evaluation of various satellite sensors for waterline extraction in a coral reef environment majuro atoll marshall islands abstract the ability of five satellite sensor bands ikonos band 4 terra aster bands 3 and 4 and landsat etm bands 4 and 5 was examined to extract the waterline at coral reef coasts majuro atoll marshall islands using different wavelength regions near infrared nir and shortwave infrared swir and different spatial resolutions 4 15 and 30 m after performing georeferencing and normalization of the images density slicing was used to extract the waterline comparisons of extracted waterline positions with ground level data for eight transects and global positioning system gps tracks of the island shorelines showed that nir bands were superior to swir bands because of the characteristics of the coral reef coasts including a lack of foam and suspended sediments which can affect the nir wavelength region if present and the presence of remnant water on reef flats during low tide which can affect the swir wavelength region a linear relationship was found between the estimation errors of waterline positions and the spatial resolutions of the nir sensors analysis on estimation errors and image costs showed that terra aster band 3 was the most cost effective sensor for extracting waterlines with reasonable accuracy the results serve as general guidelines for using satellite derived data to estimate intertidal topography and detect and monitor shorelines in coral reef environments

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

mechanisms for cessation of magmatic resurfacing on venus 1 proposed mechanisms of resurfacing on venus about 0 3 1 gyr ago usually involve either some form of tectonic resurfacing in which venusian lithosphere is recycled mechanically or magmatic resurfacing where essentially immobile lithosphere is covered by lava the focus of this study is mechanisms of magmatic resurfacing in the stagnant lid regime of mantle convection parameterized convection models suggest that cessation of magmatic resurfacing can occur in several ways 1 the mantle temperature drops sufficiently such that mantle rising adiabatically does not cross the solidus 2 the molten layer migrates below the solid melt density inversion at 250 500 km so that no melt can escape 3 sublithospheric small scale convection stops and conductive thickening of the lid suppresses melting in each case inability of magma to penetrate thickened venusian lithosphere may play a role the timing of melt cessation and duration of late stage resurfacing depend on various factors such as mantle rheology degree of mantle depletion and depth of resurfacing the models indicate that the waning stages of resurfacing which result in surface age variations can last 0 1 1 gyr a general trend of recent lithospheric thickening is also predicted some cases exhibit a rapid transition from relatively thin to thick lid as suggested by previous authors

accurate and efficient determination of the shoreline in ers 1 sar images extraction of the shoreline in sar images is a difficult task to perform using simple image processing operations such as grey value thresholding due to the presence of speckle and because the signal returned from the sea surface may be similar to that from the land a semiautomatic method for detecting the shoreline accurately and efficiently in ers 1 sar images is presented this is aimed primarily at a particular application namely the construction of a digital elevation model of an intertidal zone using sar images and hydrodynamic model output but could be carried over to other applications a coarse fine resolution processing approach is employed in which sea regions are first detected as regions of low edge density in a low resolution image then image areas near the shoreline are subjected to more elaborate processing at high resolution using an active contour model over 90 of the shoreline detected by the automatic delineation process appear visually correct

interpolation of an intertidal digital elevation model from heighted shorelines a case study in the western wash abstract a method of interpolating a spatiotemporal digital elevation model dem of an intertidal zone from a set of heighted shorelines is described the heighted shorelines are obtained by the water line method using remote sensing and hydrodynamic modelling this enables an intertidal dem to be constructed over a large area relatively quickly as well as frequent subsequent updating of the dem to detect changes the dem may be used for studying sediment transport in the intertidal zone and for the development of improved hydrodynamic models the spatiotemporal interpolator is based on universal block kriging and its results are illustrated using data from the wrangle flats area of the western wash u k for a kriging block size of 50 m 50 m 1100 days an average block height estimation standard deviation of 14 3 cm is achieved using shorelines from 33 images collected over the period 1992 1994 this height accuracy is compared with those obtained from dems constructed from beach transect data and is shown to be an improvement over the transect dem accuracies away from transects the construction of dems of improved accuracy 13 cm from combined shoreline and beach transect data using cokriging is discussed ways in which the shoreline accuracies may be improved are also considered

geologic history of the mead impact basin venus the geologic history of the mead impact basin on venus a basin similar in size to chicxulub mexico may be a guide of what to expect from future exploration of chicxulub during the collapse phase of crater formation in the mead basin radar bright impact melt material was deposited as a topographically flat surface within a large central area burying the transient cavity rim and other underlying structures the central area is not flat now and has been modified by viscous relaxation and thermal cooling effects substantial parts of the ejecta deposits have been covered by postimpact volcanic flows that are not obvious without the topographic data previous global surveys of venusian impact craters using only image data may have underestimated the number of craters embayed by volcanism

floor subsidence and rebound of large venus craters the topography and geology of large craters on venus reveal no evidence for floor rebound or relaxation rather the floors have subsided depressions of the floor centers relative to the margins are evident in topography and floor faulting is interpreted as contractional failure of the likely processes responsible only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas assuming volcanism occurred within a few tens of millions of years after the impact thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere further the predicted stresses are generally consistent with observed floor fracturing we constrain the impact heat deposited in the lithosphere to be less than roughly 5 1023 j for diameters of 100 km the absence of perceptible floor subsidence at craters this size on the moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter but also on gravity and target density the unfractured melt sheets of three large young bright floored craters imply sufficient lithospheric rigidity to support the crater cavities an elastic flexural rebound model restricts the elastic plate thickness to at least 10 15 km for the three craters corresponding to maximum geotherms of 20 30 k km 1 structural evidence for early rebound in older dark floored craters may have been buried by subsequent volcanism

the effect of wind wave growth on sar based waterline maps in this paper we investigate an issue related to the use of synthetic aperture radar imagery to detect ocean and lake waterlines in a previous publication the waterlines indicated by land p band airsar imagery of a tidal flat in south korea were compared and found to be offset from one another by approximately 80 to 170 m the authors postulated that the difference was due to depth dependent dissipation or dispersion of surface waves and the subsequent modulation of the radar backscatter as described by the bragg model in this paper we present an alternative explanation based on the growth of wind waves as a function of distance i e fetch this new explanation is more consistent with the environmental conditions radar look geometry and surface wave theory while also explaining several finer scale features observed in the imagery that are not addressed in the original publication our results indicate that the detected waterline position should be a sensitive function of radar frequency only under a restricted set of conditions namely when the body of water in question is cutoff from incoming swell and the only surface waves present are locally generated by a land breeze while such conditions may occur relatively infrequently in the coastal ocean they appear to be common when imaging the windward shores of inland lakes as illustrated by additional airsar imagery the analysis also serves as a remote sensing based validation of the existing theory for wind wave growth in a wavenumber regime not previously studied in the field

a storm surge prediction model for the northern bay of bengal with application to the cyclone disaster in april 1991 abstract a numerical model for simulating and predicting tides and storm surges in regions that include areas of open sea combined with estuarine channels and intertidal banks is described the model makes use of modified depth averaged equations with a numerical scheme in which the solution of 1d equations for narrow channels 2d equations for the open sea and an approach to modeling inundation are combined within a unified framework this simplifies the construction of realistic models for complex coastal areas and deltas the model is applied to the northern shelf of the bay of bengal and the ganges delta including the coast of bangladesh devastated by a cyclone surge in april 1991 in which about 140 000 people died hindcast and forecast simulations of this event using forcing derived from a semianalytical cyclone model with data supplied by the joint typhoon warning center jtwc are described it is shown that the timing of cyclone landfall and its coincidence with high tide determine the area

cleopatra crater on venus venera 15 16 data and impact volcanic origin controversy cleopatra structure is a 100 km diameter feature having a morphology similar to that of double ring basins of the moon and mercury and dissimilar to that of volcanic calderas on mars earth and venus the 2 4 km depth of cleopatra is anomalously large compared to venusian and terrestrial impact craters of equivalent diameters an impartial summary of the situation is as follows if cleopatra is a volcanic caldera it is a strange caldera if cleopatra is an impact crater it is a strange crater

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

topographic modification of the florida current by little bahama and great bahama banks abstract the effect of local topography in modifying the structure and variability of the florida current is examined using shipboard acoustic doppler and pegasus acoustic current profiler data pegasus absolute velocity data were obtained during 16 cruises in the florida current at 27 n as part of the subtropical atlantic climate studies stacs program the ensemble average of all pegasus velocity data shows that the effect of the constriction imposed on the mean florida current by little bahama bank can be detected up to 30 km into the straits of florida a simple model is proposed to explain how this effect can produce the subsurface maximum of northward flow commonly observed in the eastern straits pegasus and acoustic doppler data obtained during the march 1984 stacs cruise are used to describe the temporal and spatial variability of the flow it is shown that intermittent southward flow can exist in a band 10 15 km wide off little bahama bank one such event was detected during this cruise the p

measurement of recent intertidal sediment transport in morecambe bay using the waterline method abstract an illustration of how sediment transport may be measured over a large dynamic intertidal area at reasonable cost and spatiotemporal resolution using the waterline method employing remote sensing and hydrodynamic modelling is presented areas and volumes of accretion and erosion were measured in the study area of morecambe bay in north west england over the period 1992 97 tidal asymmetry is considered to be the dominant agent of sediment movement in the bay with waves being of secondary importance waterlines i e land sea boundaries were extracted from 31 synthetic aperture radar images of the bay covering a substantial fraction of the tidal range heights were attached to the waterlines using water elevations from nested hydrodynamic tide surge models interpolation in space and time was carried out to produce a continuous spatiotemporal height map of the intertidal zone height maps with a spatial resolution of about 50 m and height accuracy of about 40 cm were constructed over an area of about 350 km 2 strong temporal decorrelation in the bay limited the height accuracy achievable differences between height maps for 1992 94 and 1995 97 allowed sediment volume changes to be extracted the intertidal region showed a loss of 16 1 4 5 10 6 m 3 over the period almost all the change being due to a significant decrease between mean sea level and the low water mark changes were also measured in three of the main river estuaries within the bay the leven kent and lune these included movement of the leven north east by about 2 km cutting into cartmel wharf accretion seaward of grange over sands and changes in the low water channels at the mouth of the lune these changes could be used to validate sediment transport models of the bay

bell regio venus integration of remote sensing data and terrestrial analogs for geologic analysis the geology and surface morphology of bell regio 18 42 n 32 58 e are investigated using a combination of magellan venera and analogous terrestrial data the properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models five major volcanic sources are identified from geologic mapping tepev mons nefertiti corona a large shield volcano east of tepev and two small edifices southeast of tepev the volcano api mons lies northeast of the main bell uplift the oldest volcanic units are associated with an extensive low shield volcano east of tepev mons and a small edifice southeast of tepev the annular flow apron of tepev mons formed next with volcanism at a second small edifice on the southeast flank of tepev mons producing the youngest flow units comparisons between magellan data terrrestrial radar images and field topography profiles suggest that only three units resemble terrestrial a a flows the remainder are consistent with smoother pahoehoe type surfaces this suggests that most of the flow units were erupted at relatively low volume effusion rates 100 m3 s over long periods of time or had very low viscosities one flow unit has a knobby texture which resembles large tumuli or ridge structures much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain there are no rift zones such as those which occur at beta regio atla regio or western eistla regio tepev mons is characterized by very steep slopes up to 40 along the east flank a relatively flat summit and two large 11 and 31 km calderas though covered in places by high dielectric material the tepev mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas these calderas are quite shallow on the order of a few hundred meters at most with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature magellan and venera data suggest that the eastern caldera is covered by a layer of unconsolidated material ejecta from miriam crater has produced a halo with a dielectric constant of 6 7 surrounding nefertiti corona and potanina crater ejecta is the likely source material for radar dark wind streaks south of tepev mons a low dielectric e 2 3 triangular region on the south flank of tepev mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind this region has undergone a wide variety of volcanic processes with a major shift in eruptive style from the older low relief eastern volcanic center to the steep slopes and large summit calderas characteristic of tepev mons and the two smaller edifices

internal crater modification on venus recognizing crater centered volcanism by changes in floor morphometry and floor brightness bright floored and dark floored craters on venus show systematic differences in their size distribution and apparent modification bright floored craters exhibit the following characteristics an interior radar brightness comparable to the youngest craters on venus a tendency toward smaller crater diameters and a broad range of crater elevations in contrast the dark floored craters are darker than pristine craters on venus are typically much larger and preferentially occur at lower elevations they also have larger floors than pristine craters of the same size and are similar in many respects to floor fractured craters on venus of the four proposed origins for dark crater floors these observations are most consistent with crater centered volcanism surface weathering or eolian deposition may contribute to floor darkening in some cases but neither of these mechanisms nor impact melts can independently explain the full range of observed modifications based on the measurements of this study volcanism has affected at least 6 13 of the impact craters on venus the estimated lava fill thicknesses are of the order of 100 500 m thus if these flows are representative of eruptions elsewhere in the venusian lowlands the implied average flux of plains forming volcanism on venus for the last 300 m y is 0 01 0 02 km3 yr

patterns of shear and turbulence across the florida current abstract measurements by the multi scale profiler msp at seven stations spanning the straits of florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment contrasting properties suggest five mixing regimes the largest and most vast is the interior regime where the background flow has an inverse richardson number ri 1 ranging up to 0 55 shear is dominated by fluctuations that are 1 4 times stronger than in the open ocean and turbulent diffusivities are similarly moderate at 1 4 10 5 m2 s 1 the high velocity core of the current near the surface at midchannel is associated with weak mixing to its west is a zone of high mean shear where strong stratification results in background ri 1 of only 0 4 fluctuations are weak and diffusivity is moderate intermittent shear features beneath the core have mean ri 1 1 and strong turbulence two regimes are related to channel topography adjacent to the steep eastern slope finescale shear is

distribution and mixing of intermediate water masses in the channel of sicily mediterranean sea 1 the channel of sicily which represents the only connection between the western and the eastern mediterranean sea can be described as a three layer system where fresher water of atlantic origin modified atlantic water flows eastward in the upper layer levantine intermediate water liw leaves the eastern basin at around 250 350 m and an outflow of transitional ionian deep water tidw is identified below the liw synoptic mesoscale plankton experiment data collected during four surveys in the channel of sicily show that liw and tidw are subject to major modifications when crossing the eastern sill a significant bernoulli aspiration is found there and the bottom water at the sill is composed of modified deep ionian waters coming from 800 m an analysis of the water mass composition proves that liw mixes essentially with tidw and that the tidw flow through the channel must be larger than the expected 0 5 sv the presence of a secondary circulation related to the bottom boundary layer is demonstrated to be the primary factor leading to the dilution and cooling of liw

the accuracy of sequential aerial photography and sar data for observing urban flood dynamics a case study of the uk summer 2007 floods in this paper we examine for the first time the potential of remote sensing to monitor flood dynamics in urban areas and constrain mathematical models of these processes this is achieved through the development of a unique data set consisting of a series of eight space borne synthetic aperture radar sar and aerial photographic images of flooding of the uk town of tewkesbury acquired over an eight day period in summer 2007 previous observations of urban flooding have used single image and wrack mark data and have therefore been unable to adequately chart the propagation and recession of flood waves through complex urban topography by using a combination of space borne radar and aerial imagery we are able to show that remotely sensed imagery particularly from the new terrasar x radar can reproduce dynamics adequately and support flood modelling in urban areas we illustrate that image data from different remote sensing platforms reveal sufficient information to distinguish between models with varying degrees of channel floodplain connectivity particularly toward the end of the recession phase of the event for this test case our results also show that high resolution sar imagery even when acquired from satellites can reveal important hydraulic characteristics difficult to simulate with current dynamic flood models hence it is established at least for this test case and event that sar imagery from as far as several hundred kilometers from the earth s surface can deliver important information about floodplain dynamics that can be used to identify and help build suitable models even in built up environments

evidence for rapid regional plains emplacement on venus from the population of volcanically embayed impact craters we have reexamined the population of craters on venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater presenting nonvolcanic processes which may produce similar crater morphologies we conclude that 29 craters have been embayed by lava flows from the exterior and an additional 27 craters are ambiguous cases 3 6 of the total crater population all of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges through geologic mapping of an area covering over 8 of the surface we examined the spatial relationships between wrinkle ridged plains and stratigraphically older plains units and estimate that a large portion of the wrinkle ridged plains are thinner than 500 m which is thin enough for the rims of the largest half of preexisting craters to be preserved there are not enough craters embayed by the wrinkle ridged plains to support a model in which the plains were erupted directly onto an ancient surface given the number of large craters embayed by thin deposits of wrinkle ridged plains we develop a simple statistical model to overestimate the number of craters on the preplains surface this model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years the short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle ridged plains

tidal constituents from remote sensing image sequences abstract we address the problem of obtaining tidal constituents from a series of shoreline positions derived from time sequential airborne imagery of the intertidal zone a multi sensor experiment bachmann et al 2012 conducted at the virginia coast reserve long term ecological research vcr lter site 1 supplied in situ and airborne data hyperspectral and infrared shoreline imagery of the vcr lter barrier islands shallow lagoons and mainland coastal areas as well as topographic lidar data were obtained from the same air platform over a nine day period and lidar and in situ topographic beach surveys were used to specify the beach bathymetry and derive a digital elevation model dem over the primary area focused on in this paper at wreck island va the dem and shoreline positions are used to obtain a time sequence of tidal heights these are found to be consistent with a time series from a gps equipped tide gage float anchored nearby a least squares fit of the five largest tidal constituents m 2 n 2 s 2 o 1 k 1 to this one week record indicates that the first three can be retrieved with only a 15 20 error as determined by the one year sea surface height time series from a nearby noaa sea surface height ssh station at wachapreague va the broader question of how many images are needed to obtain tidal constituents from an annual record is also addressed using ssh values from a noaa tide gage station as a surrogate for image derived ssh values we progressively decimate a yearlong tide record calculating its tidal constituents at each level of decimation we find that that only one image every ten days is needed to retrieve the five largest tidal constituents to within a 10 error

high frequency internal waves and thick bottom mixed layers observed by gliders in the gulf stream autonomous underwater gliders are conducting high resolution surveys within the gulf stream along the u s east coast glider surveys reveal two mechanisms by which energy is extracted from the gulf stream as it flows over the blake plateau a portion of the outer continental shelf between florida and north carolina where bottom depths are less than 1000 m internal waves with vertical velocities exceeding 0 1 m s 1 and frequencies just below the local buoyancy frequency are routinely found over the blake plateau particularly near the charleston bump a prominent topographic feature these waves are likely internal lee waves generated by the subinertial gulf stream flow over the irregular bathymetry of the outer continental shelf bottom mixed layers with o 100 m thickness are also frequently encountered these thick bottom mixed layers likely form in the lee of topography due to enhanced turbulence generated by o 1 m s 1 near bottom flows

postimpact modification by volcanic or tectonic processes as the rule not the exception for venusian craters 1 we present three investigations that use the venusian impact crater population to constrain the planet s resurfacing history we evaluate stereo derived topography for 91 venusian craters that have a diameter d greater than 15 km craters with radar bright floors have greater rim floor depths and rim heights than craters with radar dark floors for the bright floored craters rim floor depths are dbf 0 483 d0 165 and rim heights are rhbf 0 056 d0 483 trends for dark floored craters are ddf 0 424 d0 108 and rhdf 0 181 d 0 025 for a 60 km crater this represents differences of 290 m in rim floor depth and 240 m in rim height we interpret these results to indicate that dark floored craters have experienced postimpact volcanic embayment and filling we examine the population of craters with d 20 km that have radar dark halos surrounding their continuous ejecta 114 craters we find that a portion of the halo has been removed for almost all dark floored craters consistent with our interpretation that dark floored craters have been affected by postimpact volcanism finally we assessed geologic histories of 12 large impact structures with stereo coverage all but one of these structures has experienced postimpact volcanism or tectonic deformation often in multiple episodes in summary widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid and the mean surface age of venus is young 150 my

impact craters on regional plains on venus age relations with wrinkle ridges and implications for the geological evolution of venus 1 a global assessment of impact craters on venus 30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges in this study we found that 10 of these craters are deformed by wrinkle ridges pre wr 93 craters are superposed on wrinkle ridges post wr and 43 craters were found to be ambiguous cases we compared our findings with the results of a similar study by mcgill 2004 done for craters of all sizes but for 1 3 of the surface of venus we found that the percentage of pre wr craters found by mcgill 2004 among the unambiguous cases 15 is close to our percentage 10 but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements we then summarize published data on the crater densities for different plains on venus we assess monte carlo modeling of volcanic resurfacing which competed with the ongoing formation of impact craters on the basis of these data and analyses we conclude that the mean age of the regional plains is close to the mean global surface age of venus and that the potential time span of regional plains emplacement was relatively short we interpret our results as evidence that wrinkle ridging on venus was a distinct tectonic phase that occurred rather early in post regional plains time probably within the first 10 although the exact duration remains uncertain regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on venus predominantly occurred during a specific phase of history the geologic record of wrinkle ridging and post plains tectonic evolution of volcanic plains on the moon and mars show interesting similarities to those of venus

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

topographic constraints on impact crater morphology on venus from high resolution stereo synthetic aperture radar digital elevation models 1 high resolution digital elevation models dems derived from magellan left left stereo synthetic aperture radar data of venus for a set of impact craters ranging in rim diameter from 5 to 300 km exhibit depths broadly as expected from theory but with significant departures for both large and small craters in craters larger than 38 km diameter rim floor depth becomes independent of diameter with a mean of about 900 m most craters smaller than 18 km diameter are also found to be unexpectedly shallow we discuss the accuracy and principal sources of error in our dems and conclude that this shallowness is real this shallowness of small craters shows that midsize bolides 200 1000 m diameter fragment in venus atmosphere and disperse over an area 10 to 20 times the original bolide diameter we identify these craters as a new class of impact feature the compound crater which may correspond to the knobby base morphological class defined by others

effects of western boundary current dynamics on the internal wave field of the southeast florida shelf 1 shelf sea internal wave fields are typically highly variable and dominated by wind and tidal forces however this is not necessarily true for outer shelf regions or very narrow shelves where remote physical processes originating over the slope or deep ocean may exert a strong influence on the internal wave climate during the summers of 2003 and 2004 observational studies were conducted to examine the effects of a western boundary current the florida current tides and wind on the mean currents and internal wave field on the outer southeast florida shelf wavelet analyses of nonstationary shelf currents coupled with altimetry images of the straits of florida reveal the strong influence of florida current fluctuations on outer shelf flows local winds were found to be a small factor in flows on the outer shelf region and tidal forcing diurnal and semidiurnal accounted for less than a third of the total barotropic variance high frequency internal waves dominate the internal wave field and result in bottom intensified currents and strong shoreward mass transport we present evidence that suggests that the florida current plays as large a role in the determination of the high frequency internal wave field as tidal forces these observations and analyses show that it is necessary to include the forcing from the florida current meanders and instabilities in order to predict accurately the episodic nature of the internal wave field on the southeast florida shelf

between the tides modelling the elevation of australia s exposed intertidal zone at continental scale abstract the intertidal zone represents a critical transition between marine and terrestrial ecosystems supporting a complex mosaic of highly productive and biologically diverse habitats however our understanding of these important coastal environments is limited by a lack of spatially consistent topographic data which can be extremely challenging and costly to obtain at continental scale satellite remote sensing represents an important resource for monitoring extensive coastal zones previous approaches to modelling the elevation of the intertidal zone using earth observation eo data have been restricted to small study regions or have relied on manual image interpretation thus limiting their ability to be applied consistently over large geographic extents in this study we present an automated open source approach to generate satellite derived elevation data for over 15 387 km2 of intertidal terrain across the entire australian coastline our approach combines global tidal modelling with a 30 year time series archive of spatially and spectrally calibrated landsat satellite data managed within the digital earth australia dea platform the resulting national intertidal digital elevation model nidem dataset provides an unprecedented three dimensional representation of australia s vast exposed intertidal zone at 25 m spatial resolution we validate our model against lidar rtk gps and multibeam bathymetry datasets finding that modelled elevations are highly accurate across sandy beach 0 41 m rmse and tidal flat environments 0 39 m rmse model performance was least accurate 2 98 m rmse within rocky shores and reefs and other complex coastal environments with extreme and variable tidal regimes we discuss key challenges associated with modelling intertidal elevation including tidal model performance and biased observations from sun synchronous satellites and suggest future directions to improve the accuracy and utility of continental scale intertidal elevation modelling our model can be applied to tidally influenced coastal environments globally addressing a key gap between the availability of sub tidal bathymetry and terrestrial elevation data

hindcasting coastal sea levels in morecambe bay a method of variational data assimilation by means of an adjoint model is used to improve the boundary conditions for a high resolution depth averaged two dimensional non linear hydrodynamic model of morecambe bay the modelled region contains a large inter tidal area and has an unusually high tidal range the assimilation procedure is robust and effective in reducing model error this system performs considerably better than the previous simple method used for assimilating data into the same numerical model height errors as low as 10 cm root mean square can be achieved

evolution of large shield volcanoes on venus 1 we studied the geologic history topographic expression and gravity signature of 29 large venusian shield volcanoes with similar morphologies in magellan synthetic aperture radar imagery while they appear similar in imagery 16 have a domical topographic expression and 13 have a central depression typical dimensions for the central depression are 150 km wide and 500 m deep the central depressions are probably not calderas resulting from collapse of a shallow magma chamber but instead are the result of a corona like sagging of a previously domical volcano the depressions all have some later volcanic filling all but one of the central depression volcanoes have been postdated by geologic features unrelated to the volcano while most of the domical volcanoes are at the top of the stratigraphic column analysis of the gravity signatures in the spatial and spectral domains shows a strong correlation between the absence of postdating features and the presence of dynamic support by an underlying plume we infer that the formation of the central depressions occurred as a result of cessation of dynamic support however there are some domical volcanoes whose geologic histories and gravity signatures also indicate that they are extinct so sagging of the central region apparently does not always occur when dynamic support is removed we suggest that the thickness of the elastic lithosphere may be a factor in determining whether a central depression forms when dynamic support is removed but the gravity data are of insufficient resolution to test this hypothesis with admittance methods

a sensitivity analysis of the waterline method of constructing a digital elevation model for intertidal areas in ers sar scene of eastern england abstract a sensitivity analysis of the waterline method of constructing a digital elevation model dem of an intertidal zone using remote sensing and hydrodynamic modelling is described variation in vertical height accuracy as a function of beach slope is investigated using a set of nine ers synthetic aperture radar sar images of the humber wash area on the english east coast acquired between 1992 and 1994 waterlines from these images are heighted using a hydrodynamic tide surge model and interpolated using block kriging on 1 500 slope beaches an average block height estimation standard deviation of 18 22 cm is achieved this rises to 27 cm on 1 100 slope beaches and 32 cm on 1 30 slope beaches the average heighting error at different slopes is decomposed into components due to waterline heighting error inadequate sensor resolution and interpolation inaccuracy it is shown that at 1 500 slope waterline heighting error and interpolation inaccuracy are the main error sources whilst at 1 30 slope errors due to inadequate sensor resolution become dominant the ability of the technique to generate intertidal dems for almost the entire coastal zone in a complete ers sar scene covering 100 100 km is demonstrated

construction of an inter tidal digital elevation model by the water line method progress in the development of a method for constructing a digital elevation model dem of an inter tidal zone using remote sensing and hydrodynamic modelling is described the method allows the construction of an inter tidal dem over large areas relatively quickly as well as frequent subsequent monitoring of the dem to detect changes and is particularly suited to sar satellite data because of the all weather day night capability of sar the resulting dem may be used for the development of improved hydrodynamic models and for studying sediment mass transport in the inter tidal zone

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

impact craters on geologic units of northern venus implications for the duration of the transition from tessera to regional plains using magellan sar images and the schaber et al 1998 crater data base we examined impact craters in the area north of 35 n and determined the geologic units on which they are superposed the crater density of the regional plains with wrinkle ridges pwr was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet t about 80 to 97 of the craters superposed on a composite unit that includes materials of tessera terrain tt densely fractured plains pdf fractured and ridged plains pfr and fracture belts fb also postdate the regional plains thus the time interval between the formation of these older units and emplacement of the regional plains t should be geologically short from a few percent to about 20 of t or approximately 40 to 150 m y this means that in the area under study volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of venus about the last 750 m y was much more active than in the subsequent time

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

resurfacing styles and rates on venus assessment of 18 venusian quadrangles abstract we have quantitatively assessed the resurfacing sources and styles in eighteen mapped venusian quadrangles about 30 of the venusian surface each quadrangle was split into 0 5 by 0 5 boxes which were then identified as corona materials large volcano materials 100 km diameter intermediate volcano materials 10 100 km small edifice materials 2 large edifices resurface areas of 10 4 10 5 km 2 coronae have greatly varying amounts of associated volcanism with some coronae producing negligible flow deposits and others producing deposits of 10 4 10 6 km 2 the areas identified as plains with no visible source occur on small scales 10 2 km 2 to large scales 10 5 km 2 our results indicate that the majority of plains resurfacing by volcanism can be tied to an identifiable source that fields of small edifices contribute more to resurfacing than we had anticipated and that resurfacing styles do not appear to have evolved over the time period represented by the surface geology in the mapped quadrangles all of the units that we quantified occur throughout the histories of the regions mapped we favor plains resurfacing to have occurred over at least 100 myr which implies terrestrially reasonable resurfacing rates

topographic mapping on large scale tidal flats with an iterative approach on the waterline method abstract tidal flats which are both a natural ecosystem and a type of landscape are of significant importance to ecosystem function and land resource potential morphologic monitoring of tidal flats has become increasingly important with respect to achieving sustainable development targets remote sensing is an established technique for the measurement of topography over tidal flats of the available methods the waterline method is particularly effective for constructing a digital elevation model dem of intertidal areas however application of the waterline method is more limited in large scale shifting tidal flats areas where the tides are not synchronized and the waterline is not a quasi contour line for this study a topographical map of the intertidal regions within the radial sand ridges rsr along the jiangsu coast china was generated using an iterative approach on the waterline method a series of 21 multi temporal satellite images 18 hj 1a b ccd and three landsat tm oli of the rsr area collected at different water levels within a five month period 31 december 2013 28 may 2014 was used to extract waterlines based on feature extraction techniques and artificial further modification these remotely sensed waterlines were combined with the corresponding water levels from the model waterlines simulated by a hydrodynamic model with an initial generalized dem of exposed tidal flats based on the 21 heighted remotely sensed waterlines a dem was constructed using the anudem interpolation method using this new dem as the input data it was re entered into the hydrodynamic model and a new round of water level assignment of waterlines was performed a third and final output dem was generated covering an area of approximately 1900 km2 of tidal flats in the rsr the water level simulation accuracy of the hydrodynamic model was within 0 15 m based on five real time tide stations and the height accuracy root mean square error of the final dem was 0 182 m based on six transects of measured data this study aimed at construction of an accurate dem for a large scale high variable zone within a short timespan based on an iterative way of the waterline method

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

a benthic front in the straits of florida and its relationship to the structure of the florida current abstract observations from ctd tow yos and microstructure profiles indicate the presence of a benthic front near 450 m depth on the western side of the straits of florida at 27 n the front is at midslope and approximately beneath the axis of the florida current there is a 0 2 kg m 3 jump in density across the front and the bottom boundary layer changes thickness from 10 20 m upslope to 50 70 m downslope of the front axial currents exhibit very strong vertical shear and cyclonic vertical vorticity upslope of the front in contrast to the significantly weaker vertical shears and anticyclonic vertical vorticity observed downslope all these features with the exception of the anticyclonic vorticity downslope of the front are consistent with a model of barotropically forced flow through a stratified channel with a sloping sidewall an arrested boundary layer regime is predicted to develop on the sloping sidewall upslope of a height hp fl n where f is the coriolis parameter l is the width of the flat b

florida current salinity and salinity transport mean and decadal changes the florida current fc contributes to atlantic circulation by carrying the western boundary flow of the subtropical gyre and the upper branch of meridional overturning circulation repeated fc hydrographic velocity salinity and temperature sections during 1982 1987 and 2001 2015 characterize its water mass structure and associated transport variability on average fc volume transport comes from subtropical north atlantic water naw 44 antarctic intermediate water aaiw 14 surface water sw 27 and an indistinct source rem 15 while salinity transport relative to the average salinity along 26 n comes from naw 55 aaiw 0 2 sw 30 and rem 15 from 1982 1987 to 2001 2015 naw aaiw and rem salinified by 0 03 0 16 g kg 1 and increased the salinity anomaly transport by 3 these patterns imply that advective salt transport by the fc 1 is sensitive to subtropical north atlantic variability and 2 is partially decoupled from the volumetric pathway of the upper overturning branch

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

the shape and appearance of craters formed by oblique impact on the moon and venus we surveyed the impact crater populations of venus and the moon dry targets with and without an atmosphere to characterize how the 3 dimensional shape of a crater and the appearance of the ejecta blanket varies with impact angle an empirical estimate of the impact angle below which particular phenomena occur was inferred from the cumulative percentage of impact craters exhibiting different traits the results of the surveys were mostly consistent with predictions from experimental work assuming a sin2 dependence for the cumulative fraction of craters forming below angle on the moon the following transitions occur 45 degrees the ejecta blanket becomes asymmetric 25 degrees a forbidden zone develops in the uprange portion of the ejecta blanket and the crater rim is depressed in that direction 15 degrees the rim becomes saddle shaped 10 degrees the rim becomes elongated in the direction of impact and the ejecta forms a butterfly pattern on venus the atmosphere causes asymmetries in the ejecta blanket to occur at higher impact angles the transitions on venus are 55 degrees the ejecta becomes heavily concentrated downrange 40 degrees a notch in the ejecta that extends to the rim appears and as impact angle decreases the notch develops into a larger forbidden zone 10 degrees a fly wing pattern develops where material is ejected in the crossrange direction but gets swept downrange no relationship between location or shape of the central structure and impact angle was observed on either planet no uprange steepening and no variation in internal slope or crater depth could be associated with impact angle on the moon for both planets as the impact angle decreases from vertical first the uprange and then the downrange rim decreases in elevation while the remainder of the rim stays at a constant elevation for craters on venus 15 km in diameter a variety of crater shapes are observed because meteoroid fragment dispersal is a significant fraction of crater diameter the longer path length for oblique impacts causes a correlation of clustered impact effects with oblique impact effects one consequence of this correlation is a shallowing of the crater with decreasing impact angle for small craters

topographic mapping of the german tidal flats analyzing sar images with the waterline method the waterline method is used to derive the topography of the tidal flats along the german coast by evaluation of synthetic aperture radar sar images a series of about 70 european remote sensing satellites sar images of the german bight taken at different water levels within four years is analyzed to detect the borderline between tidal flats and adjacent water areas using a wavelet based edge detection algorithm after geocoding the waterlines are combined with the corresponding water levels to represent the topography on an irregularly spaced grid the water levels are taken from a numerical tide model and corrected with the measured gauge data interpolation of these data into a regular grid yields a topographic map of the intertidal zone while the general practicability of this method has been demonstrated in previous studies for smaller test areas this paper is the first attempt to generate maps of a large area on a yearly basis