A new era in measuring hurricane winds began on December 15, 2016, when a Pegasus XL rocket launched by Stargazer—Orbital ATK’s L-1011 carrier aircraft—put into orbit a constellation of eight micro-satellites, the core of a NASA mission
(Cyclone Global Navigation Satellite System). Each CYGNSS micro-satellite uses less power than a 50-watt light bulb, weighs about 64 pounds, and is the size of a full-grown swan when the solar panels are extended. Each satellite is equipped with a scatterometer-like device called a Delay Doppler Mapping Instrument (DDMI). This instrument studies Global Positioning System (GPS) signals emitted by the existing constellation of 30 GPS satellites in medium Earth orbit when these signals are reflected off of the ocean (each CYGNSS satellite works with four GPS satellites at a time.) The amount of scattering of the GPS signals is related to the wind speed at the surface, allowing the CYGNSS satellites to measure the surface wind speed. GPS signals are not affected by heavy rain, which will allow CYGNSS to measure winds in hurricane eyewalls—something a previous scatterometer instrument, QuikSCAT, could not reliably do. Figure 1.
A NASA depiction
of one of the CYGNSS satellites.
The CYGNSS satellite constellation joins the European Space Agency’s two ASCAT
instruments, launched in 2007, as the only space-borne scatterometer instruments for measuring surface winds. The RapidScat instrument, which was mounted on the International Space Station, failed in December 2016, and QuikScat has been dead since 2009.
There are other spaceborne sensors that can measure winds using passive microwave methods (e.g. SSM/I, WindSat, GMI), but measurement from these sensors are subject to interference from rain, and are not as useful in a hurricane environment. Both CYGNSS and ASCAT have the same resolution, 25 km, but the eight CYGNSS satellites allow for much better coverage of the globe than a single satellite, since the eight satellites will pass over the ocean more frequently than a single satellite would. The complete constellation provides nearly gap-free coverage with an average revisit time of seven hours over the prime hurricane breeding grounds between 35 degrees north and south latitude (as far north as North Carolina and as far south as southern Australia.) The CYGNSS satellites fly in Low Earth Orbit at an altitude of about 300 miles and cannot see poleward of 38.5 degrees of latitude. ASCAT measures global winds only twice per day along two parallel swaths 550 km wide, separated by a 720 km gap. This means that ASCAT passes miss the center of circulation of a hurricane more than half the time. However, ASCAT is in polar orbit and can see the entire globe.
CYNGSS costs about $157 million, and is designed to operate for a minimum of two years. The price is relatively cheap for a weather satellite; the highly advanced Geostationary Operational Environmental Satellite-R (GOES-R) is part of a weather and solar activity monitoring program estimated to cost $11 billion. However, funding does not currently exist to provide the CYGNSS data in near real time to hurricane forecasters; up to a six day lag is anticipated between the time a measurement is taken and when the final data is processed and made available. Thus, CYGNSS is a proof-of-concept research mission intended to pave the way for future operational missions that can provide real-time wind data to hurricane forecasters. According to lead mission scientist Dr. Chris Ruf of the University of Michigan’s Climate and Space Sciences and Engineering Program, “discussions have started with NOAA about possibly supplementing our current data downlinks to improve data latency. So we might not have to wait for another mission. But this is still to be determined.” Figure 2.
Flying over the Atlantic Ocean offshore from Daytona Beach, Florida, a Pegasus XL rocket with eight Cyclone Global Navigation Satellite System, or CYGNSS, spacecraft was released from the Orbital ATK L-1011 Stargazer aircraft and the first stage ignited at 8:37 a.m. EST December 15, 2016. Image credit: NASA
.Scatterometer data: a valuable tool for hurricane forecasters
Scatterometer data are extremely valuable for many aspects of hurricane forecasting. It makes intensity estimates more accurate, provides early detection of surface circulations in developing tropical depressions, and helps define gale (34 kts, minimal tropical storm strength) and storm-force (50 kts) wind radii. The information on wind radii from scatterometer data is especially important for tropical storms and hurricanes outside the range of aircraft reconnaissance flights conducted in the Atlantic and Eastern Pacific basins, and for the regions where there are no reconnaissance flights (Central Pacific, Western Pacific, and Indian Ocean). Accurate wind radii are critical to the National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and Guam Weather Forecast Office (WFO) watch and warning process, since they affect the size of tropical storm and hurricane watch and warning areas. Between 2003 and 2006, QuikSCAT data were used at NHC 17% of the time to determine the wind radii, 21% of the time for center fixing, and 62% of the time for storm intensity estimates.
While there is evidence that satellite scatterometer data improves hurricane track forecasts of some computer models, data from CYGNSS may not significantly improve official hurricane track forecasts from NHC, though. NHC uses many models to make hurricane track forecasts, and some of these models may not be helped by scatterometer data. For example, a 2009 model study
by Dr. Jim Goerss of the Naval Research Lab found that QuikSCAT winds made no improvement to hurricane track forecasts of the NOGAPS model, one of the key models used by NHC to predict hurricane tracks. Former National Hurricane Center director Bill Proenza laudably made a big push in 2007 for a replacement of the failing QuikSCAT satellite, but unfortunately made claims about the usefulness of QuikSCAT for improving hurricane track forecasts that were not supported by scientific research
, an error that may have ultimately led to his downfall.
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