2014-02-25: Retrieving Hurricane Wind Speeds using Cross Polarization C-band measurements

Hurricane-force wind speeds can have a large societal impact and in the following paper microwave C-band cross-polarized (VH) signals are investigated to assess if they can be used to derive extreme wind speed conditions.
European satellite scatterometers have excellent hurricane penetration capability at C-band, but the vertically (VV) polarized signals become insensitive above 25 m s-1. The Meteorological Operational satellite programme Second Generation (MetOp-SG) satellites will replace the current MetOp system in the 2020+ timeframe. One instrument to be carried on-board the MetOP-SG is a C-band scatterometer (SCA), similar to ASCAT of MetOp, but with a higher spatial resolution and stability. The prime objective of SCA, (herein referred to as ASCAT-SG), will be to measure wind speed and direction over the oceans which are an important input to global and regional Numerical Weather Prediction (NWP). A new ASCAT-SG option is the inclusion of cross polarization (VH) channels on the scatterometer mid beams. These VH channels should not only improve the retrieval of extreme (hurricane) winds, but also provide improved soil moisture index and sea ice (drift) characterization. The presented work was requested by ESA in order to provide a scientific background to assist ESA and EUMETSAT in the decision making process on including the VH channels to ASCAT-SG instrument.

In this work a new wind speed geophysical model function (GMF) is presented using VH backscatter for strong to severe wind speeds (i.e. 20 m/s < U10 < 45 m/s). The VH backscatter increases exponentially with respect to wind speed (linear against VH [dB]) (Figure 1). The GMF is based on the cross-polarized RADARSAT-2 C-band SAR imagery acquired during severe wind speed (hurricane) events and compared to the ECMWF forecasts and the co-located and time co-incident SFMR wind measurements acquired using NOAA’s hurricane hunter aircraft.

Within the available VH backscatter signals no wind direction dependence can be determined. At maximum the wind direction dependence is small in comparison to the wind speed dependence. This means that severe ocean wind speed images can be directly retrieved using a VH channel from a single viewing direction.

There is a strong correlation between the highest VH measurements within a hurricane SAR image and the one-minute maximum sustained wind speeds from the NOAA best track product (Figure 2), indicating that the VH signals are sensitive to severe wind speeds up to at least ~60m/s (Category 4) .

Analysis of the three comparison data sets confirm that cross-polarized signals from satellites will enable the retrieval of strong-to-severe wind speeds where VV or horizontal (HH) polarization data has saturated. Next to this a near real time assessment of maximum sustained wind speed is possible using VH measurements. VH measurements thus would be an extremely valuable complement on next-generation scatterometers for Hurricane forecast warnings and hurricane model initialization.

Read the full article here.
The above graph combines the ECMWF-VH (represented by the grayscale contours) and the SFMR-VH distributions (represented by the contoured lines). The four lines overlaid depict the four linear fits defining the VH-GMF. The dashed red line is a linear fit based on collocation of VH with buoy data, the two blue lines are the linear fits based on the ECMWF data and the solid red line the linear fit based on the SFMR data.
Comparison of the maximum 1 minute sustained surface wind speed best track estimates versus derived by NOAA for each hurricane and the averaged sum of the highest VH signals for each hurricane image used in this study. The error estimates in the x-direction show two percentile values of the measured signals and represent the variability close to the eye within the image, the y-direction error bar shows the standard deviation within a 24 hr window around the RADARSAT-2 overpass time. The red color indicates those hurricanes which did not have collocated SFMR flights around the time of the satellite overpass, the remaining (blue) points indicate the images which were used to create the SFMR-VH distribution (Figure 1).