on estimating the surface wind stress over the sea abstractour study analyzes measurements primarily from two floating instrument platform flip field programs and from the air sea interaction tower asit site to examine the relationship between

measurements of the angle between the wind vector and wind stress vector in the surface layer over the north sea measurements of surface layer turbulence in addition to full meteorological and oceanography quantities over the north sea indicated that the stress vector on long time scales is often aligned with a direction slightly different from the mean wind flow when stratifications were near neutral the angle difference between the stress and wind exhibited a dependence on the heat and momentum fluxes in general the stress vector was observed to be to the left of the flow during stable stratifications while for unstable stratifications it was to the right this finding was consistent using two independent sets of wind stress data i e from marsen 1979 and the north sea platform winter exercise 1985

wind wave nonlinearity observed at the sea floor part ii wavenumbers and third order statistics abstract this is part 2 of a study of nonlinear effects on natural wind generated surface gravity waves in 13 m depth 30 km offshore of virginia at the sea floor in this depth free surface gravity waves are only weakly attenuated at sea and swell frequencies 0 05 0 30 hz but are very strongly attenuated at frequencies higher than about 0 35 hz hence above 0 35 hz relatively long wavelength forced waves excited by nonlinear interactions between directionally opposing free wind waves are exposed at the sea floor an array of pressure transducers at middepth was used to estimate the frequency directional spectrum of free primary sea and swell waves and the associated forced secondary pressure fluctuations were measured with an array on the sea floor in part 1 it was shown that forced wave energy levels at the sea floor increase sharply in response to directionally opposing wind waves in agreement with weakly nonlinear theory in part 2 wavelengths propagation directions and non gaussian

idealized modelling of landfalling cold fronts the effects of land surfaces sea surfaces and coastlines on the dynamics of cold fronts are examined in an idealised two dimensional confluent deformation model of frontogenesis when the deformation field is stationary increasing the surface roughness weakens the fronts while increasing the cross front ageostrophic wind and frontal updraughts the frontogenetic effect of deformation balances the frontolytic effect of turbulent mixing resulting in near steady state fronts in steady state the surface fronts are located near the point at which the cross front flow vanishes this equilibrium point moves towards the warm air as the surface roughness is increased because the cross front ageostophic wind also increases with increasing surface roughness surface sensible heating weakens and slows the fronts during the day whereas surface sensible cooling strengthens and accelerates them at night adding a coastline to the model results in very strong gravity current like fronts in the afternoon with a relative flow of cool air towards their leading edge above the boundary layer the synoptic fronts remain unaffected by the coastline as the turbulent mixing weakens in the late afternoon the coastal fronts surge inland advancing faster than the wind speed in the boundary layer when the deformation field is allowed to translate the fronts advance across the ocean towards land if the fronts reach the coastline between mid morning and late afternoon the daytime heating over the land opposes the onshore advection of cold air retarding the fronts at the coastline if the fronts reach the coastline in the evening or early morning they advance onshore relatively unimpeded hence the fronts arrive onshore preferentially in the late afternoon or evening copyright 2010 royal meteorological society

effects of stratification on surface frontogenesis warm and cold fronts abstract the role of the ambient stratification in semigeostrophic surface frontogenesis is examined model fronts forming in regions of large static stability 1 are weaker 2 are tilted more toward the horizontal and 3 propagate more slowly toward the warm air than fronts forming in regions of small static stability these results are discussed in light of the differences between warm and cold fronts

the horizontal momentum balance in the marine atmospheric boundary layer during code 2 abstract the horizontal momentum balance in the marine atmospheric boundary layer during the coastal ocean dynamics experiment code is analyzed using meteorological data from an array of surface moorings previous studies have indicated the presence of orographically generated mesoscale features that are induced by strong southward flow around point arena the present analysis demonstrates that during periods of strong southward flow the cross shore momentum equation is dominated by a balance between the ageostrophic acceleration associated with the flow curvature around point arena and the cross shore pressure gradient while the along shore momentum equation is dominated by a balance between vertical stress divergence and alongshore pressure gradient these balances are consistent with results from a shallow water model of the marine layer the calculations provide evidence for orographic modification of the horizontal structure of the boundary layer under a broader range of southward flow conditio

a review of cold fronts with prefrontal troughs and wind shifts the conceptual model of a classical surface based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface many cold fronts however do not conform to this model time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front hereafter called a prefrontal trough and prefrontal wind shift respectively although many authors have recognized these prefrontal features previously a review of the responsible mechanisms has not been performed to date this paper presents such a review ten disparate mechanisms with different frontal structures have been identified from the previous literature these mechanisms include those external to the front i e those not directly associated with the cold front itself synoptic scale forcing interaction with lee troughs drylines interaction with fronts in the mid and upper troposphere and frontogenesis associated with inhomogeneities in the prefrontal air mechanisms internal to the front i e those directly associated with the structure and dynamics of the front include the following surface friction frontogenesis acting on alongfront temperature gradients moist processes descent of air ascent of air at the front and generation of prefrontal bores gravity waves given the gaps in our knowledge of the structure evolution and dynamics of surface cold fronts this paper closes with an admonition for improving the links between theory observations and modeling to advance understanding and develop better conceptual models of cold fronts with the goal of improving both scientific understanding and operational forecasting

chemical analysis of ice vein microenvironments ii analysis of glacial samples from greenland and antarctica chemical constituents trapped within glacial ice provide a unique record of climate as well as repositories for biological material such as pollen grains fungal spores viruses bacteria and dissolved organic carbon past research suggests that the veins of polycrystalline ice may provide a liquid microenvironment for active microbial metabolism fueled by concentrated impurities in the veins despite these claims no direct measurements of impurity concentration in ice veins have been made using micro raman spectroscopy we show that sulfate and nitrate concentrations in the veins of glacial ice from greenland greenland ice sheet project 2 and antarctic newall glacier and a dominion range glacier core samples were 10 4 and 10 5 times greater than the concentrations measured in melted bulk core water methanesulfonate was not found in the veins consistent with its presence as particulate matter within the ice the measured vein concentration of molecular anions implies a highly acidic ph 3 vein environment with high ionic strength mm m we estimate that the vein volume provides 16 7 and 576 km 3 of habitable space within the greenland and antarctic ice sheets respectively which could support the metabolism of organisms that are capable of growing in cold high ionic strength solutions with low ph

airborne scatterometer detection of winds and sea surface roughness changes across the gulf stream front abstract this article investigates the scatterometer radar cross section distributions and wind stress spatial variability across a sea surface temperature sst front under different wind directions and synoptic scale atmospheric forcings shipboard meteorological measurements in concert with airborne k u band 14 0 ghz rotating scatterometer data show evidence of mesoscale circulation near the front during low ambient winds 4 m s blowing from the cold side of the sst front to the warm side the mesoscale signature is characterized by a 25 35 counterclockwise shift in the wind direction and a maximum of 4 9 db increase in the backscattering cross section across the front the mesoscale circulation is reduced when the wind direction reverses and the ambient wind flow increases to 13 6 m s the maximum cross section change across the front is reduced to 3 6 db the azimuthal characteristics of the scatterometer data are compared with the predictions of two scatterometer model functions according to the models the anisotropy of the short waves is inversely related to the wind speed such that the short wave directional spreads tend to broaden as the wind speed increases the scatterometer data however show greater short wave anisotropy at the higher wind speeds this measured difference could be caused by current velocity changes or by longwave directional changes across the front

surface stress and atmospheric boundary layer response to mesoscale sst structure in coupled simulations of the northern california current system abstractthe influence of mesoscale sea surface temperature sst variations on wind stress and boundary layer winds is examined from coupled ocean atmosphere numerical simulations and satellite obs

momentum transfer over the coastal zone spatial variations of surface stress over the coastal shoaling zone are studied offshore of duck north carolina by the longez research aircraft equipped to measure both atmospheric turbulence and oceanic waves we find that the spatial variation of the friction velocity with offshore distance is much larger with offshore flow than with onshore flow in general the mean square slope of the short waves wavelength shorter than 2 m decreases with offshore distance while the mean square slope of the long waves wavelength longer than 2 m increases with offshore distance with onshore flow the friction velocity is strongly correlated with surface waves in addition the variation of the neutral drag coefficient is well correlated with the atmospheric bulk richardson number with offshore flow the observed momentum flux significantly decreases with offshore distance within the first few kilometer offshore the relationship between the friction velocity and the mean square slope of the short waves and the relationship between the neutral drag coefficient and the atmospheric bulk richardson number are obscured by the direct influence of the upstream land surface on the measured turbulence these relationships for offshore flow agree well with those for onshore flow if the fetch is beyond the immediate influence of the land surface the results in this study suggests that the effects of the strong turbulence advected from over the nearby land surface in offshore flow may lead to ambiguous physical interpretation of the correlation between the momentum flux and the wave state

midlatitude wind stress sea surface temperature coupling in the vicinity of oceanic fronts the influences of strong gradients in sea surface temperature on near surface cross front winds are explored in a series of idealized numerical modeling experiments the atmospheric model is the naval research laboratory coupled ocean atmosphere mesoscale prediction system coamps model which is fully coupled to the regional ocean modeling system roms ocean model a series of idealized two dimensional model calculations is carried out in which the wind blows from the warm to cold side or the cold to warm side of an initially prescribed ocean front the evolution of the near surface winds boundary layer and thermal structure is described and the balances in the momentum equation are diagnosed the changes in surface winds across the front are consistent with previous models and observations showing a strong positive correlation with the sea surface temperature and boundary layer thickness the coupling arises mainly as a result of changes in the flux richardson number across the front and the strength of the coupling coefficient grows quadratically with the strength of the cross front geostrophic wind the acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced coriolis force in the vicinity of the front much of the loss gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration deceleration of the flow parallel to the front via the coriolis term this mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient and is active for flows off the equator with sufficiently strong winds although the main focus of this work is on the midlatitude strong wind regime calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross front momentum budget consistent with previous work

directional attributes of the ocean surface wind stress vector in a recent paper by geernaert 1988 it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux it was argued that since cold air advection over the open ocean generally corresponds to upward heat flux the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications conversely observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection or downward heat flux in that study only 30 of the statistical variance of the angle between the wind and wind stress vectors could be explained by air sea heat flux and it was suggested that the orbital velocity spectrum of surface waves i e data unavailable in that study could be important for explaining at least part of the remaining variance in this study we report on measurements of the wind stress vector and wave directional spectrum collected during the synthetic aperture radar x band ocean nonlinearities experiment conducted on the chesapeake light tower during 1988 the experiment yielded data reported herein that provide convincing evidence that the stress vector direction during light winds and near zero air sea heat flux is governed in large part by the direction of long waves on the ocean surface further the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction

effect of large eddies on atmospheric surface layer turbulence and the underlying wave field to improve existing models for air sea interaction a better understanding of the energy transfer across the boundary layer and in particular of the coupling of large atmospheric eddies with the air sea interface is needed recent investigations have already shown a possible coupling of large structures in atmospheric turbulence and surface ripples this was done using synthetic aperture radar sar imagery of ocean surface and almost simultaneous advanced very high resolution radiometer avhrr imagery of cloud streets at a cold air outbreak the intent of our study is to further validate this hypothesis in a general case of coastal circulation for this purpose we analyze a suite of collocated simultaneous records of airflow radar return and surface elevations from a coastal platform we investigate the influence of large eddies 20 60 min on the turbulent properties of the airflow in the first 2 m above the ocean surface the analysis shows very prominent peaks in the magnitude of 12 to 16 min fluctuations which are further modulated in 20 to 40 min intervals these scales and modulations are characteristic for all variables of interest here the detected scales and their modulation suggest significant interaction of surface layer within the first 1 2 m with large eddies of scales of o 1 and o 10 km the intermittent structure of turbulence responds by alternating contributions from bursts and sweeps the frequency of occurrence of bursts and sweeps also reveals the influence of large structures the instantaneous cross correlation between the shorter scales of momentum flux and radar return corresponding to the individual burst events can be 4 times as strong as the overall cross correlation

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

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

detailed description of deformation induced semi geostrophic frontogenesis examen dans la limite de la semi geostrophie d une variante du probleme classique de la frontogenese de surface induite par la deformation le systeme particulier d ecoulement considere constitue la reponse a un champ de deformation donne d une atmosphere barocline semi infinie stratifiee et a vorticite potentielle uniforme resultats obtention d une classe de solutions analytiques simples dans l espace geostrophique obtention d une solution explicite de l equation de sawyer eliassen relative a la circulation ageostrophique obtention d une equation autonome non lineaire dans l espace reel relative au developpement a la surface de la composante du vent alignee le long du front application aux divers aspects frontogenes a savoir l echelle temporelle du developpement du front la co occurrence des differents types frontaux la pente et le mouvement des figures frontales la structure et la force de la circulation ageostrophique et le degre de validite de l approximation semi geostrophique

mesoscale simulation of supercritical subcritical and transcritical flow along coastal topography abstract a mesoscale atmospheric model is used to address the characteristics of stratified flow bounded by a side wall along a varying coastline initial froude number values are varied through alteration of marine inversion strength permitting examination of supercritical subcritical and transcritical flow regimes encountering several coastal configurations consistent with shallow water models sharp drops in boundary layer depth and flow acceleration occur in flow rounding convex bends however significant flow response occurs in the stratified layer aloft which is unexplained by conventional shallow water theory the strongest flow acceleration occurs in the transcritical case while regardless of inversion strength the deformation of the isentropes aloft shows general structural similarity advection of horizontal momentum is an important component of the horizontal force balance a simulation having several coastline bends exhibits a detached oblique hydraulic jump upwind of a concave bend t

variation of the drag coefficient and its dependence on sea state using a gill propeller vane anemometer and resistance wave wires over a water column depth of 15 m simultaneous measurements of the momentum flux and sea surface wave spectra were acquired from the pisa mast 28 km offshore in the german bight during autumn and winter 1979 these data were analyzed to identify the relationship between wind stress and surface waves it was found that wind stresses for wind speeds above 15 m s were regularly higher than open ocean wind stresses as reported by smith 1980 and by large and pond 1981 for the same mean wind speed these results when described in terms of the drag coefficient compared closely with the results of sheppard et al 1972 who collected surface layer statistics over lough neagh northern ireland after modeling the surface waves of the north sea as a function of wave saturation or wave age it became evident that variations in the magnitude of the drag coefficient could be explained by coincident variations in the surface wave energy spectrum by applying the wave dependent roughness length model described by kitaigorodskii 1973 the north sea drag coefficient was predicted to be larger than drag coefficients reported from the open sea

evidence of kelvin waves in california s marine layer and related eddy generation abstract evidence suggests that an internal solitary kelvin wave exists in the marine layer along california the marine layer is lifted over the central coast by a weak cyclonic circulation this bump initially 850 m high moves to the north along the coast at 6 m s 1 the undisturbed layer depth is 100 200 m thick the crest height of the wave decreases to 500 m farther north winds under the raised marine layer are southerly the leading edge of the wave is easily followed by satellite as the thickened marine layer is marked by overcast stratus a greatly curved offshore leading edge indicates that nonlinear effects are important offshore scale in the overcast is about 300 km in the south and 50 km in the north surface pressure gradient alongshore is closely related to the marine layer depth the surface wind shifts when the leading and trailing edge of the wave passes northerly wave progression ceases at the sharp bend formed by cape mendocino at this time a vortex is formed in the marine laye

structure of offshore flow abstract the horizontal and vertical structure of the mean flow and turbulent fluxes are examined using aircraft observations taken near a barrier island on the east coast of the united states during offshore flow periods the spatial structure is strongly influenced by the surface roughness and surface temperature discontinuities at the coast with offshore flow of warm air over cool water the sea surface momentum flux is large near the coast and decreases rapidly with increasing offshore distance or travel time the decrease is attributed to advection and decay of turbulence from land the rate of decrease is dependent on the characteristic timescale of the eddies in the upstream land based boundary layer that are advected over the ocean as a consequence the air sea momentum exchange near the coast is influenced by upstream conditions and similarity theory is not adequate to predict the flux the vertical structure reveals an elevated layer of downward momentum flux and turbulence energy maxima over

evolution of stable internal boundary layers over a cold sea the situation studied in this paper relates to air flow from a heated land surface out over a water surface with much lower temperature observations from the baltic sea where this kind of situati

an analysis of frontogenesis in numerical simulations of baroclinic waves abstract using a primitive equation pe model we revisit two canonical flows that were previously studied using a semigeostrophic equation sg model in a previous paper the authors showed that the pe and the sg models can have significantly different versions of the large scale dynamics here they report on the implications of this difference for frontogenesis the program for the study of frontogenesis developed by b j hoskins and collaborators is followed to show how in the pe version of the canonical cases the surface warm front develops before the cold front and why the upper level front is a long nearly continuous feature going from ridge to trough the frontogenesis experienced by an air parcel is computed following the parcel to illustrate better the mechanisms involved as the present calculations are carried out longer than most previous ones the relation of the upper frontogenesis to the formation of the upper level cutoff cyclone is also examined trajectory and three dimensional g

fastex iop 18 a very deep tropopause fold ii quasi geostrophic omega diagnoses a set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold 820 hpa during the intensive observing period 18 of the fronts and atlantic storm track experiment fastex in this second part diagnoses of vertical motion using a q vector partitioning in the natural coordinate system that follows the geostrophic wind are analysed the partitioning allows the evaluation of vertical motions associated with forcing mechanisms such as confluence and diffluence thermal advection by the horizontal geostrophic shear shear advection and curvature of the flow the synoptic situation involves the formation of an intense upper level jet streak when an arctic trough and a southern ridge move into phase with each other results assessed in the course of the tropopause fold life cycle show that subsiding vertical motions associated with each of the forcing mechanisms confluence shear advection and curvature overlap on the cyclonic shear side of the entrance region of the jet streak it is shown that an additional effect of shear advection over the confluence a necessary ingredient for the development of deep tropopause folds in two dimensional contexts takes place in the present case study however the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper level frontal zone i e with a maximum frontogenetic effect are in order of importance the shear advection and the curvature

observational study of marine atmospheric boundary layer characteristics during swell abstract by combining simultaneous data from an instrumented air sea interaction spar asis buoy and a 30 m tower profiles of wind and turbulence characteristics have been obtained at several heights from about 1 to 30 m above the water surface during swell conditions five cases formed as averages over time periods ranging from 2 5 to 9 5 h representing quasi steady conditions have been selected they represent a range of typical wave age and include wind following swell cases and cross swell cases for relatively large wave age the wind profile exhibits a well defined maximum in the height range 5 10 m for more modest wave age this maximum turns into a sharp knee in the wind profile below the maximum or knee the wind increases rapidly with height above that point the wind is very nearly constant up to the highest measuring level on the tower 30 m analysis of balloon data from one day with swell indicates that the layer with constant wind in fact extends to the top of the boundary layer

the australian summertime cool change part ii mesoscale aspects abstract observations of four cold frontal systems traversing the coastal region of southeast australia in late spring and early summer are described in terms of process occurring on the mesoscale a conceptual model is presented which summarizes the main results of the data analysis features found in common with other studies of cold fronts include i the multiple line nature of the frontal transition zone ftz ii concentration of cyclonic relative vorticity at a height z 1 to 1 5 km in the rear of the ftz and iii the existence of a prefrontal jet at z 1 5 km northerly in our case southerly in the northern hemisphere the change lines within the ftz and at the leading edge if there is no sea breeze are most probably convective instability lines whose alignment and movement depend on the large scale cloud layer winds the lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar at least one of these develops into a vigorous squall line whose cold outflo

dmsp removal and dmso production in sedimenting particulate matter in the northern north sea work was conducted to establish if the downward flux of sedimenting material represents a sink for particulate dimethylsulphoniopropionate dmsp and to investigate the fate of this dmsp within the sedimenting material four sediment traps were successfully deployed for a maximum of 24 h in the northern north sea between the 5th june and the 1st july 1999 after recovery samples were taken immediately and analysed for levels of dimethylsulphide dms dimethylsulphoxide dmso and dmsp any remaining trap material was incubated in the dark at in situ seawater temperature for a maximum of 120 h results from the sediment traps showed that only 1 of the dmsp standing stock was lost daily due to sedimentation however results from the incubation experiment demonstrated not only that the downward flux of dmsp is underestimated due to its degradation within the trap material with between 30 and 47 of the dmsp being lost over 48 h but also that up to 21 of the dmsp lost may be converted to dmso these results clearly demonstrate that dmso is generated within sedimenting material

the palette of fronts and cyclones within a baroclinic wave development abstract a study is undertaken of the concurrent development of fronts and cyclones within a growing baroclinic wave the dynamical framework for the study is that of the semigeostrophic adiabatic flow of a uniform potential vorticity atmosphere that is sandwiched between two horizontal plates on an f plane a three dimensional spectral model is used to examine the growth to finite amplitude of the normal modes of this system for basic states that comprise of a symmetric jetlike baroclinic flow with and without a superposition of a uniform barotropic shear of either cyclonic or anticyclonic vorticity it is demonstrated that the presence of such lateral shear in the ambient environment exerts a profound influence upon the ensuing palette of fronts and cyclones on the synoptic scale this flow component influences the scale and strength of the individual surface highs and lows and also modifies the extent of their lateral drift away from and their relative movement along the baroclinic zone aloft t

observations of summer turbulent surface fluxes in a high arctic fjord the summer atmospheric boundary layer over a fjord in the high arctic has been investigated during three consecutive years 2008 2010 measurements of turbulent surface layer fluxes of momentum and sensible heat using a sonic anemometer and slow response instruments were taken from a tower on the coast of isfjorden svalbard and analysed for seasonal variability and differing fetch conditions the results resembled the atmospheric boundary layer characteristics previously found for ice free winter conditions the momentum flux was usually directed downwards but for low wind speeds there was a possible contribution of swell giving an upward directed momentum flux the cross wind component of the momentum flux sometimes contributed significantly to the total flux the sensible heat flux was very dependent on the origin of air whether it had a long over water fetch or originated from land areas with or without glaciers in addition to non stationarity of the flow due to the influence of the fjord s shape and its surrounding topography low wind speeds questioned the validity of the monin obukhov similarity theory whereas the over water fetch and off wind angle were of minor importance moreover some results especially the off wind angle were very sensitive to how the raw data were treated in particular which coordinate rotation method was employed the double rotation or the planar fit method

estimation of wind friction velocity and direction at the ocean surface from physical models and space borne radar scatterometer measurements this paper presents a new technique for estimating the wind friction velocity at the ocean surface from c band radar scatterometer measurements this technique uses physical models of ocean surface waves and electromagnetic backscattering from a rough surface at intermediate angles of incidence to generate predictions of the normalized radar cross section nrcs or 0 of the ocean surface for a given wind friction velocity and observational geometry the ocean spectral model used in this technique has been developed specifically for this application it combines in situ wave measurements at low wave numbers with the phillips 1985 equilibrium spectral model this choice of ocean wave model is supported by a set of open ocean wave measurements summarized in this paper a suite of models derived from both in situ and remote measurements of the sea surface is used to characterize the directional spreading of ocean waves relative to the wind direction the resulting two dimensional ocean wave spectra are used with a composite surface model to predict radar backscattering from the ocean surface at c band these radar cross section predictions are combined with ers 1 scatterometer measurements in a cost function minimization scheme to yield estimates of the friction velocity vector at the ocean surface we present examples of this technique and compare friction velocity retrievals obtained via this scheme with buoy based measurements under a variety of wind and wave conditions on the basis of the analysis of a limited number of cases this technique yielded friction velocity estimates for which the magnitude was within 22 and the direction was within 25 given that scientific applications require magnitude estimates within 10 15 and directional estimates within 20 of in situ measurements these preliminary results suggest that this is a promising approach to wind retrieval

observed directional characteristics of the wind wind stress and surface waves on the open ocean sonic anemometer data were taken during the surface waves processes program swapp in march 1990 in the north pacific measurements of the wind stress vector span several strong wind events significant angles between the wind stress vector and the mean wind vector are seen simultaneous measurements of the directional wave field were made with a surface scanning doppler sonar the data suggest that the wind stress direction is influenced by the direction of the surface waves especially for stronger winds overall the stress vector lies between the mean wind and the mean wave directions at the higher wind speeds over 8 6 m s there is a nonzero correlation between the variations in wave directions and stress directions as well finally the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible suggesting a dynamic link

air sea fluxes and surface layer turbulence around a sea surface temperature front the observed effects of sharp changes in sea surface temperature sst on the air sea fluxes surface roughness and the turbulence structure in the surface layer and the marine atmospheric boundary layer are discussed in situ flux and turbulence observations were carried out from three aircraft and two ships within the fasinex framework three other aircraft used remote sensors to measure waves microwave backscatter and lidar signatures of cloud tops descriptions of the techniques intercomparison of aircraft and ship flux data and use of different methods for analyzing the fluxes from the aircraft data are described changing synoptic weather on three successive days yielded cases of wind direction both approximately parallel and perpendicular to a surface temperature front for the wind perpendicular to the front wind over both cold to warm and warm to cold surface temperatures occurred model results consistent with the observations suggest that an internal boundary layer forms at the sst

the marine layer off northern california an example of supercritical channel flow abstract during the spring and summer northerly winds driven by the north pacific high pressure system are prevalent over the northern california continental shelf only interrupted for periods of a few days when weak or southerly winds occur in the course of the coastal ocean dynamics experiment code fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast these observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories the surface wind can be everywhere weak pattern 1 it can blow at large speeds in a uniform pattern pattern 2 or finally the structure of the northerly surface wind can be complex with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales pattern 3 the latter pattern which o

atmospheric response to atlantic tropical instability waves in community atmosphere model version 3 1 tropical instability waves tiws are 20 to 40 day waves that appear along the sea surface temperature sst front of the equatorial cold tongues in this study we investigate the atmospheric planetary boundary layer response to tiw induced sst variations in the tropical atlantic ocean using community atmosphere model version 3 cam3 developed at the national center for atmospheric research and compare the model simulation with available satellite measurements cam3 is used to simulate the response to atlantic tiws with t85 and t42 horizontal resolutions forced with daily tropical rainfall measuring mission microwave imager tmi ssts and t85 horizontal resolution forced with weekly tmi and reynolds ssts all four runs exhibit an atmospheric response to the sst anomalies but the amplitude of the response is different in each case the model simulates winds convergence column integrated water vapor and precipitation perturbations associated with tiws that are similar in many respects to the satellite observations however the wind convergence and divergence anomalies in the satellite observations are located west of the sst anomalies in the cam3 simulations the wind convergence is more in phase with sst anomalies differences between the observations and the model simulations appear to be due to both differences in the boundary layer response and differences in the horizontal advection process

turbulence adjustment and scaling in an offshore convective internal boundary layer a casper case study abstractthe characteristics of a convective internal boundary layer cibl documented offshore during the east coast phase of the coupled air sea processes and electromagnetic ducting research cas

the influence of oblique waves on the azimuthal response of a ku band scatterometer a laboratory study the authors conducted experiments in the large wind wave tank at irphe to examine the influence of longer waves propagating at oblique angles to the wind direction on microwave backscattering measurements were made of wind wave directional spectra and cross sections from a 13 5 ghz scatterometer at 30 spl deg from nadir incidence angle the authors characterize the scatterometric azimuthal scans with respect to the magnitude and the direction of the maximum backscattered power scans of solely wind waves are used to assess the effects of oblique waves for which the scans yield trends that are related to the oblique wave direction relative to the wind and the oblique wave steepness in particular cross section maxima are enhanced by oblique waves and the direction of maxima rotates from the wind axis toward the oblique wave direction these scatterometer data in addition to other oblique wave studies led the authors to conclude that wind vectors produced from standard retrieval algorithms for scatterometers should be used with prudence in regions with significant veering winds such as near meteorological fronts

the stable internal boundary layer over a coastal sea part i airborne measurements of the mean and turbulence structure abstract it has been recognized that progress toward understanding the mesoscale structure of the ocean requires more knowledge of the small scale structure of the marine atmosphere and the processes that couple the air and the sea the coastal ocean is characterized by large variations in sea surface temperature and surface roughness that affect the structure of the atmosphere stable layers are frequent features of the coastal marine atmospheric boundary layer their main effect is the formation of a discontinuity between the sea surface and the upper part of the boundary layer that supports gravity waves wind speed jets and large wind shear the general structure of a stable internal boundary layer ibl that forms over the sea downstream of a warm landmass is discussed aircraft data are presented from the internal boundary layer experiment iblex conducted over the irish sea in 1990 with the airflow from the land to the sea thermodynamic profiles were obtained perpendicular to the coast to inv

frontogenesis over a mountain ridge abstract the interaction of developing two dimensional cold and warm frontal systems with a mesoscale mountain ridge is examined the flow of the rotating model atmosphere is assumed to be inviscid adiabatic and boussinesq the geostrophic momentum approximation is made an imposed horizontal deformation field forces the frontogenesis the nonlinear model equations are solved numerically in physical space using terrain following coordinates which incorporate the fully nonlinear lower boundary condition comparison of the model results with and without topography enables assessment of the impact of the mountain on the frontogenesis a scale analysis indicates that the most general problem of frontal interaction with an infinite mountain ridge encompasses a seven dimensional parameter space the scale analysis provides justification for die two dimensional geostrophic momentum approximation and defines an inverse richardson number as a measure of the importance of the ageostrophic advection sensitivity

a comparison of the finescale structures of a prefrontal wind shift line and a strong cold front in the southern plains of the united states abstractthe objectives of this study are to determine the finescale characteristics of the wind and temperature fields associated with a prefrontal wind shift line and to contrast them with those associated with a strong cold front data from a mobile polarimetric x band doppler radar and from a surveillance s band radar temperature profiles retrieved from a thermodynamic sounder and surface observations from the oklahoma mesonet are used to analyze a prefrontal wind shift line in oklahoma on 11 november 2013 data from the same mobile radar and the oklahoma mesonet are used to identify the finescale characteristics of the wind field associated with a strong surface cold front in oklahoma on 9 april 2013 it is shown that the prefrontal wind shift line has a kinematic and thermodynamic structure similar to that of an intrusion elevated density current while the cold front has a kinematic structure similar to that of a classic density current other characteristics of the prefrontal wind shift line

use of vertical temperature gradients for prediction of tidal flat sediment characteristics 1 sediment characteristics largely govern tidal flat morphologic evolution however conventional methods of investigating spatial variability in lithology on tidal flats are difficult to employ in these highly dynamic regions in response a series of laboratory experiments was designed to investigate the use of temperature diffusion toward sediment characterization a vertical thermistor array was used to quantify temperature gradients in simulated tidal flat sediments of varying compositions thermal conductivity estimates derived from these arrays were similar to measurements from a standard heated needle probe which substantiates the thermistor methodology while the thermal diffusivities of dry homogeneous sediments were similar diffusivities for saturated homogeneous sediments ranged approximately one order of magnitude the thermal diffusivity of saturated sand was five times the thermal diffusivity of saturated kaolin and more than eight times the thermal diffusivity of saturated bentonite this suggests that vertical temperature gradients can be used for distinguishing homogeneous saturated sands from homogeneous saturated clays and perhaps even between homogeneous saturated clay types however experiments with more realistic tidal flat mixtures were less discriminating relationships between thermal diffusivity and percent fines for saturated mixtures varied depending upon clay composition indicating that clay hydration and or water content controls thermal gradients furthermore existing models for the bulk conductivity of sediment mixtures were improved only through the use of calibrated estimates of homogeneous end member conductivity and water content values our findings suggest that remotely sensed observations of water content and thermal diffusivity could only be used to qualitatively estimate tidal flat sediment characteristics

a numerical modeling study of warm offshore flow over cool water abstract numerical simulations of boundary layer evolution in offshore flow of warm air over cool water are conducted and compared with aircraft observations of mean and turbulent fields made at duck north carolina two models are used a two dimensional high resolution mesoscale model with a turbulent kinetic energy closure scheme and a three dimensional large eddy simulation les model that explicitly resolves the largest turbulent scales both models simulate general aspects of the decoupling of the weakly convective boundary layer from the surface as it is advected offshore and the formation of an internal boundary layer over the cool water two sets of experiments are performed which indicate that complexities in upstream surface conditions play an important role in controlling the observed structure the first land sea experiments examine the transition from a rough surface having the same temperature as the ambient lower atmosphere to a smooth ocean surface that is 5 c cooler in the seco

elevated levels of dimethylated sulfur compounds in lake bonney a poorly ventilated antarctic lake lake bonney is a permanently ice covered lake in the mcmurdo dry valleys antarctica the lake has two chemically stratified lobes referred to as the east and west lobes each with distinct biogenic sulfur profiles dimethylsulfoniopropionate dmspp and dimethylsulfoxide dmsop exceeded 32 and 2 nmol l 21 respectively in the photic surface waters of the lake maximum dmspp levels occurred in the deep chlorophyll layer of both lobes a zone dominated by chrysophytes and chlorophytes which are thought to be the source of dimethylated sulfur in the deep waters of the lake following sedimentation and biogeochemical processing waters beneath the chemoclines of both lobes are cold 08c saline 3 times seawater suboxic and devoid of phytoplankton biomass and activity dimethylsulfide dms levels 330 nmol l 21 in the deep west lobe are among the highest recorded in a natural aquatic ecosystem in contrast saline waters of the deep east lobe contain relatively little dms 70 nmol l 21 but high dmsod 270 nmol l 21 with the latter being the highest observed in any natural aquatic ecosystem we argue that the differences in the biogenic sulfur profiles between the deep waters of the two lobes arise principally from subtle differences in the redox conditions found in each lobe

quasi geostrophic frontogenesis over topography abstract the problem of quasi geostrophic frontogenesis due to a horizontal deformation field is re examined exact analytic solutions of all flow fields for all times are found for the case of a vertically semi infinite uniformly stratified boussinesq atmosphere the imposed horizontal deformation field is assumed independent of height but may translate horizontally relative to the initial potential temperature distribution and to the variable bottom topography only straight infinitely long fronts and ridge like topographies are considered the solutions in the absence of orography confirm and extend earlier investigations for surface and occluded fronts it is shown that the presence of monotonically sloping topography below a region of deformation leads to the formation of a surface discontinuity in potential temperature in the absence of an initial horizontal thermal gradient the associated secondary circulation is the sum of a closed thermally direct and indirect component the analysis for a tr

frontogenesis and frontal motion due to confluent deformation with a translating dilatation axis a two dimensional model of primitive equation frontogenesis is formulated in which the forcing is due to a basic state field of confluent deformation with a dilatation axis that translates in the cross front direction although the translation speed and direction of motion of the basic state dilatation axis have no effect on frontal structure when the total deformation is spatially uniform they can influence considerably the motion of the front at the surface this motion may be separated into two components i internal motion directed towards the warm air which is associated with the secondary ageostrophic circulation of the front itself and ii external motion either towards cold or warm air which is associated with the initial location and translation speed of the basic state dilatation axis the latter component ultimately dominates and thus the motion of a mature front may be thought of as being controlled by the large scale flow however the former may dominate early on in the frontal evolution resulting in significantly different frontal trajectories depending on the initial location of the basic state dilatation axis and on whether this axis is moving towards the cold or the warm air the relative locations of various conventional frontal measures are examined in the special case of a uniformly translating dilatation axis of these measures the axes of maximum baroclinicity and vorticity become collocated at the surface as the front intensifies however the basic state dilatation axis and the confluence axis the latter being defined as the location of zero total wind in the cross front direction may be displaced considerably from this merger point with this displacement depending on the initial location and subsequent motion of the basic state dilatation axis potential extensions of the model are discussed with regard to improved understanding of the difference in structure and motion between cold and warm fronts

physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research abstract recently ultrasonic anemometers uas have become available for precise high frequency measurement of three dimensional velocity and turbulence properties except for a few wind tunnel and computational fluid dynamics cfd simulations advances in aeolian sediment transport and bedform research have been limited to field studies using instrumentation that is either incapable of measuring turbulence e g cup anemometers or unable to withstand sediment laden airflow e g hotfilms in contrast extensive progress has occurred in fluvial research where turbulence instrumentation has been available for some time this paper provides a pragmatic discussion on using uas in aeolian research recent advances using this technology are reviewed and key physical and logistical considerations for measuring airflow properties and near surface shear stress using uas over complex terrain are discussed physical considerations include limitations of applying boundary layer theory to flow over natural surfaces such as non logarithmic velocity profiles resulting from roughness and topographically induced effects and the inability of instrumentation to measure within the thin constant stress region these constraints hinder accurate shear velocity u shear stress and sand transport estimation uas allow measurement of turbulent reynolds stress rs that in theory should equal profile derived shear stress discrepancies often exist between these quantities however due to three dimensional spanwise flow components and rapid distortion effects i e unbalanced production and dissipation of turbulence common in flow over complex terrain while the rs approach yields information on turbulent contributions to near surface stress generation little evidence exists showing that rs is a better measure of forces responsible for sediment transport consequently predictive equations for sediment transport using rs do not exist there is also a need to identify the role of micro turbulent events e g burst sweep cycles and macro turbulent structures e g separation cells shear layers in aeolian dynamics in field settings to validate recent wind tunnel and cfd simulations a conundrum exists regarding whether velocity data should be rotated to correct for potential sensor misalignment effects in unsteady non uniform flow over complex terrain streamline angles vary spatially and temporally with height and location thus determination of and correction to true streamline coordinates is difficult caution should be exercised with correction methods that remove implicit vertical velocity trends as this may preclude detection of geomorphically important flow behaviour e g lift at a dune crest and may complicate interpretations of rs instruments should be aligned with the underlying surface and flow visualization should be used to adjust sensor measurement planes as close as possible to local streamlines logistical considerations include sensor design advantages and operational limitations data communication formats and field deployment strategies each can affect measurement accuracy and are easily overlooked sampling range frequency and period are also important as they limit the range of velocities and scales of turbulence that can be characterized ultrasonic anemometers offer a new sampling resolution to measure turbulent airflow properties in field settings with proper considerations of their limitations uas may allow researchers to close the gap between fluvial research and develop more robust models of aeolian processes and morphodynamics

coastal atmospheric circulation around an idealized cape during wind driven upwelling studied from a coupled ocean atmosphere model abstract the study analyzes atmospheric circulation around an idealized coastal cape during summertime upwelling favorable wind conditions simulated by a mesoscale coupled ocean atmosphere model the domain resembles an eastern ocean boundary with a single cape protruding into the ocean in the center of a coastline the model predicts the formation of an orographic wind intensification area on the lee side of the cape extending a few hundred kilometers downstream and seaward imposed initial conditions do not contain a low level temperature inversion which nevertheless forms on the lee side of the cape during the simulation and which is accompanied by high froude numbers diagnosed in that area suggesting the presence of the supercritical flow formation of such an inversion is likely caused by average easterly winds resulting on the lee side that bring warm air masses originating over land as well as by air warming during adiabatic descent on the lee side of the topographic obstacle mountain leeside

variability in the summertime coastal marine atmospheric boundary layer off california usa an analysis of boundary layer structure and surface layer turbulence from measurements off the california coast is presented from data collected by research aircraft during two field experiments the coastal waves 1996 cw96 and the monterey area ship track mast experiments cw96 covers the near coast region in particular in the vicinity of major headlands whereas mast extends offshore along the us west coast coastal modification of the along coast flow occurs on two main horizontal scales firstly a large scale variability is due to the interplay between the shallow near coast marine atmospheric boundary layer mabl and the coastal terrain typically higher than the mabl depth the mabl depth decreases smoothly towards the coast while the wind speed increases to a coastal jet in response to the sloping mabl inversion secondly the flow is affected by supercritical flow dynamics as the wind speed increases and the mabl depth decreases towards the coast the mabl flow becomes supercritical in a shallow water sense as supercritical shallow water flow interacts with major headlands expansion fans form affecting both the wind speed and the mabl depth the combination of cw96 and mast data reveals significant differences between the flow along the coast and that far offshore mabl winds are stronger near the coast whereas aloft the winds are weaker than offshore the near coast mabl is also better mixed turbulence increases towards the coast while the sensible heat flux decreases and often changes sign leading to stable stratification near the coast a length scale determining the across coast influence of the expansion fans is defined from simple inviscid and irrotational shallow water theory data from four days of the cw96 experiment show that this simple theory describes the low level wind speed adequately surface layer turbulence also scales with this simple length scale but the results are complicated by upwelling of cold water giving rise to strong internal boundary layers from terms in the turbulent energy budgets it appears that dissipation of turbulence exceeds local production copyright 2004 royal meteorological society

revisiting the definition of the drag coefficient in the marine atmospheric boundary layer abstract a new functional form of the neutral drag coefficient for moderate to high wind speeds in the marine atmospheric boundary layer for a range of field measurements as reported in the literature is proposed this new form is found to describe a wide variety of measurements recorded in the open ocean coast fetch limited seas and lakes with almost one and the same set of parameters this is the result of a reanalysis of the definition of the drag coefficient in the marine boundary layer which finds that a constant is missing from the traditional definition of the drag coefficient the constant arises because the neutral friction velocity over water surfaces is not directly proportional to the 10 m wind speed a consequence of the transition to rough flow at low wind speeds within the rough flow regime the neutral friction velocity is linearly dependent on the 10 m wind speed consequently within this rough regime the new definition of the drag coefficient is not a function of the wind speed

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

surface stress in offshore flow and quasi frictional decoupling aircraft data collected at approximately 15 m above the sea surface in the coastal zone are analyzed to examine the spatial distribution of surface stress advection of stronger turbulence from land dominates the near surface turbulence for the first few kilometers offshore with offshore flow of warm air over cold water strong stratification leads to very small surface stress because the stability restricts the momentum transfer to the waves the aerodynamic surface roughness decreases to very small values which in turn decreases atmospheric mixing the redevelopment of the boundary layer farther downstream is examined computation of fluxes from observations for stable cases is difficult due to a variety of errors including large random flux errors possible instrumental loss of small scale flux difference between the surface flux and that at the observational level and inadvertent capture of mesoscale motions in the computed turbulent fluctuations although the errors appear to be substantial the aircraft momentum fluxes compare favorably with those from sonic anemometers on two buoys and a tower at the end of a 570 m pier even with near collapse of the turbulence

some evidence of colinear wind stress and wave breaking abstract data collected during the surface waves and processes program are employed to investigate a possible interrelation between wind stress and surface wave breaking from comparison of data from 15 half hour long time segments the directions of the wind stress and the whitecap motion are observed to be generally colinear with both lying between the mean wind and the swell as well a nondimensionalized whitecap speed is found to correlate with the drag coefficient these results suggest that the magnitude and direction of the wind stress might be estimated from wave breaking information