QuikSCAT satellite showing its age; ASCAT satellite helping out
The now-famous QuikSCAT satellite, which measures winds at the ocean surface world-wide twice per day, was launched in 1999, and has now exceeded its expected lifetime by several years. A reminder of this satellite's age came during the week of November 21-28, when one of the cells on the satellite's battery went bad, forcing engineers to shut off data gathering on the satellite for about 10-15 minutes as it crossed over land in the Arctic. As a result, QuikSCAT provided only half of its usual data on winds and sea ice in the Arctic during that week. Fortunately, engineers were able to swap in a spare battery cell on November 28, and QuikSCAT is now back at full operation. This is good news, since QuikSCAT is a huge help for marine forecasts, sea ice forecasts, and predictions of tropical storms.
QuikSCAT now has help. An important new source of QuikSCAT-like data has been made available by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT). They launched their first polar-orbiting satellite, Metop-A, in October 2006, and declared the satellite ready for routine operations as of May 2007. This satellite carries a scatterometer called ASCAT which, like QuikSCAT, measures the winds at the ocean surface. ASCAT doesn't "see" the Earth's surface as well as QuikSCAT can--ASCAT sees chunks of the surface 25 km by 25 km, while QuikSCAT has a resolution twice as good--12.5 km. In addition, ASCAT only sees 60% of what QuikSCAT sees of the Earth's surface--QuikSCAT sees a swath of ocean 1800 km wide, while ASCAT sees two parallel swaths 550 km wide, separated by a 720 km gap. I found it frustrating to use ASCAT much this hurricane season, since it seemed that the passes missed the center of circulation of a storm of interest about 75% of the time.
Figure 1. Comparison of the coverage pattern of the QuikSCAT and ASCAT satellites, from December 4, 2007. Image credit: NOAA.
However, ASCAT has an important advantage--it can measure ocean surface winds where heavy rain is occurring, something QuikSCAT cannot. Both instruments carry an "active" radar (also called a scatterometer)--an instrument that emits a pulse of microwave energy that bounces off the ocean surface and returns to the satellite. The amount of microwave energy bounced back to the instrument is inversely proportional to how rough the sea surface is, and one can compute the wind speed and direction at the ocean surface based on this information. QuikSCAT uses microwave energy with a wavelength of about 2 cm (Ku-band), which is significantly affected by heavy rain. Microwave radiation from ASCAT uses a wavelength of about 5 cm (C-band), which is much less affected by rain. Thus, ASCAT can retrieve winds more accurately in the heavier precipitation environments such as those found in hurricanes. However, QuikSCAT does have finer spatial resolution and better sensitivity to high winds than ASCAT. Another minor advantage of ASCAT is that the winds across the entire swath of ocean it looks at are of uniform accuracy. QuikSCAT, on the other hand, has a bit larger errors at the edge of its 1800 km-wide swath, and in the middle, making it more difficult to interpret the data in some cases.
QuikSCAT data is routinely ingested into all of the major computer models that forecast hurricanes. ASCAT data is not yet used in this way, since ASCAT is currently still in its calibration and validation phase. However, by the 2008 hurricane season, ASCAT data will probably be used in this fashion. Having ASCAT to complement QuikSCAT will be a big help to NHC forecasters, particularly for those storms far out at sea where the Hurricane Hunters cannot reach.
ASCAT data is available from the ASCAT web page. QuikSCAT expert Dr. Paul Chang of NOAA also has ASCAT data available on his NOAA Marine Observing Systems web page.
This Friday, the Colorado State University team issues its first forecast for the 2008 Atlantic hurricane season. I'll present an analysis of the forecast Friday afternoon.