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Stepped Frequency Microwave Radiometer (SFMR) development

Principal Investigator: Eric Uhlhorn

Collaborating scientist(s):
Peter Black
Alan Goldstein (AOC)
Ivan Popstefanija (ProSensing Inc.)
Jim Carswell (Remote Sensing Solutions Inc.)
Jimmy Franklin (NHC)

Reports and presentations:

Objective:
Measurement of the hurricane surface wind field, and in particular the estimation of wind maxima, has long been a requirement of the Tropical Prediction Center/OAR (TPC/NHC). The NOAA/Hurricane Research Division's (HRD) Stepped-Frequency Microwave Radiometer (SFMR) ,built by Prosensing Inc., is the prototype for a new generation of airborne remote sensing instruments designed for operational surface wind estimation in hurricanes. The first experimental SFMR surface wind measurements were made in Hurricane Allen in 1980, the first real-time retrieval of winds on board the aircraft in Hurricane Earl in 1985, and the first operational transmission of winds to TPC/NHC in Hurricane Dennis in 1999.

Background:
Since hurricane reconnaissance began in 1947, numerous methods have been employed to estimate the distribution of surface winds in hurricanes. Sea-state catalogs have provided a subjective guide for the determination of the wind speed. For many years surface winds have been estimated by flight-level measurements using various extrapolation algorithms. Maximum sustained winds have also been estimated using pressure-wind relationships. Studies prior to 1980 have shown that passive microwave emissions from the sea surface are also strongly correlated with wind speed.

The concept for the first experimental SFMR was proposed by C. T. Swift at the University of Massachusetts Microwave Remote Sensing Laboratory (UMASS/MIRSL) and built by NASA's Langley Research Center in 1978. The SFMR design involved a single nadir-viewing antenna and receiver capable of making measurements of radio emission from the sea surface at four selectable frequencies between 4.5 and 7.2 GHz. The "stepping" procedure allowed for estimating the surface wind speed in hurricanes by correcting for rain-induced effects in the measurements, and therefore enabling recovery of the rainrate. The first measurements by the original SFMR were made from the NOAA WC-130 aircraft in Hurricane Allen in 1980. By making assumptions about the vertical structure of the atmosphere together with sea surface temperature (SST) measurements by a downward-looking airborne infrared radiometer, reasonable estimates of the ocean surface brightness temperature (Tb) were made at 4.5, 5.0, 5.6, and 6.6 GHz. Wind speeds were then calculated assuming a linear increase in wind speed with Tb, independent of frequency. Agreement between surface (20 m) winds extrapolated from the 1500 m flight-level and the SFMR estimates for independent flight legs were within +/-10 %. Despite the success in Allen, this instrument was never again flown into a hurricane.

A second SFMR was designed and built in 1982 under the supervision of C. T. Swift. The number of frequencies was expanded to six between 4.6 and 7.2 GHz, and the instrument integration time was reduced to less than one second resulting in improved spatial resolution. A new retrieval algorithm was also implemented. This instrument was flown on board the NOAA WP-3D in 1984, and during 12 flights during the 1985 hurricane season. The SFMR was further modified in 1986 and initially used for studies of sea-ice structure. Using data obtained in Hurricanes Earl (1985), Gilbert (1988), and Hugo (1989), the empirical emissivity/wind speed relationships were refined to include winds over 60 m/s.

With support from the Office of the Federal Coordinator for Meteorology (OFCM) the existing horn antenna was replaced with a dipole array antenna in 1993. The new antenna with a new set of six frequencies was flown in Hurricane Olivia (1994) and retrieved high-quality wind estimates. Further funds were provided by OFCM for an upgrade of the SFMR's receiver, which allowed for increased calibration stability. The reconfigured SFMR was first flown in Hurricane Jerry in 1995. Minor modifications were made to reduce background noise levels after the 1995 season, and since then the SFMR has flown under this configuration. Following component failures in 2000, the NOAA Office of Oceanic and Atmospheric Research (OAR) supported an instrument repair and again the SFMR returned surface winds during the 2001 hurricane season. Since 1980, the SFMR has flown on over 150 flights in 50 tropical cyclones.

SFMR surface winds (dashed red) and flight level winds (solid black) for the first WP-3D pass through Hurricane Katrina on Aug 28 from northeast (left) to southwest (right). SFMR rain rate is also shown in dotted green.

Accomplishments:

  • Obtain concurrent/collocated surface wind estimates from HRD and AOC SFMRs and GPS dropwindsondes -- Status: completed 16 missions in 3 hurricanes (Frances/Ivan/Jeanne)
  • Send report to NHC on performance of SFMR during landfalls of Hurricanes Ivan and Jeanne -- Status: completed Oct. 2004
  • Obtain raw measurements for both SFMR systems from AOC -- Status: completed Dec. 2004
  • Post-processing of HRD SFMR measurements -- Status: completed Jan. 2005
  • Post-processing of AOC SFMR measurements -- Status: completed Feb. 2005
  • Presentation of initial results of intercomparison between HRD and AOC SFMR wind measurements at 59th IHC -- Status: completed March 2005
  • Updated calibration for AOC SFMR and testing of alternative calibration procedures -- Status: completed April 2005
  • Post-processing of GPS sonde surface wind estimates -- Status : completed April 2005
Goals:

  • Indentifying collocated SFMR and GPS surface wind estimates and performing statistical analyses -- Status: Expected completion 3rd Q 2005
  • Calibration flights to test results in prepartation for 2005 season -- Status: Expected completion 3rd Q 2005
  • Execute 2005 flight missions to collect additional data and test new calibration procedures -- Status: Expected completion 4th Q 2005
  • Prepare final report documenting results of evaluation report -- Status: expected completion 4th Q 2005

Key references:

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Last modified: 2/23/2005
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