principles of early development of karst conduits under natural and man made conditions revealed by mathematical analysis of numerical models numerical models of the enlargement of primary fissures in limestone by calcite aggressive water show a complex behavior if the lengths of the fractures are large and hydraulic heads are low as is the case in nature dissolution rates at the exit of the channel determine its development by causing a slow increase of water flow which after a long gestation time by positive feedback accelerates dramatically within a short time span mathematical analysis of simplified approximations yields an analytical expression for the breakthrough time when this happens in excellent agreement with the results of a numerical model this expression quantifies the geometrical hydraulic and chemical parameters determining such karst processes if the lengths of the enlarging channels are small but hydraulic heads are high as is the case for artificial hydraulic structures such as dams it is the widening at the entrance of the flow path which determines the enlargement of the conduit within the lifetime of the dam this can cause serious water losses this can also be explained by mathematical analysis of simplified approximations which yield an analytical threshold condition from which the safety of a dam can be judged thus in both cases the dynamic processes of karstification are revealed to gain a deeper understanding of the early development of karst systems as a further important result one finds that minimum conditions below which karstification cannot develop do not exist

review and test of reproducibility of subdecadal resolution palaeoenvironmental reconstructions from microfossil assemblages abstract very high even annual resolution quantitative palaeoenvironmental reconstructions have been generated from microfossil assemblage data using transfer functions evidence of the utility of some of these reconstructions is given by the high correlations between the reconstructions and instrumental records such performance is despite several ecological and taphonomic issues that can become more problematic at subdecadal resolution than at multidecadal resolution this paper explores these timescale dependent issues and tests the reproducibility of the several of published reconstructions it demonstrates that the previously reported exceptionally good performance of several published chironomid based reconstructions is not reproducible

wellen radar wera a new ground wave hf radar for ocean remote sensing hf radars can be used to measure surface currents and wave spectra the coastal radar codar used by the university of hamburg was designed for current mapping only it has been operated for 15 field experiments during the past 15 years recently a new hf radar called wellen radar wera has been developed at the university of hamburg one main advantage of the system is the possibility of connecting different configurations of receive antennas when operated with a linear array information on the sea state can be obtained via second order spectral bands a further advantage is the flexibility in range resolution between 0 3 and 1 2 km instead of the fixed resolution of about 2 km of codar this is achieved by transmitting frequency modulated continuous wave fmcw chirps instead of continuous wave cw pulses in addition this technique avoids the blind range of about 3 km in front of the codar the technical design of wera is described and first experimental results are presented

evolution of near inertial waves during an upwelling event on the new jersey inner shelf abstract a 1996 field program provided a nearly three dimensional view of the temporal evolution of near inertial motion toward the end of an upwelling event on new jersey s inner shelf the appearance of near inertial motion is marked by a rapid rise in kinetic energy at the surface that is of the same magnitude and temporal structure of the work done by the wind indicating that the inertial motions are forced by local winds the incipient near inertial motion is surface intensified and spatially coherent it has a horizontal wavelength of 300 km and a mean kinetic energy of 0 04 m2 s 2 both of which decrease to less than 100 km and 0 01 m2 s 2 within two inertial periods an energy budget suggests that the rapid decline in surface kinetic energy is primarily due to vertical propagation into the thermocline near inertial motion in the thermocline is heterogeneous suggesting interaction between near inertial motion and subinertial shears the heterogeneous nature of near inertial waves poses a practica

remote sensing of shelf sea currents using a high frequency ocean surface current radar system in a deployment of an ocean surface current radar system made during may 1985 currents were measured in a region of sea to the northeast of anglesey wales and compared with data obtained from three current meters the uppermost current meter was fixed at 5 m from the surface in a water column of 40 m depth with tidal range of 9 m in the east west direction close to the orientation of the major axis of the tidal ellipse the radar and top current meter show good agreement to within an rms value of 0 16 m s 1 consistent with earlier comparative studies for the weaker north south components of the flow however differences between the radar results and those from the current meters are apparent a larger amplitude north south signal is recorded by the radar in terms of the m2 rotary clockwise and anticlockwise components of the flow the radar data contain amplitude and phase anomalies with respect to a predicted surface behavior deduced from the current meter records and theory in particular anticlockwise phase at the surface is advanced considerably by 20 further analysis shows that the flow field recorded by the radar has two components the predicted tidal response and an additional elliptical term of lower eccentricity than the main east west tidal signal and with major axis directed close to north hourly synoptic maps of tidal circulation patterns derived from the radar show a smooth transition from near westward flows on the ebb to near eastward flows on the flood the variation of north south residual currents with height can be interpreted in terms of a mean state consistent with geostrophic shear and associated with the roughly east west density gradient

direct measurements of ocean surface velocity from space interpretation and validation the median doppler shift of radar echoes is analyzed in measurements by envisat s advanced synthetic aperture radar asar over the ocean this doppler centroid differs from a predicted signal based on the predicted motion of the satellite and earth this anomaly converted to a surface doppler velocity u d appears to be of geophysical origin two wide swath images over the gulf stream around cape hatteras suggest that u d contains high resolution information on surface currents while on a global scale u d is found to vary with the wind speed in the range direction a simple quantitative forward model is proposed based on a practical two scale decomposition of the surface geometry and kinematics the model represents the effect of the wind through the wave spectrum and gives u d u 10 u c with u 10 and u c as the 10 m wind speed and quasi eulerian current in the line of sight of the radar projected on the sea surface respectively and as a coefficient function of the wind speed wave development and radar geometry it is found that for an incidence angle of 23 0 3 for moderate winds and fully developed seas this model is validated with a global data set of asar wave mode observations with colocated model winds acquired over the global ocean during the years 2003 and 2004 the doppler signal therefore provides the signed parameter u d that can be used to reduce the wind direction ambiguity in the inversion of high resolution wind fields from sar imagery a qualitative validation of current effects is shown for the english channel where tidal currents dominate thus it should be possible to combine this previously ignored geophysical doppler signal with traditional information on sea surface roughness in order to provide very high resolution wind and current fields

characterizing long island sound outflows from hf radar using self organizing maps abstract the surface outflows from the long island sound are examined from one year records of hf radar codar observations synoptic flow patterns are identified using manual classification empirical orthogonal function eof decomposition and self organizing maps som four characteristic flow patterns for the spring summer and fall winter seasons each are obtained through a 2 2 som array the som is confirmed by comparison with manual classification and is shown to be a significant improvement over eof classification it is suggested that the degrees of freedom of the leading eof modes can be used as a constraint on the otherwise arbitrary som dimension the relationship between the flow patterns and the winds also can be conveniently examined in som the outflows are shown to interact strongly with the ambient coastal currents both of which are under the influence of the winds this result challenges the conventional wisdom which often treats the outflows independent of the ambient currents the advantage of using som in synthesizing and interpreting synoptic hf radar observations is clearly demonstrated

mopod a generic model of porosity development a code mopod has been developed to investigate general relationships between simple porosity growth laws and pore growth phenomena mopod has been formulated as an initial value problem and to date investigations have focused on a very simple porosity growth law of the form dai t dt vei where e is the aperture growth rate exponent a range of qualitatively distinct evolved geometries have been described for porosity growth on 2d and 3d arrays of varying geometries and connectivities as a function of the exponent e of the aperture growth rate law and the width of the initial aperture distribution z at low growth rate exponents and moderate values of z over time there is a homogenization of apertures oriented sub parallel to the head gradient at moderate growth rate exponents these apertures become increasingly heterogeneous in evolved arrays with planar heterogeneities developing sub parallel to the head gradient for low values of z while anastomosing structures develop at higher values of z for larger growth rate exponents preferentially enlarged array spanning paths develop no selforganization phenomena have been observed because periodic or cyclic behaviour is not inherent in the simple growth laws investigated to date

remote sensing of ocean wave spectra by interferometric synthetic aperture radar the two dimensional power spectra of ocean waves are of great interest not only to oceanographers but also in practical applications such as wave forecasting trans oceanic ship routing and design of coast and offshore installations remote sensing of ocean surface waves can be difficult using conventional synthetic aperture radar sar techniques but waves can be observed clearly by sar in the interferometric configuration insar 1 2 this improvement is due to the ability of insar to provide images of the local surface velocity field in contrast to conventional sar for which the imaging process is related indirectly to the complex modulation of the surface reflectivity by longer waves and currents here we show that insar can be used to obtain wavenumber spectra that are in agreement with power spectra measured in situ this new method thus has considerable potential to provide instantaneous spatial information about the structutre of ocean wave fields

the free surface turbulent shear layer abstract in applying the wall layer analogy to a wind blown free surface it is necessary to decide in what coordinate system such an anology is realistic a smoothed sea surface is taken to be that produced by the nearly irrotational components of the wave field relegating irregular dissipative wavelets to turbulence it is then possible to regard wave motion and shear flow as independent except for the vortex force effect of stokes drift discussed by leibovich an analysts or the available observations shows that the free surface shear layer has many of the characteristics of the wall layer a major difference is a much larger roughness parameter arising presumably from direct energy input to surface turbulence by the wind velocity gradients near the free surface are much smaller then over a solid wall under otherwise comparable conditions

on the statistics of the phase of microwave backscatter from the ocean surface in this paper we describe what we believe is the first study of the distribution of the phase of microwave fields backscattered from the ocean surface scatterometry data from the synthetic aperture radar and x band ocean nonlinearities experiment conducted on the german north sea research platform forschungsplatform nordsee in november 1990 have been analyzed to reveal the distribution of phase differences as a function of time lag a theoretical model for these statistics is presented based on modulated gaussian fields this theoretical model is shown to be in good agreement with the measured statistics from this agreement we conclude that the backscattered fields have a gaussian distribution on short time scales but are modulated in amplitude and frequency by the long surface waves these results are of more than purely academic interest with direct applications to the design and analysis of interferometric synthetic aperture radars a relatively new class of instruments that may be capable of providing high resolution maps of ocean surface currents from aircraft or satellites

on the accuracy of hf radar measurement in the tsushima strait 1 the accuracy of hf radar measurement in the tsushima strait is investigated a comparison between radial velocities measured by hf radar and acoustic doppler current profiler adcp which provides an upper bound of hf radar measurement error shows that the root mean square rms velocity difference obtained from the principal component analysis is 6 62 11 3 cm s 1 a comparison of velocities measured by two facing hf radars which provides a lower bound of hf radar measurement error shows that the variance error of hourly radial velocity is 5 75 13 3 cm s 1 the bias error of hf radar measurement is also found to be reasonably small through a comparison of tidal ellipses estimated from 1 year of hf radar data with those from 5 years of adcp data these results suggest that the variance error of hf radar measurement is the dominant source of the velocity difference found between adcp and hf radar

observation of tropical cyclones by high resolution scatterometry unprecedented views of surface wind fields in tropical cyclones hereafter tcs are provided by the european remote sensing satellite ers c band scatterometer scatterometer measurements at c band are able to penetrate convective storms clouds observing the surface wind fields with good accuracy however the resolution of the measurements 50x50 km 2 limits the interpretation of the scatterometer signals in such mesoscale events the strong gradients of the surface wind existing at scales of a few kms are smoothed in the measured features such as the intensity and location of the wind maxima and the position of the center beyond the ers systems the scatterometers on board the adeos and metop satellites designed by the jet propulsion laboratory and by the european space agency respectively will be able to produce measurements of the backscattering coefficient at about 25x25 km 2 resolution a few sets of ers 1 orbits sampling tc events were produced with an experimental 25x25 km 2 resolution enhancing the resolution by a factor of 2 allows location of the wind maxima and minima in a tc with a much better accuracy than at 50 km resolution in addition a better resolution reduces the geophysical noise variability of wind speed within the cell and effect of rain that dominates the radiometric noise and hence improves the definition of the backscattering measurements a comprehensive analysis of the backscattering measurements in the case of high winds and high sea states obtained within tcs is proposed in order to refine the interpretation of the wind vector derived from a backscattering model that is currently only calibrated up to moderate winds 20 m s in neutral conditions observations of the topex poseidon dual frequency altimeter are also used for that purpose patterns of the surface winds in tcs are described and characteristic features concerning asymmetries in the maximum winds and in the divergence field are discussed

the impact of the laacher see tephra on terrestrial and aquatic ecosystems in the black forest southern germany late glacial lake sediments containing the laacher see tephra lst 11 000 yr bp have been analysed for pollen and diatoms at two sites hirschenmoor rotmeer in the montane belt of the black forest southern germany in order to detect the possible impact of this major volcanic eruption on terrestrial and aquatic biota the pollen assemblages at both sites show a minor increase in grasses and sedges following the tephra deposition whereas the pollen of aquatic macrophytes do not show any consistent pattern partial redundancy analysis and monte carlo permutation tests suggest however that the lst had no statistically significant effect either on the terrestrial or aquatic pollen assemblages at either site when the effects of time and climatic change are allowed for statistically the diatom assemblages at both sites changed after the deposition of the lst and diatom inferred ph estimates suggest a short but non significant excursion towards lower values multivariate analyses of the diatom data indicate a statistically significant impact of the deposition of the lst and associated changes in sediment lithology on the diatom assemblages when the effects of climatic change and time are allowed for statistically however owing to the interaction between tephra and lithology it is not possible to conclude if the diatom changes were caused by the deposition of the lst or by lithological changes

shelf circulation induced by an orographic wind jet the dynamical response to cross shelf wind jet episodes is investigated the study area is located at the northern margin of the ebro shelf in the northwestern nw mediterranean sea where episodes of strong northwesterly wind occur in this case the wind is channeled through the ebro valley and intensifies upon reaching the sea resulting in a wind jet the wind jet response in terms of water circulation and vertical density structure is investigated using a numerical model the numerical outputs agree with water current observations from a high frequency radar additionally temperature sea level and wind measurements are also used for the skill assessment of the model for the wind jet episodes the numerical results show a well defined two layer circulation in the cross shelf direction with the surface currents in the direction of the wind this pattern is consistent with sea level set down due to the wind effect the comparison of the vertical structure response for different episodes revealed that the increase of stratification leads to an onshore displacement of the transition from inner shelf to mid shelf in general the cross shelf momentum balance during a wind jet episode exhibits a balance between the frictional terms and the pressure gradient in shallow waters shifting to a balance between the coriolis force and the wind stress terms in deeper waters

satellite measurements reveal persistent small scale features in ocean winds four year averages of 25 kilometer resolution measurements of near surface wind speed and direction over the global ocean from the quikscat satellite radar scatterometer reveal the existence of surprisingly persistent small scale features in the dynamically and thermodynamically important curl and divergence of the wind stress air sea interaction over sea surface temperature fronts throughout the world ocean is evident in both the curl and divergence fields as are the influences of islands and coastal mountains ocean currents such as the gulf stream generate distinctive patterns in the curl field these previously unresolved features have important implications for oceanographic and air sea interaction research

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

early development of karst aquifers on percolation networks of fractures in limestone we have modeled flow and dissolution processes within percolation networks representing stochastic primary fracture systems in limestone at the beginning of karstification flow is evenly distributed on all fractures available as the system develops by dissolutional widening of the fractures preferred flow pathways evolve which attract more and more flow until at breakthrough the total flow rate increases dramatically these breakthrough times have been investigated with regard to their dependence on the initial fracture aperture width the dimension of the evolving karst aquifer hydraulic gradients connectivity of the percolation network and the chemical parameters such as higher order rate constants and exponents and equilibrium concentration they exhibit a behavior similar to that of breakthrough times for one dimensional conduits dreybrodt 1996 our results show that the structure of the evolving karst aquifer is determined by the initial geological setting however the final conduits are selected from competing pathways with potential breakthrough times close to each other whereby the details of the distribution of undersaturation with respect to calcite play an important role

on wind wave current interactions during the shoaling waves experiment 1 this paper presents a case study of wind wave current interaction during the shoaling waves experiment showex surface current fields off duck north carolina were measured by a high frequency ocean surface current radar oscr wind wind stress and directional wave data were obtained from several air sea interaction spar asis buoys moored in the oscr scanning domain at several times during the experiment significant coastal currents entered the experimental area high horizontal shears at the current edge resulted in the waves at the peak of wind sea spectra but not those in the higher frequency equilibrium range being shifted away from the mean wind direction this led to a significant turning of the wind stress vector away from the mean wind direction the interactions presented here have important applications in radar remote sensing and are discussed in the context of recent radar imaging models of the ocean surface

seasonal variations in the speed factor and deflection angle of the wind driven surface flow in the tsushima strait 1 the speed factor and deflection angle of the wind driven surface flow were estimated for the tsushima strait from long 3 5 years records of surface velocities measured with hf radar geostrophic velocities estimated from sea level differences and analyzed wind velocities large seasonal variations in both the speed factor and deflection angle were found the speed factor which is defined here as the ratio of the wind driven surface flow to the waterside friction velocity is 7 7 9 5 in winter and 11 4 14 3 in summer while the deflection angle is 17 7 27 3 in winter and increases to 48 5 67 3 in summer a likely cause of these seasonal variations is the seasonal variation of the eddy viscosity profile it was also found that the speed factor in winter increases with the friction velocity this dependence is possibly explained by deepening of the wind driven surface flow with increasing friction velocity as the friction velocity increases hf radar whose measurement depth is fixed tends to measure a shallower part of the wind driven surface flow whose speed factor is larger

the mid holocene ulmus decline at diss mere norfolk uk a year by year pollen stratigraphy from annual laminations 3 m of annually laminated sediments were recovered from a small steep sided lake diss mere norfolk uk in an 8 cm section known to include the mid holocene elm decline the laminae were sampl

early development of karst systems 1 preferential flow path enlargement under laminar flow modeling of flow and solutional processes within networks of interconnected conduits in limestone aquifers indicates that enlargement occurs very selectively during the early stages of karst aquifer development under laminar flow if initial flow paths are uniform in size almost all enlargement occurs along a single set of connected conduits that lie along a direct path between recharge and discharge locations and are aligned along the hydraulic gradient with a sufficiently large variation in initial aperture widths enlargement occurs along the flow path offering the least resistance to flow but since flow rates in laminar flow are proportional to the fourth power of diameter but only linearly proportional to hydraulic gradient the preferentially enlarged set of fractures may follow an indirect path results disfavor earlier suggestions that nonselective cave patterns result from artesian flows at least under laminar flow conditions and that all passages should be competitive until the onset of turbulent flow

testing the integrity of fine spatial resolution palaeoecological records microcharcoal data from near duplicate peat profiles from the north york moors uk microcharcoal data which cover the millennium preceding the ulmus decline from near replicate peat profiles 30 cm apart are presented from north gill an intensively studied upland spring head site in north east england with an established detailed palaeoecological record the ulmus decline at this site has been consistently dated to ca 4700 14c years bp the microcharcoal records from these two very closely adjacent points which must have had virtually identical microcharcoal source areas are assessed for their degrees of similarity and variability at 1 cm vertical sampling intervals in an investigation which parallels a previous comparative pollen study of the two profiles the major trends and frequency peaks and troughs of the twin microcharcoal curves were found to correspond well suggesting that microcharcoal records from small peat mires within forested catchments have fine spatial integrity are reproducible and can be interpreted securely as reliable records of fire history in their small source areas much variation occurred at the level of individual spectra within coeval zones which must record the same fire history and ecology however producing differently shaped microcharcoal curves while the record of major trends will be accurate therefore too precise an interpretation of microcharcoal data at this temporal scale may not be justified micro spatial variations in frequencies of fungal spores associated with local burning are also presented and evaluated

karst aquifer evolution in fractured rocks we study the large scale evolution and flow in a fractured karst aquifer by means of a newly developed numerical method a karst aquifer is discretized into a set of irregularly spaced nodal points which are connected to their set of natural neighbors to simulate a network of interconnected conduits in two dimensions the conduits are allowed to enlarge by solutional widening the geometric flexibility of this method along with a simplified model for the dissolution kinetics within the system water carbon dioxide calcite enables us to study both laminar and turbulent flow in a karst aquifer during its early phase of evolution a sensitivity analysis is conducted for parameters such as conduit diameter hydraulic pressure differences and recharge conditions along the surface of the aquifer and shows that passage evolution depends strongly on the recharge condition and the amount of water available under fixed hydraulic head boundary conditions an early single passage system develops under laminar conditions and is transformed into a maze like passage system after the onset of turbulence fixed recharge boundary conditions are more likely to result in a branchwork like passage system although the addition of distributed recharge may lead to a maze like system of secondary passages

surface current variability east of okinawa island obtained from remotely sensed and in situ observational data we investigated ocean surface current variability in the region east of okinawa island in the spring of 1998 using in situ and remotely sensed data including surface velocity estimates inferred from hf ocean radars most previous studies utilizing hf ocean radars were limited to shallow continental shelves in contrast most of the observation area in this study was in the open ocean during the observation period the hf ocean radars sampled a region where anticyclonic and cyclonic mesoscale eddies were adjacent to each other the hf radar derived ocean currents agree with those measured by a current meter the surface currents were highly variable in both time and space and were affected by the eddy field in the offshore region the current field was related to the near surface water temperature for example a local temperature rise was often associated with northeastward flows the surface currents in regions shallower than about 1000 m had a significant correlation with local wind forcing our hf ocean radars often detected a strongly convergent zone which was different in character from those described in previous studies in that it was in the deep ocean from other data such as national oceanic and atmospheric administration advanced very high resolution radiometer imagery we concluded that the convergent zone was due to mesoscale eddy fronts

ocean surface features and currents measured with synthetic aperture radar interferometry and hf radar this paper describes the results of the first quantitative comparison between high resolution ocean surface current fields extracted from interferometric synthetic aperture radar insar measurements and those from a high frequency hf ocean surface current radar oscr system data from each of these radar systems along with supporting measurements from shipboard and buoy mounted sensors were collected during the high resolution remote sensing experiment high res on june 20 1993 on the continental shelf off the coast of cape hatteras north carolina both components of the surface current were obtained from the insar system at roughly 10 m resolution from two orthogonal flight legs over the region separated in time by about 30 min the oscr system measured two dimensional surface current vectors at about 1 km resolution over this same region while the usns bartlett was collecting hydrographic samples and near surface current measurements two dimensional wave spectra as well as meteorological and additional current measurements were collected at two buoys within the experimental area we discuss in the paper two techniques for eliminating the effect of surface wave motion on the insar current estimates one method relies on some knowledge of the local wind and wave field and the use of a microwave scattering model the other method makes use of a few in situ current measurements spaced at different range locations across the insar image using either of these techniques we find that the agreement between the insar and oscr current estimates is generally very good furthermore the insar current and magnitude imagery show the presence of undulating surface features where abrupt changes in the current speed and direction occurred the ship surveys indicate that these features were caused by the collision of water masses of different density we show for the first time in this paper a high resolution area extensive vector surface current map derived from the insar of the two dimensional flow in the vicinity of these features our results demonstrate convincingly that high resolution oceanic surface current vectors can be derived from insar current measurements and that these measurements may be very beneficial for detailed studies of the dynamics of small scale surface features in regions of strong current divergences or shears

the surface velocity structure of the florida current in a jet coordinate frame the structure and variability of the florida current between 25 26 n are investigated using hf radar ocean current measurements to provide the most detailed view of the surface jet to date a 2 d jet coordinate analysis is performed to define lateral displacements of the jet in time meandering and associated structural variations over a 2 year period 2005 2006 in the jet coordinate frame core speed has a median value of 160 cm s 1 at the central latitude of the array 25 4 n with a standard deviation std of 35 cm s 1 the jet meanders at timescales of 3 30 days with a std of 8 km and a downstream phase speed of 80 km day 1 meandering accounts for 45 of eddy kinetic energy computed in a fixed geographical reference frame core speed width and shear undergo the same dominant 3 30 day variability plus an annual cycle that matches seasonality of alongshore wind stress jet transport at 25 5 n exhibits a different seasonality to volume transport at 27 n most likely driven by input from the northwest providence channel core speed correlates inversely with miami sea level fluctuations such that a 40 cm s 1 deceleration is associated with a 10 cm elevation in sea level although there is no correlation of sea level to jet meandering or width accurately quantifying the florida current s variability is critical to understand and forecast future changes in the climate system of the north atlantic as well as local impacts on coastal circulation and sea level variability along south florida s coastline

estimates of currents in the nearshore ocean region using interferometric synthetic aperture radar microwave remote sensing of the ocean surface using synthetic aperture radar sar is of great interest due to its high resolution and the potential of nearly instantaneous coverage of large areas the experience accumulated in ocean imaging by sar indicates that there are substantial difficulties in interpretation of the obtained images of ocean wave systems ocean surface currents are almost impossible to obtain from sar imagery the present paper reports on measurements of ocean surface currents based on imaging of the nearshore regions of the monterey bay using an interferometric sar the method is a modification of the conventional sar which employs two spatially separated antennas this modification allows one to construct an interferogram from the two complex images obtained by each antenna which provides direct mapping of the observed surface velocities the present results demonstrate the advantages of this novel technique in studying nearshore ocean surface dynamics in particular spatial distribution of the longshore current velocity is presented

on the accuracy of hf radar surface current measurements intercomparisons with ship based sensors high frequency hf radar systems can provide periodic two dimensional vector current estimates over an area approaching 1000 km as the use of these hf systems has gained wider acceptance a number of attempts have been made to estimate the accuracy of such systems however comparisons of hf radar current estimates with in situ sensors are difficult to interpret since hf systems measure currents averaged over an area of 1 km 2 and to a depth of only 50 cm while in situ sensors measure currents at a point and somewhat greater depths 1 to 10 m previous studies of the accuracy of hf radar technology have thus attributed the differences observed between hf radar and in situ sensors to an unknown combination of vertical shear horizontal inhomogeneity in situ instrument errors and hf radar system errors this study examines the accuracy of hf radar current measurements using data from the 1993 high resolution remote sensing experiment conducted off cape hatteras north carolina data from four shipborne in situ current meters are compared with data from an ocean surface current radar oscr a commercial current measuring radar we attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location the results suggest an upper bound on the accuracy of the oscr derived radial currents of 7 to 8 cm s the measurement of near surface currents in the coastal environ ment is an important problem

a new view of near shore dynamics based on observations from hf radar abstract since 1984 the oscr hf radar system has been used in over 50 deployments to measure near shore surface currents for both scientific and engineering applications the enhanced scope resolution and accuracy of these measurements have yielded new insights into the tidal wind and density driven dynamics of the near shore zone tidal current ellipses obtained from these radar measurements have been shown to be in good aggrement with values calculated by numerical models both for the predominant constituents and also for higher harmonics coherent patterns of wind forced currents ahve been determined with strong evidence of a slab like surface response in one deployment with offshore winds blowing over relatively deep water this slab rotated clockwise at near inertial frequency strong up to 20cm s 1 persistent surface residual currents are commonly observed these are almost certainly generated by small horizontal density gradients these observed surface residuals provide ideal data for rigorous testing of 3 d numerical models with a threatened rise in sea level hf radar is well suited for observing the expected changes in the dynamics of near shore regions continuing development of these radar systems offers exciting prospects of remote sensing of both surface waves and currents future applications may extend beyond the near shore region to measurements along the shelf edge in oceanic gyres and for beach processes

evidence for the possible role of beaver castor fiber in the prehistoric ontogenesis of a mire in northwest england uk excavation of the relict stratigraphy of a drained agriculturally reclaimed mire in lancashire northwest england has revealed evidence for prehistoric i e c 820 520 cal bc beaver activity in the form of gnawed timber stratified in the basal sediments the lowest horizons of the peat deposits contained the intact root systems of a dense hazel wood indicating that the mire probably owed its origins to the rapid flooding of a relatively dry landscape the exact cause of this inundation remains uncertain although beaver damming is a strong candidate the possibility that beaver activity is implicated in the flooding of the site is potentially of great significance both palaeoecologically and archaeologically the site may represent the first recorded example in british prehistory of a non human animal playing a role in inducing mire development it also adds weight to the possibility that some small scale woodland clearances identified in british pollen diagrams which have been attributed to anthropogenic activity may be due to the effects of beaver

some examples of detection of oceanic mesoscale eddies by the seasat synthetic aperture radar this note presents images of three dynamically different regions in the ocean to demonstrate the variety of mesoscale variabilities detected by the seasat synthetic aperture radar sar south of the grand banks of newfoundland a cold eddy is observed to form as the result of the southward intrusion of labrador sea water perhaps having led to the birth of a gulf stream extension ring off the northern coast of california features resulting from the offshore intrusions of cold upwelling water are observed two topographically generated eddies are detected near misteriosa bank on the cayman ridge in the northwestern caribbean comparisons are made with concurrent noaa 5 infrared images whenever the eddies have thermal signatures

hf radar data assimilation in the monterey bay area 1 the utility of high frequency hf radar data for improving numerical circulation model predictions is evaluated comparisons of the statistical properties of the codar type hf radar data and the observed wind indicate a strong correlation between the dominant alongshore upwelling favoring wind forcing and hf radar derived surface currents along the central california coastline because inadequate knowledge of the wind stress is probably a significant source of error in the model solutions the idea of using hf radar data to provide corrections to the model wind forcing is promising different hf radar data assimilation schemes are compared and judged based on the correlations observed between model currents and independent observations from two mooring sites analysis of correlation maps between model predicted and observed currents indicates a spatial and temporal shift between modeled and observed features however the impact of hf radar data assimilation reduces these spatial and temporal shifts a significant improvement in the correlation between the model and observed subsurface currents is achieved when an ekman layer projection of the corrections is included in this approach assimilation of hf radar data produces additional ekman pumping vertical velocity based on the horizontal pattern of model observed velocity mismatch at the surface

lagrangian and eulerian characterization of two counter rotating submesoscale eddies in a western boundary current in recent decades high spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies in the surface velocity temperature and chlorophyll a fields we use a suite of high resolution data to characterize two counter rotating short lived eddies formed at the front between the warm east australian current eac and temperate coastal waters 308s eastern australia in this region submesoscale filaments and short lived eddies are dynamically generated and decay at time scales of hours to days dominant cyclonic filaments of o 1 rossby number formed along frontal jets and eddy boundaries generating localized ageostrophic circulations at the submesoscale measurements of over ocean wind direction and surface currents from high frequency radars reveal the influence of the short term small scale wind forcing on the surface circulation enhancement of the horizontal shear frontal jet destabilization and the generation and decay of the cyclonic eddy by contrast the anticyclonic eddy formation was most likely associated with eac mesoscale instability and anticyclonic vorticity lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones limiting their transport mixing between eac derived and coastal waters was increased along the frontal regions and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales the cyclonic cold core eddy entrained high chlorophyll a shelf waters on its convergent side suggesting spiral eddy cyclogenesis

the fine structure of nearshore tidal and residual circulations revealed by h f radar surface current measurements abstract using the ocean surface current radar oscr developed by the rutherford appleton laboratory uk 30 days of synoptic hourly surface current vectors were obtained for 84 locations within a nearshore region some 18 kms square tidal analyses of these data show that the currents associated with the predominant m2 constituent sweep smoothly and regularly through the area unaffected by the finer topographic features moreover contours of the amplitude of the m2 semi major axis are mutually consistent to a precision of better than 0 5 cm s 1 statistical analyses of these data indicate that the standard error of oscr current measurements is less than 4 cm s 1 by contrast the major higher harmonic constituent m4 shows pronounced but ordered spatial variability relating the observed distributions for m4 and m2 is foreseen as an instructive modeling problem that should advance our knowledge of shallow water tidal interaction processes standard relationships between residual surface currents and

interferometric radar measurement of ocean surface currents we describe a new method of measuring surface currents using an interferometric synthetic aperture radar an airborne implementation has been tested over the san francisco bay near the time of maximum tidal flow resulting in a map of the east west component of the current only the line of sight component of velocity is measured by this technique where the signal to noise ratio was strongest statistical fluctuations of less than 4 cm s 1 were observed for ocean patches of 60 60 m

in search of finiteness the limits of fine resolution palynology of sphagnum peat high resolution analysis of ombrogenous peats often reveals pollen frequencies that change markedly from one sample to the next indicating limited postdepositional mixing this raises the question whether with sufficiently thin samples seasonal differences in pollen deposition can be observed seasonal resolution in peat would provide a dating technique with an accuracy similar to that of other annually banded archives to study the limits of temporal resolution contiguous 0 5 mm thick slices of slightly humified sphagnum section cuspidata peat were palynologically analysed no seasonal patterns in the pollen stratigraphy could be observed this is ascribed to the mixing of pollen grains as a result of small scaled vertical water movements and to the horizontal mode of shoot layering in the sphagnum species the latter process may also be the cause of a smoothed annual signal because of the absence of seasonal differentiation the palynological distinction of annual peat layers is not feasible

hf radar wave and wind measurement over the eastern china sea high frequency hf radar can be employed to measure sea surface state parameters such as waveheight wind field and surface current velocity this paper describes the application of the hf ground wave radar in remote sensing the surface conditions over the eastern china sea in october 2000 the radar referred to as the osmar2000 was developed by wuhan university preliminary wave spectra waveheights and wind fields estimated from the collected data are presented and compared with ship recorded measurements where such are available the range for wind direction sensing is up to 200 km wave information and wind speed can be provided up to a range of 120 km the mean difference between radar and ship measured significant waveheight is 0 323 m wind direction is measured within 20 spl deg and wind speed to within 0 6 m s with such agreement being fairly reasonable the feasibility of the inversion algorithm and the ocean state real time sensing capability of osmar2000 are demonstrated

surface circulation at the strait of gibraltar a combined hf radar and high resolution model study observations from a high frequency radar system and outputs from a high resolution operational ocean model working at the strait of gibraltar have been analyzed and compared during the period february 2013 to september 2014 in order to evaluate their capability to resolve the surface circulation of the region the description of the mean circulation patterns has been statistically assessed showing good agreement particularly in the central region of the strait corresponding with the atlantic jet aj stream although some short scale features are not reproduced by the model in the frequency domain very high concordance is observed tidal maps of diurnal and semidiurnal constituents are in good agreement with previous observations the analysis of the model and radar response to the wind forcing reveals that the low resolution of the model wind forcing field and its deeper superficial level smoothes the wind effect on the simulated currents the first three eof modes account for the 86 of model and radar variances the coincidence between the observed and simulated patterns is very significant for the first two modes which account for the mean velocity field and the latitudinal shifting of the aj consequence of the flow topography interaction the third mode captures the wind induced circulation and greater discrepancies are found in this case results underline the complementary character of both systems radar observations improve the model description resolving short scale processes while the model completes the radar information when the time or spatial coverage is poorer

aqueous surface layer flows induced by microscale breaking wind waves microscale breaking wind waves cover much of the surface of open waters exposed to moderate wind forcing recent studies indicate that understanding the nature and key features of the surface skin flows associated with these small waves is fundamental to explaining the dramatic enhancement of constituent exchange that occurs in their presence we describe a laboratory study in which velocity measurements were made within a few hundred micrometres of the surface of microscale breaking wind waves without bubble entrainment using flow visualization and particle image velocimetry piv techniques for a range of wind speed and fetch conditions our measurements show that for each experiment the mean surface drift directly induced by the wind on the upwind faces and crests of these waves is 0 23 0 02 u a in the trough increasing to 0 33 0 07 u a at the crest where u a is the wind friction velocity about these mean values there is substantial variability in the instantaneous surface velocity up to approximately 0 17u a in the trough and 0 37u a at the crest this variability can be attributed primarily to the modulation of the wave field with additional contributions arising from fluctuations in wind forcing and near surface turbulence generated by shear in the drift layer or by the influence of transient microscale breaking we observed that in a frame of reference travelling with a microscale breaking wave the transport in the aqueous surface layer is rearward along its entire surface except within and immediately upwind of the spilling region moreover we found that transport of surface fluid rarely occurs forward over the crest and into the spilling region this is in marked contrast with previously envisaged wind drift layer flow structures visualizations and piv measurements demonstrate the important role of microscale wind wave breaking in the direct transport of fluid from the surface to the highly turbulent domain below many hundreds of surface flow visualization images were carefully examined these showed that at the toe of each microscale breaker spilling region there is an intense and highly localized convergence of surface fluid with convergence rates generally exceeding 100 s 1 by comparison observations of surface convergence attributable to parasitic capillary activity are modest the changes in mean surface drift along the upwind faces of the waves are equivalent to mean surface divergences of between 0 2 and 1 3 s 1 flow visualizations of the surface layer along the upwind windward faces of these waves revealed the occurrence of locally intense flow divergence however maximum values of the divergence rate were observed to be only of order 10 s 1 hence these divergence zones are much more diffuse than the convergence zones at the toes of spilling regions overall our measurements strongly support the view that microscale breaking is likely to be the dominant process in the enhancement of sea surface exchange at moderate wind speeds as has been suggested by a number of previous investigators using a simple model based on our observations it is shown that microscale breaking is potentially a very effective process in the observed enhancement of constituent transfer for u 10 4 m s 1 where u 10 is wind speed measured 10 m above the surface and this mechanism exceeds by a wide margin the strength of other previously proposed mechanisms

the importance of lateral variability on exchange across the inner shelf south of martha s vineyard ma this study examines the spatial variability of transport within the inner shelf south of martha s vineyard massachusetts its time and space dependence and its importance to the total volume exchanged between the nearshore and the coastal ocean the exchange of water across the inner shelf is often considered to be driven primarily by wind forcing yet the effects of small scale o 1 10 km variability on the total exchange have not been well quantified using a combination of high resolution hf radar based surface currents and a dense array of moorings to document the lateral variability of across shelf exchange the cumulative wind driven across shelf transport over the summer stratified period was less than the volume of the inner shelf onshore of the 25 m isobath along shelf variations in the wind driven exchange were as large as the spatial mean of the wind driven exchange a spatially varying time averaged circulation caused by tidal rectification resulted in across shelf exchange larger in magnitude than and independent of the integrated wind forced exchange coherent submesoscale eddies also occurred frequently within the domain due to flow topography effects onshore and horizontal density gradients offshore generally with lifespans shorter than 10 h diameters smaller than 6 km and vertical depths shallower than 10 m the across shelf volume transport due to eddies estimated by seeding particles within the surface current fields was more than half the wind driven depth dependent exchange thus accounting for the potential coherent along shelf variability present over the inner shelf can significantly increase estimates of the across shelf transfer of water masses and particles

radar derived interferometric surface currents and their relationship to subsurface current structure radar derived ocean surface currents are analyzed in conjunction with in situ acoustic doppler current profiler adcp measurements the interferometric measurements were collected by an x band imaging doppler radar in a manner analogous to those of along track interferometric synthetic aperture radar ati sar while the advent of ati sar has provided a new potentially powerful technique for current mapping the relationship between surface currents and interferometric velocity measurements is not yet clearly understood this paper presents comparisons between radar derived and in situ current measurements to develop a precise method for estimating the surface current from interferometric measurements the influence of long wave orbital velocities and the influence of bragg resonant waves are studied we find that coupling between the orbital velocity and backscattered power i e the modulation transfer function can bias surface current estimates potentially by up to 20 cm s 1 in an upwind viewing orientation furthermore experimental observations verify a cos 2n 0 2 analytical model for the directional spreading of bragg resonant waves extending our analysis to include subsurface currents case studies are presented under varying environmental conditions for which the vertical current structure changes considerably analysis of radar imagery yields both radial surface currents and vector subsurface current estimates derived from long wave dispersion characteristics combining these with coincident adcp measurements yields a vertical profile of current using these measurement techniques we make several observations within the upper meter of the ocean these profiles reveal the sensitivity of x band interferometric measurements to wind drift and the near surface current structure

a new interpretation of multifrequency multipolarization radar signatures of the gulf stream front radar signatures which are observed on sir c x sar multifrequency multipolarization synthetic aperture radar images of the gulf stream off the u s east coast are compared with results of simulations with a numerical radar imaging model based on in situ data current and wind variations are included into the model as well as a variation of the thermal stability of the marine atmospheric boundary layer across the gulf stream front according to our model predictions all of these parameter variations can cause radar signatures of similar shape and modulation depth but due to specific dependencies of radar signatures on variations of surface currents and winds we show that it is possible to distinguish between radar signatures of oceanic and atmospheric origin in multifrequency multipolarization images and to estimate the corresponding current and wind variations independently for one set of radar images we derive a most likely scenario of oceanic and atmospheric parameters during the time of the image acquisition for which good overall agreement between observed and simulated radar signatures is obtained at most radar channels

southeast florida shelf circulation and volume exchange observations of km scale variability abstract tides wind and the florida current affect the dynamics of the south florida continental shelf over a range of scales two field experiments in the winter largo1 and summer largo2 employed a high frequency hf ocean surface current radar oscr moored current meters and a boat mounted acoustic doppler current profiler adcp to study the circulation over the southeast florida shelf the oscr measured near surface currents over an approximately 30 km by 30 km grid with 1 km horizontal resolution covering the shelf and extending offshore of the shelf break by using observed 20 min surface vector field time series to backward track patches with defined volumes of shelf water their recent origin was identified the proportion of surface volume flux past various points on the shelf that originated offshore along the shelf edge and inshore was then estimated onshore winds were linked to periods of transport of offshore water all the way to the inner shelf during both the summer and winter seasons in largo2 the exchange of offshore water with water over the middle and inner shelf was closely linked to onshore and downcoast winds simulated particle trajectories revealed that florida current spin off eddies caused off shelf water to be advected onto the middle largo1 or outer largo2 shelf however because eddies had short residence times and small spatial scales their contribution to the overall exchange was much smaller than the wind

holocene vegetation dynamics of anticosti island quebec and consequences of remoteness on ecological succession abstract the postglacial vegetation history of anticosti island in the gulf of st lawrence quebec was reconstructed from pollen analysis of three sites providing the first paleoecological data for the island the earliest plant communities 8550 7770 14c yr b p 9500 8500 cal yr b p were characterized by a high representation of green alder alnus crispa indicating an open environment with sparse spruce picea between 7770 and 3700 14c yr b p 8500 4000 cal yr b p the island was colonized by mixed deciduous and coniferous forests dominated by paper birch betula papyrifera and spruce a major increase in the abundance of balsam fir abies balsamea occurred ca 3700 14c yr b p 4000 cal yr b p indicating development of the closed fir spruce forests as they exist today the island s remoteness from continental seed plant sources influenced plant colonization afforestation and ecological succession the gulf of st lawrence slowed plant migration and tree species had to spread from the gaspe peninsula repeated drops in fir pollen percentages during the last 4 000 years may reflect the activity of defoliating insects

effects of low frequency current variability on near inertial submesoscale vortices from september 30 to october 14 1993 and from may 18 to june 13 1994 ocean surface currents along the inshore boundary of the florida current were measured using a land based ocean surface current radar oscr deployed along the florida keys a dual station oscr mapped the current fields at 20 min intervals with a horizontal resolution of 1 2 km over a 30 44 km domain using the high frequency hf 25 4 mhz mode an upward looking narrowband acoustic doppler current profiler adcp was moored in 150 m of water seaward of the shelf break concurrently sampling the three dimensional current vector between 15 and 130 m at 30 min intervals regression analyses between surface and subsurface currents at 15 m depth from the adcp indicated biases of 3 12 cm s 1 and slopes of o 0 8 1 root mean square differences were about 18 cm s 1 because of a 15 m depth separation between the two measurements and bulk current shears were o 10 3 s 1 in the florida current where maximum velocities exceeded 150 cm s 1 between 40 and 70 m surface tidal currents were 2 10 cm s 1 in the diurnal and semidiurnal bands and revealed fine scale variability in the tidal ellipses due to topographical variations differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s 1 in these constituents and explained more of the current variance with depth during the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s 1 this feature was embedded in the near inertial wave band where the local inertial period was 29 hours that may have been forced by an abrupt change in the wind direction this transient feature was trapped and advected along the inshore edge of the florida current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f these energetic near inertial currents of 15 20 cm s 1 had horizontal and vertical wavelengths of 40 km and 50 100 m respectively

comparison of observed hf radar adcp and model barotropic tidal currents in the new york bight and block island sound abstract a three dimensional primitive equation model is used to simulate the barotropically forced semidiurnal tidal currents in the new york bight nyb and block island sound bis model tidal velocity ellipses are verified at historical current meter locations and with measurements from acoustic doppler current profilers adcps and hf radar codar the model reproduces well the regional tidal flow pattern as well as the vertical tidal velocity structure the model misfits typically are 2 4 cm s in amplitude and 5 7 in orientation through comprehensive skill assessment the model s capacity of simulating the three dimensional barotropic tidal currents is clearly demonstrated moreover it is shown that the integrity of hf radar measurements can be assured by verifying with model tidal ellipses

late and post glacial evolution of lake steisslingen i abstract piston cores from lake steisslingen a small hard water lake situated 20 km west of lake constance and mainly supplied by submerged springs reveal a record of the environmental history going back to 15 600 cal yr bp from the bolling to ca 9800 cal yr bp the sediments are partly seasonally laminated from then on up to the middle ages the lamination is mostly undisturbed providing a high resolution time scale by varve counting a detailed pollen profile and 13 ams 14c analyses all investigated sediment sections show evidence of exceptionally high sulphate concentrations in the lake water which lead to the fixation of all reactive iron as pyrite the high sulphur content can only be attributed to s rich water supplied by the submerged springs thus indicating the miocene obere meeres molasse as the dominant aquifer since soil erosion is strongly reduced due to increased forest density in the preboreal boreal the submerged springs are the main nutrient source for the lake mixing or stable stratification of the waterbody mostly depends on nutrient availability and bioproductivity in the epilimnion the controlling external factors are wind and changes in water level and or temperature at the end of the younger dryas and during the preboreal low water levels and precipitation rates are assumed the water column mixing was regular bioproductivity high and high degradation activity of organic matter led to a redissolution of calcite the hydrological separation of a northern shallow lake seeried probably fell into the period of this low lake level around 10 450 10 300 cal yr bp the lake level rose but never again reached the former level between 10 000 and 7200 cal yr bp non glacial varves are even found in shallower parts of the lake indicating a stable anoxic monimolimnion with a rising chemocline bioproductivity was low wind protection provided by the now dense forestation and favourable climatic conditions e g high summer temperatures and low winter temperatures seem to be the responsible factors fluctuating human impact is identified from about 7400 cal yr bp until permanent settlement by the alemanni about 1500 cal yr bp strong human influence is observed from 2000 cal yr bp onwards when agricultural activity and clearing of the shoreline vegetation by man caused high soil erosion rates and thus eutrophication at about 800 cal yr bp the recent hydrological conditions were established by implementing an artificial drain thus lowering the lake level

the influence of fetch on the response of surface currents to wind studied by hf ocean surface radar the momentum transfer from wind to sea generates surface currents through both the wind shear stress and the stokes drift induced by waves this paper addresses issues in the interpretation of hf radar measurements of surface currents and momentum transfer from air to sea surface current data over a 30 day period from hf ocean surface radar are used to study the response of surface currents to wind two periods of relatively constant wind are identified one for the short fetch condition and the other for the long fetch condition results suggest that the ratio of surface current speed to wind speed is larger under the long fetch condition while the angle between the surface current vector and wind vector is larger under the short fetch condition data analysis shows that the stokes drift dominates the surface currents under the long fetch condition when the sea state is more mature while the stokes drifts and ekman type currents play almost equally important roles in the total currents under the short fetch condition the ratios of stokes drift to wind speed under these two fetch conditions are shown to agree well with results derived from the empirical wave growth function these results suggest that fetch and therefore sea state significantly influences the total response of surface current to wind in both the magnitude and direction by variations in the significance of stokes drift furthermore this work provides observational evidence that surface currents measured by hf radar include stokes drift it demonstrates the potential of hf radar in addressing the issue of momentum transfer from air to sea under various environmental conditions

accuracy of bathymetry and current retrievals from airborne optical time series imaging of shoaling waves a sequence of visual images of shoaling ocean waves collected from an aircraft can be used to retrieve maps of water depth and or currents the data are mapped to rectilinear coordinates on the mean ocean surface and three dimensional 3d cubes of these data typically 2 min dwell and 256 m spl times 256 m square area are fourier transformed to provide the 3d frequency wavenumber spectrum of the modulations the spectrum is fitted by the theoretical two dimensional dispersion surface for linear gravity waves with the depth and current vector as free parameters and the errors determined by comparison with simultaneously collected in situ data very little attention has been focused on the limitations and inherent errors in this approach for retrieving these parameters in the past this paper examines the dependence of these errors upon various instrumental and collection geometry parameters in order to estimate the achievable retrieval accuracy and provide collection guidelines significant errors occur when the mapping is inaccurate and when the data cubes are too small either spatially or temporally data cubes of the size given above or larger are required to contain the relative errors within 5 to 10 of the ground truth values the spatial resolution and temporal sampling rate are less critical and pixel sizes of spl sim 3 4 m and framing rates of spl sim 0 5 hz for depth and 1 2 hz for currents are sufficient for providing adequate results with the data examined here finally we show that the covariance matrix from the functional fit provides a quantitative measure of the retrieval accuracy independent of in situ data

mapping near inertial variability in the se bay of biscay from hf radar data and two offshore moored buoys 1 hf radar surface current data together with data from two operational offshore oceanographic buoys located over the slope are used to map the variability associated with the near inertial waves during a target year 2009 in the se bay of biscay the results obtained show the complex 4d distribution of inertial oscillations in this area we find a very pronounced horizontal structure across the area with ranges of a factor 5 in near inertial kinetic energy this pattern presents also strong seasonal variability with a peak in ke closer to the shelf break in summer whereas winter maximum is weaker and located further to the north east the mooring data indicate more trapping near the surface in summer these patterns are discussed in relation to the known seasonal differences in atmospheric buoyancy forcing and the characteristics of the sub inertial surface velocity field

remote radar measurement of shelf currents off key largo florida u s a abstract the coastal shelf of the florida keys is characterized by shallow and highly variable topography where currents are influenced by tides wind and the very energetic offshore florida current system the transient dynamics of variable scale are not easily captured with traditional fixed mooring observation platforms consequently an experiment combining moorings acoustic doppler current profiler adcp transects and a shore based high frequency hf radar system oscr was conducted to monitor the spatial and temporal variability of the flow field over the shelf and off shore deeper waters both the wind and tide driven flows in the middle to inner shelf called hawk channel and two offshore eddy events of the florida current were sampled during the experiment qualitative agreement between oscr current meter moorings and adcp transects was good in the shelf however at the inshore edge of the florida current there were significant differences the adcp captured the strong horizontal shear at the shelf edge whereas oscr smoothed the front over two kilometres due to its measurement cell size these differences in the spatial sampling resolution in the high shear region prevented direct quantitative comparisons although there were significant local variations linked to topography a major portion of the subtidal flow in the shelf was related to wind within the shelf both cross shelf and along shelf flows were highly spatially correlated with r 0 82 and r 0 86 respectively two sub mesoscale eddies that had characteristics consistent with the spin off eddies described by lee 1975 were observed to advect at velocities ranging from 0 53 ms 1 to 0 80 ms 1 along the inshore edge of the florida current the two eddies had along shelf length scales from 19 km to 47 km and cross shelf scales of 15 km and 25 km each eddy consisted of a single cyclonic vortex rotating with tangential velocities of 0 30 ms 1 and 0 50 ms 1 respectively the eddies had a strong signature at the shelf break however they did not have any observable influence on flows over the inner shelf

insar imagery of surface currents wave fields and fronts the authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean absolute phase calibration is accomplished by noting the azimuthal displacement of range travelling targets demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open ocean in the absence of ground truth the high resolution of radar imagery permits observation of sharp velocity discontinuities e g the gulf stream boundary and the wave field the recent sir c x sar shuttle missions dramatically emphasize the experimental and observational aspects of space based radar the combination of absolute velocities high spatial resolution and wide area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope shelf water interactions with ocean currents in particular the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front the results of this paper are compared with available ground truth

synthetic aperture radar imaging of upper ocean circulation features and wind fronts a sharp transition from dark to brighter backscatter regions the corresponding profiles of backscatter modulation across these features are expressed by a peak of about 2 db in contrast to a steplike increase of 5 8 db this suggests that sar image expressions of upper ocean circulation features and wind fronts can be distinguished and classified on the basis of this classification we attempt to quantify the dominating marine geophysical variables this method for systematic interpretation of sar images should be further validated with the use of airborne or satellite data such as from the first

surface current patterns in front of the venice lagoon abstract surface patterns of the low frequency current in a 20 30 km region in front of the venice lagoon were analysed from a 13 month long hf radar data set surface circulation was related to prevalent wind regimes in the area and to the tidal flow through the lagoon inlet three different categories of wind forcing were defined bora ne wind sirocco se wind and finally the category containing all other wind directions and calms winds lower than 3 m sec mean flow and vorticity spatial distributions were discussed for different wind conditions the coastal area about 5 km wide is characterized by a flow field with maximum vorticity outside the coastal boundary layer the interior flow is part of the adriatic basin wide cyclonic circulation two counter rotating vortices of the dimension of about 4 5 km were evidenced in the average flow field in front of the lagoon inlet malamocco inlet in all situations except for the bora the vortex pair is probably associated with the tidal flow through the inlet the bora wind induces a strong southward coastal jet detached from the coast by about 5 km homogenizing the flow and eliminating the dipole the average coastal flow pattern in calm wind conditions was also analysed as a function of the strong inflow outflow currents in the inlet 0 7 m s 1 from the lagoon inlet in both cases the vorticity pattern is similar with the negative vorticity to the left of the inlet and positive to the right looking seaward

measurements of upper ocean surface current shear with high frequency radar estimates of the variation with depth of both the speed and direction of near surface ocean currents within the top meter have been made using time series data collected from a dual frequency high frequency radar located on the california coast south of monterey long term averages derived from this data set over 3 different years consistently revealed an expected reduction in the speed of the near surface current as a function of depth consistent with a logarithmic vertical current profile an unexpected and previously unmeasured clockwise rotation with increasing depth of the near surface current was also observed hypotheses are suggested for the apparent rotation

decomposing observations of high frequency radar derived surface currents by their forcing mechanisms locally wind driven surface currents 1 the wind impulse response function and transfer function for high frequency radar derived surface currents off southern san diego are calculated using several local wind observations the spatial map of the transfer function reflects the influence of the coast on wind current dynamics near the coast within 20 km from the shoreline the amplitudes of the transfer function at inertial and diurnal frequencies are reduced due to effects of coastline and bottom bathymetry meanwhile the amplitude of low frequency currents increases near the coast which is attributed to the local geostrophic balance between cross shore pressure gradients against the coast and currents locally wind driven surface currents are estimated from the data derived response function and their power spectrum shows a strong diurnal peak superposed on a red spectrum similar to the spectra of observed winds current magnitudes and veering angles to a quasi steady wind are typically 2 5 of the wind speed and vary 50 90 to the right of the wind respectively a wind skill map is introduced to present the fractional variance of surface currents explained by local winds as a verification tool for wind data quality and relevance moreover the transfer functions in summer and winter are presented to examine the seasonal variation in ocean surface current response to the wind associated with stratification change

wind relaxations and poleward flow events in a coastal upwelling system on the central california coast 1 when upwelling favorable winds weaken or relax in coastal upwelling systems prevailing equatorward currents often weaken and then reverse to form propagating poleward currents here the statistics of wind relaxations and poleward flow events along the south central california coast are derived using meteorological and oceanographic times series during 2000 2006 over the 7 year record 169 wind relaxations were observed about 1 every 2 weeks and 127 of these were followed by poleward flow events thermistor moorings and current profilers along the 15 m isobath at alegria point arguello point purisima and point sal recorded the poleward flows the poleward flows propagate northward at 10 30 km d 1 and appear as sequential temperature increases at the moorings wind relaxations occur throughout the year but are most frequent in september and least frequent in april when upwelling winds are strong and persistent poleward flows follow wind relaxations frequently during may through november and rarely in december and january sea level differences between santa monica and port san luis california decreased as winds relaxed consistent with forcing by alongshore pressure gradients temperature distributions at point arguello point purisima and point sal were skewed toward higher values because of the poleward flows the alongshore distance traveled by the poleward flows increased with duration of the wind relaxations and magnitudes of alongshore temperature and sea level differences prior to the relaxations

coastal ocean fronts and eddies imaged with ers 1 synthetic aperture radar ers 1 c band synthetic aperture radar sar data were collected during the norwegian continental shelf experiment norcsex both in november 1991 during the ers 1 commissioning phase and at different seasons in 1992 and 1993 characteristic sar image expressions are observed in relation to perturbation of the surface current short wave interaction across the norwegian coastal current front for winds less than 10 12 rn s 1 in situ measurements document the existence of alternating zones of convergence and divergence coexisting with a strong near surface current shear of nearly 4f where f is the coriolis parameter across a distance of a few kilometers under calm to moderate winds i e 4 7 rn s 1 characteristic expressions of upper ocean circulation features also include the manifestation of eddies through the presence of surface film which damps the bragg waves comparison of a near coincident national oceanic and atmospheric administration advanced very high resolution radiometer image and an ers 1 sar image supports the interpretation that surface current fronts are imaged by sar in combination with an sar image simulation model the relative quantitative importance of shear convergence and divergence along the front is examined although the model formulation is simple and the absolute magnitude of the perturbations is uncertain the study shows that the sar images can sometimes be used to interpret frontal dynamics including growth and decay of meanders