Antonio Reppucci

Tropical Cyclone Intensity Estimated From Wide-Swath SAR Images

Due to the relatively small amount of in situ data available for the open oceans, remote sensing techniques take an important role in the retrieval of geophysical information, particularly during extreme events. The work presented here aims at the improvement of prediction of cyclone intensity using synthetic aperture radar (SAR) images. A new method to measure the hurricane intensity using SAR images, in combination with a parametric Holland-type model of wind speed, is presented. The algorithm is based on a least square minimization of the difference between the parametric model results and the SAR measurement. The radius of the maximum wind speed, required as input for the minimization procedure, is estimated from the SAR image using wavelet analysis. Information on wind direction is extracted from the SAR image through analysis of image features caused by boundary layer rolls. The root-mean-square error of the suggested method has been validated to be equal to 3.9 m/s. The study is based on a data set of wide-swath SAR images of about 400 km × 400 km coverage, acquired by the European Envisat satellite, over tropical cyclones. As a case study, hurricane Katrina is investigated in detail. A total of five tropical cyclone images will be used to validate the results of the new algorithm.

Extreme wind conditions observed by satellite synthetic aperture radar in the North West Pacific

In this paper some recent results on SAR observation of extreme surface wind conditions are summarized. Particular emphasis is put on the investigation of typhoons occurring in the North West Pacific. The study is based on the use of ENVISAT ASAR wide swath images (400 km×400 km), which allow synoptic measurements of the complete mesoscale system at high resolution (150 m). Surface wind speed for typhoon cases is determined from SAR measurements using the geophysical model function CMOD5. Further structures observed in the image like streaks indicating wind direction and the ring of maximum wind speed are additionally taken into account to reconstruct the typhoon wind field. The influence of heavy rain on the radar cross section is estimated from an existing radiative transfer model and compared to the SAR measurements. A new technique for the estimation of typhoon intensity from SAR data is presented, which makes use of a parametric type model. The main goal of the paper is the improvement of the estimation of maximum typhoon intensity using SAR data.

Use of tandem pairs of ERS-2 and ENVISAT SAR data for the analysis of oceanographic and atmospheric processes

Currently the European satellites ERS-2 and ENVISAT are flying on the same orbit with a time separation of 30 minutes. In this presentation pairs of the respective synthetic aperture radar data are analyzed with respect to different atmospheric and oceanic processes. The presented results were obtained in the framework of the ESA AO project COTAR. The tandem configuration exists since the launch of ENVISAT in 2002. In the presentation an overview will be given of the available image pairs acquired over the ocean on a global scale. Combinations of ERS-2 SAR data with both image mode and wide swath mode scenes provided by the ENVISAT ASAR are considered. The two SAR images enable the analysis of the change of radar cross section within half an hour. This temporal separation is very interesting for oceanographic applications because there are many processes like atmospheric fronts, convective cells, ocean tides, etc., which are detectable on this time scale. In the presentation tandem pairs acquired over the research platform FINO west of the island Borkum in the North Sea will be presented. The area is of high practical interest because of the planned Offshore Windpark Borkum West. The platform provides wind measurements at different heights and additional oceanic information to support the planning activities for this windpark. The dynamics of atmospheric structures is analysed. It is well known that the near surface wind field is a dominating factor for the normalized radar cross section of the sea surface. For this reason SAR scenes are well suited to study atmospheric effects with high spatial resolution. It is shown that the use of tandem pairs enables the study of processes like the propagation of an atmospheric fronts or the evolution of convective cells. Both effects are illustrated with different examples using additional information from in situ measurements. Furthermore existing techniques for the estimation of the wind field in 10 m height from SAR data are applied to both images. The evolution of the spatial structure of both wind speed and wind direction is analysed. The observations are related to some theoretical issues like, e.g., the Taylor hypothesis. Particular emphasize is put on the connection between the spatial and the temporal structure of the wind field. This topic is of high practical relevance, e.g., in the context of offshore windenergy exploitation. Additional applications like ship tracking, oil spill detection and the study of ocean wave field dynamics are briefly discussed as well.

Tropical Cyclone Parameters Derived from Synthetic Aperture Radar (SAR) Images

Ocean waves play an important role in the dynamics of extreme events like hurricanes or typhoons by conditioning the air/sea fluxes of momentum, heat and moisture. In this study a data set of ENVISAT ASAR Scan SAR images (400 times 400 km coverage) are used to observe the structure of tropical cyclones and typhoons at the sea surface. The following features of hurricanes were determined: wavelength and direction of boundary layer rolls for information of mixed boundary layer depth, radius of maximum wind speed, sea state in terms of wavelength, and direction. These image parameters are related to parametric models of hurricanes and validated by aircraft measurements from the National Hurricane Center (NHC). The work aims at the improvement of prediction of the cyclone track, intensity and sea state at these high wind speed conditions. The paper focuses on two illustrative case studies from August 2005, e.g., images of Hurricane Katrina acquired over the Gulf of Mexico and of Typhoon Talim acquired over the NW Pacific.

Extreme wind conditions in tropical cyclones observed from synthetic aperture radar images

Both atmospheric and oceanic processes play an important role in the dynamics of tropical cyclones. Due to the relatively small amount of in situ data available for extreme events like hurricanes or typhoons remote sensing techniques play an important role in the measurement of the relevant geophysical parameters. In this paper some recent results on SAR observation of extreme wind conditions are summarized. The study is based on the use of ENVISAT ASAR wide swath images. The objective of this study is to investigate the performance of SAR to improve the existing model for the retrieval of information on wind field under extreme wind and wave conditions using information from a parametric Holland type model.

Severe Weather Applications over the Oceans using ERS SAR Wavemode Data

Due to the relatively small amount of in situ data available for the open oceans, particularly during extreme events, under such conditions remote sensing techniques take an important role in the retrieval of geophysical information. Up to now the only remote sensing system capable of providing information on two dimensional sea state on a global and continuous scale and under all weather conditions is the Synthetic Aperture Radar (SAR). In the scope of the project Wave Atlas, ESA provided a two years wave-mode dataset of ERS-2 SAR raw data, mainly collected during 1999 and 2000, which was reprocessed to single-look- complex imagettes at DLR using the BSAR processor. In this study the data were used for a statistical evaluation resulting in global ocean maps of different basic image parameters and oceanic parameters like wind speed, significant wave height, mean period and their respective regional and temporal variability during the seasons of the year using the empirical algorithms CWIND1.0 and CWAVE1.0. Global statistics are given for the time frame of 1998 to 2000 and examples of severe storms are analysed in detail. In future, the reprocessing of wavemode data is planned to be extended to the full lifetime of ERS-1 and ERS-2, which is at least 1991-2006. As wave mode data are also available from the ENVISAT mission, there is the possibility for future expansion.