a strong wind event on the ross ice shelf antarctica a case study of scale interactions abstractin situ observations satellite imagery numerical weather prediction and reanalysis fields are used to investigate the synoptic and mesoscale environment of a strong wind event swe at mcmurdo station ross island region on the ross ice shelf antarctica on 10 october 2003 the swe occurred during the passage of a sequence of three mesoscale low pressure systems from the central ross ice shelf to the southwest ross sea a potential vorticity pv analysis showed that the lows drew air of continental origin down the glacial valleys of the transantarctic mountains and onto the ice shelf as a katabatic drainage flow however the analysis indicated that the air mass associated with the swe was of recurved maritime origin drawn in by the second mesoscale low l2 this air mass approached mcmurdo station from the south where interactions with the orography played a critical role in the early stages of the event when the wind speed was less than 10 m s 1 the air was deflected around the topograph

structure of a frontal cyclone a diagnostic study of a mid latitude cyclone has been carried out using routinely available numerical weather prediction model products and imagery the cyclone was intense but not exceptionally so and the results are believed to have some generality the detailed description of the cyclone structure just before it began to deepen rapidly integrates concepts from a number of researchers into a common framework a dominant feature of the cyclone close to its centre was the cloud head a region of cloud with a sharp convex outer edge which formed poleward of the main polar front cloud band the cloud head was caused by two flows that entered it from the east ascending and fanning out within it one flow the cold conveyor belt brought low wet bulb potential temperature w air back into the cloud head from low levels ahead of the warm front the other flow was due to high w air that peeled off from the base of the main warm sector airflow i e part of the warm conveyor belt and travelled in the boundary layer back towards the cyclone centre first undercutting dry air that had earlier descended from the upper troposphere called a dry intrusion and then ascending at the upper boundary of the cloud head above the cold conveyor belt the transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts with a more diffuse frontal region in between frontal fracture the two sharp surface cold fronts were associated with narrow cold frontal rainbands line convection one of the line convection segments forming the southern edge of the cloud head the overrunning of the dry intrusion in the region of the frontal fracture led to a structure known as a split front in which an upper level humidity front began to run ahead of the position of the surface cold front a feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air the cold conveyor belt in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper level jet

assimilation of scatterometer winds into surface pressure fields using a variational method a variational formulation was used to assimilate seasat a scatterometer sass surface wind measurements near and during a severe storm in the north atlantic into conventional national meteorological center sea level pressure fields an estimate of the relative vorticity at every point on a grid was calculated using each of these two data sets a solution to a modified geostrophic stream function is found subject to the constraints that 1 the relative vorticities calculated from the data agree as closely as possible with the relative vorticities from the variational solution and that 2 the average kinetic energy is a minimum results are obtained which support the idea that averaged satellite data can be treated as synoptic data direct substitution rather than a time weighted insertion made from sass winds generally resulted in more accurate pressure analyses in addition this relatively simple model provides surface pressure fields which agree extremely well with surface truth and the results of other investigators who required additional sources of input data into more complex models it will be possible to obtain improved wind field maps from future scatterometer pressure fields in mid latitudes

the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

autumnal precipitation distribution on the southern flank of the alps a numerical model study of the mechanisms during the autumn there are frequent events of heavy precipitation on the southern slopes of the meso scale alpine chain moreover the accompanying climatology exhibits a distinctive spatial distribution with some meso scale regions of enhanced precipitation such as the wet spot in the vicinity of lago maggiore possible mechanisms to account for the observed spatial distribution are established using a heuristic model based approach it is shown using a mesoscale model that the response to a steady horizontally uniform and moisture laden flow impinging upon a comparatively realistic representation of the alpine chain can reproduce both the overall meso scale precipitation distribution and its meso scale sub structure the nature of the simulated flow regime over around or intermediate is sensitive to the strength of the ambient geostrophic flow and its modification in the planetary boundary layer contrariwise the simulated precipitation distribution in the lago maggiore wet spot is comparatively insensitive to the changes in direction of the incident geostrophic flow from south to south west and in strength in the range 10 30 m s 1 although there is a marked change in the structure of the near alpine flow this systematic precipitation enhancement appears to be related to the relative strength and compensating effects of an easterly barrier jet tracking along the zonally elongated part of the alps and an air stream channelled between the maritime alps and the apennines further evidence to the above interpretation of the meso scale precipitation distribution is given with simulations conducted using a smoothed and schematic representation of the alpine chain likewise the sensitivity to the presence of a lago maggiore like indentation of the topography is tested and evidence is provided of a further indentation induced precipitation enhancement at the local scale copyright 2004 royal meteorological society

lower mantle velocity inhomogeneity observed at grf array grf broadband array recordings provide strong support for the existence of anomalous p coda phases along the kuriles european path first observed using wwssn data the data suggest that the phase termed here pdp arrives 3 5s after the direct p wave and has a slowness 0 7 0 8 s deg smaller the latter measurement precludes pdp s misidentification as a source stopping phase and given the magnitude of the slowness difference slab multipathing and diffraction are equally unlikely because the phase is clearly not observed for all kurile events and from timing and geometric considerations crustal reverberations are also untenable as an explanation for pdp s appearance the hypothesis which best explains the pdp arrival time arrival angle and waveform is the presence of a 3 velocity jump in the lower mantle approximately 290 km above the core mantle boundary because of the failure of other studies to observe pdp under favorable recording conditions the reflector must be of limited regional extent beneath northern siberia

effects of wind stress and wind stress curl variability on coastal upwelling abstract aircraft measurements during the winter 1989 shelf mixed layer experiment smile and summer 1982 coastal ocean dynamics experiment code were used to characterize the spatial variation of the low level wind and wind stress over the northern california shelf the curl of the wind stress was calculated from directly measured turbulent stress components the accuracy of the computed curl was estimated to be adequate to map the spatial structure wintertime measurements showed a concentration of large positive curl over 1 pa 100 km 1 west of point arena regardless of wind direction due to the effects of the coastal topography on the wind fields results from summertime measurements showed a similar local maximum of positive curl west of point arena larger curl values over 3 5 pa 100 km 1 however were observed across a hydraulic jump propagating from stewarts point for highly supercritical marine boundary layer flow a two layer vertically integrated numerical model of coastal upwellin

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

the structure of the marine inversion layer off the central california coast mesoscale conditions abstract data collected on board an instrumented aircraft during the marine atmospheric boundary layer experiments west coast mables wc field study are examined the data were collected at several levels in the marine and inversion layers approximately 200 km west of the central california coast during august 1978 composite sounding and cross sections of wind and temperature variables are presented to provide an overview of the mesoscale structure of the offshore marine and inversion layers at the time of mables wc under typical summertime synoptic conditions which prevailed early in the study period three flights examined a strong inversion is found base heights and temperature increases across the inversion base are comparable to values observed in the san francisco bay area the topography of the inversion base shows a slope downward toward the east during all three of these flights and also suggests the presence of convective activity gravity wave activity or both during two of the three

observed dynamics of coastal flow at cape mendocino during coastal waves 1996 abstract airborne in situ and remote meteorological measurements from around cape mendocino california sampled during the coastal waves 1996 field program are analyzed for three different days 7 12 and 26 june 1996 two days conformed to typical summertime conditions with a strong northerly downcoast flow while the wind on the third day was weaker on the first 2 days the flow was supercritical in the sense that the froude number was larger than unity and these 2 days feature expansion fans in the lee of the cape on the third day no such phenomenon was observed all 3 days had a strong thermal wind caused by the marine inversion sloping down toward the coast on the first 2 days the flow aloft was westerly or northerly so that the thermal wind added to the background flow results in a strong jet on the third day the flow aloft was southerly and consequently even with the added thermal wind the northerly flow in the marine layer was too weak to be supercritical the main difference between t

observations of large wind shear above the marine boundary layer near point buchon california abstractparticularly strong winds along the coast of southern california on 24 may 2012 were measured by the wyoming king air research aircraft during the precision atmospheric marine boundary layer experiment preamble the fast flow is bounded laterally by the coastal topography and vertically by a pronounced temperature inversion separating the cool moist air in the marine boundary layer mbl from the warm dry air aloft many studies have investigated the response of this two layer flow to changes in the coastline by invoking hydraulic theory which explains the essential characteristics including changes in mbl depth and the attendant wind processes occurring just above the mbl are important to the low level thermodynamic and kinematic structure observations on this day demonstrate how the large shear above the mbl can impact the lower atmosphere a typical two layer system was observed north of point buchon which was supercritical around point buchon the depth of the mbl decreased and wind i

multiple phase analysis of the shear velocity structure in the d region beneath alaska a large data set more than 990 total waveforms comprised of north american station recordings of a variety of long period s phases s scs sdiff ss sscs sks skks from 36 earthquakes in western pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the d region beneath alaska three hypotheses that have been proposed to explain observed scs precursors and s waveform distortions for this region are appraised with the multiple phase data set and the radial and lateral variations of the shear velocity structure in the 1500 km broad portion of d sampled are also assessed the observed travel time and waveform variations may result from diffraction by near source slab structure scattering by low velocity heterogeneity in the lower mantle or interaction with complex velocity gradients in d associated with a deep mantle velocity discontinuity we utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques and find that the discontinuity model is most consistent with the data subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior which is consistent with extensive local stratification of d the data show sufficient coherence over the 1500 km broad region that we find it useful to present a local average radial model the preferred model sylo has a 2 75 velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the d layer analysis of sks s sks scs and sks skks differential times also indicates that the outermost 100 km of the core below the fast d region underlying alaska must be slower than the prem structure the observations exhibit significant scatter most of which can be attributed to unmodeled heterogeneity in the midmantle and near source regions as well as lateral variations in the d region the complex structure of d that we prefer may be associated with large scale downwelling in the lower mantle but it is not certain whether it is a purely thermal or a combined chemical and thermal effect

adjustment of the summertime marine atmospheric boundary layer to the western iberia coastal morphology during summer june july and august northerly winds driven by the azores anticyclone are prevalent over western iberia the quick scatterometer satellite 2000 to 2009 summertime estimates reveal a broad high wind speed 7 ms 1 area extending about 300 km from shore and along the entire iberian west coast nested in this large high speed region preferred maximum regions anchored in the iberian major capes finisterre roca and s vicente are found composite analyses of wind maxima were performed to diagnose the typical summertime synoptic scale pressure distribution associated with these smaller size high speed regions the flow low level structure was further studied with a mesoscale numerical prediction model for three northerly events characterized by typical summertime synoptic conditions a low level coastal jet setting the background conditions to the marine atmospheric boundary layer mabl response to topography was found in the three cases the causes for wind maximum downwind capes were investigated focusing on the hypothesis that western iberia mabl responds to hydraulic forcing for the three events supercritical and transcritical flow conditions were identified and expansion fan signatures were found downwind each cape aircraft measurements performed during one of the events gave additional evidence of the expansion fan leeward cape roca the importance of other forcing mechanisms was also assessed by considering the hypothesis of downslope wind acceleration and found to be in direct conflict with soundings and surface observations

spectral analysis of quikscat surface winds and two dimensional turbulence a spectral decomposition of quikscat surface wind vectors reveals different levels of variance and different values of the spectral slope in various regions of the world ocean for the 12 months investigated the traditional considerations on the factors affecting the shape of the spectra are reviewed and compared to the results in particular the influence of large scale synoptic systems is shown by comparing the steeper and more energetic spectra of the midlatitudes to the shallower spectra of the tropics similarly the signature of convection is investigated by comparing spectra in the tropical convectively active and dry zones of the pacific ocean spectra of vorticity and divergence are calculated along with spectral vorticity to divergence ratios their spatial and temporal variations are discussed it is hypothesized that when convection is enhanced in the tropics the spectral analysis captures the mesoscale synoptic structures in which convection is embedded and that the spectra exhibit some of the characteristics of their midlatitude counterparts i e steeper and more energetic

another look at downslope windstorms part i the development of analogs to supercritical flow in an infinitely deep continuously stratified fluid abstract numerical simulations are conducted to examine the role played by different amplification mechanisms in the development of large amplitude mountain waves it is shown that when the static stability has a two layer structure the nonlinear response can differ significantly from the solution to the equivalent linear problem when the parameter nh u is as small as 0 3 in the cases where the nonlinear waves are much larger than their linear counterparts the highest stability is found in the lower layer and the flow resembles a hydraulic jump simulations of the 11 january 1972 boulder windstorm are presented which suggest that the transition to supercritical flow forced by the presence of a low level inversion plays an essential role in triggering the windstorm the similarities between breaking waves and nonbreaking waves which undergo a transition to supercritical flow are discussed

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

transcritical flows in the coastal marine atmospheric boundary layer abstract numerical solutions of shallow water flow in a variable width channel are computed to model the summertime marine atmospheric boundary layer off the u s west coast using an idealization of the coastline in the vicinity of point arena california as an example several steady state base flows are presented that are hydraulically transcritical these flows are strongly nonlinear weakly rotational and are forced by constant pressure gradients and damped by nonlinear bottom drag at the sea surface the transcritical base flows are supercritical in the vicinity of bends in the coastline but are subcritical to the north upstream and to the south downstream where the coastline is straight within the supercritical region orographic bends in the coastline produce expansion fans and compression jumps the same structures found in globally supercritical flows when the imposed pressure gradient forcing is increased the resulting base flow has a supercritical to subcritical transitional jump that

gap flow in an alpine valley during a shallow south fohn event observations numerical simulations and hydraulic analogue this paper examines the three dimensional structure and dynamics of southerly hybrid gap mountain flow through the wipp valley wipptal austria observed on 30 october 1999 using high resolution observations and model simulations the observations were obtained during a shallow south fohn event documented in the framework of the mesoscale alpine programme map three important data sources were used the airborne differential absorption lidar leandre 2 the ground based doppler lidar teaco2 and in situ measurements from the national oceanic and atmospheric administration p 3 aircraft this event was simulated down to 2 km horizontal resolution using the non hydrostatic mesoscale model meso nh the structure and dynamics of the flow were realistically simulated the combination of high resolution observations and numerical simulations provided a comprehensive three dimensional picture of the flow through the wipptal in the gap entrance region brenner pass austria the low level jet was not solely due to the channelling of the southerly synoptic flow through the elevated gap part of the wipptal flow originated as a mountain wave at the valley head wall of the brenner pass downstream of the pass the shallow fohn flow had the characteristics of a downslope windstorm as it rushed down towards the inn valley inntal and the city of innsbruck austria downhill of the brenner pass the strongest flow was observed over a small obstacle along the western side wall the nosslachjoch rather than channelled in the deeper part of the valley just to the east further north the low level jet was observed in the centre of the valley approximately halfway between brenner pass and innsbruck where the along axis direction of the valley changes from north to north north west the low level jet was observed to be deflected to the eastern side wall of the wipptal interaction between the stubaier alpen the largest and highest topographic feature to the west of the wipptal and the south westerly synoptic flow was found to be the primary mechanism responsible for the deflection the along and cross valley structure and dynamics of the flow were observed to be highly variable due to the influence of surrounding mountains localized steep slopes within the valley and outflows from tributaries the gschnitztal and the stubaital to the west of the wipptal for that shallow fohn case observations and simulations provided a large body of evidence that downslope flow created thinning thickening fluid and accelerations decelerations reminiscent of mountain wave hydraulic theory along the wipptal two hydraulic jump like transitions were observed and simulated i on the lee slope of the nosslachjoch and ii in the gschnitztal exit region a hydraulic solution of the flow was calculated in the framework of reduced gravity shallow water theory the down valley evolution of the froude number computed using leandre 2 p 3 flight level and teaco2 measurements confirmed that these transitions were associated with super to subcritical transitions copyright 2002 royal meteorological society

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

low level jets orographic effects and extreme events in nares strait a model based mesoscale climatology abstract a mesoscale atmospheric model nested in operational global numerical weather prediction fields is used to estimate low level winds and surface wind stress through nares strait between ellesmere island and greenland during 2 yr from august 2003 to july 2005 during most of the year the model low level winds are dominated by intense southward along strait flow with monthly mean southward 10 m winds reaching 10 m s 1 in winter summertime flow is weak and distributions of hourly along strait winds during the 2 yr period are strongly bimodal the strong southward low level winds are associated with ageostrophic orographically channeled flow down the pressure gradient from the lincoln sea to baffin bay and are highly correlated with the pressure difference along nares strait the 2 yr means and leading eofs of monthly mean 10 m winds and wind stress place the strongest winds and stress in the southern parts of smith sound and of kennedy channel at the openings to baffin bay and kane basin at

compressible convection with constant and variable viscosity the effect on slab formation geoid and topography two dimensional cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented density variations according to the adams williamson equation of state are included in the case of constant viscosity convection viscous and adiabatic heating damp the flow compressible density variations hardly influence geoid undulations however the lower thermal boundary layer becomes thinner and the mean cell temperature increases in the case of variable viscosity the nonlinear coupling between compression adiabatic and viscous heating and a temperature pressure and stress dependent rheology leads to important consequences the upwelling flow broadens and plumes are retarded the flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low viscosity region this mechanism seems to be important for the formation of subducting slabs extrapolated to mantle conditions two low viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle this process might be aided by an increase of negative buoyancy forces with depth as observed in the models further results are as follows increasing the dissipation number in variable viscosity convection may either damp or speed up convection depending on the rheology models with internal heating and a fixed bottom temperature show that the threshold to time dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended boussinesq approximation

diurnal cycle of supercritical along coast flows abstract in this study a three dimensional hydrostatic mesoscale model is used to address the transient behavior of supercritical along coast flow a control experiment and several sensitivity tests are performed in order to investigate the diurnal cycle of flow characteristics an idealized representation of the northern california terrain is used and the model results are interpreted within the reduced gravity shallow water concept in two preceding studies by the authors this theory appeared to be violated since the flow accelerated along the coastline upstream of the change in coastline orientation even though the flow was supercritical here it is shown that the criticality of the flow for typical summertime conditions along the california coast actually varies diurnally the gradual acceleration of the flow along the upstream coastline is established during a subcritical phase of the simulation thus the shallow water concept is not violated because the along coast jet is primarily driven by

observations simulations and analysis of nonstationary trapped lee waves abstract although considerable understanding of mountain waves has been gained with the aid of the steady state assumption it is clear that mountain waves evolve over time group velocity arguments indicate that this evolution can occur in less than 1 h this study uses observations of trapped lee waves to measure the rate at which their horizontal wavelengths change including a detailed analysis of two events in which such changes are clearly documented in one case doppler lidar observations show a steady increase in horizontal wavelength of 6 h 1 over 4 h and clearly illustrates the relationship between the wave clouds and wave motions in a second case visible satellite imagery reveals an increase in wavelength of 14 h 1 which is related to temporal changes in vertical air motions measured by wind profilers within the wave field hourly vertical profiles of wind and virtual temperature measured by radio acoustic sounding systems rass and wind profilers reveal important changes in the wave env

amplitudes of diffracted long period p and s waves and the velocities and q structure at the base of the mantle frequency and time domain analysis of amplitudes of long period 20 s p and s waves diffracted around the earth s core and registered at the wwnss and csn stations show that the velocities at the base of the mantle have slightly positive gradients this result substantiates the previous ray parameter study of mula and muller 1980 such velocity gradients at the base of the mantle imply that there is no significant departure from homogeneity and or adiabaticity of the base of the mantle from the rest of the lower mantle as suggested by for example bolt 1972 jones 1977 jeanloz and richter 1979 and doornbos and mondt 1979b in addition to this the q factor for the base of the mantle is found not to be less than 250

interpretation of the d zone at the base of the mantle dependence on assumed values of thermal conductivity a common assumption in modeling dynamic processes in the earth s lower mantle is that thermal conductivity is sensibly constant throughout the lower mantle and equal to approximately 4 w m 1 k 1 this value is the result of extrapolation using a linearized theory with estimated parameters for lattice contributions to thermal conductivity furthermore this value is highly dependent on the assumed geotherm excluding theoretical uncertainties in the extrapolation recent experimental data for the pressure dependence of conductivity coupled with a revised geotherm lead to substantially higher estimated values for the lower mantle the hypothesis that the seismically resolved anomalous zone at the base of the mantle d is a thermal boundary layer can be questioned in this case of a higher estimation for thermal conductivity

diagnosis of frontal instabilities over the southern ocean the development of three fronts over the southern ocean is described using seawinds on quikscat scatterometer surface winds and an attribution technique to partition the wind field in three components nondivergent and irrotational components at the scale of the front and the remaining harmonic component or environmental flow induced by the synoptic scale flow the front and the environment in which the front is embedded are analyzed separately a frontal wave is shown to develop out of the first front when the large scale alongfront stretching decreases the environmental flow becomes frontolytic and a connection with the upper levels is established in the second case the stretching remains relatively strong and no frontal wave develops the third front exhibits a developing wave but is not in a favorable configuration with the upper levels the frontal wave does not deepen significantly

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

frequency dependence of q in the mantle underlying the shield areas of eurasia part iii the q model summary a large set of teleseismic body wave data covering a broad range in frequency was analysed to determine the frequency arid depth dependence of q for p and s waves under the northern shield areas of eurasia based on numerous f estimates for p and s waves covering the seismic band between 0 02 and 8 hz a q model of the eurasian shield was constructed the data require a model in which q increases with frequency and which is characterized by q values in the upper mantle that are generally higher than those of global average models the model with the best fit includes a minimum in q between about 100 and 200 km depth and high q values on the order of thousands throughout most of the deeper mantle these results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth preliminary results suggest that t under tectonic regions is higher than t under shield regions for all frequencies over the 0 02 to 8 hz frequency range suggesting that q varies regionally as well as with frequency and depth

an extreme wind event at casey station antarctica model output satellite data and in situ observations are used to investigate the conditions that gave rise to an extreme wind event at the australian casey station 66 27 s 110 53 e on the coast of east antarctica the event took place over the period march 20 22 1992 and resulted in casey station s highest ever wind gust for march 66 9 m s 1 130 knots and 10 m mean winds of near 50 m s 1 the event occurred when a deep low was located just north of the coast and there was high surface pressure inland the rapid deepening of the low took place within a strong baroclinic zone lying north south between a cold trough and a ridge bringing very warm air southward a conceptual model is proposed for the very strong winds experienced at casey station key elements of the model are 1 a synoptic scale high low pressure couplet providing a strengthening pressure gradient 2 entrainment of radiatively cooled air by the supercritical synoptic gradient leading to downslope flow 3 the acceleration of the wind down the lee slope of law dome occurring primarily in response to a topographically induced long period vertically propagating gravity wave and 4 sources of negative buoyancy including prestorm radiatively cooled air and later in the storm maritime air cooled by heat flux to the ice surface the topographically induced gravity wave increases the horizontal temperature difference thus increasing the negative buoyancy of the surface airflow

the dynamics of northwest summer winds over the santa barbara channel a mesoscale model is used to examine the dynamics of northwest flow over the santa barbara channel region three cases are considered each characterized by typical summertime synoptic conditions but with differences in pressure gradient strength and marine boundary layer depth mbl the first case examines a relatively deep mbl and strong pressure gradient case 2 is characterized by a more shallow mbl and weaker pressure gradient and case 3 represents a transition from a deep mbl to shallow conditions in all cases simulated surface winds show reasonable agreement with observations over most of the model domain with the exception of regions near abrupt terrain changes results from the model indicate that the flow with a deep mbl 400 m and strong pressure gradient case 1 is supercritical causing regions of acceleration and expansion in the lee of point conception when the mbl is shallow 150 m case 2 a transcritical flow scenario exists with subcritical flow upstream from point conception and a supercritical flow region over the santa barbara channel and downstream from the channel islands flow over the channel is strongly affected by diurnal heating in shallow mbl cases reversing direction in step with a land breeze circulation induced by nighttime cooling the land breeze forces an internal wave disturbance that propagates westward across the channel eliminating the supercritical flow region in the lee of point conception conditions with a deep mbl 400 m produce less variability in the surface winds except for the region sheltered by the santa ynez mountains an expansion fan is still evident in this case but it is produced by the interaction of the flow with higher terrain north and east of the channel the low hills on point conception and the channel islands do not have a large blocking effect on the surface flow when the mbl is deep analysis of the momentum budget supports the conclusion that the boundary layer behaves like a transcritical hydraulic flow when the mbl is shallow except for the open ocean region the coriolis term is minor in comparison with the pressure and advection terms diurnal heating effects are evident in the nearshore pressure term which varies from offshore during the late evening to onshore in the afternoon these effects are most significant when the mbl is shallow and can augment the hydraulically forced pressure pattern causing a stronger expansion fan in the late afternoon and a collapse of the expansion fan during the early morning

scatterometer and ecmwf derived wind vorticity over the mediterranean basin this article investigates the spatial and temporal structures of the monthly mean vorticity fields and over the mediterranean basin derived respectively from satellite scatterometers and ecmwf global atmospheric model t799 wind fields from february 2006 to january 2010 statistics show a mean difference between and over the whole mediterranean sea relative bias of 0 108 10 5 s 1 furthermore both the cyclonic and the anticyclonic components of are stronger than those of the analysis of the spatial features of and shows that the greatest discrepancies occur along coasts and in the adriatic sea due to the ability of to describe vorticity features smaller than about 6 9 103 km2 roughly corresponding to square areas of side 83 km or circular areas of radius 46 km that are not present in given the effective spatial resolution of the model the temporal features of and have shown a substantially similar trend in anticyclonic and a deep difference in cyclonic circulations again related to the limited ability of to describe small scale vorticity patches as the main source of the differences local characterization of and along six transects across areas where the wind vorticity features are well known has confirmed the results of substantial similarity of and at spatial scales larger than 100 km or so and remarkable differences in those regions such as the adriatic and crete seas affected by vorticity variability at smaller scales copyright 2012 royal meteorological society

coastal jet adjustment near point conception california with opposing wind in the bight abstracttypical spring and summer conditions offshore of california consist of strong northerly low level wind contained within the cool well mixed marine boundary layer mbl that is separated from the warm and dry free troposphere by a sharp temperature inversion this system is often represented by two layers constrained by a lateral boundary aircraft measurements near point conception california on 3 june 2012 during the precision atmospheric mbl experiment preamble captured small scale features associated with northerly flow approaching the point with the added complexity of encountering opposing wind in the santa barbara channel an extremely sharp cloud edge extends south southwest of point conception and the flight strategy consisted of a spoke pattern to map the features across the cloud edge lidar and in situ measurements reveal a nearly vertical jump in the mbl from 500 to 100 m close to the coast and a sharp edge at least 70 km away from the coast in this case it is hypothesized that

a nonhydrostatic mesoscale simulation of the 10 11 june 1994 coastally trapped wind reversal abstract during the summer months the california coast is under the influence of persistent northwesterly flow several times each summer this regime is disrupted by coastally trapped wind reversals ctwr in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast controversy exists as to the physical mechanisms responsible for initiation and maintenance of ctwrs while it is clear that coastal terrain is important in creating the trapped response the precise role played by terrain is unclear in the present study these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10 11 june 1994 ctwr event the results show that the model successfully reproduces many of the observed features of this event including anomalous vertical structure involving the relatively shallow boundary layer with a warm nearly neutral layer above the northward propagation of southerly flow in advance of a tongue of coastal stratus fog and a substantial reduction i

climatology of cyclogenesis mechanisms in the mediterranean abstract a general climatology of the main mechanisms involved in mediterranean cyclogenesis is presented a diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons and in different cyclogenetic regions within the same season it is shown that cyclones that developed over the three most active areas in winter the gulf of genoa the aegean sea and the black sea are essentially subsynoptic lows triggered by the major north atlantic synoptic systems being affected by local orography and or low level baroclinicity over the northern mediterranean coast it is also suggested that cyclones in two or all three of these regions often occur consecutively linked to the same synoptic system in spring and summer thermally induced lows become progressively more important despite the existence of other factors such as the atlas mountains contributing to lee cyclogenesis in northern africa or the extension of the asian monsoon into the eastern

a scheme for improving scatterometer surface wind fields a method is presented for improving quikscat surface wind fields the university of washington planetary boundary layer model is used to retrieve a surface pressure field from any swath of quikscat surface wind vectors an alternate set of surface wind vectors is computed from the newly calculated pressure field the latter can be smoothed and the process can be iterated new surface wind vectors can be calculated where ambiguity removal fails and where measurements are missing the present methodology preserves boundary layer dynamics and is an improvement over a statistical filter

airflow over a two dimensional escarpment i observations observations of forced flow over the highly asymmetric topography of the darling scarp western australia are presented these are typified by shallow surface gradient winds of 10 20 m s 1 with an inherent critical level aloft and a stable layer produced through nocturnal radiational cooling the observations highlight a topographically induced hydraulic jump lee waves or vertically propagating hydrostatic waves depending on the upwind stability and wind profile the vertically propagating waves and hydraulic jump are observed to produce accelerated flow down the escarpment

ocean surface pressure fields from satellite sensed winds abstract the university of washington s planetary boundary layer model is inverted to use remotely sensed satellite scatterometer derived surface winds as input to calculate maritime surface pressure fields the analysis of three different synoptic storm situations is performed using the model and is then compared to conventional national weather service analyses agreement is good isolation of the pbl secondary flow stratification and thermal wind effects in the model revealed that each may be significant under certain conditions however the model shows sensitivity to the thermodynamic features only in a general sense and even a neutral stratification solution gives a good approximation the high density of the scatterometer data produces mesoscale hundreds of kilometers dynamic details which cannot be confirmed by conventional data at this stage

the influence of mesoscale orography on a coastal jet and rainband abstract the role of mesoscale orography along the central california coast in the development and evolution of a coastal jet and rainband is investigated using a high resolution triply nested nonhydrostatic numerical model comparison of the model simulations which use horizontal grid increments of 5 and 2 km on the inner computational meshes with a coastal mesoscale observation network indicates that the finescale structure of the jet and rainband dynamics are adequately simulated although phase and orientation errors occur the observed and simulated near surface winds have maximum speeds that exceed 22 m s 1 and a direction nearly parallel to the coastline and topography force balance analysis indicates that blocking in the lowest 500 m and flow over the coastal range above this layer contribute to mesoscale pressure perturbations including pressure ridging upstream of the coastal mountains which forces the ageostrophic dynamics of the coastal jet pressure perturbations associated with the to

evidence for a p wave velocity anomaly in d a number of strong p coda phases have been observed on european wwssn station short period recordings of intermediate magnitude kurile arc earthquakes which occurred at several different depths of focus these phases have been observed for several different events and cannot be explained by known phases such as pcp or source multiplicity a lower mantle model containing a first order discontinuity with a 2 75 p wave velocity contrast and located about 344 km above the core mantle boundary produces synthetic seismograms which approximately reproduce the observed amplitudes and arrival times of the coda phases however the fact that these phases are not always observed and that their amplitudes vary significantly for different events and stations suggests that the discontinuity at the top of d may be a transition zone and may be laterally variable in thickness and contrast

applicability of reduced gravity shallow water theory to atmospheric flow over topography abstractapplicability of the reduced gravity shallow water rgsw theory to a shallow atmospheric layer capped by an inversion underneath a deep stratified atmosphere over a two dimensional ridge has been investigated using linear analysis and nonlinear numerical simulations two key nondimensional parameters are identified namely and where g is the reduced gravity acceleration h0 is the rgsw layer depth and n and u are the buoyancy frequency and wind speed respectively in the layer above the inversion if j and are around unity or larger the response of the rgsw flow over the ridge can be significantly modified by pressure perturbations aloft any jumplike perturbations in the rgsw layer rapidly decay while propagating away from the ridge as the perturbation energy radiates into the upper layer with j and much less than unity rgsw theory is more adequate for describing rgsw flows in addition inversion splitting occurs downstream of a jump when where ni is the buoyancy frequency in the

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

the summertime low level jet and marine boundary layer structure along the california coast abstract this paper examines the strong summertime northerly low level jet llj that frequently exists along the california coast the persistent synoptic scale pressure distribution north pacific high to the west thermal low to the east and baroclinity created by the juxtaposition of the heated continent and the cool marine layer produce the mean structure of this llj strong diurnal thermal forcing coupled with topographic influences on the flow modulate the jet structure position and intensity a mesoscale model is used to examine many of the complex facets of the llj flow dynamics several sensitivity studies in addition to a control experiment aid in this investigation principal findings of this study include the following the pronounced east west slope of the marine planetary boundary layer mpbl is not due primarily to colder sst values along the coast dynamically forced low level coastal divergence coupled with synoptic scale divergence appears to be dominant in determining mpbl i

supercritical flow interaction within the cape blanco cape mendocino orographic complex abstract supercritical flow interaction occurring in the marine boundary layer between closely spaced coastal capes is investigated with a mesoscale numerical prediction model as an extension of previous work the u s navy s coupled ocean atmosphere mesoscale prediction system coamps is used to perform idealized model simulations with marine layers of varying upstream froude number to elucidate the different flow responses for a single convex bend the impact upon the supercritical flow of introducing a series of closely spaced coastal bends is then investigated the expansion fan is significantly reduced in magnitude and size by the formation of a compression wave at a blocking concave bend approximately 150 km downstream building upon the idealized marine layer response real data forecasts are then examined for several time periods of supercritical flow interaction between cape blanco oregon and cape mendocino california observations from the coastal waves 1996 cw96 field program were colle

melting temperature of the earth s mantle and core earth s thermal structure abstract although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs the temperature distribution in the lower mantle is poorly known models depend strongly on the assumptions of the amount of internal heating and the viscosity profile new melting data on iron to pressures of the outer core 2 mbar and the observed strong decrease of eutectic melting depression in the fe feo fes system with increasing pressure however tightly constrain the temperature at the inner core boundary to slightly less than 5000 k this estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements the resulting temperature at the core mantle boundary of about 4000 k then requires a large temperature gradient at the bottom of the lower mantle of about 1500 k recent findings of the very high melting temperatures o

evidence for a shear velocity discontinuity in the lower mantle beneath india and the indian ocean abstract sh and ssh seismograms are modeled to determine the shear velocity structure in the d region beneath india and the indian ocean the signals show waveform complexities similar to those observed in data sampling the d region beneath alaska the caribbean and eurasia lay and helmberger which have been attributed to a 2 7 shear velocity discontinuity 280 km above the core mantle boundary the new data set consists of long period tangential component recordings at wwssn stations in africa the middle east and europe for 11 intermediate and deep focus indonesian earthquakes in the distance range 70 82 the waveforms show an arrival between sh and scsh with systematic moveout from 89 to 94 there is a strong distortion of the sh waveforms indicating the arrival of several phases closely spaced in time the relative time shifts of similar complexity in the corresponding ssh phases requires a deep mantle origin the depth dependence and moveout of the interference effects are well predicted for both sh and ssh phases by a model with a lower mantle discontinuity alternative explanations of the interference as resulting from receiver reverberations sks contamination multiple source complexity or near source multipathing are ruled out by systematic tests while it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data synthetic waveform modeling using generalized ray theory and reflectivity shows that the data can be well fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions lay and helmberger but with a negative velocity gradient within the d layer

the response of bora type flow to sea surface temperature a non linear two dimensional hydrostatic incompressible numerical model with a higher order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the adriatic coast a non linear large amplitude mountain wave is generated and is broken beneath and within its critical layer due to resonant tuning between the initially single layer atmosphere and the terrain the tuning is governed by the froude number a qualitative and sometimes quantitative analogy exists between the wave breaking unsteady stratified flow and the hydraulic jump steady two layer flow it is also known that the strongest adriatic bora appears during the winter season when the sea surface temperature is typically larger than the ground surface temperature firstly a relatively higher lower sea surface temperature means an additional distortion moderation of the mountain wave and consequently a larger smaller area with bora wind maxima for a relatively higher sea surface temperature a propagating hydraulic jump occurs typically bora maxima are about three to four times larger than the related geostrophic wind 8 m s 1 secondly the presence and importance of the inertial oscillation are indicated since the wave breaking is the vital component of the strongest bora cases there is a relatively large elevated area i e the critical layer with substantial flow decelerations and generally low wind speeds the wave breaking area has a rossby number o 1 hence the earth s rotation appears to be an important part of bora evolution the simulations presented consider generalized bora cases which may pertain to other similar orographic flows

using ascat scatterometer winds to evaluate relative biases in the quikscat derived wind vorticity relative wind vorticity hereafter vorticity is a crucial parameter to understand the spatial features of the wind field in the mediterranean sea which is the area where this study is focused these are particularly interesting because they are often the effects of the interaction between the airflow and the orography wind vorticity has been derived from both quick scatterometer quikscat and advanced scatterometer ascat 12 5 km scatterometer data downloaded from the national aeronautics and space administration physical oceanography distributed active archive center data archive and compared in the period from march to november 2009 the monthly mean fields of the vorticity show discrepancies which need to be understood this paper thus aims to understand the nature of these differences to make the two vorticity data sets compatible and usable as a common data set results have been provided in terms of the relative bias in vorticity which is the mean difference between the ascat a and quikscat q monthly mean vorticities averaged over the entire mediterranean basin and the entire study period this difference 0 093 10 5 0 05 10 5s 1 is mainly due to a relative vorticity bias in the cyclonic component of rather than in the anticyclonic component whose bias is four times smaller this bias does not depend significantly on the variable accuracy of the wind speed and direction across the quikscat swath this study led us to define and analyze the so called vorticity noise which is present particularly in the quikscat derived vorticity to understand if and how it can contribute to the relative bias in vorticity the contribution of this kind of noise on has been found relevant only for the cyclonic vorticity of q by applying a cyclonic denoising to each swath of quikscat 0 016 10 5 0 05 10 5s 1 is obtained drastically reduced with respect to the initial value this may be considered the typical bias over the mediterranean sea between a and q derived from the 12 5 km data after applying the cyclonic denoising to quikscat vorticity fields

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

seismological mapping of fine structure near the base of the earth s mantle the earth s core mantle boundary cmb juxtaposes liquid iron and crystalline silicates and is a region of large vertical thermal gradients the d region which extends up to 200 300 km above the cmb often has elevated shear wave velocity and suggestions of lateral variations in structure1 recent improvements in our ability to assemble and analyse records from regional seismic networks have allowed us to examine long profiles of travel times amplitudes and waveforms from more than a thousand short period seismometers2 we observe across canada and the united states p waves that have grazed the cmb from the powerful nuclear test in lop nor china on 21 may 1992 first arrival travel times and large secondary arrivals are consistent with a 1 5 compressional velocity increase with depth 130 km above the cmb about half the thickness of d in this locality3 our observations together with evidence for the absence of such a thin fast layer in neighbouring regions suggest the presence of lateral heterogeneity in composition or phase at the base of the mantle

strong downslope winds and application of hydraulic like theory an experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions using combined remote and insitu instrumentation the examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability such as at the top of a temperature inversion at the proper height above the mountain top strong downslope winds could occur even for wind directions with a deviation of 60 off the axis perpendicular to the ridgeline as long as the cross mountain wind has a significant value at least 7 ms 1 the developed disturbances are associated with intense downdrafts of the order of 4 5 ms 1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar the same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer abl albeit with weaker intensity fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min further evidence is also provided for the application of the hydraulic like theory under real atmospheric conditions

evidence against a discontinuity at the top of d summary we investigate a recent suggestion of lay helmberger that there is a world wide discontinuity in 5 wave velocity about 250 300 km above the core mantle boundary with an increase of 2 5 3 per cent from above to below their model predicts pronounced pulse form changes of long period sh body waves in the epicentral distance range 95 120 a range which has not been studied by them similar pulse form changes are expected in p wave seismograms under the reasonable assumption of a similar increase in p velocity nine long period seismogram sections of sh and p waves from deepfocus earthquakes below fiji west brazil the sea of japan the sea of okhotsk and the banda sea are presented with practically no indication of the complications expected also the decay constants of pdiff as predicted by the new model differ strongly from observed values thus there is clear evidence against this model the pulse form peculiarities between mantle s and scs in transverse component seismograms at distances from 70 to 80 which lay helmberger interpreted as reflections from the discontinuity of their model must have a different cause

the use of structure functions and spectra from numerical model output to determine effective model resolution abstract the effective model resolution of three numerical weather prediction nwp models is determined from analyses of spatial structure functions and spatial spectra in this paper the effective resolution is defined as the dimensions of the rectangular spatial filter that describes the net effect of all of the nwp model s numerical filtering and smoothing effects these effects are determined by comparison of spatial statistics of the nwp model output with statistical climatologies derived from aircraft data for the upper troposphere and lower stratosphere the comparisons are based on both spatial structure functions and spatial spectra the structure function approach has fewer assumptions and fewer numerical artifacts accurate estimates of nwp effective model resolution require a robust climatology of the spatial statistics which are a function of latitude and location such as over mountainous regions an artifact in the climatology of the velocity statistics resulting from mountain waves is i

numerical study of the diurnal cycle along the central oregon coast during summertime northerly flow abstract a triply nested mesoscale atmospheric numerical model is used to study the dynamics of the diurnal cycle of the summertime lower atmosphere along the central oregon coast simulations of four consecutive days in september 1998 during which the winds were strong and northerly are analyzed comparisons with profiler observations suggest that the model performed well enough to provide a useful estimate of the diurnal circulation during the four days of interest the low level wind pattern has a broad maximum between cape blanco and cape mendocino with a large north south gradient along the oregon coast the low level jet undergoes diurnal horizontal and vertical displacements which partially resemble previous observational and modeling results along the california coast in both the model and the profiler data there is a minimum in northerly wind between 1500 and 1800 utc 0700 and 1000 local time and a double maximum in offshore flow above the marine boundary layer with peaks near 0700 and

evidence of a lower mantle shear velocity discontinuity in s and ss phases teleseismic recordings of direct s and ss body wave phases and their core reflected counterparts scs and sscs from intermediate and deep focus earthquakes are used to analyze the lowermost mantle shear velocity structure beneath alaska a model with a 2 75 shear velocity increase 280 km above the core mantle boundary accurately matches waveform complexi ties in both the s and ss wavetrains variations in source depth produce systematic shifts in the timing of the triplication arrivals between the s and ss travel time branches that are readily ob served in long period wwssn tangential component recordings the systematic range and depth dependence of the observed shifts are well predicted by the discontinuity model and preclude explanations of the waveform complexity as resulting from multi ple ruptures at the source receiver reverberations or near source scattering from slab structure accurate models of the seismic velocity structure in the lower most 200 km of the mantle d region are essential for calcula tions of the thermal history and dynamics of the core and mantle several recent investigations of seismic body waves have indi cated the presence of velocity stratification above the core mantle boundary lay and helmberger 1983 wright et al 1985 while models of long wavelength lateral velocity variations in the lower mantle indicate strong heterogeneities in the d region dziewon ski 1984 clayton and comer 1983 current thermal models indicate a large temperature contrast of about 1000 o between the core and mantle requiring a thermal boundary layer at the base of the mantle jeanloz and richter 1979 loper 1984 this is consistent with the general decrease in velocity gradients observed in the lowermost mantle doornbos 1983 doornbos et al 1986 the combined effects of stratification lateral hetero geneity and decreased velocity gradients complicate interpreta tion of seismic data sampling the d region it is particularly important to resolve whether velocity discontinuities are actually present within d since compositional stratification would strongly affect the thermal boundary layer dynamics as well as the interpretation of the lateral velocity variations this paper presents an analysis of a combination of seismic body waves that is particularly sensitive to the presence of a velocity discontinuity in d lay and helmberger 1983 found evidence for a 2 75 shear velocity discontinuity about 280 km above the core in s waves from intermediate and deep focus earthquakes the long period tangential component seismograms they analyzed show wave form complexities in the distance range 75 o to 95 o that were attributed to a triplication produced by an abrupt velocity in crease this interpretation was criticized by schlittenhardt et al 1985 who argue that such a discontinuity is incompatible with

airflow over a two dimensional escarpment ii hydrostatic flow hydrostatically dominated forced flow over the asymmetric topography of the darling scarp western australia is modelled using a hydrostatic numerical mesoscale model and compared with observations simulations reproduce the essential features of wave overturning and the development of a shooting hydraulic flow this response is dependent on the wind profile but not on the existence of a critical level the high backsheared environment leads to a high ratio of the brunt vaisala frequency to wind speed throughout the profile resulting in short wavelengths high nonlinearity and overturning with the ratio of the halfwidth of the topography to the boundary layer depth being small the flow fields are shown to be sensitive to the boundary layer

the core shadow zone boundary and lateral variations of the p velocity structure of the lowermost mantle abstract the recent determination of high quality short period p wave amplitude profiles near the core shadow zone for three source receiver combinations allows an exploration of lateral variations in p velocity structure at the base of the mantle various radially symmetric models are tested by comparison of the data with amplitudes measured from generalized ray theory synthetics the assumption necessary to justify one dimensional modeling that each of the profiles is most sensitive to scales of heterogeneity larger than the discrete regions sampled is well supported by the coherence of the individual profiles the observed amplitude versus distance profiles exhibit significant regional variations of the apparent shadow zone boundary with as much as a 5 shift in onset distance but it is possible to model the overall behavior using simple regionally varying positive p velocity gradients in the lowermost mantle velocity models with pervasive negative velocity gradients in the d layer are not consistent with the data the modeling indicates that d velocities beneath the north pole are 2 slower than those beneath the central pacific while velocities beneath the north pacific are 1 faster although the simplest class of successful models begins to deviate from the prem reference earth model as much as 690 km above the core mantle boundary these models do not violate global mantle velocity constraints and the theoretical slowness values calculated for the three models are consistent with slowness measurements for the same general regions more complex multiple gradient lower mantle velocity structures with a thinner zone of lateral heterogeneity may be compatible with the p wave data but such detailed structures cannot be resolved by our modeling though bounds can be placed on viable structures the strong lateral variations required by the data support the presence of compositional heterogeneity in d and the data require that at least in several locations the predominant p velocity gradients in d are positive

the novaya zemlya bora and its impact on barents sea air sea interaction 1 novaya zemlya is a mountainous archipelago in the eastern arctic weather station data indicates that southeasterly high speed winds are frequent especially during the winter although it has been proposed that a bora is responsible for these winds there has been no quantitative analysis of the flow in the region here we present the first high resolution climatology of the three dimensional wind field near novaya zemlya the high speed wind events are shown to share many characteristics with bora as well as models of downslope windstorms the highest wind speeds occur where barents sea dense water is observed to form and we show that there is a doubling in the turbulent heat transfer from the ocean to the atmosphere during bora it is argued that the bora plays an important role in the transformation of the atlantic water that passes by the archipelago en route to the central arctic ocean

generalized hydraulic solutions pertaining to severe downslope winds abstract nonlinear steady state solutions for stratified two layer flow over a ridge are found in parameter space these solutions lie between the interfacial case of long and the constant stratification case of smith the solutions predict the depth of the layer which will descend and accelerate qualitative agreement with bora and boulder observations is found

satellite based midlatitude cyclone statistics over the southern ocean 1 scatterometer derived pressure fields and storm tracking 1 a wavelet based method is described for incorporating swaths of surface pressure derived from scatterometer measurements into surface pressure analyses obtained from the european centre for medium range weather forecasts ecmwf the resulting modified pressure fields are used to identify low pressure centers over the southern ocean and to build statistics of midlatitude cyclones during 7 years of the seawinds on quikscat operational period july 1999 to june 2006 the impact of the scatterometer derived pressure swaths is assessed with a statistical analysis of cyclone characteristics central pressure radius depth performed in parallel on the ecmwf and on the modified pressure fields more low pressure centers 5 10 depending on the season are identified with the modified pressure fields in particular incipient lows captured earlier than ecmwf and more short lived mesoscale cyclones with a life span less than 4 days the cyclones identified with the modified pressure fields are characterized by lower central pressure and tighter isobars on average a parallel spectral analysis reveals 1 additional energy at scales less than 2000 km in the modified pressure fields

the impact of seasat a scatterometer data on the numerical prediction of the queen elizabeth ii storm the development of the queen elizabeth ii storm from 1200 ut on september 9 1978 to 0000 ut on september 11 1978 is forecast with a limited area fine resolution 100 km numerical weather prediction model forecasts from initial conditions that include and exclude seasat scatterometer surface winds are performed when the seasat winds are allowed to influence the upper levels of the atmosphere a large positive impact is found

origin of short period signals following p diffracted waves a case study of the 1994 bolivian deep earthquake abstract seismograms of the june 9 1994 bolivian deep earthquake recorded at epicentral distances from 100 to 122 show a train of signals with predominant frequencies between 1 and 2 hz after the arrivals of short period diffracted p waves p diff we investigate the origin of these signals following p diff by analyzing a total of 20 records from the iris broad band network and the short period network of new zealand the arrivals of late signals continue for over 100 s that is two times longer than the estimated source duration of this event subsequent aftershocks which cause the following signals are not expected from the long period records these results indicate that the long continuation of short period signals is not due to the source complexities the signals following p diff have small incident angles and their spectra show peaks at about the same frequencies these characteristics of the following signals exclude the possibility that their origin is shallow structure such as the heterogeneities beneath the stations or upper mantle p diff propagates a long distance within the heterogeneous region near the core mantle boundary we conclude that the short period signals following the main p diff are scattered waves caused by small scale heterogeneities near the core mantle boundary

supercritical marine layer flow along a smoothly varying coastline abstract a model for hydraulically supercritical atmospheric marine layer flow along a smoothly varying coastline is formulated and solved numerically the model is motivated by a recent comparison of code observations to a simple hydraulic theory which suggested the presence of an expansion fan and a compression jump downstream of topographic features the marine layer is modeled as a homogeneous rotating fluid layer decelerated by surface friction and forced by imposed upper level pressure gradients the equations are solved by a characteristic based gridpoint scheme the results indicate that the expansion fan is a robust feature that persists under most conditions in the present more realistic model but is dramatically altered in structure by the presence of friction while the jump may weaken rapidly offshore due mainly to offshore variations of the layer height upstream of the jump the agreement between observations and model predictions is good enough to suggest that a first order description of

present seismic evidence for a boundary layer at the base of the mantle to improve the global coverage needed in investigating the observational consequences of boundary layer models the existing data set has been extended with long period diffracted p and sh from the sro network and short period diffracted p from the norsar array it was found that on average the amplitude decay of diffracted sh is relatively small in accordance with the majority of previously published wwssn data and consistent with the observed weakness in dispersion of this phase these results in combination with the apparent absence of anomalously high dt d can be explained by a relatively thin 100 km low velocity zone at the base of the mantle long period diffracted p wave data probably lack the required resolution for detecting a relatively thin transition zone such a zone with relatively low velocity is allowed by an average of travel time and amplitude data across the core shadow boundary and appears to be required by the norsar observations of short period diffraction far into the shadow however recent results of individual short period p wave studies pertaining to geographically limited regions only including also the norsar observations are partly conflicting unless lateral variations exist on balance current thermal models appear to be consistent with but not required by the present seismic evidence

ocean model validation of the nasa scatterometer winds the suitability of basin scale satellite based scatterometer winds for forcing of numerical ocean models is examined using a reduced gravity primitive equation model of the tropical pacific ocean three surface forcing fields are validated in a comparison of upper layer thickness ult from the ocean model with observed sea level data the forcing fields are the florida state university observed winds winds derived from the nasa scatterometer nscat and stresses derived directly from nscat the sea level data sets are the world ocean circulation experiment fast sea level data set from island measurements and sea level anomalies from topex poseidon results of this comparison demonstrate that while the three model results are qualitatively similar the results are quantitatively better when forcing with the nscat derived stresses this is particularly true in the eastern tropical pacific and in convergent zones where forcing with the nscat stresses can lead to large differences in ult 40 m compared with results from the other two wind products

the structure of the lowermost mantle determined by short period p wave amplitudes summary the seismic velocities in the d region lowermost 200 km of the mantle are recognized to be anomalously low though the details of the velocity structure are not known the structure of d is important in particular whether a smooth velocity model is appropriate or not a smooth decrease in the seismic velocities would be consistent with a thermal boundary layer at the base of the mantle we have used the amplitudes of short period t 1s p and diffracted p waves to investigate the internal structure of d a short period amplitude data set is obtained by using underground nuclear events as sources and applying receiver corrections to the amplitudes receiver effects are largely responsible for the factor of 8 scatter in the amplitudes of the north american wwssn stations applying receiver corrections reduces the scatter to a factor of 2 thereby providing a quantitatively useful amplitude profile into the core shadow using soviet events and north american wwssn stations the d layer beneath the north polar region is well sampled the core shadow at t 1 s begins sharply at a distance of 95 5 and the slope of the amplitude decay is well defined also the amplitudes decrease slightly from 87 to 90 then increase to 95 synthetic seismograms are used to test various earth models with the important conclusion that the amplitudes from smooth d models with a small velocity gradient in d decay too slowly in the shadow this mismatch cannot be satisfactorily explained by random forward scattering or a thin low q layer within d anelastic calculations show that a thin low q layer in d decreases the amplitudes before the shadow with little effect on the decay slope in the shadow all of the features of the observed amplitude profile can be explained as the interference effects of a model that has a low velocity zone in the upper part of d followed by a normal velocity gradient in the lower part of d this type of model polar series also explains the scatter often observed in dt d beyond 90 the interference effects and required velocity changes in d are small and long period amplitudes will respond only to the averaged velocity gradient in d the polar models imply a compositional and or phase change at the top of d thus the preferred seismological model does not allow the d region to be interpreted as a single thermal boundary layer between the mantle and core

cyclone surface pressure fields and frontogenesis from nasa scatterometer nscat winds two extratropical marine cyclones and their associated frontal features are examined by computing surface pressure fields from nasa scatterometer nscat winds a variational method solves for a new surface pressure field by blending high resolution 25 km relative vorticity computed along the satellite track with an initial geostrophic vorticity field employing this method with each successive pass of the satellite over the study area allows this surface pressure field to evolve as dictated by the relative vorticity patterns computed from nscat winds the result is a high resolution surface pressure field that captures features such as fronts and low pressure centers in more detail than national centers for environmental prediction ncep reanalyses while using the actual relative vorticity to adjust the geostrophic vorticity ignores the ageostrophy of surface winds which can be significant in the vicinity of fronts and jet streaks it is a necessary approximation given that the technique uses only surface data the nscat surface pressure fields prove to be nearly as accurate as ncep reanalyses when compared to ship and buoy observations which is an encouraging result given that ncep reanalyses incorporate a myriad of data sources and the nscat fields rely primarily on one source in addition the high resolution relative vorticity fields computed from nscat winds reveal the location of surface fronts in great detail these fronts are verified using ncep analyses in situ data and satellite imagery

quasi periodic bora gusts related to the structure of the troposphere this study provides a new insight into the behaviour of the bora wind gusts wind speed and direction measured with a 1 s sampling interval between 1 december 2001 and 31 january 2002 at senj east adriatic provided a sufficiently large database for a study of the gusts behaviour the performed spectral analysis confirmed the occurrence of pulsations in the bora flow moreover a new type of dynamics involving the onset cessation and reappearance of the pulsations within a single episode has been observed in several cases suggested mechanisms responsible for generating these phenomena have been determined from comparisons of surface wind data with the upstream tropospheric thermodynamical structure derived from zagreb radiosonde data in particular it has been shown that the appearance of an upper tropospheric jet stream results in cessation of the pulsations and the decrease in the jet stream supports quasi periodic gust behaviour as the pulsations are generated by wave breaking the jet stream appearance has been related to the disappearance of the wave breaking region in the lee of the mountain which is in accordance with previous studies of downslope windstorms copyright 2004 royal meteorological society