atmospheric rotors and severe turbulence in a long deep valley abstractthe conceptual model of an atmospheric rotor is reexamined in the context of a valley using data from the terrain induced rotor experiment t rex conducted in 2006 in the southern sierra nevada and owens valley california all t rex cases with strong mountain wave activity have been investigated and four of them iops 1 4 6 and 13 are presented in detail their analysis reveals a rich variety of rotorlike turbulent flow structures that may form in the valley during periods of strong cross mountain winds typical flow scenarios in the valley include elevated turbulence zones downslope flow separation at a valley inversion turbulent interaction of in valley westerlies and along valley flows and highly transient mountain waves and rotors the scenarios can be related to different stages of the passage of midlatitude frontal systems across the region the observations from owens valley show that the elements of the classic rotor concept are modulated and at times almost completely offset

mountain waves over mont blanc influence of a stagnant boundary layer abstract a stationary mountain wave embedded in southwesterly flow over mont blanc in the alps was observed simultaneously by three research aircraft and three types of remote sensing gps dropsondes airborne light detecting and ranging lidar and rapid scan satellite imagery these observations provide a basis for testing linear and nonlinear theories of how mountain waves over complex terrain are controlled by the ambient wind profile especially the effects of a low level stagnant layer and the jet stream aloft the layer of blocked flow near the ground reduced the amplitude of the wave generation the strong wind and weak stability in the upper troposphere forced the wave into a decaying evanescent state in spite of this evanescent condition no lee waves were observed the authors resolve this paradox by demonstrating that the stagnant layer below 3 km played an additional role it was able to absorb downward reflected waves preventing the formation of a resonant cavity linear theory inclu

gravity wave excitation during the coastal transition of an extreme katabatic flow in antarctica abstractthe offshore extent of antarctic katabatic winds exerts a strong control on the production of sea ice and the formation of polynyas in this study we make use of a combination of ground ba

emulating sentinel 1 doppler radial ice drift measurements using envisat asar data using data from the envisat advanced synthetic aperture radar asar instrument this paper demonstrates how the high precision radial surface velocity product which will become available with the european space agency s sentinel 1 satellite can complement the analysis of sea ice motion high resolution doppler frequency measurements are used to estimate the subsecond line of sight motion of drifting sea ice in fram strait we compare the method with buoy measurements and a recent cross correlation algorithm for tracking ice between pairs of images maximum speeds measured from the time series were on the order of 20 cm s using our method we measured instantaneous speeds reaching 40 60 cm s

katabatic jumps in the martian northern polar regions abstract martian polar regions host active regional wind circulations such as the downslope katabatic winds which develop owing to near surface radiative cooling and sloped topography many observations stratigraphy from radar profiling frost streaks spectral analysis of ices concur to show that aeolian processes play a key role in glacial processes in martian polar regions a spectacular manifestation of this resides in elongated clouds that forms within the polar spiral troughs a series of geological depressions in mars polar caps here we report mesoscale atmospheric modeling in martian polar regions making use of five nested domains operating a model downscaling from horizontal resolutions of twenty kilometers to 200 m in a typical polar trough we show that strong katabatic jumps form at the bottom of polar troughs with an horizontal morphology and location similar to trough clouds large vertical velocity up to 3 m s and temperature perturbations up to 20 k propitious to cloud formation this strongly suggests that trough clouds on mars are caused by katabatic jumps forming within polar troughs this phenomena is analogous to the terrestrial loewe phenomena over antarctica s slopes and coastlines resulting in a distinctive wall of snow during katabatic events our mesoscale modeling results thereby suggest that trough clouds might be present manifestations of the ice migration processes that yielded the internal cap structure discovered by radar observations as part of a cyclic step process this has important implications for the stability and possible migration over geological timescales of water ice surface reservoirs and overall for the evolution of mars polar caps over geological timescales

observations of downslope winds and rotors in the falkland islands a field campaign aimed at observing the near surface flow field across and downwind of a mountain range on the falkland islands south atlantic is described the objective was to understand and eventually predict orographically generated turbulence the instrumentation was based primarily on an array of automatic weather stations awss which recorded 30 s mean surface pressure wind speed and direction at 2 m temperature and relative humidity for approximately one year these measurements were supported by twice daily radiosonde releases the densest part of the aws array was located to the south of the wickham mountain range across mount pleasant airfield mpa in northerly flow the array provides a detailed study of the flow downwind of the mountain range the dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow during some of these episodes short lived typically 1 hour periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft such events present a significant hazard to aviation at mpa examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors the data are used to show that the downwind fractional speed up is proportional to the non dimensional mountain height based on upstream near surface winds and a depth averaged brunt vaisala frequency diagnosed from radiosonde data similarly a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth averaged brunt vaisala frequency the dependence of the flow behaviour on the froude number defined in the usual way for two layer shallow water flow and ratio of mountain height to inversion height is presented in terms of a flow regime diagram

direct measurements of ocean surface velocity from space interpretation and validation the median doppler shift of radar echoes is analyzed in measurements by envisat s advanced synthetic aperture radar asar over the ocean this doppler centroid differs from a predicted signal based on the predicted motion of the satellite and earth this anomaly converted to a surface doppler velocity u d appears to be of geophysical origin two wide swath images over the gulf stream around cape hatteras suggest that u d contains high resolution information on surface currents while on a global scale u d is found to vary with the wind speed in the range direction a simple quantitative forward model is proposed based on a practical two scale decomposition of the surface geometry and kinematics the model represents the effect of the wind through the wave spectrum and gives u d u 10 u c with u 10 and u c as the 10 m wind speed and quasi eulerian current in the line of sight of the radar projected on the sea surface respectively and as a coefficient function of the wind speed wave development and radar geometry it is found that for an incidence angle of 23 0 3 for moderate winds and fully developed seas this model is validated with a global data set of asar wave mode observations with colocated model winds acquired over the global ocean during the years 2003 and 2004 the doppler signal therefore provides the signed parameter u d that can be used to reduce the wind direction ambiguity in the inversion of high resolution wind fields from sar imagery a qualitative validation of current effects is shown for the english channel where tidal currents dominate thus it should be possible to combine this previously ignored geophysical doppler signal with traditional information on sea surface roughness in order to provide very high resolution wind and current fields

motion tracking of discontinuous sea ice the purpose of this paper is twofold 1 to develop a high resolution sea ice motion tracking system at the geospatial mesoscale 1 100 km2 and 2 to propose an algorithm that measures motion at close proximity to discontinuous regions here we present a motion tracking system that computes differential motion at 400 m resolution and validate the accuracy precision of this system via four studies the first study measures the accuracy against displacements measured from in situ global positioning system gps buoys deployed at the sea ice experiment dynamic nature of the arctic sedna and the surface heat budget of the arctic ocean sheba experiments the estimates are found to be statistically comparable with gps with an average error of 361 9 and 600 6 m for the experiments respectively the second study compares the estimated displacements to those measured by the radarsat geophysical processing system a precision error of 75 7 m is found between the two motion tracking systems the third study uses intensity warping of randomly sampled measurements to evaluate discontinuous motion tracking a one tailed wilcoxon signed rank test is used to validate these measurements at 0 01 results from this paper prove that anisotropic smoothing produces significantly smaller errors at discontinuous locations w 4240 and p 0 001 over conventional isotropic smoothing the fourth study compares displacements measured by anisotropic smoothing against manual measurements this paper demonstrates an average reduction of the estimation error by 50 m with the use of anisotropic smoothing over the conventional isotropic smoothing

some aspects of the effect of surface friction on flows over mountains the effect of surface friction on flow over mountains in stratified fluids is investigated using a two dimensional version of the naval research laboratory s non hydrostatic coupled ocean atmosphere mesoscale prediction system coamps without surface friction flow over a mountain attains a maximum speed at the surface on the lee slope with surface friction the flow speed vanishes at the surface and reaches a maximum near the top of the boundary layer bl corresponding to this wind distribution is a shallow layer of very large vertical wind shear beneath the jet maximum and a weaker but deeper sheared layer elsewhere this strong shear generates maximum turbulent kinetic energy tke at the jet maximum region meanwhile the buoyancy effect associated with the mountain gravity wave inhibits the production of the tke above the jet maximum region the combined effects of shear and buoyancy lead to a very shallow bl on the lee slope in general surface friction reduces the amplitude of mountain gravity waves comparison of the flow fields with the outline of the bl height indicates that flow characteristics correspond closely to the distribution of the bl height it is hypothesized that the reduction in mountain wave amplitude in the presence of surface friction is due to the reduction in the slope of the bl height as compared to the terrain height for narrower and or higher mountains the bl height exhibits a secondary peak on the lee side that further modifies the wave simulations using the bl height as a solid mountain without surface friction confirm that characteristics of flow over the mountain are associated with the distribution of bl height rather than the terrain height field as the non dimensional mountain height increases the magnitude of the effect of surface friction decreases due to decreasing bl depth on the lee side with the result that the bl height profile becomes closer to the terrain height field the effect of surface friction on asymmetric mountains is also investigated without surface friction the pressure drag shows distinctive reduction with a secondary peak on the downstream side but the effect is less when the secondary peak is on the upstream side with surface friction all three mountains have similar pressure drag corresponding to their similar bl height profiles copyright 2003 royal meteorological society

diurnal variation of downslope winds in owens valley during the sierra rotor experiment abstract the impact of diurnal forcing on a downslope wind event that occurred in owens valley in california during the sierra rotors project srp in the spring of 2004 has been examined based on observational analysis and diagnosis of numerical simulations the observations indicate that while the upstream flow was characterized by persistent westerlies at and above the mountaintop level the cross valley winds in owens valley exhibited strong diurnal variation the numerical simulations using the coupled ocean atmosphere mesoscale prediction system coamps capture many of the observed salient features and indicate that the in valley flow evolved among three states during a diurnal cycle before sunrise moderate downslope winds were confined to the western slope of owens valley shallow penetration state surface heating after sunrise weakened the downslope winds and mountain waves and eventually led to the decoupling of the well mixed valley air from the westerlies aloft around local noon decoupled

nonlinear processes in earthquake foci the results of laboratory experiments on biaxially compressed physical models of a seismic source are presented discussed and interpreted in terms of nonlinear dynamics the relation is shown between the degree of seismic pulse coherency expressed through the amplitude frequency spectra development and energy or magnitude in a series of model experiments it has been ascertained that the degree of radiated waves coherency plays a more important role concerning the seismic energy release than the size radius of the seismic source

effects of inversion height and surface heat flux on downslope windstorms abstractsimulations are presented focusing on the role of temperature inversions in controlling the formation and strength of downslope wind storms three mechanisms are examined depending on the relative height of the inversion with respect to the mountain and the stability of vertically propagating mountain waves for low level inversions flows are generated that closely resemble a reduced gravity shallow water hydraulic response with a large vertical displacement of the inversion on the lee side of the mountain for higher level inversion cases simulated flows more closely followed a stratified hydraulic behavior with the inversion acting as a rigid reflective lid in the third mechanism downslope winds were forced by a self induced critical layer located below the inversion height the presence of the inversion in this case had little effect on the resulting downslope winds observations made on the falkland islands show that downslope windstorms may preferentially occur in early morning even withou

source complexity of small earthquakes near matsushiro japan abstract broad band seismograms from a suite of small strike slip earthquakes which occurred in the matsushiro region of honshu japan during the period august 1971 september 1974 and recorded on carnegie seismograph mat are used to study source properties for events ranging in size from 10 11 10 15 nm in an earlier study dysart et al modelled frequency spectra of these events with a single brune pulse and made estimates of moment corner frequency and stress drop they employed cepstral filtering to suppress the effects of structure and to increase the stability of parameter estimates the same 32 waveforms considered here display a notable variation in complexity which cannot be explained by size local structure or stopping phases in order to understand better the time history of the complex events within the suite the sh waveforms were divided into successive time segments and the same modelling procedure was applied to the segments as was applied to the initial sh arrivals cepstral filtering was particularly helpful in reducing the interference effects of the s coda the results indicate that corner frequency changes little from subevent to subevent this suggests that small earthquakes proceed by incremental slippage on the same fault in a sequence of partial stress releases

a new theory for downslope windstorms and trapped mountain waves abstractlinear mountain gravity waves forced by a nonlinear surface boundary condition are derived for a background wind that is null at the surface and increases smoothly to reach a constant value aloft and for a constant buoyancy frequency in this configuration the mountain waves have a critical level just below the surface that is dynamically controlled by the surface and minimum richardson number j when the flow is very stable and when the depth over which dissipations act is smaller than the mountain height this critical level dynamics easily produces large downslope winds and foehns the downslope winds are more intense when the stability increases and much less pronounced when it decreases when j goes below 1 in contrast the trapped lee waves are very small when the flow is very stable start to appear when and can become pure trapped waves e g not decaying downstream when the flow is unstable for for the trapped waves these results are explained by the fact that the critical l

the evolution of lee wave rotor activity in the lee of pike s peak under the influence of a cold frontal passage implications for aircraft safety abstract a lee wave rotor system interacting with an approaching cold front in the lee of pike s peak near colorado springs colorado on 1 april 1997 is studied observationally and numerically dynamical effects associated with the approaching cold front caused the amplification of the evolving lee wave and rotor creating increasingly more hazardous flight conditions for nearby airports the rapidly evolving winds measured by a doppler lidar and 915 mhz wind profilers and simulated by the regional atmospheric modeling system rams produced light to moderate turbulence for a research aircraft making missed approaches at the colorado springs airport during the wave amplification phase as the cold front approached the foothills the lee wave rotor system ended abruptly reducing hazardous flight conditions the doppler lidar s detailed measurements of the lee wave rotor system allowed for an evaluation of rams ability to capture these complex wind features qualitative and quantitative comparisons betw

high resolution sea ice motion estimation with optical flow using satellite spectroradiometer data the purpose of this paper is twofold 1 to propose an approach based on optical flow for the estimation of sea ice motion as an accurate dense and computationally efficient alternative to state of the art pattern matching approaches and 2 to investigate the potential of moderate resolution imaging spectroradiometer modis optical satellite data for combined daily and high resolution motion estimation a series of modis image pairs for a selected region in arctic ocean is employed and sparse pairwise correspondences between nonrigid patches are calculated an edge preserving sparse to dense interpolation is applied followed by variational energy minimization to compute the final optical flow a state of the art multiresolution pattern matching method based on phase correlation and normalized cross correlation is also implemented and evaluated for comparison thorough experimentation with different settings of input data preprocessing as well as varying scales of motion is performed the derived motion vectors are compared with coarser resolution operational sea ice motion vector products from combined buoy and microwave satellite data the proposed optical flow approach clearly outperforms the pattern matching method in most cases both in terms of accuracy and motion vector consistency the estimated motion vectors from the modis images highly correlate with the operational vector products in addition modis provides a vector field with spatial resolution two orders of magnitude higher than the operational products that are able to detect significantly smaller sea ice drifts

topographic control of stratified flows upstream jets blocking and isolating layers optimal solutions to the nonlinear hydrostatic boussinesq equations are developed for steady density stratified topographically controlled flows characterized by blocking and upstream influence these flows are jet like upstream of an isolated obstacle and are contained within an asymmetric thinning stream tube that is accelerated as it passes over the crest a stagnant nearly uniform density isolating layer surrounded by a bifurcated uppermost streamline separates the accelerated flow from an uncoupled flow above the flows are optimal in the sense that for a given stratification the solutions maximize the topographic rise above the blocking level required for hydraulic control while minimizing the total energy of the flow hydraulic control is defined mathematically by the asymmetry of the accelerated flow as it passes the crest a subsequent analysis of the taylor goldstein equation shows that these sheared non uniformly stratified flows are indeed subcritical upstream critical at the crest and supercritical downstream with respect to gravest mode long internal waves the flows obtained are relevant to arrested wedge flows selective withdrawal stratified towing experiments tidal flow over topography and atmospheric flows over mountains

on the diurnal variation of mountain waves abstract the diurnal variation of mountain waves and wave drag associated with flow past mesoscale ridges has been examined using the coupled ocean atmosphere mesoscale prediction system coamps and an analytical boundary layer bl model the wave drag exhibits substantial diurnal variation in response to the change in the atmospheric bl characteristics such as the bl depth shape factor and stability during daytime a convective bl develops characterized by a shallow shear layer near the surface and a deep well mixed layer aloft both of which tend to decrease the wave drag as a result the convective bl could significantly weaken mountain waves and reduce the momentum flux by up to 90 near the surface the flow pattern resembles a potential flow with a surface wind maximum located near the ridge crest during nighttime a shallow stable bl develops and the modulation of wave drag by the stable nocturnal bl is governed by the bl froude number fr if the bl flow is supercritical the drag incr

the lithosphere of the earth as a nonlinear system with implications for earthquake prediction the lithosphere of the earth can be viewed as a hierarchy of volumes from tectonic plates to grains of rock their relative movement against the forces of friction and cohesion is realized to a large extent through earthquakes the movement is controlled by a wide variety of independent processes concentrated in the thin boundary zones between the volumes a boundary zone has a similar hierarchical structure consisting of volumes separated by boundary zones etc altogether these processes transform the lithosphere into a large nonlinear system featuring instability and deterministic chaos from this background some integral grossly averaged empirical regularities emerge indicating a wide range of similarity collective behavior and the possibility of intermediate term earthquake prediction

on the use of doppler shift for sea surface wind retrieval from sar the synthetic aperture radar sar doppler centroid has been used to estimate the scatter line of sight radar velocity in weak to moderate ocean surface current environment the sar doppler centroid is dominated by the directionality and strength of wave induced ocean surface displacements in this paper we show how this sea state signature can be used to improve surface wind retrieval from sar doppler shifts of c band radar return signals from the ocean are thoroughly investigated by colocating wind measurements from the ascat scatterometer with doppler centroid anomalies retrieved from envisat asar an empirical geophysical model function cdop is derived predicting doppler shifts at both vv and hh polarization as function of wind speed radar incidence angle and wind direction with respect to radar look direction this function is used into a bayesian inversion scheme in combination with wind from a priori forecast model and the normalized radar cross section nrcs the benefit of doppler for sar wind retrieval is shown in complex meteorological situations such as atmospheric fronts or low pressure systems using in situ information validation reveals that this method helps to improve the wind direction retrieval uncertainty of the calibration of doppler shift from envisat asar hampers the inversion scheme in cases where nrcs and model wind are accurate and in close agreement the method is however very promising with respect of future sar missions in particular sentinel 1 where the doppler centroid anomaly will be more robustly retrieved

application of catastrophe theory in earthquake hazard assessment and earthquake prediction research abstract catastrophe theory is a mathematical theory for the description of natural and social catastrophic phenomena however its application in the fields of earthquake hazard assessment and earthquake prediction research has not been reported based on real seismicity time series data in a specific region a method for earthquake hazard assessment and earthquake prediction by incorporating cusp catastrophe a 3 is presented through experimental computation of real earthquake sequences the effectiveness of the method is demonstrated it is therefore shown that the catastrophe theory has potential for application in the fields of earthquake hazard assessment and earthquake prediction research

a laboratory study of the effect of frost flowers on c band radar backscatter from sea ice c band images of arctic sea ice taken by the ers 1 synthetic aperture radar show transitory regions of enhanced radar backscatter from young sea ice published field observations associate this increase with frost flower growth and the capture of blowing snow by the flowers to investigate the first part of this phenomenon we carried out a laboratory experiment on the response of c band radar backscatter to frost flowers growing on the surface of newly formed saline ice the experiment took place in a 5 m by 7 m by 1 2 m deep saline water pool located in a two story indoor refrigerated facility at the cold regions research and engineering laboratory sodium chloride ice was grown in this pool at an air temperature of 28 c the frost flowers first appeared on the ice surface as dendrites and then changed to needles as the ice sheet grew thicker and the surface temperatures became colder the frost flowers reached to a height of 10 15 mm and beneath each cluster of frost flowers a slush layer formed to a thickness of approximately 4 mm far field radar measurements of the backscatter from the ice were made at incident angles from 20 to 40 and at approximately 6 hour intervals throughout the 3 day period of the experiment a backscatter minimum occurred early in the flower growth at the time coincident with an abrupt doubling in the ice surface salinity once the full flower coverage was achieved we removed first the crystal flowers and then the slush layer from the ice surface the results for these cases show that the crystals have little impact on the backscatter while the underlying slush patches yield a backscatter increase of 3 5 db over that of bare ice the laboratory results suggest that this relative backscatter increase of approximately 5 db can be used as an index to mark the full areal coverage of frost flowers

sea ice motion tracking from sequential dual polarization radarsat 2 images a new sea ice motion tracking algorithm that operates with two sequential synthetic aperture radar sar radarsat 2 scansar images is presented the feature tracking approach is based on the combination of the phase correlation and cross correlation methods an algorithm for selecting control points a matching technique an approach for filtering out error vectors and a confidence levels setting for output drift vectors were specifically developed in order to increase the system s robustness and accuracy we evaluated ice motion tracking results derived from hh and hv channels of radarsat 2 scansar imagery and formulated a condition where the hv channel is more reliable than the hh channel for ice tracking furthermore we found that the ice motion tracking from the hv channel is not affected by noise floor stripes which are prominent in the cross polarization radarsat 2 scansar images the developed sea ice tracking technology was implemented at the canadian ice service environment canada for operational use the system was successfully run to provide operational support of field work in the arctic ocean in compliance with the united nations convention on the law of the sea in the spring of 2010

mapping the agulhas current from space an assessment of asar surface current velocities 1 over 2 years of surface current information collected in the agulhas current region and derived from the doppler centroid anomalies of envisat s advanced synthetic aperture radar asar are examined the sources of errors and potential use of asar surface current velocities for oceanographic research are assessed asar surface current velocities are compared to surface drifter data and merged altimetry observations maps of sea surface temperature are used to establish the asar s capacity to capture the synoptic circulation discrepancies between observed and predicted asar velocities result predominantly from inadequate wind corrections combined with radar incidence angles below 30 occasionally observed wind induced outliers cause a bias in the estimated asar velocities but do not affect the ability of the asar to systematically image regions of strong surface current flow and shear time averaged maps of asar derived surface current velocity seem able to accurately capture the position as well as the intensity of the agulhas current the ability of the asar to pick up the smaller features of the circulation along the shelf break also shows that variability along the agulhas bank is of the same order of magnitude as that observed in the agulhas retroflection asar surface current velocities offer a very good complement to altimetry in regions where the mean dynamic topography is poorly resolved the quasi synoptic nature of asar acquisitions combined with the relatively high resolution of asar surface current velocities also make it attractive for studies of submesoscale processes and western boundary current dynamics

turbulence characteristics in an elevated shear layer over owens valley abstract characteristics of turbulence in the lower and middle troposphere over owens valley have been examined using aircraft in situ measurements obtained from the terrain induced rotor experiment the two events analyzed in this study are characterized by a deep turbulent layer from the valley floor up to the midtroposphere associated with the interaction between trapped waves and an elevated shear layer kelvin helmholtz kh instability develops above the mountaintop level and often along the wave crests where the richardson number is reduced the turbulence induced by kh instability is characterized by a progressive downscale energy cascade a well defined inertial subrange up to 1 km and large eddies with vertical to horizontal aspect ratios less than unity the turbulence below the mountaintop level is largely shear induced associated with wave steepening and breaking and is more isotropic evaluation of structure functions indicates that while the turbulence energy cascade is predominately dow

regional spectral scaling relations of source parameters for earthquakes in the baltic shield abstract spectral analysis of regionally recorded lg waves is performed to determine source parameters such as seismic moment source radius and stress drop of earthquakes in the baltic shield and to derive regional spectral scaling relations the data consist of about 350 lg phases on short period vertical component analog seismograms from earthquakes with magnitude m l ranging from 2 to 5 2 source radii estimated from the corner frequency show only a slight increase with increasing seismic moment in the range 3 10 18 to 5 10 20 dyn cm while the increase is more rapid for larger events baltic shield earthquakes show increasing stress drops ranging from about 0 1 to 10 bars for increasing seismic moment the relatively low stress drops could in part be explained by bias due to band limited analog data the slope of the curve relating seismic moment to corner frequency is steeper than 3 which suggests a departure from a constant stress drop scaling relation proposed for large earthquakes the derived relationship between the seismic moment m o and magnitude m l is log m o 16 93 1 01 m l for 2 m l 5 2

comparing sar based short time lag cross correlation and doppler derived sea ice drift velocities this paper shows initial results from estimating doppler radial surface velocities rvls over arctic sea ice using the sentinel 1a s1a satellite our study presents the first quantitative comparison between ice drift derived from the doppler shifts and drift derived using time series methods over comparable time scales we compare the doppler derived ice velocities with global positioning system tracks from a drifting ice station as well as vector fields derived using traditional cross correlation between a pair of s1a and radarsat 2 images with a time lag of only 25 min a strategy is provided for precise calibration of the doppler values in the context of the s1a level 2 ocean rvl product when comparing the two methods root mean squared errors rmses of 7 cm s were found for the extra wide ew4 and ew5 swaths while the highest rmse of 32 cm s was obtained for the ew1 swath though the agreement is not perfect our experiment demonstrates that the doppler technique is capable of measuring a signal from the ice if the ice is fast moving however for typical ice speeds the uncertainties quickly grow beyond the speeds we are trying to measure finally we show how the application of an antenna pattern correction reduces a bias in the estimated doppler offsets

an intercomparison of t rex mountain wave simulations and implications for mesoscale predictability abstractnumerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed a basic benchmark and five other test cases are simulated in a two dimensional framework using the same initial state which is based on conditions during intensive observation period iop 6 of the terrain induced rotor experiment t rex in which intense mountain wave activity was observed all of the models use an identical horizontal resolution of 1 km and the same vertical resolution the six simulated test cases use various terrain heights a 100 m bell shaped hill a 1000 m idealized ridge that is steeper on the lee slope a 2500 m ridge with the same terrain shape and a cross sierra terrain profile the models are tested with both free slip and no slip lower boundary conditions the results indicate a surprisingly diverse spectrum of simulated mountain wave characteristics including lee waves hydraulic like jump features and gravity wave breaking the vertical v

lee wave resonances over double bell shaped obstacles lee wave resonance over double bell shaped obstacles is investigated through a series of idealized highresolution numerical simulations with the nonhydrostatic coupled ocean atmosphere mesoscale prediction system coamps model using a free slip lower boundary condition the profiles of wind speed and stability as well as terrain derive from observations of lee wave events over the sierra nevada and inyo mountains from the recently completed terrain induced rotor experiment t rex numerical experiments show that double bell shaped obstacles promote trapped lee waves that are in general shorter than those excited by an isolated ridge while the permissible trapped lee wave modes are determined by the upstream atmospheric structure primarily vertical wind shear the selected lee wave wavelengths for two obstacles that are close or equal in height are dictated by the discrete terrain spectrum and correspond to higher harmonics of the primary orographic wavelength which is equal to the ridge separation distance the exception is the smallest ridge separation distance examined one that corresponds to the owens valley width and is closest to the wavelength determined by the given upstream atmospheric structure for which the primary lee wave and orographic wavelengths were found to nearly coincide the influence two mountains exert on the overall lee wave field is found to persist at very large ridge separation distances for the nonlinear nonhydrostatic waves examined the ridge separation distance is found to exert a much stronger control over the lee wave wavelengths than the mountain half width positive and negative interferences of lee waves which can be detected through their imprint on wave drag and wave amplitudes were found to produce appreciable differences in the flow structure mainly over the downstream peak with negative interference characterized by a highly symmetric flow pattern leading to a low drag state

the intense lee wave rotor event of sierra rotors iop 8 abstract a large amplitude lee wave rotor event observationally documented during sierra rotors project intensive observing period iop 8 on 24 26 march 2004 in the lee of the southern sierra nevada is examined mountain waves and rotors occurred over owens valley in a pre cold frontal environment in this study the evolution and structure of the observed and numerically simulated mountain waves and rotors during the event on 25 march in which the horizontal circulation associated with the rotor was observed as an opposing easterly flow by the mesonetwork of surface stations in owens valley are analyzed the high resolution numerical simulations of this case performed with the coupled ocean atmosphere mesoscale prediction system coamps run with multiple nested grid domains the finest grid having 333 m horizontal spacing reproduced many of the observed features of this event these include small amplitude waves above the sierra ridge decoupled from thermally forced flow within the valley and a l

observations and numerical simulations of subrotor vortices during t rex high resolution observations from scanning doppler and aerosol lidars wind profiler radars as well as surface and aircraft measurements during the terrain induced rotor experiment t rex provide the first comprehensive documentation of small scale intense vortices associated with atmospheric rotors that form in the lee of mountainous terrain although rotors are already recognized as potential hazards for aircraft it is proposed that these small scale vortices or subrotors are the most dangerous features because of strong wind shear and the transient nature of the vortices a life cycle of a subrotor event is captured by scanning doppler and aerosol lidars over a 5 min period the lidars depict an amplifying vortex with a characteristic length scale of 500 1000 m that overturns and intensifies to a maximum spanwise vorticity greater than 0 2 s 1 radar wind profiler observations document a series of vortices characterized by updraft downdraft couplets and regions of enhanced reversed flow that are generated in a layer of strong vertical wind shear and subcritical richardson number the observations and numerical simulations reveal that turbulent subrotors occur most frequently along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes as the subrotors break from the vortex sheet intensification occurs through vortex stretching and in some cases tilting processes related to three dimensional turbulent mixing the subrotors and ambient vortex sheet are shown to intensify through a modest increase in the upstream inversion strength which illustrates the predictability challenges for the turbulent characterization of rotors

initial condition sensitivities and the predictability of downslope winds the sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70 memberensemblesimulationsoftwoprototypicalwindstormsobservedduringtheterrain inducedrotor experiment t rex the10 weakestand 10 strongest ensemblemembersare compositedand compared for each event in the first case the 6 h ensemble mean forecast shows a large amplitude breaking mountain wave and severe downslope winds nevertheless the forecasts are very sensitive to the initial conditions because the difference in the downslope wind speeds predicted by the strong and weak member composites grows to larger than 28 m s 21 over the 6 h forecast the structure of the synoptic scale flow one hour prior to the windstorm and during the windstorm is very similar in both the weak and strong member composites wave breaking is not a significant factor in the second case in which the strong winds are generated by a layer of high static stability flowing beneath a layer of weaker mid and upper tropospheric stability in this case thesensitivitytoinitialconditionsisweakerbutstillsignificant thedifferenceindownslopewindspeeds between the weak and strong member composites grows to 22 m s 21 over 12 h during and one hour before the windstorm the synoptic scale flow exhibits appreciable differences between the strong and weakmember composites although this case appears to be more predictable than the wave breaking event neither case suggests that much confidence should be placed in the intensity of downslope winds forecast 12 or more hours in advance

implications of regime transitions for mountain wave breaking predictability abstract a suite of high resolution two dimensional ensemble simulations are used to investigate the predictability of mountain waves wave breaking and downslope windstorms for relatively low hills and mountains perturbation growth is weak and ensemble spread is small gravity waves and wave breaking associated with higher mountains exhibit rapid perturbation growth and large ensemble variance near the regime boundary between mountain waves and wave breaking a bimodal response is apparent with large ensemble variance several ensemble members exhibit a trapped wave response and others reveal a hydraulic jump and large amplitude breaking in the stratosphere the bimodality of the wave response brings into question the appropriateness of commonly used ensemble statistics such as the ensemble mean in these situations small uncertainties in the initial state within observational error limits result in significant ensemble spread in the strength of the downslope wind speed wave breaking and wave mom

retrieval of sea surface range velocities from envisat asar doppler centroid measurements the processing steps and error corrections needed to retrieve estimates of sea surface range doppler velocities from envisat advanced synthetic aperture radar wide swath medium resolution image products are presented retrieval accuracies based on examination of the corrected doppler shift measurements are assessed the root mean square errors of the doppler shift after bias corrections are found to be 4 7 and 3 9 hz in vv and hh polarizations respectively at 35 incidence angle this corresponds to horizontal doppler velocities of 23 and 19 cm s

influence of lee waves on the near surface flow downwind of the pennines the results of a recent field experiment focusing on the near surface pressure and flow fields downstream of the pennines in northern england are presented the main aim of the experiment is the improvement of wind forecasts downstream of orography trapped lee waves commonly occur in westerly flow in this region and during the experiment there were numerous instances of apparent flow separation indicating the formation of lee wave rotors the spatial variability of the near surface flow in these circumstances is closely linked to the positions of lee wave crests and troughs aloft and appears to be a response to pressure gradients induced by the lee waves for large amplitude waves it has been possible to demonstrate a correlation between the fractional change of the flow speed across the measurement array which if large enough may lead to flow separation and a normalized pressure perturbation amplitude for a group of lee wave cases during which the cross mountain flow is strong a rapid decrease in the scorer parameter within the lower portion of the troposphere appears to be a prerequisite for rotors to form however this does not guarantee their occurrence for a fixed scorer parameter profile idealized two dimensional simulations indicate that the lee wave induced pressure perturbation amplitude and hence the occurrence of rotors is controlled largely by the strength of the wind upstream close to the mountain top level it seems that the combination of a favourable scorer parameter profile and sufficiently strong low level winds is required for rotors to develop crown copyright 2007 reproduced with the permission of her majesty s stationery office published by john wiley sons ltd

observations of the temporal evolution and spatial structure of the gap flow in the wipp valley on 2 and 3 october 1999 the investigation of gap flow in the wipp valley gap project is one of the objectives of the mesoscale alpine programme map the valley runs south north across the brenner pass from italy to austria the pass is the lowest one of the main alpine ridge and is therefore a favorable location for a gap flow which is called foehn based on the extensive dataset of map this study gives a detailed analysis of foehn on 2 and 3 october 1999 intensive observation period 5 iop 5 the foehn event began as a gap flow that was separated from midlevel winds by a strong temperature inversion during the night of 1 2 october 1999 on the next night 2 3 october the inversion dissipated and the gap flow was combined with strong midlevel cross ridge flow on 3 october 1999 this study shows that the existence of a temperature inversion above the gap flow has a crucial impact on the flow structure another emphasis of the study was the investigation of the small scale flow structure downstream of the gap jumplike features and regions with flow reversals which were interpreted as rollers reversed rotors could be observed in the wipp valley a jet layer with a wavy structure indicated a gravity wave in the southern part of the valley in the northern part the flow showed strong asymmetry with wind speeds nearly twice as strong on the eastern side as on the western one