roll convection within an arctic cold air outbreak interpretation of in situ aircraft measurements and spaceborne sar imagery by a three dimensional atmospheric model abstract atmospheric roll convection within an arctic cold air outbreak was observed over the greenland sea during the arktis 1993 experiment on 24 march 1993 by in situ aircraft measurements and synthetic aperture radar sar imagery from the first european remote sensing satellite ers 1 inside a boundary layer heated from below two kinds of rolls were observed one aligned parallel and the other perpendicular to the mean wind direction the wind parallel rolls occupied the entire boundary layer whereas the wind perpendicular rolls were confined to a region around the top of the boundary layer where a strong vertical shear in the downstream wind component was observed a three dimensional numerical model has been applied to simulate the observed convective pattern it is shown that the model does not reproduce the observed pattern when using a height constant geostrophic wind profile however when adjusting the vertical wind profile to the one measured from the aircraft the model reproduces buoya

near real time mapping of peak ground acceleration and peak ground velocity following a strong earthquake during a disastrous earthquake the early assessment and timely reporting of the peak ground acceleration pga and peak ground velocity pgv maps will be crucial in an effective emergency response operation in this study we first derive an empirical relationship between m l and m w the pga and pgv attenuation relationships are deduced with data from the taiwan strong motion instrumentation program tsmip and the taiwan rapid earthquake information release system treirs site corrections of the attenuation relationships for shallow and large earthquakes in taiwan region are also obtained peak values of earthquake strong ground motion can be well determined in taiwan as soon as the earthquake location is determined and magnitudes are calculated by the treirs this peak ground motion value information can be immediately turned into the calculated pga and pgv maps that can be issued within two minutes of the earthquake origin time during any disastrous earthquake these maps are found to be very useful for immediate seismic damage assessment and dispatching of emergency response missions manuscript received 1 december 2000

inferring marine atmospheric boundary layer properties from spectral characteristics of satellite borne sar imagery abstract for the commonly observed range of air sea temperature difference and surface wind speed the static stability of the atmospheric surface layer can have a significant effect on the mean surface stress and its turbulence scale horizontal variability while traditional satellite borne scatterometers have insufficient horizontal resolution to map this turbulence scale horizontal variability satellite borne synthetic aperture radars sar can this paper explores the potential for applying existing boundary layer similarity theories to these sar derived maps of turbulence scale horizontal variability in air sea stress two potential approaches for deriving boundary layer turbulence and stability statistics from sar backscatter imagery are considered the first approach employs monin obukhov similarity theory mixed layer similarity theory and a sar based estimate of the atmospheric boundary layer depth to relate the ratio of the mean and standard deviation of the sar derived wind speed field to the

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

the seasat a satellite scatterometer the geophysical evaluation of remotely sensed wind vectors over the ocean on the seasat a satellite a microwave scatterometer was used to determine the vector wind over the world s oceans the technique is based on the sensitivity of microwave radar backscatter to the centimeter length ocean waves created by the action of the surface wind this paper describes the algorithm used to convert the scatterometer s measurements of ocean normalized radar cross section to the neutral stability vector wind at 19 5 m height and the comparison of these winds with high quality surface observations the wind vector algorithm used an empirical model function to describe the dependence of the ocean 0 on the 19 5 m neutral stability wind vector two model functions developed from a limited base of aircraft and satellite measurements were evaluated by using an independent set of in situ surface wind observations from the joint air sea interaction experiment jasin although these model functions were found to have some weaknesses the results of these comparisons produced better results than the seasat specifications of wind speed accuracy of 2 m s and wind direction accuracy of 20 over the 0 16 m s range of winds observed during jasin an improved model function was later developed by tuning to these jasin data so that the remaining biases between the observed surface winds and the scatterometer derived winds were minimized results are presented for this model function compared against other surface wind observations from the gulf of alaska seasat experiment and the seasat storms hurricane experiment

frequency dependent site amplifications with f 0 01 hz evaluated from velocity and density models in central taiwan the frequency dependent site amplifications at 87 free field strong motion station sites in central taiwan are evaluated from the velocity and density structures constructed from borehole data measured at shallow depths and the veloc ity models inferred from earthquake data at great depths results based on the quarter wavelength approximation method proposed by boore and joyner 1997 show that the site amplifications increase with frequency and are larger in the western plain with thick holocene alluvium than in the western foothill with pleistocene and miocene formations considering wave attenuation site amplification first increases and then decreases with increasing frequency the turning frequency ft associated with the maximum amplification varies site by site

on the origin of atmospheric frontal lines off the east coast of taiwan observed on spaceborne synthetic aperture radar images abstract frontal lines having offshore distances typically between 40 and 80 km are often visible on synthetic aperture radar sar images acquired over the east coast of taiwan by the european remote sensing satellites 1 and 2 ers 1 and ers 2 and envisat in a previous paper the authors showed that they are of atmospheric and not of oceanic origin however in that paper they did not give a definite answer to the question of which physical mechanism causes them in this paper the authors present simulations carried out with the fifth generation pennsylvania state university national center for atmospheric research mesoscale model which shows that the frontal lines are associated with a quasi stationary low level convergence zone generated by the dynamic interaction of onshore airflow of the synoptic scale wind with the coastal mountain range of the island of taiwan reversed airflow collides with the onshore flowing air leading to an uplift of air which is often accompanied by the formation of bands

estimates of shear wave q and κ0 for unconsolidated and semiconsolidated sediments in eastern north america measured and calculated values of the effective quality factor qef and the site attenuation parameter 0 for unconsolidated and semiconsolidated sediments in eastern north america ena indicate that the latter is strongly dependent on sediment thickness estimates of 0 for national earthquake hazard reduction program nehrp bc site profiles sediment plus hard rock in the mississippi embayment and the atlantic coastal plain were found to increase from about 9 to 31 msec for sedi ment thicknesses ranging from 116 to 600 m stochastic simulations using the 175 m thick hypothetical nehrp bc site profile used to estimate ena ground motions in the nationalseismichazardmapsbytheu s geologicalsurvey usgs indicatethat 0 20 msec provides a smaller estimate of amplification that agrees more closely with the low strain short period site coefficients in the nehrp recommended provisions for seismic regulations for new buildings and other structures nehrpprovisions than the10msecvalueusedbytheusgs alinearregressionofthe 0 estimatescompiledin this study indicates that 0 20 msec corresponds to a relatively thick bc sediment thickness of 460 116 m these same stochastic simulations indicate that the rela tivelyshallowusgssiteprofileprovidesestimatesofamplificationthataresmallerthan thelow strainlong periodsitecoefficientsinthenehrpprovisions thedependenceof bothsiteattenuationandsiteamplificationonsedimentthicknesssuggeststhattheuseof a single reference site condition for hazard mapping might not be appropriate instead theseresultsimplythateitheraregionalsetofreferencesiteprofilesshouldbedeveloped or that a more uniform site condition such as hard rock should be used to define a more stable reference site condition in ena

atmospheric front over the east china sea studied by multisensor satellite and in situ data 1 a frontal feature visible on a synthetic aperture radar sar image acquired by the radarsat satellite over the east china sea on 19 november 2000 is analyzed in conjunction with data acquired by quikscat topex poseidon tropical rain measurement mission trmm defense meteorological satellite program dmsp and national oceanic and atmospheric administration noaa satellites and with data obtained from ship measurements although this frontal feature is located close to the kuroshio front it is demonstrated that it is not a sea surface manifestation of an oceanic front but rather of an atmospheric front extending over 800 km from an area of the pacific ocean northeast of taiwan to the southern coast of korea it is a cold front moving in the southeast direction with a speed of approximately 45 50 km hour and associated with a 40 km wide rainband trailing the front the radarsat image which has a resolution of 50 m reveals fine scale structures of the atmospheric front in particular small scale convective rain cells embedded in the front conclusion is drawn that accurate interpretation of frontal features in sar images requires use of additional meteorological and remote sensing data and information

the rhine outflow plume studied by the analysis of synthetic aperture radar data and numerical simulations the dynamics of the rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations the remote sensing data consist of 41 synthetic aperture radar sar images acquired by the first and second european remote sensing satellites ers 1 and ers 2 over the outflow region of the river rhine most of them show sea surface signatures of oceanic phenomena for example surface current and wind variations ship wakes and oil slicks in particular in 36 of these images pronounced frontal features are visible as narrow zones of mainly enhanced sometimes enhanced reduced radar backscatter that can be associated with the rhine surface front within the area enclosed by the frontal line large zones characterized by a lower radar backscatter than in the outer area are often visible the analysis of the ers sar images suggests that the form and the location of the frontal features are mainly linked to the semidiurnal tidal phase in the outflow region although their variability suggests also that they weakly depend on river discharge residual currents and neap spring tidal cycle in order to test this observational hypothesis the results obtained from the analysis of the ers sar images are compared with the results obtained from the numerical simulation of the hydrodynamics of the rhine outflow region carried out using a two layer frontal model which is based on the nonlinear hydrostatic shallow water equations on an f plane the model is forced by prescribing tidal and residual currents and river discharge at the open boundaries several simulations are performed by varying the values of these forcing parameters the numerical results corroborate the observational conjecture it is found that the form and the location of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the outflow region and that river discharge residual currents and neap spring tidal cycle contribute only secondarily to their determination inserting the simulated surface velocity field into a simple radar imaging model that relates the modulation of the backscattered radar power to the surface velocity convergence in radar look direction narrow elongated bands of enhanced radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated rhine plume area the consistency of the model results with the results obtained from the analysis of the sar images enables one to infer a mean spatial and temporal evolution of the rhine outflow plume over a semidiurnal tidal cycle from the analysis of spaceborne sar images acquired during different tidal cycles over the rhine outflow area and suggests the possibility of using numerical modeling in conjunction with the analysis of spaceborne measurements for monitoring the oceanic variability in the rhine outflow area

the relationship between wind vector and normalized radar cross section used to derive seasat a satellite scatterometer winds the dependence of ocean normalized radar cross section nrcs or 0 on the 19 5 m neutral stability wind vector for the seasat a satellite scatterometer sass is described for the sass wind vector algorithm this dependence is specified in the form nrcs u where is the radar incidence angle is the angle between wind direction and radar azimuth and u is the neutral stability wind speed in meters per second at a height of 19 5 m the development of models to express this relationship and to provide the basis for inversion of nrcs to sass winds is traced from its initial form based on aircraft scatterometer measurements through the seasat field validation experiments goasex storms and jasin to its final form comparison of the model nrcs versus surface wind speed with sass data from these experiments are made

measurements of sea spikes in microwave backscatter at moderate incidence previous authors have associated large events or sea spikes in the radar cross section of the sea surface with steep or breaking waves here we present a method based on an intensity threshold and the mean doppler frequency to identify and quantify sea spikes likely to be associated with large scale wave breaking this method is applied to microwave measurements of ku band made with a dual polarized doppler scatterometer at a moderate incidence angle 45 from a fixed platform in the north sea for vv and hh polarization the mean normalized radar cross section 0 the frequency of sea spike occurrence n and the sea spike contribution to the mean normalized radar cross section ss0 are correlated with direct measurements of the friction velocity u in the range 13 u 49 cm s 1 the friction velocity exponents for both n and ss0 indicate an approximately cubic dependence while the residual radar cross section res0 0 ss is approximately proportional to friction velocity for u greater than approximately 20 cm s 1 at high friction velocities u 40 50 cm s 1 these data show that the contribution of large sea spikes to the mean radar cross section for measurements between 25 and 45 relative to the wind is in the range of 5 10 for vv polarization and 10 20 for hh polarization

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

ers 1 scatterometer measurements i the relationship between radar cross section and buoy wind in two oceanic regions extensive european remote sensing satellite ers 1 spacecraft scatterometer measurements were collocated within spl plusmn 25 km of buoy measurements at midlatitudes and in the equatorial pacific during 1992 1994 two different directional functional forms for the geophysical model were fit to these measurements described by cross section maxima aligned and offset with respect to the mean wind direction the two models exhibit fits with similar residuals for each of three years in the equatorial region thus neither of these two model functions can convincingly be identified as more accurate than the other unless additional factors are considered nevertheless the offset model results in an average directional offset near 10 spl deg for each season during the three year period whereas no such offset was inferred at midlatitudes the cross section dependence on wind direction also exhibits another unexpected average property a larger downwind compared to upwind cross section a sub 10 spl ap 0 25 db in the equatorial pacific this characteristic could be caused by a concentration of the short bragg waves on the backside of the long wave crests both of these unexpected characteristics are likely related to a dominant systematic swell traveling nearly perpendicular to the prevailing easterly winds across the equatorial pacific ocean the possible cross section dependence on wind stress and wind speed is due to long waves when adjusting the buoy measurements to a common height are discussed

observations of bora events over the adriatic sea and black sea by spaceborne synthetic aperture radar abstract bora events over the adriatic sea and black sea are investigated by using synthetic aperture radar sar images acquired by the advanced sar asar on board the european satellite envisat it is shown that the sea surface roughness patterns associated with bora events which are captured by sar yield information on the finescale structure of the bora wind field that cannot be obtained by other spaceborne instruments in particular sar is capable of resolving 1 bora induced wind jets and wakes that are organized in bands normal to the coastline 2 atmospheric gravity waves and 3 boundaries between the bora wind fields and ambient wind fields quantitative information on the sea surface wind field is extracted from the envisat asar images by inferring the wind direction from wind induced streaks visible on sar images and by using the c band wind scatterometer model cmod ifr2 to convert normalized cross sections into wind speeds it is argued that spaceborne sar images acquired over the east c

atmospheric boundary layer rolls observed by the synthetic aperture radar aboard the ers 1 satellite two synthetic aperture radar sar images acquired by the european remote sensing satellite ers 1 over the jade weser estuary in the german bight of the north sea on january 2 and 20 1992 are analyzed the images show sea surface manifestations of atmospheric boundary layer rolls this is inferred from the orientation of the quasi periodic sea surface patterns which are aligned approximately with the wind direction from the ratio of the wavelength of the patterns to the height of the boundary layer and from the conditions encountered in the atmospheric boundary layer as measured quasi simultaneously by radiosondes the atmospheric boundary layer rolls were generated by a dynamic instability on january 2 and by a thermal instability on january 20 for the first time quantitative estimates of variations of the wind velocity at the sea surface associated with the atmospheric rolls are extracted from a spaceborne radar sar image it is shown that wind velocities derived from sar image intensity variations are in agreement with theoretical estimates

attenuation of seismic shear wave energy in switzerland summary modelling the attenuation of shear wave energy is an important component of seismic hazard analysis previous studies have shown how attenuation particularly in the uppermost layers of the crust is regionally dependent the impact of this is that the decay of energy radiating from an earthquake will vary from place to place to quantify the regional attenuation in switzerland we model the fourier spectral amplitude of small to moderate earthquakes recorded on the local seismic networks high frequency decay is parametrized by q and while apparent geometrical spreading models account for the frequency independent decay of energy we analyse ground motion encompassing the significant duration of shaking to provide models that are useful for the purpose of seismic hazard analysis two methods are used to estimate the whole path attenuation parameter t first a simultaneous fit of the source model and attenuation effects across the entire spectral bandwidth for earthquakes with m 2 and secondly a linear fit of an attenuation model to the high frequency part of the spectrum for earthquakes with m 3 5 the t parameter is found to vary with hypocentral distance consistent with a weakly attenuating crust and strongly attenuating uppermost layer 1 d tomographic inversions indicate a profile of increasing q with depth down to the moho frequency independent decay is parametrized using a three part model which allows for the inclusion of moho reflection phases in the spectrum in the range of 20 140 km in the swiss foreland and from 70 to 140 km in the swiss alps

tower based measurements of normalized radar cross section from lake ontario evidence of wind stress dependence we report here the dependence of the normalized radar cross section nrcs on incidence angle azimuth angle wind speed wind stress and atmospheric stratification for ku band microwave backscatter from a lake the measurements were made in autumn 1987 on lake ontario using a rotating microwave system mounted on a research tower operated by the canada centre for inland waters the results show that at intermediate incidence angles the nrcs on the lake generally increases faster with wind speed than it does on the ocean we attribute this to the larger atmospheric drag coefficients which exist on the lake compared with the ocean and we show that the results are more consistent with a dependence of the nrcs on wind stress than on wind speed near the surface we find a stratification dependence of the nrcs similar to that previously reported at c band and show that at 40 and 60 incidence angles this dependence can be removed by parameterizing the nrcs in terms of either the friction velocity or neutral wind speed at a 20 incidence angle the stratification dependence is not removed by this procedure

ocean wind fields retrieved from radar image sequences 1 the dependency of radar backscatter on the surface wind field is investigated using a nautical x band radar operating at grazing incidence and horizontal polarization in transmit and receive this resulted in development of an algorithm for wind retrieval from nautical radar image sequences the algorithm consists of two parts in the first part wind directions are extracted from wind induced streaks that are in line with the mean surface wind direction these streaks are visible in the temporal integrated radar images at scales between 100 and 500 m the orientation of the streaks is determined from the local gradients which are derived from the radar images smoothed and reduced to the appropriate scales in the second part wind speeds are derived from the backscatter of the temporal integrated radar image sequence and the radar retrieved wind direction the dependency of the radar backscatter on the local surface wind and geometry of the radar is parameterized by training a neural network the algorithm is applied to radar image sequences acquired by a nautical x band radar mounted aboard an offshore platform in the north sea the radar derived winds are validated by comparison to in situ wind data measured at the platform the comparison of wind directions resulted in a correlation of 0 99 with a standard deviation of 14 2 for wind speeds the correlation is 0 97 with a standard deviation of 0 85 m s 1 in contrast to traditional offshore wind sensors the retrieval of the wind field from the backscatter of the ocean surface makes the system independent of the sensors motion and installation height and reduces the effects due to platform induced blockage and turbulence effects

detection and classification of mesoscale atmospheric phenomena above sea in sar imagery abstract satellite radar observations above sea are dedicated to small scale and mesoscale phenomena in the ocean and the marine boundary layer what the synthetic aperture radar sar sensors see on the ocean surface is primarily the variation of the bragg resonant waves providing signatures of local wind changes and imprints of atmospheric phenomena and the areas of enhanced wave breaking associated with wave current interaction for oceanographic processes like currents fronts and eddies sar images contain information about the processes in the marine atmosphere boundary layer mabl in particular when variation of wind speed and direction is significant several publications are available in the literature modeling and explaining each phenomenon separately however only a few focus on the identification of each phenomenon in an automatic or semi automatic way the objective of the present paper is to detect and to classify five atmospheric phenomena in sar images low wind areas katabatic wind island wakes atmospheric gravity waves and atmospheric convective cells an object based environment was used to identify the phenomena and fuzzy logic used to classify them several features extracted for each phenomenon were used for feeding the classifier the proposed method used 16 sar images and accuracy was examined in another 16 sar images using 457 object samples the method proved to be successful with an overall accuracy of 80 and kappa index of 0 75 the class of low wind areas negatively affected the overall accuracy due to overlapping feature values with the rest of the categories

three dimensional simulations of seismic wave propagation in the taipei basin with realistic topography based upon the spectral element method we use the spectral element method to simulate strong ground motion throughout the taipei metropolitan area mesh generation for the taipei basin poses two main challenges 1 the basin is surrounded by steep mountains and 2 the city is located on top of a shallow low wave speed sedimentary basin to accommodate the steep and rapidly varying topography we introduce a thin high resolution mesh layer near the surface the mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave speed contrasts variations in moho thickness beneath northern taiwan are also incorporated in the mesh spectral element simulations show that ground motion in the taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains the amplification of ground motion is mainly controlled by basin depth and shallow shear wave speeds although surface topography also serves to amplify and prolong seismic shaking

simultaneous observations of ocean surface winds and waves by geosat radar altimeter and airborne synthetic aperture radar during the 1988 norwegian continental shelf experiment quasi simultaneous measurements of the ocean surface by the canada centre for remote sensing ccrs convair 580 cv 580 synthetic aperture radar sar were acquired on march 20 1988 along an ascending pass of the geosat radar altimeter as part of the prelaunch ers 1 norwegian continental shelf experiment norcsex 88 over a region where the sar look direction is parallel to the wind vector a relationship between the ocean scattering cross sections measured by the geosat altimeter and the sar is obtained in the regions where the wind direction is changing an estimate of the wind direction is derived from the differences measured between the altimeter and the sar scattering cross sections wavelength and wave directions derived from the sar wave spectra are in good agreement with the sea truth data obtained with the wave directional buoys and with the altimeter derived swell estimates significant wave height measured by the geosat altimeter is compared with the buoy measurements and is used to assess the validity of the significant wave height deduced from the sar

on a satellite scatterometer as an anemometer the seasat a satellite scatterometer sass wind algorithm was tested against surface wind fields three classes of surface wind fields were used point measurements an isotach field over an intensively measured triangle with 200 km sides and synoptic scale winds determined from pressure and temperature fields and a planetary boundary layer model these comparisons can verify optimal capabilities of sass at accuracies of 2 m s 1 and 20 sass ability to locate fronts accurately is also established complications in nonsteady highly variable fields indicate problems exist in unqualified application of the sass algorithm to all conditions

numerical modelling of ground motion in the taipei basin basin and source effects summary the taipei basin in northern taiwan is located in a high seismicity region and was affected by several earthquakes in the past ml 7 3 on 1909 april 15 ml 6 8 on 1986 november 15 the chi chi ml 7 3 earthquake on 1999 september 21 and ml 6 8 on 2002 march 31 the main characteristic of the taipei basin is its complex shape with a deep western and shallow eastern part the uppermost sungshan formation with its low shear wave velocities 90 200 m s 1 is also a distinct feature of the basin based on the large data base of earthquake records obtained from the taiwan strong motion instrumentation program network many studies on ground motion within the taipei basin exist however the influence of the various subsurface structures on the observed ground motions as well as the variability of ground motion with respect to earthquake location is not fully understood we apply a 3 d finite difference method to simulate wave propagation up to 1 hz for a small earthquake close to the basin in order to resolve these open questions by varying source and structural parameters we explore the variability of ground motion our study includes a subsurface model that is based on recent studies on the basin structure and on the crustal structure of taiwan from our simulations we find a good fit between simulated and observed waveforms and peak ground accelerations for the considered small earthquake near the basin we also explore the influence of fault plane orientation hypocentre location deep basin structure and soft soil surface layers of the sungshan formation by varying the subsurface structure and earthquake position our studies reveal that the basin structure produces an amplification factor of about 4 compared to hard rock conditions additionally the soft soil sungshan formation produce amplification of a factor of 2 this results in a maximum amplification of the basin structure of about 8 which is in good comparison with amplification values larger than 5 found from the analysis of observed earthquakes these values clearly exceed the amplification values of about 2 3 obtained when applying standard 1 d site effect analysis our simulations for different earthquake positions show that ground motion depends strongly on earthquake location and fault orientation therefore the application of average values of spectral amplification obtained from the analysis of recorded data from distant earthquakes with different azimuths and fault planes may significantly underestimate future ground motions of possible earthquakes on known faults close to the taipei basin the simulation of a small earthquake near the taipei basin presented in this study will help to set up adequate simulation parameters for a possible large earthquake close to the taipei basin such a simulation of a scenario earthquake close to the taipei basin would allow to significantly improve hazard assessment as no observations of strong earthquakes in the vicinity of the basin exist

attenuation of body waves in the central new madrid seismic zone abstract data from the portable array for numerical data acquisition panda deployment in the central part of the new madrid seismic zone have been used to study the attenuation of body waves the spectral decay technique is used to analyze acceleration amplitude spectra of both p and s waves in a frequency range of 5 to 25 hz in order to obtain the high frequency attenuation parameter a multiple weighted least squares method is applied to obtain the site attenuation factor 0 at each station and to get d dr which were interpreted in terms of a two layer q structure consisting of a sedimentary layer overlying the upper crust the 0 values for s waves are stable and show a positive correlation with sediment thickness the 0 values for p waves in the sediments show significant scatter which may be due to greater sensitivity of p wave attenuation to water content of the soils than that of s waves which gave more stable 0 values consistency checks on the assumptions made in obtaining the p and s values are made by studying the source spectrum of several events and the differential sp s from the spectral ratio of the converted sp to s phases the three sets of values are found to be internally consistent but our results are limited to deep sediment sites since the complete vertical distribution of q with sediment depth is not known

tower based measurements of the ocean c band radar backscattering cross section continuous measurements of the c band normalized radar cross section nrcs at vertical vertical polarization as a function of wind speed and direction were carried out from a research platform in the north sea during a period of 11 months in 1984 the nrcss have been correlated with other environmental data routinely collected at the platform like air water temperature difference water temperature and ocean wave slope the wind speed exponent of the c band nrcs at 45 incidence angle was determined as 1 50 valid in the wind speed range 2 19 m s our data set shows that the c band nrcs depends strongly on the air sea temperature difference for example when the air sea temperature difference decreases from 5 c to 5 c the nrcs increases by 6 db at a wind speed of 8 9 m s and an incidence angle of 45 for stable air sea interfaces and low wind speeds the nrcs increases with increasing root mean square wave slope however no systematic dependence on water temperature was observed

sar retrieved wind in polar regions comparison with in situ data and atmospheric model output european remote sensing ers satellites synthetic aperture radar sar wind retrievals using cmod ifr2 are for the first time retrieved in the marginal ice zone miz and in arctic coastal areas and compared with in situ observations from reseach vessels rvs and output from a high resolution atmospheric model the root mean squares rms of the comparisons were 1 6 m s sup 1 and 2 m s sup 1 respectively the spatial variation of the sar wind fields established a decrease in wind speed close to the ice edge for the late summer situations where the wind was along the ice edge with the ice to the left this decrease is believed to be due to changes in atmospheric stability possibly through development of an internal boundary layer caused by the cold ice cover and melt water lower wind speed near the ice edge is confirmed by the atmospheric model and the in situ observations furthermore good results are obtained from sar wind retrieval in leads when compared with model output during a cold air outbreak routine measurements in the miz are useful for estimating the wind stress and therefore sar may play an important role in this region finally the identification of a jet out from hinlopen strait in the svalbard region and low wind wakes along the coast in the sar retrieved wind field is confirmed by in situ observations as the rv moves through the region the jet is also confirmed by the atmospheric model which is able to reproduce the situation

katabatic wind fields in coastal areas studied by ers 1 synthetic aperture radar imagery and numerical modeling synthetic aperture radar sar images acquired over mediterranean coastal waters from the first european remote sensing satellite ers 1 showing sea surface manifestations of katabatic wind fields are presented in particular sea surface roughness patterns generated by katabatic winds blowing from 1800 in high mountains through a broad valley at the western coast of calabria southern italy into the gulf of gioia is investigated the ers 1 sar images show that their areal extent and shape vary strongly depending on the meteorological conditions the roughness pattern sometimes has the form of a mushroom an elongated tongue a broad blob or a narrow truncated band for one event september 8 1992 we have simulated the wind field at the sea surface by using a nonhydrostatic mesoscale atmospheric model and then compared it with the wind field derived from the ers 1 sar image by using the c band wind scatterometer model cmod4 the comparison shows that the atmospheric model reproduces quite well the mushroom like form of the wind field pattern while the wind speed is obtained somewhat lower than the one inferred from the sar image this study demonstrates that sar images acquired over coastal waters are well suited to study local wind fields in coastal areas and to test numerical models that describe local wind fields which extend from the coast onto the sea

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

a small scale oceanic eddy off the coast of west africa studied by multi sensor satellite and surface drifter data a small scale oceanic eddy which was generated in autumn 2011 at the headland of cap vert off the coast of senegal west africa and then propagated westward into the open north atlantic ocean is studied by multi sensor satellite and surface drifter data the eddy was generated after a sudden increase of the trade winds causing an enhanced southward flow and upwelling at the coast of senegal after this wind burst event an extremely nonlinear cyclonic eddy with a radius of about 10 to 20 km evolved downstream of cap vert with rossby number larger than one our analysis suggests that the eddy was generated by flow separation at the headland of cap vert the eddy was tracked on its way into the open north atlantic ocean from satellites over 31 days via its sea surface temperature and chlorophyll a chl signature and by a satellite tracked surface drifter the satellite images show that this small scale eddy transported nutrients from the upwelling region westward into the oligotrophic north atlantic thus giving rise to enhanced chl concentration there maximum chl concentration was encountered few days after vortex generation which is consistent with a delayed plankton growth following nutrient supply into the euphotic zone within the eddy furthermore the eddy was imaged by the synthetic aperture radar sar onboard the envisat satellite it is shown that the radar signatures of cold eddies result from damping of short surface waves by biogenic surface films which arise from surface active material secreted by the biota in the cold eddy as well as by the change of the stability of the air sea interface highlights processes during the onset of coastal upwelling off west africa multi sensor remote sensing and in situ observations highly nonlinear small scale eddy generation at cap vert nrcs reduction in parts of the eddy due to biogenic surface films

the high cut parameter kappa for the near surface geology in and around the taipei basin taiwan abstract the decay parameter at the high frequency kappa value introduced by anderson and hough 1984 is measured from 54 stations situated in and around the taipei basin northern taiwan different frequency windows for the s wave amplitude spectra are used to measure the r value for each individual record the preferred r values are determined by the least squared fitting with the best correlation coefficient the site specific 0 values which are computed by removing the anelastic effect of regional geological structure by grouping r into the crustal and subduction events according to the focal depth and site classification i e class b c d and e sites are calculated to be in the range of 0 034 0 066 s we correlate the 0 values with the averaged shear wave velocity of the topmost 30 m layer v s 30 and find the independence of 0 on v s 30 in addition 28 stations lie on sediment in the taipei basin the gradient of 0 versus the sedimentary thickness to the tertiary base in the basin is calculated and then used to estimate the effective quality factor q ef of the layer q ef is 75 3 for the whole sedimentary layers above the tertiary base given one standard deviation q ef are in the 41 9 376 7 range q ef can provide an accurate referred parameter for the future ground motion simulation applications

strong ground motion in the taipei basin from the 1999 chi chi taiwan earthquake the taipei basin located in northwest taiwan about 160 km from the epicenter of the chi chi earthquake is a shallow triangular shaped basin filled with low velocity fluvial deposits there is a strong velocity contrast across the basement interface of about 600 m sec at a depth of about 600 700 m in the deeper section of the basin suggesting that ground motion should be amplified at sites in the basin in this article the ground motion recordings are analyzed to determine the effect of the basin both in terms of amplifications expected from a 1d model of the sediments in the basin and in terms of the 3d structure of the basin residuals determined for peak acceleration from attenuation curves are more positive amplified in the basin average of 5 3 cm sec 2 compared to 24 2 cm sec 2 for those stations outside the basin and between 75 and 110 km from the surface projection of the faulted area a 40 increase in peak ground acceleration residuals for peak velocity are also significantly more positive at stations in the basin 31 8 cm sec compared to 20 0 cm sec out the correlation of peak motion with depth to basement while minor in peak acceleration is stronger in the peak velocities record sections of ground motion from stations in and around the taipei basin show that the largest long period arrival which is coherent across the region is strongest on the vertical component and has a period of about 10 12 sec this phase appears to be a rayleigh wave probably associated with rupture at the north end of the chelungpu fault records of strong motion from stations in and near the basin have an additional higher frequency signal nearest the deepest point in the basin the signal is characterized by frequencies of about 0 3 0 4 hz these frequencies are close to simple predictions using horizontal layers and the velocity structure of the basin polarizations of the s wave are mostly coherent across the array although there are significant differences along the northwest edge that may indicate large strains across that edge of the basin the length of each record after the main s wave are all longer at basin stations compared to those outside this increase in duration of ground shaking is probably caused by amplification of ground motion at basin stations although coda q 0 67 1 30 hz is slightly larger inside the basin compared to those at local stations outside the basin durations correlate with depth to basement these motions are in the range that can induce damage in buildings and may have contributed to the structural collapse of multistory buildings in the taipei basin

modification of surface winds near ocean fronts effects of gulf stream rings on scatterometer quikscat nscat wind observations 1 modifications to surface winds by currents and sea surface temperature sst gradients near frontal boundaries of gulf stream rings are analyzed using satellite sst and scatterometer nasa s quick scatterometer quikscat nasa scatterometer nscat wind observations a component of scatterometer wind approximately equal and opposite to the surface current vector is observed and attributed to the fact that scatterometers detect relative motion of water and air warm core ring wcr ssts act to destabilize the marine atmospheric boundary layer mabl increasing surface wind magnitude by 10 15 and decreasing veering angle by 5 15 relative to large scale mean winds cold core ring ccr ssts cause impacts of similar magnitude and opposite sense magnitudes and directions of modifications are accounted for by mabl dynamics of a nonlinear planetary boundary layer model forced by air sea temperature differences wind modifications occur within tens of kilometers of sst fronts implying a wind response timescale of order 1 hour by contrast uniformity of modified winds across the larger area within rings suggests the response time for the mabl to return to equilibrium downstream from a front exceeds 10 hours over wcrs strong divergence convergence occurs on the upwind downwind side curl is strongly negative positive to the right left side facing downwind opposite patterns are generally seen over ccrs divergence curl peaks where winds blow perpendicular parallel to sst fronts sst image analysis indicates enhanced cloudiness occurs with downwind convergence over wcrs wind stress curl due to ring modifications causes dipolar ekman pumping sufficient to influence ring translation and decay processes

frequency dependent sites amplifications evaluated from well logging data in central taiwan 1 based on the quarter wavelength approximation by boore and joyner 1997 the frequent dependent site amplifications at 87 free field strong motion stations in central taiwan from borehole data are evaluated results show that the amplifications increase with frequency and the amplification is larger at the sites in the western plain with holocene alluvium than at those in the western foothill with pleistocene and miocene formations

radar scattering behavior of estuarine outflow plumes we present results of dual polarized radar scattering measurements of the chesapeake bay outflow plume near unity polarization ratios ratios of horizontally polarized radar echoes over vertically polarized ones are observed in large incidence angle 60 spl deg to 80 spl deg radar echoes from the outflow plume and its frontal boundary normally referred to as a front under strong surface current convergence 0 008 0 02 s sup 1 suggesting the existence of steepened and breaking waves in the regions cumulative distribution functions of the horizontally polarized radar returns from the front show approximately 90 of the radar echoes are from steepened and breaking waves vertically polarized echoes do not show this effect these experimental results substantiate early modeling investigators speculation of featured scattering contributing to horizontally polarized radar signatures of oceanic fronts our results also suggest that horizontal radar polarization can be used to remotely sense additional hydrodynamic processes such as wave trapping blocking and breaking near oceanic fronts better than what is possible with only vertical polarization

ocean wave spectral distortion in airborne synthetic aperture radar imagery during the norwegian continental shelf experiment of 1988 c band radar images of ocean gravity waves off the norwegian coast were processed into one dimensional azimuth spectra these spectra were used to measure the azimuth spectral width cutoff on the basis of a least squares fit to a gaussian spectral shape the widths were calculated for a range of wave heights 2 5 m and wind speeds 2 18 m s during 3 days in march 1988 velocity smearing estimates were extracted independent of r v and incidence angle based on an imaging model and the measured azimuth cutoffs with values varying from 0 4 to 0 7 m s quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery we propose a first order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m this model is offered as a first estimate of when ocean wave swell will be detected by the c band sar on board the ers 1 spacecraft the model predicts that this swell will be imaged under light winds of the order of 2 4 m s higher wind speeds cause larger smearing which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track

understanding the physics of kappa κ insights from a downhole array at high frequencies the acceleration spectral amplitude decreases rapidly this has been modelled with the spectral decay factor its site component 0 is used widely today in ground motion prediction and simulation and numerous approaches have been proposed to compute it in this study we estimate for the euroseistest valley a geologically complex and seismically active region with a permanent strong motion array consisting of 14 surface and 6 downhole stations site conditions range from soft sediments to hard rock first we use the classical approach to separate local and regional attenuation and measure 0 second we take advantage of the existing knowledge of the geological profile and material properties to examine the correlation of 0 with different site characterization parameters 0 correlates well with vs30 as expected indicating a strong effect from the geological structure in the upper 30 m but it correlates equally well with the resonant frequency and depth to bedrock of the stations which indicates strong effects from the entire sedimentary column down to 400 m third we use our results to improve our physical understanding of 0 we propose a conceptual model of 0 with vs comprising two new notions on the one hand and contrary to existing correlations we observe that 0 stabilizes for high vs values this may indicate the existence of regional values for hard rock 0 if so we propose that borehole measurements almost never used up to now for 0 may be useful in determining these values on the other hand we find that material damping as expressed through travel times may not suffice to account for the total 0 measured at the surface we propose that apart from material damping additional site attenuation may be caused by scattering from small scale variability in the profile if this is so then geotechnical damping measurements may not suffice to infer the overall crustal attenuation under a site but starting with a regional value possibly from a borehole and adding damping we might define a lower bound for site specific 0 more precise estimates would necessitate seismological site instrumentation

growth of wind waves with fetch observed by the geosat altimeter in the japan sea under winter monsoon the wind speeds and the significant wave heights derived from the geosat altimeter are validated by comparing with the data obtained at japanese ocean data buoy stations a systematic error is found in the altimeter wind speeds at high winds an experimental equation is derived in order to correct this error for the present study by using the corrected wind speeds and the wave heights growth of wind waves with fetch in the japan sea under winter monsoon is investigated the altimeter derived nondimensional significant wave height and nondimensional fetch show good agreement with the empirical fetch graph formulas fetch variation of the wind speeds is also discussed in relation to the evolution of the turbulent boundary layer over wind waves by using the altimeter data