Wind speed retrieval from the Gaofen-3 synthetic aperture radar for VV- and HH-polarization using a re-tuned algorithm

In this study, a re-tuned algorithm based on the geophysical model function (GMF) C-SARMOD2 is proposed to retrieve wind speed from Synthetic Aperture Radar (SAR) imagery collected by the Chinese C-band Gaofen-3 (GF-3) SAR. More than 10,000 Vertical-Vertical (VV) and Horizontal-Horizontal (HH) polarization GF-3 images acquired in quad-polarization stripmap (QPS) and wave (WV) modes have been collected during the last three years, in which wind patterns are observed over open seas with incidence angles ranging from 18° to 52°. These images, collocated with wind vectors from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis at 0.125° resolution, are used to re-tune the C-SARMOD2 algorithm to specialize it for the GF-3 SAR (CSARMOD-GF). In particular, the CSARMOD-GF performs differently from the C-SARMOD2 at low-to-moderate incidence angles smaller than about 34°. Comparisons with wind speed data from the Advanced Scatterometer (ASCAT), Chinese Haiyang-2B (HY-2B) and buoys from the National Data Buoy Center (NDBC) show that the root-mean-square error (RMSE) of the retrieved wind speed is approximately 1.8 m/s. Additionally, the CSARMOD-GF algorithm outperforms three state-of-the-art methods – C-SARMOD, C-SARMOD2, and CMOD7 – that, when applied to GF-3 SAR imagery, generating a RMSE of approximately 2.0–2.4 m/s

Risk of appendiceal neoplasm after interval appendectomy for complicated appendicitis: A systematic review and meta-analysis

Background: Non-operative management is often the treatment of choice in cases of complicated appendicitis and routine interval appendectomy is not usually recommended. Actually, recent studies show an alarming number of appendiceal neoplasms following interval appendectomy. The aim of this study is to evaluate the prevalence of appendiceal neoplasms and their histological types after interval appendectomy for complicated appendicitis in adults. Methods: A comprehensive literature search of the PubMed, Scopus and Web of Science databases was conducted according to the PRISMA statement. Studies reporting appendiceal neoplasm rates after interval appendectomy and histopathological characteristics were included. The most recent World Health Organization (WHO) classification of malignant tumours was considered. A pooled prevalence analysis for both prevalence and pathology was performed. Results: A total of eight studies was included: seven retrospective series and one randomized controlled trial. The pooled prevalence of neoplasms after interval appendectomy was 11% (95% CI 7-15; I2 = 37.5%, p = 0.13). Appendiceal mucinous neoplasms occurred in 43% (95% CI 19-68), adenocarcinoma in 29% (95% CI 6-51), appendiceal neuroendocrine neoplasm in 21% (95% CI 6-36), globet cell carcinoma in 13% (95% CI -2-28), adenoma or serrated lesions in 20% (95% CI -0-41) of cases. Conclusion: The risk of appendiceal neoplasm in patients treated with interval appendectomy for complicated appendicitis is 11%; mucinous neoplasm is the most common histopathological type. Further studies should investigate this association in order to clarify the biological pathway and clinical implications

Analysis of waves observed by synthetic aperture radar across ocean fronts

In this study, synthetic aperture radar (SAR) imaging of waves across ocean fronts was investigated using C-band Sentinel-1 VV-polarized SAR imagery collected over the Yangtze and the Zhujiang estuaries. The presence of ocean fronts in the study area was confirmed by collocated sea surface temperature (SST) data provided by the Advanced Very High Resolution Radiometer (AVHRR) and sea surface current information from the National Ocean Partnership Program (NOPP) based on the HYbrid Coordinate Ocean Model (HYCOM). The experimental results revealed that as the current speed increased, the cut-off wavelength (λc) increased as well. The effect of the increasing azimuth cut-off wavelength, however, was relatively weak in terms of variations of the normalized radar cross-section (NRCS), i.e., it was within 2 dB for λc ≤ 60 m. Hence, it was weaker than the NRCS variation related to SST. Larger NRCS variations (i.e., within 5 dB) occurred for λc values up to 120 m. In addition, the experimental results also demonstrated that the parameterized first-guess spectrum method (PFSM) wave retrieval performance was affected by ocean fronts. In particular, overestimations occurred when ocean fronts were present and λc was < 100

Megi Typhoon Monitoring by X-Band Synthetic Aperture Radar Measurements

In this study, typhoon monitoring is addressed using X-band synthetic aperture radar (SAR) imagery collected by the German TerraSAR-X mission and the Italian COSMO-SkyMed constellation during the typhoon Megi. Geometrical features, rain rate, and wind speed associated with the typhoon are retrieved by the SAR data set. One of the key benefits of the X-band observations relies in their sensitivity to rain that can be exploited to provide an estimate of geometrical features and rain rate by analyzing attenuation bands present in the SAR data. In addition, wind speed is retrieved using a rain-free model based on two geophysical model functions (GMFs) and experimental results show that the nonlinear relationship between normalized radar cross section and wind speed provided by one of the GMFs can be exploited to provide a rough estimate of high wind speeds

A Sensitivity Analysis on the Spectral Signatures of Low-Backscattering Sea Areas in Sentinel-1 SAR Images

Satellite synthetic aperture radar (SAR) is a unique tool to collect measurements over sea surface but the physical interpretation of such data is not always straightforward. Among the different sea targets of interest, low-backscattering areas are often associated to marine oil pollution even if several physical phenomena may also result in low-backscattering patches at sea. In this study, the effects of low-backscattering areas of anthropogenic and natural origin on the azimuth autocorrelation function (AACF) are analyzed using VV-polarized SAR measurements. Two objective metrics are introduced to quantify the deviation of the AACF evaluated over low-backscattering areas with reference to slick-free sea surface. Experiments, undertaken on six Sentinel-1 SAR scenes, collected in Interferometric Wide Swath VV+VH imaging mode over large low-backscattering areas of different origin under low-to-moderate wind conditions (speed ≤ 7 m/s), spanning a wide range of incidence angles (from about 30° up to 46°), demonstrated that the AACF evaluated within low-backscattering sea areas remarkably deviates from the slick-free sea surface one and the largest deviation is observed over oil slicks

Extreme wind speed retrieval from sar azimuth cut-off

2019 Living Planet Symposium, 13-17 May 2019, Milan, ItalyNowadays, wind speed retrieval is a topic of great interest. Most of the remote-sensing satellite radar systems are able to provide sea-surface wind field information, and they can be considered the main near sea surface wind information source. Within this context, active microwave remote sensing, in particular the scatterometer and the Synthetic Aperture Radar (SAR), is worldwide recognized as one of the best suitable tools to perform a reliable sea-surface wind speed retrieval. Radar backscatter intensity and its statistical properties contain quantitative information about sea surface roughness and, therefore, can be used to derive sea-surface wind information. Empirically derived Geophysical Model Functions (GMF) are used to relate the calibrated radar return to the wind speed at a height of 10m. GMFs usually relate the radar backscatter to wind speed and wind direction. An alternative method to retrieve wind speed from SAR is represented by the spectral based approaches, i.e.; the azimuth cut-off procedure. When dealing with SAR microwave sensors, Doppler misregistration along the azimuth is induced by gravity wave orbital motion. This issue is the major responsible of a distortion of the imaged wave spectrum and of a strong cut-off in the azimuthal direction: this is the azimuth cut-off. Originally, this technique was proposed in [1] to retrieve significant wave height (SWH), without the knowledge of any a priori wind direction information. The theory that is at the basis of the azimuth cut-off method is related to the effect of the orbital motion related to the surface waves in the SAR imaging of the ocean surface. This orbital motion results in additional Doppler shifts that distort the phase history of the backscattered signal used to synthetize the resolution in azimuth. The result is a low-pass filtered SAR image in the azimuth direction. The azimuth cut-off well correlates to the SWH because of its sensitivity to the long waves. In this study, the ACF-based λc approach is extended to high wind speed regimes, e.g.; extreme weather conditions. The key issues that need to be tackled concern the tuning with respect to pixel spacing, box size and the homogeneity of the SAR imagery. Preliminary results obtained processing a large data set of Sentinle-1 SAR imagery collected under a broad range of wind conditions show that the box size and the median filter window should be set at 1 km × 1 km and 90-120 m, respectively. Then, tailored experiments have been undertaken an actual Sentinel-1 dataset collected under high wind regimes, i.e.; tropical cyclones. The obtained λc values are compared both with the ECMWF forecasted information and with wind speed maps provided by Ifremer. Preliminary results show that there is a good agreement between λc and wind speed in some areas of the cyclone, i.e.; further enough from the cyclone eye. [1] V. Kerbaol, B. Chapron, and P.W. Vachon, “Analysis of ERS-1/2 synthetic aperture radar wave mode imagettes”, Journal of Geophysical Reserarch, vol. 103, no. C4, pp. 7833-7846, 1998

A novel azimuth cut-off implementation to improve sea surface wind retrievals from SAR images

International Ocean Vector Winds Science Team Workshop (2018 IOVWST), 24-26 April 2018, BarcelonaPeer Reviewe

Assimilation of SAR-Derived Sea Surface Winds Into Typhoon Forecast Model

2018 IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2018), Observing, Understanding And ForecastingThe Dynamics Of Our Planet, 22-27 July 2018, Valencia, SpainTyphoon is one of the most powerful and destructive natural disasters. Accurate forecasting of Typhoon track and intensity is very important to disaster prevention and reduction. Satellite observations can effectively compensate for the shortcomings of traditional methods of sea surface measurement and provide all-weather observation over the sea surface, which is of great significance to improve the numerical prediction of strong convective weather over ocean. The spaceborne radar observes the backscattering caused by the sea surface roughness, and then, the sea surface wind can be retrieved. The Synthetic Aperture Radar (SAR) is an important data source for sea surface monitoring. A variety of meteorological hydrological elements can be retrieved by SAR observation, and it has been used in data assimilation in recent years [1]. SAR imagery is also used to monitor strength and structure of typhoons [2]. The accuracy of sea surface winds retrieved from SAR has been found to be comparable to that of scatterometer data [3], and these wind fields can be used with a data assimilation system to provide the initial conditions for the numerical weather prediction (NWP) model [4]Peer Reviewe

On the assimilation of SAR-derived sea surface winds into typhoon forecast model

2018 Dragon 4 Mid-term Results Symposium, 19-22 June 2018, Xi’an, P.R. ChinaPeer Reviewe