A New SAR Retrieval Method for Hurricane Wind Parameters
Antonio Reppucci(1), Susanne Lehner(1) and Johannes Schulz-Stellenfleth(1)
(1) German Aerospace Center (DLR), Oberpfaffenhofen, 82234 Wessling, Germany
Synthetic Aperture Radar (SAR) has the unique capability to image the normalized radar cross section (NRCS) of the ocean surface with high spatial resolution. It has been demonstrated that using the same wind retrieval technique as used for Scatterometer it is possible to retrieve high resolution surface wind fields on a kilometre scale.
When used with tropical cyclone images, although the procedure is able to provide a smooth and consistent wind field, saturation of the radar backscatter and damping due to heavy rain leads to an underestimation of the maximum measured wind speed.
A new method to estimate the hurricane maximum wind speed and central pressure, and so its strength, using SAR images in combination with a parametric Holland type model is presented. The work aims at the improvement of the prediction of cyclone intensity and track.
The algorithm presented is based on a last square minimization of the difference between the parametric model and the SAR measurement in the range where the wind speed is below 20 m/s. The radius of maximum wind speed, required as input for the minimization procedure, is measured applying an image analysis method.
The algorithm is applied on a dataset of wide swath (SAR) images (400 km x 400 km coverage), acquired by the European ENVISAT satellite.
Several examples with cyclone center observed at different incidence angles are presented. The observed tropical cyclones, like, e.g., Hurricanes Katrina and Rita and Typhoon Kiko in 2005, cover a range between Category I and V according to the Saffir-Simpson scale.
The retrieved values of maximum wind speed and central pressure agree well with the measurements provided by NOAA Hurricane Research Division and the Japan Meteorological Agency.
In addition the effect of heavy rain on the radar backscattering and on the retrieved wind field is investigated theoretically.
The potential and limitations of the new retrieval technique are discussed.
In the next step the method will be extended for the use with horizontally polarized RADARSAT-1 images as well as X-band TerraSAR-X data.
A strategy to include the technique into an operational hurricane forecast system is discussed.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,