less than 8 mph about 50% of the time. Thus, we can estimate the surface winds in a hurricane to an accuracy of about 10% using the SFMR.

The SFMR instrument requires about 10 seconds to make a measurement. At the typical flight speed of a C-130 or P-3 hurricane hunter aircraft, the winds are effectively averaged over about 1.5 km along the flight track. According to Uhlhorn and Black (2003), the SFMR instrument underestimates the winds in the right-rear quadrant of northern hemisphere storms. This occurs because the wind is aligned with the direction the waves propagate, resulting in building waves that do not produce very large foam patches in their wake after breaking.

Other hurricane news
Another piece of good news this hurricane season--both of NOAA's P-3 aircraft will both be available for hurricane duty. Last year, only one of these aircraft was available. A third P-3 is on order and due to be operational by 2009, but the latest budget has no money to crew the aircraft. There's other bad news to report on funding for hurricane research, which I'll report in a later blog.

Jeff Masters