摘要:

In recent years, particular efforts have been made to derive wind fields over the oceans from synthetic aperture radar (SAR) images. In contrast with the scatterometer, the SAR has a higher spatial resolution and therefore has the potential to provide higher resolution wind information. Since there are at least two geophysical parameters (wind speed and wind direction) modulating the single SAR backscatter measurements, the inversion of wind fields from SAR observations has an inherent problem of underdetermination. Moreover, this modulation is highly nonlinear, further complicating the inversion.Lorenc[1986] presented a general statistical approach to solve inversion problems (including underdetermined problems) in meteorological analysis. We propose a SAR wind retrieval method based on this general approach. This simplified method combines the SAR information with some background information coming from high‐resolution limited area model to retrieve the most probable wind vector, assuming that all sources of information contain errors and that these are well characterized. We then evaluate two different SAR wind retrieval methods. The first one is commonly used by the SAR community and is based on a combination of a wind streak detection algorithm (wind direction retrieval) and a C band model inversion (wind speed retrieval). The second one is the new method we propose, based on the general statistical approach. We show the potential problems and limitations of using any of these methods and show how the second method can potentially contribute to a significant improvement in SAR wind retrieval. The new method prepares the ground for the assimilation of SAR data in high‐resolution numerical weather prediction mod

ISSN:0148-0227    

DOI:10.1029/2001JC000925

年代:2002

数据来源: WILEY

摘要:

Surface and subsurface currents and stratification were observed on and near the narrow shelf off Fort Lauderdale, Florida, in June–August 1999. The Florida Current flowed past, occasionally on the shelf, with speeds at times exceeding 2 m s−1only 8 km offshore. The typical vorticity associated with the lateral shear of the Florida Current was 4f, wherefis the local Coriolis parameter. Two dominant modes of higher frequency current variability were embedded in this low‐frequency flow, a 10‐hour signal with amplitudes of up to 0.5 m s−1and an equally as strong, almost rectilinear signal with a 27‐hour period, the same as the local inertial period. Both signals were propagating northward along the shore with wavelengths of ∼27 and ∼170 km for the 10‐ and 27‐hour signals, respectively, and with corresponding phase speeds of 0.85 and 1.7 m s−1. The phase trend in east‐west direction of the 10‐hour signal is consistent with unstable, growing waves. The 10‐ and 27‐hour signals appear dominantly barotropic near the coast out to depths of at least 50 m, but the 10‐hour signal displayed a 180° phase change between surface and bottom farther away from the shelf break at 160 m depth. The two dominant signals cannot be attr

ISSN:0148-0227    

DOI:10.1029/2001JC000813

年代:2002

数据来源: WILEY

摘要:

Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high‐resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high‐resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the south of Kuroshio (140°E, 30°N) by means of sequential data assimilation of TOPEX/Poseidon altimeter data of October 1992 for 30 days. The strength of the assimilation was varied to produce five eddies (Rossby number ϵ ∼ 0.012–0.014) comparable in scale to those observed in this region. The two results (ϵ = 0.0131, 0.0141) resemble the observed onset of short‐term meandering events in 1993 and 1998. The westward propagating anticyclonic eddy collides with the Kuroshio southeast of Kyushu, propagates downstream, and triggers the short‐term Kuroshio meander (occurring between the Kii Peninsula and Izu ridge with duration of half a year). The sequence resembles the scenario hypothesized on the basis of altimeter observations and other in situ measurements during Tokyo Ogasawara Line Experiment. The description of these cases is important, but we focus here on the differentiation between the strong eddy cases (ϵ = 0.0131, 0.0141) and the weak eddy cases (ϵ = 0.0118, 0.0124, 0.0125) classified on the basis of their subsequent evolutions: the strong geddy (meandering case) propagates west as it elongates zonally and barotropic tripolar vortices form, and the weak eddy (nonmeandering case) makes an abrupt southward migration while the eddy core splits as a result of advection by lower layer geostrophic motions. Such distinct evolutions are conjectured a result of a competition of the effects of nonlinearity, dispersion, and barotropicity of which their relative importance varied among the simulated eddies. Because the scale variation of the injected eddies are natural, we expect that even the weak‐eddy case exists in nature; in fact, such a case w

ISSN:0148-0227    

DOI:10.1029/2000JC000756

年代:2002

数据来源: WILEY

摘要:

Measurements of suspended sediment concentration, and the associated bedform morphology, were made beneath regular and irregular waves in a large‐scale flume. The bed forms were measured using an acoustic ripple profiler, and the suspended sediments were measured using an acoustic backscatter system, together with pumped sampling. Using the measured bed form dimensions and the flow as inputs, standard approaches have been used to predict the reference concentration and vertical distribution of suspended sediment concentration.Nielsen's [1986] empirical expression for the reference concentration,C0, shows reasonable overall agreement with the present measurements in respect of the dependence ofC0on the Shields parameter (skin friction); but the formula somewhat overestimates the measured concentrations. To analyze the form of the measured mean concentration profiles, comparisons have been made with the simple one‐dimensional (vertical) formulations, and extensions thereof, proposed byNielsen[1992] on the basis of pure diffusion, pure convection, and combined convection and diffusion. It is concluded that in a near‐bed layer of thickness about two ripple heights (i.e., the layer dominated by vortex formation and shedding above ripples), pure diffusion characterized by a height‐independent sediment diffusivity provides a good representation of the measured profiles. Above this,Nielsen's [1992] convection‐diffusion solution provides a better representation. It is shown, however, that by use of pure turbulent diffusion modeling concepts, the same profile can also be obtained by the use of a height‐varying, “constant + linear,” sediment diffusion model. This diffusivity represents the enhanced mixing in the outer part of the oscillatory boundary layer caused by the breakdown of coherent vortex structures into random turbulence. The relative merits of convection and diffusion scheme

ISSN:0148-0227    

DOI:10.1029/2001JC000988

年代:2002

数据来源: WILEY

摘要:

A primitive equation model is used to examine the structure and energetics of M2internal tides generated at the Hawaiian Ridge. Recent estimates based on altimeter data suggest that 20 GW of barotropic tidal dissipation occurs at the ridge, with conversion to internal tides believed to be the dominant dissipation mechanism. The model simulates an internal tide that accounts for 9.7 GW of radiated energy away from the ridge in the northeast and southwest directions. The strongest generation occurs at three sites where enhanced barotropic currents flow across elongated topographic features. The depth‐integrated baroclinic energy flux and energy densities at these sites are on the order of 104W m−1and 104J m−2, respectively. A modal decomposition indicates that 62% of the outgoing energy flux is accounted for by the first internal mode, 15% is accounted for by the second mode, and less than 5% is accounted for by each subsequent higher mode. The tidal dissipation due to bottom friction along the ridge is estimated to be 0.1 GW. The level of turbulent dissipation near the ridge owing to tidal energy conversion remains to be determined to assess fully the barotropic‐baroclinic energy

ISSN:0148-0227    

DOI:10.1029/2001JC000996

年代:2002

数据来源: WILEY

摘要:

Changes in visible region (400–700 nm) absorption spectra of chromophoric dissolved organic matter (CDOM) resulting from natural photobleaching have not been well characterized in aquatic systems. Photobleaching of CDOM absorption and the resultant changes in the exponential spectral slope in the visible domain are documented in a coastal fjord. Measurements of spectral CDOM absorption were made with a WET Labs ac‐9 in situ and in surface water incubated in UV‐transparent Tedlar bags at 1 m for 6 days. The exponential slope of the absorption spectrum between 412–560 nm increased as a function of radiation dosage in the first 72 h of the incubations. Higher spectral slope values were observed in surface waters relative to bottom waters, in agreement with our experimental results. The extent to which a CDOM absorption spectrum altered by photobleaching can serve as a signature of the process itself, containing information about absorption loss, was investigated. From these observations, we propose that higher spectral slopes typically associated with oceanic CDOM relative to coastal CDOM may be due to increased cumulative photobleaching as opposed to a shift in the spectral signature of CDOM sources as has been hypot

ISSN:0148-0227    

DOI:10.1029/1999JC000281

年代:2002

数据来源: WILEY

摘要:

Objectively analyzed fields of satellite sea surface temperature (SST, advanced very high resolution radiometer (AVHRR) Pathfinder) and sea surface height anomaly (SSHA, combined TOPEX/Poseidon–ERS‐1/2) are used to characterize, statistically, the mesoscale variability about the U.S. Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time‐Series Study (BATS) site. These results are applied to the in situ BATS time series data and a local one‐dimensional (1‐D) physical upper ocean model to better understand the contribution of mesoscale eddies to the time series record and the model‐data mismatch. Using a low‐pass spatial filter, we decompose the anomalies from the seasonal cycle into two components: the large‐scale, regional climate variability and a mesoscale signal. The mesoscale SST and SSHA fields are positively cross‐correlated at a statistically significant level, consistent with near‐surface isotherm displacements for cyclonic and anticyclonic eddies. The results from time‐lagged cross‐correlation analysis show that detectable eddy signatures exist in the in situ SST data and that eddies are a noticeable (∼10%) but not dominant error source for the 1‐D model solution. Several factors may be at work: the 1‐D model captures a more regional signal, whereas the BATS in situ data include small‐scale spatial heterogeneity; the satellite data and 1‐D model are indirectly coupled via the National Centers for Environmental Prediction (NCEP) reanalysis forcing data; and the satellite‐based mesoscale variability estimates are also missing specific events because of the sparse space‐time sampling of a polar orbiting, visible/infrared wavelength sensor. The mesoscale eddy cross‐correlation signature did not show up clearly in a similar analysis conducted on the original anomaly fields, highlighting the fact that climate scale variability needs to be carefull

ISSN:0148-0227    

DOI:10.1029/2000JC000589

年代:2002

数据来源: WILEY

摘要:

Observational (Tropical Atmosphere‐Ocean array) data on the annual cycle of upper ocean zonal currents on the equator are analyzed using a simple dynamical ocean model in order to investigate underlying dynamics. The model, by treating linear and nonlinear terms semi‐independently, allows a separation of various linear and nonlinear effects. The model focuses on linear dynamics of low‐order baroclinic modes. By realistically simulating the vertical structure of annual cycle, the model shows that linear dynamics determines the vertical and meridional structure of the annual cycle. Nonlinearity is weak and only important in the undercurrent, where it provides a simple mechanism for the annual cycle: mean meridional advection of the annual cycle north of the equator onto the equator, with the boreal springtime surge in the undercurrent being a direct result of a surge centered at 2°N. Model results show that annual variations in zonal currents are out of phase across the equator, surging in the corresponding spring. This behavior is a response to trade wind variations, which are also equatorially antisymmetric, and is generated by the second meridional mode Rossb

ISSN:0148-0227    

DOI:10.1029/2000JC000711

年代:2002

数据来源: WILEY

摘要:

The three‐dimensional Princeton Ocean Model is used to investigate the mean flow and the hydraulics regime in the Strait of Gibraltar. The model makes use of a coast‐following curvilinear orthogonal grid that includes the Gulf of Cadiz and the Alboran Sea, with very high resolution in the strait (∼500 m). A lock‐exchange initial condition is used: The western part of the model domain is filled with Atlantic water, whereas the eastern part is filled with Mediterranean water. A conservative, nondiffusive, and nondispersive numerical scheme for the horizontal tracers advection has been implemented to simulate the free‐flow adjustment for the lock‐exchange initial condition. Predicted velocities, interface depth, water, and salinity transports are comparable with observed data. Model results describe circulation of the Mediterranean outflow in detail and demonstrate the presence of a mixing layer between the Atlantic and Mediterranean water. The hydraulic regime is analyzed, calculating the composite Froude number for three layers within the strait. Results indicate the presence of a hydraulic control at Camarinal Sill and in the northern part of Tarifa Narrows. In order to understand if the Strait of Gibraltar is in the submaximal or maximal regime we have formulated a cross‐strait mean composite Froude number. This formulation allowed us to limit the values of the composite Froude number in the range of 1.2–1.5 at Camarinal Sill and between 0.4 and 0.8 at Tarifa Narrows. We have concluded that the mean exchange simulated by the model is in su

ISSN:0148-0227    

DOI:10.1029/2001JC000929

年代:2002

数据来源: WILEY

摘要:

The variation of seaward and shoreward infragravity energy fluxes across the shoaling and surf zones of a gently sloping sandy beach is estimated from field observations and related to forcing by groups of sea and swell, dissipation, and shoreline reflection. Data from collocated pressure and velocity sensors deployed between 1 and 6 m water depth are combined, using the assumption of cross‐shore propagation, to decompose the infragravity wave field into shoreward and seaward propagating components. Seaward of the surf zone, shoreward propagating infragravity waves are amplified by nonlinear interactions with groups of sea and swell, and the shoreward infragravity energy flux increases in the onshore direction. In the surf zone, nonlinear phase coupling between infragravity waves and groups of sea and swell decreases, as does the shoreward infragravity energy flux, consistent with the cessation of nonlinear forcing and the increased importance of infragravity wave dissipation. Seaward propagating infragravity waves are not phase coupled to incident wave groups, and their energy levels suggest strong infragravity wave reflection near the shoreline. The cross‐shore variation of the seaward energy flux is weaker than that of the shoreward flux, resulting in cross‐shore variation of the squared infragravity reflection coefficient (ratio of seaward to shoreward energy flux) between about 0.4 an

ISSN:0148-0227    

DOI:10.1029/2001JC000970

年代:2002

数据来源: WILEY