low latitude mesospheric turbulence investigated using coordinated mst radar and rocket borne observations from india 1 in this paper we present and discuss observational results on low latitude mesospheric turbulence obtained from a coordinated experiment made using a langmuir probe lp onboard a rocket launched from sriharikota 13 6 n 80 2 e and the mesosphere stratosphere troposphere mst radar from gadanki 13 5 n 79 2 e on 8 april 2005 the lp detected electron density irregularities with scale sizes in the range of about 1 m to 1 km in three height regions one region in between 69 6 and 72 km and the other two around 75 km and 78 km each having thickness of less than 1 km the mst radar observations however showed two distinct scattering layers one around 66 km and another around 75 km the wave number spectra of the in situ observations except for those of 69 6 72 km and radar observed spectral parameters clearly suggest that the electron density fluctuations detected by the lp and those responsible for the radar echoes are of turbulence origin energy dissipation rates estimated from both rocket borne in situ measurements and radar observed spectral width are found to be in the range of 1 70 mwkg 1 the rms turbulent velocities estimated from the two observations are found to be 3 m s 1 further the energy dissipation rates and rms turbulent velocities estimated for the height region of 75 km which is common in the in situ and radar data and not reported earlier are found to be in good agreement with each other the energy dissipation rates are compared critically with those reported earlier and the radar rocket observations are discussed in the light of current understanding of the low latitude mesospheric echoes

gravity wave propagation characteristics 60 120 km as determined by the saskatoon mf radar gravnet system 1983 85 at 52 n 107 w abstract gravity waves gw have been detected and their characteristics measured by observations with the saskatoon multiple bistatic system gravnet data are available from 50 days for two height ranges 64 97 km 100 115 km and for the four seasons of 1983 85 wave characteristics include horizontal wavelength phase velocity period and amplitude background wind data allows the corresponding intrinsic parameters to be calculated many waves are doppler shifted to near their critical levels altitude and seasonal variations in the gw characteristics are shown the strongest variation is in the horizontal direction of wave propagation with southward directions dominating but significant eastward westward fluxes in summer winter below 100 km

modis based mosaic of antarctica moa data sets continent wide surface morphology and snow grain size abstract we present digital image mosaics of surface morphology and optical snow grain size for the antarctic continent and surrounding islands assembled from 260 moderate resolution imaging spectroradiometer modis images the products are derived from modis band 1 red light 647 nm and band 2 infrared 857 nm orbit swath data acquired during the 2003 2004 austral summer multiple images of all areas are combined in a data cumulation scheme to improve spatial resolution and increase radiometric content of the mosaics the component images were de striped geo registered and re sampled to the projection grid using the modis swath to grid toolbox ms2gt software a 125 m ground equivalent polar stereographic projection was used identical to previous radar image mosaics and similar to several other continent wide data sets geo location accuracy of the final mosaics is better than 125 m swath acquisition times were limited to the range 05 00 13 30 ut to provide broad but uniform mosaic illumination regions of cloud cover blowing snow and intense forward scattering were masked prior to compositing the moa snow grain size data set image provides the first continent wide semi quantitative map of mean summer snow firn and blue ice grain size optical grain size radius was determined using a normalized difference index derived from modis band 1 and band 2 rad values atmospheric correction adjustments for directional reflectance variation and snow grain size estimation were accomplished by a look up table generated by a combination of published models of snow spectral reflectance and atmosphere radiative transfer validation of the snow grain size image is provided by in situ spectra of snow covered sea ice made in october 2003 we use the new data sets to determine major ice shelf areas and shelf edge coastline and ice grounding line vector files and determine areas of selected blue ice regions these data sets are available for download at http nsidc org data moa

satellite observations of atmospheric variances a possible indication of gravity waves the microwave limb sounder mls on the upper atmosphere research satellite has now produced the first global maps of small scale variances in the middle atmosphere initial analyses are presented here that suggest these variances are due to gravity waves and the technique used to extract gravity wave information from saturated radiance measurements is described observations at 30 80km altitudes show that the variances of horizontal scales less than 100km are strongly correlated with upper tropospheric convection surface topography and stratospheric jetstreams mls monthly averages during solstice periods suggest that the normalized variance amplitude grows exponentially with height in the stratosphere and saturates in the mesosphere as expected from wave breaking and dissipation at these altitudes

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

tidally driven stick slip motion in the mouth of whillans ice stream antarctica we show that the ice plain in the mouth of whillans ice stream formerly ice stream b antarctica moves by stick slip motion during a spring tide period rapid motions regularly occur near high tide and during falling tide this correlation is weaker during a neap tide period when the tidal magnitudes are less precise timing of these motion events suggests that they propagate through the region with a mean velocity of 88 ms 1 we hypothesize that this speed is associated with the propagation of shear waves through a wet subglacial till motion events are also seen on more smoothly flowing floating ice event delays are very short between grounded and floating stations suggesting the events propagate through the ice shelf as an elastic wave we further hypothesize the events are caused by the interaction of a sticky bed the accumulation of stored elastic strain through the compression of ice by upstream inflow and tidal forcing motion events seem to be triggered either by reduction of vertical normal stresses at high tide or by the increase of shear stresses from sub shelf ocean currents during falling tide event magnitudes are not related to the length of the preceding quiescent period suggesting significant viscous dissipation within the till

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

seasonal variation of momentum flux in the mesosphere observed with the mu radar we have regularly monitored mesospheric winds at 60 85km for three years with the mu radar 35 n 136 e and have found that the variance of wind velocity fluctuations for periods from 5 min to 2 hr shows a semi annual variation with a significant peak in summer centered in july and irregular enhancements in winter the zonal momentum flux induced by gravity waves shows eastward and westward maxima in july august and december february respectively typical eastward accelerations at the 70km altitude range from 7 to 13 m s day and from 8 to 11 m s day in summer and winter respectively which agree fairly well with theoretical predictions

first observations of turbulence parameters in the troposphere over the bay of bengal and the arabian sea using radiosonde 1 this paper discusses the turbulence characteristics of the free atmosphere over the bay of bengal and the arabian sea during the premonsoon season of 2006 the turbulence scaling concepts developed over the past decades for application to oceanic mixing are used for retrieving turbulence properties in the free atmosphere from high resolution soundings as described by clayson and kantha 2008 the variability in the vertical turbulence structure over a large spatial region is studied using this novel method for the first time investigations revealed the presence of a highly turbulent band at upper troposphere below the cold point tropopause cpt with significant enhancements in thorpe length and eddy diffusion coefficient this increased turbulence is studied in conjunction with the observations of the tropical tropopause layer ttl it is seen that the lower and upper boundaries of this turbulence band nearly coincide with the height where the potential temperature lapse rate begins to show a decrease lrhmin and cold point tropopause respectively the ttl is the transition regime from convectively dominated troposphere to radiatively dominated stratosphere and is characterized by a decrease in the static stability the enhanced zonal and meridional wind shear observed in this region shows that the stability in this region is further weakened because of the breaking of kelvin helmholtz waves which results in the enhancement of turbulence the study reveals the potential of high resolution soundings to be used in the investigations of turbulence in the free atmosphere

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

tide induced lateral movement of brunt ice shelf antarctica 1 accurate position data show that brunt ice shelf has an irregular three dimensional motion driven by the ocean tide a 30 day gps record from halley station shows not only the expected vertical tidal displacement but also a significant variation in horizontal velocity the maximum forward velocity occurs about four hours before the maximum tidal height a phase relationship that suggests ocean currents may be responsible however values of ice water friction coefficients needed to make this explanation plausible appear to be too high to be physically reasonable alternative explanations invoke interactions between the ice shelf and a grounded area at the ice front our findings suggest that significant errors might result if these motions are ignored when interpreting double difference satellite sar interferograms where the common assumption is that the horizontal velocity of an ice shelf has no tidal dependence

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

paleovegetation and paleoclimate in middle late pliocene shanxi central china abstract sequences of lacustrine sediments developed in intermontane basins in the middle eastern shanxi plateau of china have been investigated in order to reconstruct the paleovegetation and paleoclimate for the middle late pliocene according to the magnetostratigraphy and the fossil assemblages the lacustrine sediments of yushe and taigu basins were deposited between 5 5 and 2 5 ma bp the zhangcun and xiaobai formations are considered to cover a similar period ranging from 3 5 to 2 5 ma bp an increase of picea and abies shows that the climate began to become cold after about 4 4 ma bp a cold wet episode with relatively warm dry and warm humid intervals occurred between 3 6 and 2 5 ma bp in the yushe and taigu basins an increase artemisia chenopodiaceae and ephedra shows that the climate became drier after 2 5 ma bp the climate changes of this period probably reflect variations of the east asian winter and summer monsoons with an increase winter monsoon activity during the early pleistocene

upper tropospheric inversion and easterly jet in the tropics 1 shipboard radiosonde measurements revealed a persistent temperature inversion layer with a thickness of 200 m at 12 13 km in a nonconvective region over the tropical eastern pacific along 2 n in september 1999 simultaneous relative humidity measurements indicated that the thin inversion layer was located at the top of a very wet layer with a thickness of 3 4 km which was found to originate from the intertropical convergence zone itcz to the north radiative transfer calculations suggested that this upper tropospheric inversion uti was produced and maintained by strong longwave cooling in this wet layer a strong easterly jet stream was also observed at 12 13 km centered around 4 5 n this easterly jet was in the thermal wind balance with meridional temperature gradients produced by the cloud and radiative processes in the itcz and the wet outflow furthermore the jet in turn acted to spread inversions further downstream through the transport of radiatively active water vapor this feedback mechanism may explain the omnipresence of temperature inversions and layering structures in trace gases in the tropical troposphere examination of high resolution radiosonde data at other sites in the tropical pacific indicates that similar utis often appear around 12 15 km the uti around 12 15 km may thus be characterized as one of the climatological inversions in the tropical troposphere forming the lower boundary of the so called tropical tropopause layer where the tropospheric air is processed photochemically and microphysically before entering the stratosphere

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

diurnal lake level cycles on ice shelves driven by meltwater input and ocean tidal tilt diurnal depth cycles of decimeter scale are observed in a supraglacial lake on the mcmurdo ice shelf antarctica we evaluate two possible causes 1 tidal tilt of the ice shelf in response to the underlying ocean tide and 2 meltwater input variation we find the latter to be the most likely explanation of our observations however we do not rule out tidal tilt as a source of centimeter scale variations and point to the possibility that other larger supraglacial lake systems particularly those on ice shelves that experience higher amplitude tidal tilts such as in the weddell sea may have depth cycles driven by ocean tide the broader significance of diurnal cycles in meltwater depth is that under circumstances where the ice shelf is thin tidal tilt amplitudes are high and meltwater runoff rates are large there may be associated flexure stresses that can contribute to ice shelf fracture and destabilization for the mcmurdo ice shelf 20 50 m thickness 1 m tidal amplitude and 10 cm water depth variations these stresses amount to several 10 s of kpa

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

the separated polar winter stratopause a gravity wave driven climatological feature abstract an examination of satellite derived temperatures reveals that the winter polar stratopause is usually elevated and warmer than the adjacent midlatitude stratopause this separated stratopause occurs in both hemispheres but is more pronounced and persistent in the southern winter it descends with time towards spring and exhibits week to week variability observational diagnostics and results from a two dimensional 2 d model suggest that gravity wave driving can account for this separated polar stratopause by driving a meridional circulation with downwelling over the winter pole in the model the solar heating pattern induces stronger winter westerlies than summer easterlies which leads to a stronger gravity wave driven circulation in the winter hemisphere spherical geometry and the high latitude location of the winter westerly jet combine to yield a concentrated region of downwelling model results suggest that descent of the temperature maximum with time is probably caused by wave mean f

turbulence at the tropopause due to breaking kelvin waves observed by the equatorial atmosphere radar 1 the equatorial atmosphere radar ear installed in sumatra island indonesia observed significant enhancement of turbulence in the tropopause region 15 17 km intermittently for 5 days in november 2001 the turbulence intensity was estimated with the spectral width of the radar echo power spectrum and the turbulence during the period was a factor of up to 5 larger in kinetic energy than that in other periods further analyses confirm that the enhanced turbulence was convectively generated in the breaking phase of an equatorial kelvin wave between july and december 2001 we observed at least three more prominent cases of the turbulence generation by breaking kelvin waves in the tropopause region

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

tidal modulation of ice shelf flow a viscous model of the ross ice shelf three stations near the calving front of the ross ice shelf antarctica recorded gps data through a full spring neap tidal cycle in november 2005 the data revealed a diurnal horizontal motion that varied both along and transverse to the long term average velocity direction similar to tidal signals observed in other ice shelves and ice streams based on its periodicity it was hypothesized that the signal represents a flow response of the ross ice shelf to the diurnal tides of the ross sea to assess the influence of the tide on the ice shelf motion two hypotheses were developed the first addressed the direct response of the ice shelf to tidal forcing such as forces due to sea surface slopes or forces due to sub ice shelf currents the second involved the indirect response of ice shelf flow to the tidal signals observed in the ice streams that source the ice shelf a finite element model based on viscous creep flow was developed to test these hypotheses but succeeded only in falsifying both hypotheses i e showing that direct tidal effects produce too small a response and indirect tidal effects produce a response that is not smooth in time this nullification suggests that a combination of viscous and elastic deformation is required to explain the observations

tides and the flow of rutford ice stream west antarctica 1 surface speeds of rutford ice stream west antarctica are known to vary by around 10 20 depending on location with a fortnightly periodicity corresponding to a springneap tidal cycle the reasons for these periodic variations in flow are unclear here the possible role of tidal stress transmission upstream of the grounding line in affecting rates of basal motion is investigated it is found that nonlinear rheological effects within the till when coupled with transmission of tidal stresses within the ice that are linearly related to tidal amplitude can give rise to the type of periodic oscillations in flow observed this nonlinear interaction between tidal forcing and till deformation increases the mean ice flux across the grounding line by a few percent above what might be expected in the absence of tidal forcing periodic velocity fluctuations of this type have not been observed on other ice streams however modeling suggests that this may be due to lack of data and that such flow variations are likely to be common features of active ice streams draining into the ronne ice shelf as well as of other ice streams subjected to similar tidal forcing

simulation of large scale dynamics in the mesosphere and lower thermosphere with the doppler spread parameterization of gravity waves 2 eddy mixing and the diurnal tide turbulence is thought to be produced by gravity waves that become unstable in the upper atmosphere owing to a relatively short vertical wavelength of the solar diurnal tide turbulent viscous type mixing is expected to be particularly important for tidal dissipation in the mesosphere and lower thermosphere mlt within the doppler spread parameterization dsp of gravity waves the vertical eddy exchange coefficients may be calculated from the gw energy deposition rate the dsp as implemented in the spectral mesosphere lower thermosphere model is shown to provide realistic simulations of both the diurnal tide and turbulent mixing in the mlt in particular the vertical structure and seasonal variations of eddy mixing agree well with in situ observations and independent estimates from global thermal balance the seasonal variations of turbulence are found to be primarily responsible for a strong semiannual variation of the diurnal tide in agreement with recent satellite observations

medium scale gravity wave activity in the bottomside f region in tropical regions thermospheric gravity waves gws in the bottomside f region have been proposed to play a key role in the generation of equatorial plasma bubbles epbs however direct observations of such waves are scarce this study provides a systematic survey of medium scale 620 km neutral atmosphere perturbations at this critical altitude in the tropics using 4 years of in situ gravity field and steady state ocean circulation explorer satellite measurements of thermospheric density and zonal wind the analysis reveals pronounced features on their global distribution and seasonal variability 1 a prominent three peak longitudinal structure exists in all seasons with stronger perturbations over continents than over oceans 2 their seasonal variation consists of a primary semiannual oscillations sao and a secondary annual oscillation ao the sao component maximizes around solstices and minimizes around equinoxes while the ao component maximizes around june solstice these gw features resemble those of epbs in spatial distribution but show opposite trend in climatological variations this may imply that stronger medium scale gw activity does not always lead to more epbs possible origins of the bottomside gws are discussed among which tropical deep convection appears to be most plausible

doppler ducting of short period gravity waves by midlatitude tidal wind structure 1 multiwavelength airglow image data depicting a short period 4 9 min atmospheric gravity wave characterized by a sharp leading front have been analyzed together with synoptic meteor radar wind data recorded simultaneously from bear lake observatory utah 41 6 n 111 6 w the wind data suggest the presence of a semidiurnal tide with horizontal winds peaking at around 60 m s along the sse direction of motion 170 from north of this short period wave it was found that the gravity wave was most probably ducted because of the doppler shift imposed by this wind structure a marked 180 phase shift was observed between the near infrared oh and the oi 557 7 nm emissions numerical simulation results for similar ducted waves excited by idealized model sources suggest that the phase shift between the wave modulated airglow intensities may be explained simply by chemical processes rather than by wave dynamics phase velocities of simulated waves however appear higher than those of observed waves suggesting the importance of tidal thermal structure in determining the doppler ducted wave characteristics

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

small scale wind fluctuations in the tropical tropopause layer from aircraft measurements occurrence nature and impact on vertical mixing abstractthe contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer ttl is poorly constrained partly due to a lack of direct observations here the authors use high resolution 20 hz airborne measurements to study the occurrence and properties of small scale 100 m wind fluctuations in the ttl 14 19 km over the tropical pacific the fluctuations are highly intermittent and appear localized within shallow 100 m patches furthermore active turbulent events are more frequent at low altitude near deep convection and within layers of low gradient richardson number a case study emphasizes the link between the turbulent events and the occurrence of inertio gravity waves having small horizontal or vertical scale in order to evaluate the impact of the observed fluctuations on tracer mixing their characteristics are examined during active events they are in broad agreement with inertial range turbulence theory the motions are close to 3d isotropic and the sp

pre sunrise mesospheric echoes and turbulent wind structure observed with the mu radar summer mesospheric observations using the mu radar at shigaraki japan show that radar echoes could be detected before sunrise under some circumstances the pre sunrise radar echoes observed at heights above 80 km are weak but clearly occur in a layer strong radar echoes observed after sunrise at similar heights show an unusual broadening of the spectral width in the echo power this indicates the presence of strong atmospheric turbulence in addition frequency spectra for the vertical wind fluctuations show an enhancement of the spectral power density in a wide range of periods shorter than the local brunt vaisala one this is consistent with the occurrence of strong turbulence layer in the upper mesosphere though not immediately identified with the turbulence activity thus the presence of strong turbulence layer might yield us a possibility of detecting radar echoes before sunrise

behavior of flexural gravity waves on ice shelves application to the ross ice shelf ocean waves continuously impact floating ice shelves and affect their stress regime low frequency long period 75 400 s ocean waves are able to reach ice shelf cavities from distant sources and excite flexural gravity waves that represent coupled motion in the water of the cavity and the ice covering above analytic treatment of simplified geometric configuration and three dimensional numerical simulations of these flexural gravity waves applied to the ross ice shelf show that propagation and ice shelf flexural stresses are strongly controlled by the geometry of the system bathymetry of the ice shelf cavity and ice shelf cavity thickness the derived dispersion relationships group and phase velocities of these waves can be used to infer poorly constrained characteristics of ice shelves from field observations the results of numerical simulations show that the flexural gravity waves propagate as beams the orientation of these beams is determined by the direction of the open ocean waves incident on the ice shelf front the higher frequency ocean waves cause larger flexural stresses while lower frequency waves can propagate farther away from the ice shelf front and cause flexural stresses in the vicinity of the grounding line

the effects of basal melting on the present flow of the ross ice shelf antarctica on utilise un modele mixte aux elements finis et differences finies de l ecoulement et du transfert de chaleur avec deux hypotheses fusion nulle partout et fusion equilibrant tout epaississement a grande echelle

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

mu radar and lidar observations of clear air turbulence underneath cirrus abstract turbulence generation mechanisms prevalent in the atmosphere are mainly shear instabilities breaking of internal buoyancy waves and convective instabilities such as thermal convection due to heating of the ground in the present work clear air turbulence underneath a cirrus cloud base is described owing to coincident observations from the vhf 46 5 mhz middle and upper atmosphere mu radar a rayleigh mie raman rmr lidar and a balloon radiosonde on 7 8 june 2006 at shigaraki japan 34 85 n 136 10 e time height cross section of lidar backscatter ratio obtained at 2206 lt 7 june 2006 showed the presence of a cirrus layer between 8 0 and 12 5 km msl downward penetrating structures of ice crystals with horizontal and vertical extents of 1 0 4 0 km and 200 800 m respectively have been detected at the cirrus cloud base for about 35 min at the same time the mu radar data revealed clear air turbulence layers developing downward from the cloud base in the environment of the protuberances

measurements of vertical eddy diffusivity across the tropopause using radio acoustic sounding system rass 1 a four day high temporal resolution mu radar rass data set was obtained under light wind conditions in august 1995 enabling retrieval of virtual temperatures up to 20 km this provided a unique opportunity to study the vertical eddy diffusivity k across the tropopause using rass brunt vaisala frequency n and radar spectral width co located six hourly radiosondes enabled calculation of k using radiosonde n and radar spectral width rass k decreased across the wmo tropopause from 0 39 0 05 m2s 1 in the uppermost kilometre of the troposphere to 0 27 0 03 m2s 1 in the lowermost kilometre of the stratosphere the equivalent radiosonde k were 0 43 0 06 m2s 1 and 0 31 0 07 m2s 1 respectively the wmo tropopause seems to provide a better definition than the cold point tropopause in separating the stratosphere from the troposphere when considering the mixing of minor constituents

modelling intra annual dynamics of a major marine terminating arctic glacier significant intra annual variability in flow rates of tidewater terminating arctic glaciers has been observed in recent years these changes may result from oceanic and or atmospheric forcing through 1 perturbations at the terminus such as enhanced submarine melt and changes in sea ice buttressing or 2 increased surface melt in response to atmospheric warming reaching the bed and promoting glacier slip we examine the influence of these processes on belcher glacier a large fast flowing tidewater outlet of the devon island ice cap in the canadian arctic a hydrologically coupled higher order ice flow model is used to estimate changes in glacier flow speed as a result of changes in sea ice buttressing and hydrologically driven melt season dynamics daily run off from five sub catchments over the 2008 and 2009 melt seasons provides meltwater forcing for the model simulations model results are compared with remotely sensed and in situ ice surface velocity measurements sea ice effects are found to have a minor influence on glacier flow speed relative to that of meltwater drainage which is clearly implicated in short term velocity variations during the melt season we find that threshold drainage is essential in determining the timing of these short lived accelerations

numerical simulations of the ross sea tides tidal currents below the floating ross ice shelf are reconstructed by using a numerical tidal model they are predominantly diurnal achieve maximum strength in regions near where the ice shelf runs aground and are significantly enhanced by topographic rossby wave propagation along the ice front a comparison with observations of the vertical motion of the ice shelf surface indicates that the model reproduces the diurnal tidal characteristics within 20 similar agreement for the relatively weak semi diurnal tides was not obtained and this calls attention to possible errors of the open boundary forcing obtained from global ocean tidal simulations and to possible errors in mapping zones of ice shelf grounding air sea contact below the ice shelf is eliminated by the thick ice cover the dominant sub ice shelf circulation may thus be tidally induced a preliminary assessment of sub ice shelf conditions based on the numerical tidal simulations suggests that 1 strong barotropic circulation is driven along the ice front and 2 tidal fronts may form in the sub ice shelf cavity where the water column is thin and where the buoyancy input is weak

seasonal variability of vertical eddy diffusivity in the middle atmosphere 1 three year observations by the middle and upper atmosphere radar the vertical eddy diffusivity k due to atmospheric turbulence with spatial scales of 100 102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from january 1986 to december 1988 the method of analysis follows lilly et al 1974 sato and woodman 1982 and hocking 1983a 1985 1988 and the contamination due to beam broadening vertical shear and transience has been removed although observations for horizontal wind speeds larger than approximately 40 m s such as occur near the tropopause jet stream in winter have been omitted because of excessive beam broadening sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper troposphere and lower stratosphere 6 20 km altitude and for the mesosphere 60 82 km altitude the monthly median of k shows a local maximum near the tropopause jet stream altitude it becomes larger in the mesosphere increasing gradually with height maxima of k are observed in winter near the tropopause and in summer in the mesosphere and the seasonal variability of k reaches approximately an order of magnitude a semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons

normal modes of the world ocean part iv synthesis of diurnal and semidiurnal tides abstract diurnal and semidiurnal tides of second and third degree are synthesized from 60 normal modes with period in the range 8 to 96 h diurnal tides especially those of second degree can be represented by remarkably few modes the principal lunar diurnal constituent for example consists almost entirely of a single forced mode excited mainly in the pacific and indian oceans semidiurnals are spectrally more heterogeneous and more resonant than diurnals but some specific features can be attributed to individual modes several of the most energetic modes in the principal lunar semidiurnal constituent are prominent in the atlantic ocean together with the fact that diurnally excited modes are relatively weak in that region this presumably accounts for the observed tendency for the total tide to be predominantly semidiurnal in the atlantic but mixed diurnal and semidiurnal in many parts of the pacific and indian oceans

observations of a reversal in long term average vertical velocities near the jet stream wind maximum abstract analysis of vertical velocity measurements made for four days each month over the period from 1986 to 1988 by the mu radar in japan shows a reversal in direction near the peak in the zonal wind profile during the winter months more specifically the reversal is noted during periods when the peak horizontal wind speeds 60 m s 1 the vertical velocities associated with the circulation have magnitudes of 10 20 cm s 1 and the depth of the circulation is of the order of several kilometers in 6 out of 14 cases when the feature was observed the direction of the vertical circulation although not the magnitude could be explained by adiabatic ascent or subsidence along the average potential temperature surface slopes for the observation intervals the direction of the circulation was such that it would tend to produce cooling and heating for the ascent and subsidence respectively that would tend to strengthen or at least maintain the jet in the remaining eight cases the direction of the vertical

diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica 1 recent gps observations from a spatially extensive network across ronne ice shelf show significant daily ice flow variations at all sites the almost synchronous horizontal displacements occur at diurnal and semidiurnal tidal periods during spring tides displacements velocities and strains near the ice front have superimposed oscillations that are 300 of their mean values and occur over a six hour period resulting in regular ice shelf flow reversals close to ice stream grounding lines however the horizontal diurnal and semidiurnal signals decay and almost vanish from our analysis we conclude that ice shelves respond primarily elastically to tidal tilting thus accounting for the observed diurnal and semidiurnal flow variations and their amplification toward the ice shelf front our findings suggest that detailed modeling of these data could provide improved ice shelf and ice stream models for correctly simulating ice shelf flow and predicting future ice sheet evolution citation makinson k m a king k w nicholls and g hilmar gudmundsson 2012 diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica geophys res lett 39 l10501 doi 10 1029 2012gl051636

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

antarctic analog for dilational bands on europa europa s surface shows signs of extension which is revealed as lithospheric dilation expressed along ridges dilational bands and ridged bands ridges the most common tectonic feature on europa comprise a central crack flanked by two raised banks a few hundred meters high on each side together these three classes may represent a continuum of formation in tufts dilational model ridge formation is dominated by daily tidal cycling of a crack which can be superimposed with regional secular dilation the two sources of dilation can combine to form the various band morphologies observed new gps data along a rift on the ross ice shelf antarctica is a suitable earth analog to test the framework of tufts dilational model as predicted by tufts dilational model tensile failures in the ross ice shelf exhibit secular dilation upon which a tidal signal can be seen from this analog we conclude that tufts dilational model for europan ridges and bands may be credible and that the secular dilation is most likely from a regional source and not tidally driven

nonlinear interaction between ocean tides and the larsen c ice shelf system 1 we report on three gps records of flow of the larsen c ice shelf spanning 2 months to 2 years variations in speed are evident at periods from a few hours to 182 days including semi diurnal diurnal and 14 76 days at fortnightly periods the ice shelf varies by 10 from its long term speed but at diurnal timescales it is up to 100 a nonlinear relationship between ocean tides and velocity is required to explain the observed variations in flow as an initial examination we model flow as a non linear function of basal shear stress including tidal perturbations in the ice shelf grounding zone and show that the major features of the observations are reproduced notably the long period signal largely absent from the vertical tidal signal alternative explanations are discussed these observations demonstrate that the ice shelf system is highly sensitive to relatively modest changes in forcing at its boundaries

vertical diffusivity in the lower stratosphere from lagrangian back trajectory reconstructions of ozone profiles 1 we present a simple stochastic dynamical approach to estimate the turbulent vertical diffusivity d in the lower stratosphere from routinely observed ozone profiles first an observed ozone profile is reconstructed using three dimensional back trajectories obtained from analyzed winds and initializing the trajectories with ozone values from the output of a chemistry transport model assuming that diffusion in the vertical follows a simple random walk leading to a gaussian probability distribution for the particle displacements we perform monte carlo simulations with ensembles of particles originating along each point in the vertical profile by choosing different values of d as input in the calculations we generate different profiles that are smoothed through diffusion comparing with the observed profile we can identify that value of d which is in best agreement at an intermediate range of vertical length scales as an upper bound of the actual d for northern midlatitude lower stratospheric conditions during winter over a period of 12 days the best estimate is d 0 1 m2 s 1 or slightly larger the present results are discussed in the context of comparable estimations of vertical diffusivity in the literature