Hurricane Surface Wind Measurements from an Operational Stepped Frequency Microwave Radiometer

Abstract

For the first time, the NOAA/Aircraft Operations Center (AOC) flew stepped frequency microwave radiometers (SFMRs) on both WP-3D research aircraft for operational hurricane surface wind speed measurement in 2005. An unprecedented number of major hurricanes provided ample data to evaluate both instrument performance and surface wind speed retrieval quality up to 70 m s−1 (Saffir–Simpson category 5). To this end, a new microwave emissivity–wind speed model function based on estimates of near-surface winds in hurricanes by global positioning system (GPS) dropwindsondes is proposed. For practical purposes, utilizing this function removes a previously documented high bias in moderate SFMR-measured wind speeds (10–50 m s−1), and additionally corrects an extreme wind speed (>60 m s−1) underestimate. The AOC operational SFMRs yield retrievals that are precise to within ∼2% at 30 m s−1, which is a factor of 2 improvement over the NOAA Hurricane Research Division’s SFMR, and comparable to the precision found here for GPS dropwindsonde near-surface wind speeds. A small (1.6 m s−1), but statistically significant, overall high bias was found for independent SFMR measurements utilizing emissivity data not used for model function development. Across the range of measured wind speeds (10–70 m s−1), SFMR 10-s averaged wind speeds are within 4 m s−1 (rms) of the dropwindsonde near-surface estimate, or 5%–25% depending on speed. However, an analysis of eyewall peak wind speeds indicates an overall 2.6 m s−1 GPS low bias relative to the peak SFMR estimate on the same flight leg, suggesting a real increase in the maximum wind speed estimate due to SFMR’s high-density sampling. Through a series of statistical tests, the SFMR is shown to reduce the overall bias in the peak surface wind speed estimate by ∼50% over the current flight-level wind reduction method and is comparable at extreme wind speeds. The updated model function is demonstrated to behave differently below and above the hurricane wind speed threshold (∼32 m s−1), which may have implications for air–sea momentum and kinetic energy exchange. The change in behavior is at least qualitatively consistent with recent laboratory and field results concerning the drag coefficient in high wind speed conditions, which show a fairly clear “leveling off” of the drag coefficient with increased wind speed above ∼30 m s−1. Finally, a composite analysis of historical data indicates that the earth-relative SFMR peak wind speed is typically located in the hurricane’s right-front quadrant, which is consistent with previous observational and theoretical studies of surface wind structure.

Assimilating Global Wave Model Predictions and Deep-Water Wave Observations in Nearshore Swell Predictions

“…For uncertainty from the dropsonde-based surface wind speed estimates, we consider the 3.1 m s −1 root mean square difference reported in Fig. 3 of Uhlhorn et al (2007). Using these values together with the 6.0 m s −1 root mean square difference in the HIRAD – dropsonde comparisons gives a rough estimate of root mean square error as RMSE HIRAD = ( (6.0 m s −1 ) 2 – (3.1 m s −1 ) 2 – (2 m s −1 ) 2 ) 0.5 = 4.7 m s −1 .…”

“…The SFMR has been flown in hurricanes since 1980, with multiple generations of designs, hardware, and retrieval algorithms (Uhlhorn and Black 2003 and references therein; Uhlhorn et al 2007; Klotz and Uhlhorn 2014). Klotz and Uhlhorn (2014) reported on the SFMR algorithm versions that were operational from 2006–2014 (termed “operational” in that paper), and the current version that became operational in 2015 (termed “revised” in that paper).…”

“…This averaging removes some of the effect of gustiness in the dropsonde wind profile. Near surface wind speed is estimated from WL150 using the coefficients in Uhlhorn et al’s (2007) Fig. 2.…”

“…Dropsondes from reconnaissance or research aircraft can provide detailed vertical profiles of the wind, but are necessarily limited in their coverage. The Stepped Frequency Microwave Radiometers (SFMR) on hurricane hunter aircraft are very good at estimating surface wind speed in hurricane conditions, but only along a nadir trace directly beneath the aircraft (Uhlhorn and Black 2003; Uhlhorn et al 2007; Klotz and Uhlhorn 2014). …”

Observations of a Nondeveloping Tropical Disturbance in the Western North Pacific during TCS-08 (2008)

“…1). Both aircraft collected flight-level data and the WC-130J retrieved surface wind speeds and rain rates underneath the flight track using the onboard Stepped Frequency Microwave Radiometer (SFMR; Uhlhorn et al 2007). …”