In this paper, we study the retrieval of wind information from nautical X-band radar data. In contrast to previous studies, where data from stationary research platforms were used, this study focuses ...on data from a moving platform, encountering a larger variety of conditions than a platform at a fixed location. Compared to traditional in situ sensors, wind data derived from nautical radar images are much less susceptible to air flow distortion by the platform, since the images cover a large area around the ship. Images collected with a standard nautical HH-polarized X-band radar operating at grazing incidence exhibit a single intensity peak in upwind direction. The wind retrieval method developed here uses a harmonic function that is least-squares fitted to the radar backscatter intensity as a function of antenna look direction. The upwind direction is given by the direction that corresponds to the peak of the fitted function. An empirical model function is derived to retrieve the wind speed from the average radar backscatter intensity. Contrary to wind retrieval methods that have been proposed before, this approach is well suited for data acquired from a moving platform, as it functions well even if the radar field of view is partially shadowed and does not require ship motion correction. Here, we focus on data that were collected during two storms, using the first storm to derive and the second to test the empirical model functions. The method is validated using measurements from two ship-based anemometers.
Abstract
Estuarine intertidal flats are exposed to a wide range of changing environmental conditions, but despite many years of field studies, little is known about their geotechnical ...characteristics. This study investigates (1) the variability of sediment grain size distributions, water contents, and Atterberg limits of three distinct exposed tidal flats; and (2) the potential pathways for remote characterization of those sites. Sediment samples were collected from three flats in the Great Bay Estuary located in the Gulf of Maine at the border of New Hampshire and Maine, United States: two sites composed of primarily fine-grained soils (Adams Point and Woody) and one composed of predominately coarse-grained soils (Mast Cove). Additionally, high-resolution X-band synthetic aperture radar (SAR) images were collected concurrently to field measurements from various satellites and over different incidence angles. In each image, the footprint area corresponding to the three exposed tidal flats was extracted, and statistical properties of the backscatter intensity histograms for each region were computed and compared. Moisture contents, fines contents, and grain size distributions varied between the three sites, with the USCS classifications ranging from elastic silts (Woody) and low-plasticity silts (Adams Point) with high water contents (83.2%–172.7%) and fines contents (83.4%–99.5%) to silty sand and well-graded sands with gravels (Mast Cove) with lower water contents (17.7%–48.3%) and fines contents (4.8%–44.3%). Across the sites, the statistical properties of the SAR backscatter exhibited distinct trends regarding mean backscatter, entropy, and uniformity in relation to the Unified Soil Classification Scheme (USCS) classification and fines content. These trends were consistent across all images collected. A potential sediment classification scheme to derive the USCS classification from the X-band SAR imagery is suggested utilizing the mean, entropy, and uniformity of the backscatter coefficient.
Since the SEASAT mission in 1978, satellite-based synthetic aperture radar (SAR) images have been used to study oceanic internal waves. Internal waves become visible in SAR images because their ...orbital currents modulate the surface roughness. While this leads to an accurate spatial representation of internal wave patterns, the complexity of the imaging mechanism makes it difficult to derive actual currents and internal wave amplitudes from SAR signatures. We demonstrate in this paper how a more robust parameter retrieval is possible with along-track interferometric SAR (InSAR) data that resolve amplitudes and temporal phase changes of the backscattered signal together, the latter of which are directly related to the scatterers' line-of-sight velocities. Our example data set, which was acquired by TerraSAR-X in Dual Receive Antenna mode at Dongsha (South China Sea), exhibits strong signatures of internal waves in the interferogram amplitude and phase. We use a simple internal soliton parameterization and a numerical radar imaging model to find a plausible combination of internal wave parameters, which leads to good agreement between simulated and observed signatures. Testing the sensitivity of radar amplitude and phase signatures to various parameters, we show that along-track InSAR data should generally permit more accurate and less ambiguous internal wave parameter retrievals than conventional SAR images.
A warm jet in a cold ocean MacKinnon, Jennifer A; Simmons, Harper L; Hargrove, John ...
Nature communications,
04/2021, Letnik:
12, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn ...cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.
Abstract
The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment ...(CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V
F. G. Walton Smith
was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm
and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This paper presents a geophysical model function (GMF) that has been developed to describe the relation of the ocean surface wind with the normalized radar cross section (NRCS) at C-band cross ...polarization (cross-pol). Synthetic aperture radar (SAR) images have been simultaneously collected at copolarization (co-pol) and cross-pol at moderate to high wind speeds. Using the SAR co-pol retrieved wind fields and an uncertainty estimate of the retrieved wind speeds, the cross-pol dependencies of the NRCS are investigated with respect to wind, incidence angle, and polarization pairs. For wind speeds above 10 m/s, there is a significant dependence of the NRCS on wind speed. However, the SAR cross-pol data are also significantly affected by the noise floor and crosstalk between the channels. Estimates of the noise floor are determined and removed from the NRCS. Three GMFs are developed: the first is for transmission at horizontal (H) polarization and the second at vertical (V) polarization. A third GMF accounts for wind direction dependence. Validation of the GMFs is conducted by comparison with collocated Stepped Frequency Microwave Radiometer (SFMR) data. The resulting bias of -0.7 m/s and standard deviation of 3.7 m/s demonstrate the excellent performance for these GMFs for wind speed retrieval between 10 and 35 m/s. Furthermore, comparisons show that SAR cross-pol retrieved wind speeds are of similar quality as those of SFMR and are significantly better in the moderate to high wind speed regime than SAR co-pol retrieved winds.
A framework for estimating moisture content from satellite-based multispectral imagery of sandy beaches was tested under various site conditions and sensors. It utilizes the reflectance of dry soil ...and an empirical factor c relating reflectance and moisture content for a specific sediment. Here, c was derived two ways: first, from in situ measurements of moisture content and average NIR image reflectance; and second, from laboratory-based measurements of moisture content and spectrometer reflectance. The proposed method was tested at four sandy beaches: Duck, North Carolina; and Cannon Beach, Ocean Cape, and Point Carrew, Yakutat, Alaska. Both measured and estimated moisture content profiles were impacted by site geomorphology. For profiles with uniform slopes, moisture contents ranged from 3.0% to 8.0% (zone 1) and from 8.0% to 23.0% (zone 2). Compared to field measurements, the moisture contents estimated using c calibrated from in situ and laboratory data resulted in percent error of 3.6%–44.7% and 2.7%–58.6%, respectively. The highest percent error occurred at the transition from zone 1 to zone 2. Generally, moisture contents were overestimated in zone 1 and underestimated in zone 2, but followed the expected trends based on field measurements. When estimated moisture contents in zone 1 exceeded 10%, surface roughness, debris, geomorphology, and weather conditions were considered.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Buoy observations from a 1999 Gulf of Mexico field program (GOM99) are used to investigate the relationships among friction velocity
u
*
, wind speed
U
, and amount of swell present. A
U
–
u
...*sea
parameterization is developed for the case of pure wind sea (denoted by
u
*sea
), which is linear in
U
over the range of available winds (2–16 m s
−1
). The curve shows no sign of an inflection point near 7–8 m s
−1
as suggested in a 2012 paper by Andreas et al. on the basis of a transition from smooth to rough flow. When observations containing more than minimal swell energy are included, a different
U
–
u
*
equation for
U
< 8 m s
−1
is found, which would intersect the pure wind-sea curve about 7–8 m s
−1
. These two relationships yield a bilinear curve similar to Andreas et al. with an apparent inflection near 7–8 m s
−1
. The absence of the inflection in the GOM99 experiment pure wind-sea curve and the similarity of the GOM99 swell-dominated low wind speed to Andreas et al.’s low wind speed relationship suggest that the inflection may be due to the effect of swell and not a flow transition. Swell heights in the range of only 25–50 cm may be sufficient to impact stress at low wind speeds.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We examine the structure of turbulent airflow over ocean waves. Based on an analysis of wind and wave observations derived from a moored and floating Air–Sea Interaction Spar buoy during the Shoaling ...Waves Experiment field campaign, we show that the cospectra of momentum flux for wind–sea conditions follow established universal scaling laws. Under swell-dominant conditions, the wave boundary layer is extended and the universal cospectral scaling breaks down, as demonstrated previously. On the other hand, the use of peak wave frequency to reproduce the universal cospectra successfully explains the structure of the turbulent flow field. We quantify the wave-coherent component of the airflow and this clarifies how ocean waves affect momentum transfer through the wave boundary layer. In fact, the estimated wave-induced stresses for swell-dominant conditions explain the anomalous cospectral shapes observed near the peak wave frequency.
A large collaborative program has studied the coupled air‐ice‐ocean‐wave processes occurring in the Arctic during the autumn ice advance. The program included a field campaign in the western Arctic ...during the autumn of 2015, with in situ data collection and both aerial and satellite remote sensing. Many of the analyses have focused on using and improving forecast models. Summarizing and synthesizing the results from a series of separate papers, the overall view is of an Arctic shifting to a more seasonal system. The dramatic increase in open water extent and duration in the autumn means that large surface waves and significant surface heat fluxes are now common. When refreezing finally does occur, it is a highly variable process in space and time. Wind and wave events drive episodic advances and retreats of the ice edge, with associated variations in sea ice formation types (e.g., pancakes, nilas). This variability becomes imprinted on the winter ice cover, which in turn affects the melt season the following year.
Key Points
A large study of air‐ice‐ocean‐waves interactions was completed during the autumn of 2015 in the western Arctic
Strong wave‐ice feedbacks, including pancake ice formation and wave attenuation, were observed
Autumn refreezing of the seasonal ice cover is controlled by ocean preconditioning, atmospheric forcing (i.e., on‐ice versus off‐ice winds), and mixing events
PDF
