William Perrie

Ocean Vector Winds Retrieval From C-Band Fully Polarimetric SAR Measurements

We present an efficient algorithm for retrieving the ocean-surface wind vector from C-band Radar Satellite RADARSAT-2 fully polarimetric synthetic aperture radar (SAR) measurements based upon the copolarized geophysical model function, i.e., CMOD5.N, and the cross-polarized ocean backscatter model, i.e., C-2PO. The analysis of fine quad-polarization mode single-look complex SAR data and collocated in situ moored buoy observations reveals that the polarimetric correlation coefficient between co- and cross-polarization channels has odd symmetry with respect to the wind direction. This characteristic is different from the feature that normalized radar cross sections for quad-polarization have even symmetry regarding the wind direction. We first use the C-2PO model to directly retrieve wind speeds without any external wind-direction and radar-incidence-angle inputs. Subsequently, the retrieved wind speeds, along with incidence angles and CMOD5.N, are employed to invert the wind direction, still with ambiguities. The odd-symmetry property is then applied to remove the wind direction ambiguities. Thus, it is shown that fully polarimetric SAR measurements provide complementary directional information for the ocean-surface wind fields. This method has the potential to improve wind vector retrievals from space.

Target Detection on the Ocean With the Relative Phase of Compact Polarimetry SAR

This paper discusses the potential for automatic ocean surveillance using compact linear polarization (CL-pol) synthetic aperture radar (SAR), with large area coverage. Here, the target is a wind farm in the North Sea. The relative phase, as derived from CL-pol SAR, is employed for detection of the wind turbines, apart from the wind turbines wakes, based on fine-mode quad-polarization (quad-pol) RADARSAT-2 (RS-2) images. The relative phase of CL-pol measurements improves the contrast between the wind turbines and their wakes, because it has opposite signs for these two entities. Moreover, there is almost no variation in the relative phase with respect to wind speed or incidence angle. The results are verified by high-sea-state cases, up to 8.7-m significant wave height and 24.3-m/s wind speed, and also 641 quad-pol RS-2 SAR images collocated with 52 National Data Buoy Center buoys at different incidence angles and sea states. Thus, the relative phase of CL-pol SAR provides new light into the problem of operational autodetection of man-made targets, under high-sea-state conditions, over large areas.

Wind Speed Retrieval From VH Dual-Polarization RADARSAT-2 SAR Images

Recent studies show the promising ability of cross-polarization synthetic aperture radar (SAR) for high-wind-speed monitoring. However, no dual-polarization (dual-pol) geophysical model functions (GMFs) have thus far been established. In this paper, we first address the difference between dual-pol and quad-pol VH radar returns with respect to wind speed and show the necessity of having a VH dual-pol GMF for ScanSAR images, for remote sensing of mesoscale wind processes, such as hurricanes. By collocating VH dual-pol RADARSAT-2 SAR measurements with in situ buoy winds, a VH dual-pol GMF was developed after introducing a denoise procedure to increase the signal-to-noise ratio for SAR measurements. Study results suggest that the VH dual-pol radar returns have a linear monotonic piecewise increasing dependence on wind speed, with decreasing scatter in high winds. Successful application of the GMF to high-wind-speed retrieval for hurricane Gustav in 2008 suggests that VH mode SAR images do not exhibit signal saturation or the speed ambiguity problem that affects copolarization SAR images for high-wind-speed retrieval. This result supports the potential future application of VH dual-pol SAR images to hurricane monitoring.

Cross-Polarization Radar Backscattering From the Ocean Surface and Its Dependence on Wind Velocity

Recent wind retrieval algorithms using crosspolarization (cross-pol) radar sea return (VH) assume that VH is independent on the azimuth angle and mainly varies with the wind speed. Incidence angle dependence is either absent or is only in wind speeds less than 21 m/s. However, azimuth and incidence angle variations are expected since theory and data comparisons show the dominance of surface effects in the scattering mechanisms; thus, both co-polarization and cross-pol cross sections reflect the directional distribution of the ocean surface roughness and wave breaking. Here, the VH dependence on wind velocity is analyzed with special focus on the variations with the incidence and azimuth angles. The results show that, for the typical incidence angle range of radar images used for hurricane wind retrieval (20°-50°), the magnitude of these angular variations is equivalent to a difference of about 10 m/s in the retrieved wind speed for low-to-strong winds (<; ~21 m/s) and probably for strong-to-severe winds (> ~21 m/s) as well. It is prudent to incorporate the incidence angle dependence and the azimuth angle dependence in the wind retrieval algorithm and in the signal simulation for the design of next-generation scatterometers. The dependence on the wind speed is also examined. It reconfirms that the VH sensitivity increases toward high winds, but signal saturation may occur.

A Hurricane Tangential Wind Profile Estimation Method for C-Band Cross-Polarization SAR

Hurricane tangential wind profiles are routinely observed by aircraft reconnaissance in order to estimate the surface winds. However, these wind profile estimates are occasionally biased because along-track aircraft observations might not determine the nonaxisymmetric hurricane structure characteristics. In this paper, we resolve this problem by calculating the mean wind speed in all radial directions using cross-polarization SAR wide-swath images. Moreover, we propose a one-half modified Rankine vortex (OHMRV) model to describe the hurricane wind profile, particularly for those wind profiles with a wind speed maximum and an inflection point possibly associated with the degeneration of the inner wind maximum in the hurricane reintensification phase. OHMRV characterizes the hurricane wind profile and represents a model that complements the previously established single-modified Rankine vortex model and the double-modified Rankine vortex model. Moreover, the OHMRV-derived wind profiles are used to estimate hurricane intensity and structure parameters, such as the maximum wind speed and the radius of maximum wind. For validation of the method, the estimated ISPs are compared with measurements by the stepped-frequency microwave radiometer on board the National Oceanic and Atmospheric Administration aircraft. These parameters contribute to a description of hurricane inner-core intensity and structure associated with eyewall replacement cycles.

On the impacts of climate change and the upper ocean on midlatitude northwest Atlantic landfalling cyclones

[1]xa0The influence of climate change on midlatitude North Atlantic landfalling autumn storms is investigated using a relatively high-resolution mesoscale atmosphere-ocean coupled model system. Atmospheric boundary conditions for autumn storm simulations by this coupled model system are given by the Canadian second-generation Coupled Global Climate Model (CGCM2), following the Intergovernmental Panel on Climate Change IS92a scenario. The control and high-CO2 boundary conditions are obtained from CGCM2 simulations representing the present climate (1975–1994), and a future climate change scenario (2040–2059), corresponding to a doubling of greenhouse gases. An understanding of the possible influences of climate change on the storm climate is achieved through our simulations. The impact of climate change is seen in slightly decreased intensities in landfalling cyclones (about 5 hPa) resulting from the competition between warming provided by the climate change scenario and modest cooling around the storm center induced mainly by dynamic cooling. An additional impact is that cyclone tracks tend to shift poleward.

Oil spill detection on the ocean surface using hybrid polarimetric SAR imagery

Hybrid-polarimetric SAR (synthetic aperture radar) is a new SAR mode, with relatively simple architecture, low cost, and wide swath, which will be carried by several Earth-observing systems from now to the near future. Here, we show how the second Stokes parameter of hybrid-polarimetric SAR can be employed to detect oil on the ocean surface using the classic well-known Otsu threshold methodology, in relation to contributions from different polarizations and dampening effects on backscatter intensity, neglecting the specific scattering mechanisms and oil types for an oil-covered surface. The detection methodology is demonstrated to be reliable in three example cases: oil-on-water experiments conducted by the Norwegian Clean Seas Association, natural oil seeps from the Gulf of Mexico, and observations from the Deep Water Horizon oil spill disaster in 2010.

An analysis of the polarimetric scattering properties of oil spills on the ocean surface with hybrid polarimetry SAR

An analysis of the polarimetric scattering properties of oil-covered waters is conducted using the classic Poincaré ellipticity parameter chi (χ) from the Stokes parameters of hybrid polarized mode Synthetic Aperture Radar (SAR). For natural oil seeps, χ has a change in signs, comparing oil-covered waters with the `clean ocean surface. The χ sign reversal is basic to sign difference oil spill detection methods. However, a problem is that oil spills related to the Deep Water Horizon (DWH) disaster did not exhibit a reversal in χ signs, when `clean ocean surfaces are compared to the area contaminated by crude oil. Therefore, the scattering mechanism related to the oil seeps is different from that of the DWH oil spill; the former is dominated with even bounce scattering and the latter is dominated by Bragg scattering, similar to that of the clean oil-free ocean surface.

RADARSAT application in ocean wind measurements

SAR can penetrate clouds and precipitation and can provide accurate observations of surface features such as fronts and localized coastal marine winds. Often atmosphere-ocean interaction processes evident in SAR imagery are not present from in situ observations, the operational network, weather prediction models, or scatterometer data, especially in complex coastal areas. Timely, high-resolution SAR-derived wind data have a great potential to improve operational marine weather forecasts, particularly in coastal areas. As an outcome of NSWP the marine meteorologists and other users have an interrupted access to high resolution color-coded maps of SAR-derived wind speeds, ocean buoy observations, and surface wind predictions from NWP models. Over the last 18 months, they had access to over 13,000 such images across all Canadian coastal zones. The two-year Canadian National SAR Winds Project (2009–2011) has successfully demonstrated that quasi-operational SAR Wind retrieval system could be an important element of the operational weather analysis and prediction structure.

On SAR hurricane wind speed ambiguities

Radar backscattered signals over the ocean are dampened in extremely high winds, which leads to a wind speed ambiguity problem during the process of wind retrieval from synthetic aperture radar (SAR). This problem was firstly studied by Shen et al. (2007), where we proposed a wind speed ambiguity removal scheme for the two wind speed solutions that may exist for any given normalized radar cross section (NRCS) and wind direction. This approach is based on the operational geophysical model function (GMF) CMOD5. Recently, new C-band GMFs for high wind have been developed, among which, a HH polarized GMF was established for the first time. In this study, the wind speed ambiguity problem will be studied within the context of the available high wind GMFs, which are,CMOD5,CMOD4HW,HWGMF_V and HWGMF_H. For the wind retrieval from HH polarized SAR images, a hybrid empirical polarization ratio is generally adopted. To compare the different behavior of various GMF models, this polarization ratio is used to transform the HH polarized GMF into a W field. Although the wind speed ambiguity problem is found in most GMFs, the saturation wind speed where radar backscattered signals start to decrease is different for the various GMFs. We show that consideration of the wind speed ambiguity problem is important for high wind retrieval from SAR images, especially for category 5 hurricanes.