ALBERT

Description: [1]  We examine sea-state-dependent wind work on the oceanic general circulation, using a wave hindcast dataset, QuikSCAT winds, and geostrophic and total ocean surface currents from (1) AVISO and (2) ECCO2 model products. For wind work on surface geostrophic currents estimated from AVISO or ECCO2, sea-state-dependent wind stress increases an average of 24% (0.17TW) or 23% (0.15TW), compared with estimates that exclude sea state effects. For wind work on the total surface currents, the sea-state-dependent wind stress increases the wind work by about 24% (0.4TW). In terms of spatial distribution, the increase in wind power input occurs mainly in high-wind tropical regions and the mid-latitude storm track regions, like the Antarctic Circumpolar Current region, where the relatively rough ocean surface is characterized by young waves and high winds. By comparison, in some regions with relatively low winds and mature ocean waves, there is a slight reduction in estimated wind power input.

Description: A composite radar scattering model composed of the atmosphere radiative transfer model and the ocean surface Bragg wave theory is developed to analyze the impact of hurricane rain on the normalized radar-backscatter cross-section (NRCS) measured in the VV- and cross-polarized C-band Synthetic Aperture Radar (SAR) channels. The model results are validated against SAR and SFMR measured wind speeds and rain rates for two hurricane cases. The contribution of rain to the NRCS is backscatter from two parts: the atmosphere column and the ocean surface. In the atmosphere, microwave attenuation and the rain-induced volume backscattering are simulated by the model. We find that the impact of raindrops in the atmosphere is almost negligible for the VV polarization, but important for the cross-polarization. On the ocean surface, comparisons between our model and other existing models without rain lead to the conclusion that the VV polarization NRCS can be simulated reasonably well without considering the non-Bragg scattering mechanisms. Similar to the wave breaking mechanism, the microwave diffraction on the craters, crowns and stalks, produced by rain drops, are also negligible for VV polarization. However, the non-Bragg scattering is important for the cross-polarized NRCS simulations. Finally, we performed simulations to understand the VV-polarized NRCS behavior under different wind speeds at various rain rates. This article is protected by copyright. All rights reserved.

Description: We investigated the use of C-band RADARSAT Constellation Mission (RCM) synthetic aperture radar (SAR) for retrieval of ocean surface wind speeds by using four new channels (right circular transmit, vertical receive (RV); right circular transmit, horizontal receive (RH); right circular transmit, left circular transmit (RL); and right circular transmit, right circular receive (RR)) in compact polarimetry (CP) mode. Using 256 buoy measurements collocated with RADARSAT-2 fine beam quad-polarized scenes, RCM CP data was simulated using a “CP simulator”. Provided that the relative wind direction is known, our results demonstrate that wind speed can be retrieved from RV, RH and RL polarization channels using existing C-band model (CMOD) geophysical model function (GMF) and polarization ratio (PR) models. Simulated RR-polarized radar returns have a strong linear relationship with speed and are less sensitive to relative wind direction and incidence angle. Therefore, a model is proposed for the RR-polarized synthetic aperture radar (SAR) data. Our results show that the proposed model can provide an efficient methodology for wind speed retrieval.

Description: Polarimetric SAR decomposition parameters, average alpha angle ($\overline{\alpha}$) and entropy (H) are estimated for oil-slick contaminated sea surfaces and slick-free conditions using a RADARSAT-2 quad-polarization SAR image. The values of H and $\overline{\alpha}$ within oil slick areas are significantly higher than those of the ambient sea surface, indicating the dominance of Bragg scattering for the slick-free ocean and non-Bragg scattering for the oil-slick area. In land classification, the conformity coefficient (μ) is often used to discriminate surface scattering with double-bounce or volume scattering. Based on this rationale, we also develop a method using μ as a logical scalar descriptor to map oil slicks under low-to-moderate wind conditions. The proposed method is assessed using a RADARSAT-2 quad-polarization SAR image of oil slicks in the Gulf of Mexico. Analysis shows that when μ is positive the sea surface is slick-free, whereas μ is negative for oil-slick areas. This method provides a simple and effective mapping technique for oil slick detection.

Description: The wind speed sensitivity of cross-polarization (cross-pol) radar backscattering cross section ( VH ) from the ocean surface increases toward high winds. The signal saturation problem of VH , if it exists, occurs at a much higher wind speed compared to the co-polarization (co-pol: VV or HH ) sea returns. These properties make VH a better choice over VV or HH for monitoring severe weather. Combined with high spatial resolution of the synthetic aperture radar (SAR), the development of hurricane wind retrieval using VH is advancing rapidly. This paper describes a cross-pol C-band radar backscattering geophysical model function (GMF) with incidence angle dependence for the full wind speed range in the available datasets (up to 56m/s). The GMF is derived from RADARSAT-2 (R2) dual-polarization (dual-pol) ScanSAR modes with 300 and 500km swaths. The proposed GMF is compared to other published algorithms. The result shows that the simulated VH cross section and the retrieved wind speed with the proposed GMF is in better agreement with measurements. With careful treatment of noise, the VH retrieved wind speeds may extend to mild or moderate conditions. The higher fraction of non-Bragg contribution in VH can be exploited for analysis of surface wave breaking. This article is protected by copyright. All rights reserved.

Description: The oil–water mixture ratio for oil spills on the ocean surface is an important parameter for volume estimation of oil spills, response strategy for the oil spills, cleanup operations, and remediation planning for the impacts on wildlife. Hybrid-polarized (HP) mode compact polarization (CP) synthetic aperture radar (SAR) imagery will soon be available with the launch of the RADARSAT Constellation Mission. The advantage of the proposed new SAR system is that CP images will have wider swath and shorter revisit time compared to quad-polarization (QP) images, which are presently available from space-borne and air-borne SAR. We present a methodology to retrieve the oil–water mixture ratio at the ocean surface using CP SAR imagery. We emulated the HP mode of CP SAR image using Uninhabited Aerial Vehicle SAR (UAVSAR) L band observations collected on 23 June 2010 over the site of the Deep Water Horizon drilling rig. The gap between elements ratio of CP SAR covariance matrix and that of QP SAR Sinclair matrix is bridged. Numerical optimization and look up table methods are used to relate the oil–water mixture ratio to elements of the covariance matrix for the HP data backscatter. The mixture ratio estimates determined from the ratio of diagonal elements of the covariance matrix for HP mode CP data are compared with results retrieved from the co-polarization ratio from the original QP SAR observations. Results from the proposed methodology for SAR images captured in the HP mode of CP data are shown to compare favourably to observed in situ data of the mixture ratios.

Description: Abstract The abrupt deceleration of accelerated Greenland Ice Sheet (GrIS) melting since 2013, after a period of acceleration previously noted, is studied here. It is shown that the deceleration of GrIS melting since 2013 is due to the reduction in short‐wave solar radiation in the presence of increasing total cloud cover, which is driven by a more persistent positive summer North Atlantic Oscillation on the decadal time scale. By presenting the coherence with the temperature variability at the weather stations in Greenland, which have century‐long records, we deduce that the acceleration of GrIS melting during the early 2000s and the subsequent deceleration since 2013 will reoccur frequently on decadal time scales, with the amplitude nearly half of the multidecadal warming trend of the GrIS melt. It can reduce the mass loss from the GrIS on short to medium time scales but is unlikely to halt mass loss related to climate change in the future. This finding highlights the importance of internal climate variability on the mass budget of the GrIS and therefore on predictions of future global sea level change and may help to assist planning for associated social and economic consequences.

Description: Abstract We conduct a sensitivity study of the North Atlantic cyclone climate, with respect to the cumulus parameterization (CP) and planetary boundary layer (PBL) schemes, applied in the dynamical downscaling of HadGEM2‐ES simulations using Polar Weather Research and Forecasting. A set of seven sensitivity experiments is conducted for 1979–2004 using these CP and PBL schemes. CP schemes are Kain‐Fritsch (KF), Zhang‐McFarlane (ZM), and the modified Tiedtke (TZ). PBL schemes are the local Mellor‐Yamada‐Janjić (MYJ), Mellor‐Yamada‐Nakanishi‐Niino Level 2.5 (MYNN2) schemes, and nonlocal Yonsei University (YSU) scheme. In terms of cyclone intensities, our results show that the fine‐resolution Weather Research and Forecasting simulations capture higher intensities of summer cyclones than the HadGEM2‐ES results, in better agreement with the ERA‐Interim reanalysis, and exhibit a weak sensitivity to CP and PBL schemes. However, the frequency of summer cyclones is quite sensitive to CP and PBL schemes. Four KF experiments suggest comparable numbers of cyclone tracks as derived from ERA‐Interim reanalysis data, which is about 21% more than the ZM‐MYJ or TZ experiments and 11% more than HadGEM2‐ES results. Larger tropospheric potential vorticity and more cyclogenesis in the KF simulation than in TZ or ZM simulations are associated with different cyclone triggering functions and different vertical distributions of heating. We also find increased cyclone activities over the North Atlantic in the YSU or MYNN2 schemes compared to MYJ, but results also depend on the KF CP scheme. This can be attributed to stronger vertical mixing in YSU and MYNN schemes, resulting in smaller static stability and enhanced atmospheric moisture.

Description: Convection is an important phenomenon in the marine atmospheric boundary layer (MABL). Previous spaceborne radar studies of such have been limited to single polarization data, and therefore their focus was on the variation in intensity of the radar return, which was constrained by the existence of a single polarization image pattern, representing different atmospheric and oceanic phenomena. In this paper, we study the polarimetric characteristics of mesoscale cellular convection (MCC) in the MABL using high-resolution data from fully polarimetric (HH, VV, HV, and VH) RADARSAT-2 (RS-2) synthetic aperture radar (SAR) images, in conjunction with closely collocated mesoscale atmospheric model simulations, to identify the MCC signatures. To compare the polarimetric characteristics of MCC with those of the ocean surface, our analysis also includes 641 open ocean surface quad-polarization RS-2 SAR images collocated with 52 National Data Buoy Center buoys. The open ocean surface SAR images exhibit different polarimetric characteristics from those of MCC. Thus, we differentiate MCC from other open ocean phenomena, based on identifiable polarimetric SAR characteristics.

Description: The wide-swath mode of synthetic aperture radar (SAR) is a good way of detecting typhoon/hurricane winds with a cross-polarization mode. However, its ability to detect wind waves is restricted because of its spatial resolution and nonlinear imaging mechanisms. In this study, we use the SAR-retrieved wind speed, Sentinel-1 SAR wave mode and buoy data to examine fetch- and duration-limited parametric models (denoted H-models), to estimate the wave parameters (significant wave height Hs, dominant wave period Tp) generated by hurricanes or typhoons. Three sets of H-models, in total 6 models, are involved: The H-3Sec model simulates the wave parameters in 3 sections of a given storm (right, left and back); H-LUT models, including the H-LUTI model and H-LUTB model, provide a better resolution of the azimuthal estimation of wind waves inside the storm by analyzing the dataset from Bonnie 1998 and Ivan 2004; and the third set of models is called the H-Harm models, which consider the effects of the radius of the maximum wind speed rm on the wave simulation. In the case of typhoon Krovanh, the comparison with wave-mode measurements shows that the duration-limited models underestimate the high values for the wind-wave Hs, while the fetch models’ results are more accurate, especially for the H-LUTI model. By analyzing 86 SAR wave mode images, it is found that the H-LUTI model is the best among the 6 H-models, and can effectively simulate the wind-wave Hs, except in the center area of the typhoon; root mean square errors (rmse) can reach 0.88 m, and the coefficient correlation (R2) is 0.86. The H-Harm models add rm as an additional factor to be considered, but this does not add significant improvement in performance compared to the others. This limitation is probably due to the fact that the data sets used to develop the H-Harm models have only a limited coverage range, with respect to rm. Applying H-models to RADARSAT-2 ScanSAR mode data, we compare the retrieved wave parameters to collected buoy measurements, showing good consistency. The H-LUTI model, using a fetch-limited function, does the best among these 6 H-models, whose rmse and R2 are 0.86 m and 0.77 for Hs, and 1.06 s and 0.76 for Tp, respectively. Results indicate the potential for H-models to simulate waves generated by typhoons/hurricanes.