the hydraulics of a stratified fluid flowing through a contraction the steady hydraulics of a continuously stratified fluid flowing from a stagnant reservoir through a horizontal contraction was studied experimentally and theoretically as the channel narrows the flow accelerates through a succession of virtual controls at each of which the flow passes from subcritical to supercritical with respect to a particular wave mode when the narrowest section acts as a control the flow is asymmetric about the narrowest section supercritical in the divergent section and selfsimilar throughout the channel with increased flow rate a new enclosed self similar solution was found with level isopycnals and velocity uniform with depth

upstream effects of mesoscale mountains abstract the alpine experiment alpex has revealed that low level air is typically diverted around the alps without reaching the mountaintop in pursuit of an understanding of the physical basis of this phenomenon and of its generality we have explored the characteristics of orographic blocking of a rotating continuously stratified quid as revealed in a simple model problem retaining full nonlinear and transient effects hydrostatic dynamics is assumed and the obstacle is taken to be an infinitely long ridge with height h x the key questions treated are the strength of the upstream deceleration of cross mountain flow and the length scale over which the decelerated region extends by means of scale analysis the controlling parameters are found to be the rossby number ro u fl and the froude number fr nhm u where u is the speed of the oncoming flow f is the coriolis parameter l the mountain half width n the brunt vaisala frequency and hm is the maximum mountain height the scale analysis also

origins of aircraft damaging clear air turbulence during the 9 december 1992 colorado downslope windstorm numerical simulations and comparison with observations results from numerical simulations of the colorado front range downslope windstorm of 9 december 1992 are presented although this case was not characterized by severe surface winds the event caused extreme clearair turbulence cat aloft as indicated by the severe structural damage experienced by a dc 8 cargo jet at 9 7 km above mean sea level over the mountains detailed measurements from the national oceanic and atmospheric administration environmental research laboratories environmental technology laboratory doppler lidar and wind profilers operating on that day and from the defense meteorological satellite program satellite allow for a uniquely rich comparison between the simulations and observations four levels of grid refinement were used in the model the outer domain used national centers for environmental prediction data for initial and boundary conditions the finest grid used 200 m in all three dimensions over a 48 km by 48 km section the range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time evolving windstorm forced during a frontal passage this full range of resolution and model complexity was essential in this case many aspects of this windstorm are inherently three dimensional and are not represented in idealized models using either 2d or so called 2d 3d dynamics both the timing and location of wave breaking compared well with observations the model also reproduced cross stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar model results also show that the observed cat derives from interactions between these wavelike jet stream disturbances and mountain forced internal gravity waves due to the nearly east west orientation of the jet stream these two interacting wave modes were orthogonal to each other thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes hvts aligned with the mean flow these hvts remained aloft while they propagated downstream at about the elevation of the aircraft incident and evidence for such a vortex was seen by the lidar the model and observations suggest that one of these intense vortices may have caused the aircraft incident reports of strong surface gusts were intermittent along the front range during the period of this study the model showed that interactions between the gravity waves and flow aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations the simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic downburst of turbulence in the present case the perturbations of the jet stream provide a funnel shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves the physical picture for the upper levels is similar to the surface gusts described by clark and farley resulting from vortex tilting the cat feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind

groundwater level fluctuation in the waimea plains new zealand changes in a coastal aquifer within the last 30 years data for the waimea plains new zealand indicate that the lower confined groundwater aquifer is hydraulically homogeneous and that shallow groundwater levels inland are affected mostly by anthropogenic processes while those near the coast are affected more by sea level variation analysis of long term data for new zealand indicates that sea level has increased continuously but trends are not spatially uniform results from non parametric trend analysis show that rising trends for groundwater levels are predominant in the shallow aquifer both inland on the waimea plains and for recent years near the coast while decreasing trends are evident in the underlying confined aquifer near the coast groundwater level change in the shallow aquifer appears to be more affected by climate change than the lower confined aquifer correlation analysis indicated that groundwater levels are more affected by rainfall during the rainy season than the dry season and more influenced by rainfall inland than near the coast groundwater level declines in the lower confined aquifer near the coast which has its major recharge area inland in the catchment may be substantially affected by groundwater abstraction in inland areas as well as sea level variation but there are little evidences of seawater intrusion meanwhile groundwater recharge over the catchment area has great influence on rising groundwater levels in the shallow aquifer and its recharge is estimated to be 417 8 mm year using chloride concentrations of precipitation and groundwater

a study of mountain lee waves using clear air radar the tropospheric and lower stratospheric response to flow over complex terrain is investigated using observations of vertical and horizontal motions made by a clear air doppler radar stationed at toulon in southern france analysis of the data has disclosed that episodic enhancements in the variability of vertical and horizontal velocities are due to lee waves excited by obstacles located within 5 km of the radar three separate cases of lee wave activity are studied two dimensional linear perturbation theory and existing three dimensional simulation results for appropriate parameter values are used as guides to check against the vertical structure observed by radar the magnitudes of vertical velocities observed during lee wave incidence are well within the range reported in previous observational and numerical simulation studies from a detailed examination of one of the cases of trapped waves the time variation observed by the radar of timescales of less than an hour has emerged as the likely result of small directional changes in the upstream winds causing a bodily rotation of the downstream three dimensional wave pattern centred on the wave generating obstacle the observed tendency of the vertical velocity perturbation to change sign with time above the radar location follows as a consequence of the presence of large horizontal gradients in the motion field which are inherent in the structure of three dimensional lee waves the temporal variability of the lee wave pattern has allowed the radar to sweep out a partial horizontal cross section revealing vertical circulation features that are broadly consistent with gravity wave behaviour however analysis of the effects of finite antenna beam spacing on the sampled data indicates that a spurious phase shift might have been introduced between the vertical and horizontal velocity perturbations observed in the cross section in contrast the measured vertical velocities are not seriously affected by finite beam width effects the lee waves examined in this study are quite moderate nevertheless this circumstance has provided a favourable opportunity to conduct a critical evaluation of the capabilities and limitations of this radar technique for observing lee wave phenomena in general we have been able to show clearly how the incidence and vertical structure of the waves respond to the synoptic scale situation in particular we have been able to distinguish between trapped and vertically propagating waves and to relate a transition between these wave types to synoptic scale changes in the static stability structure

doppler lidar observations of a downslope windstorm abstract during january and february 1987 the noaa wpl pulsed doppler lidar was deployed in the foothills west of boulder colorado to study orographically induced flows over the continental divide on 29 january 1987 the lidar with its unique spatial and temporal data gathering capabilities documented a downslope windstorm affecting the boulder area and the rest of the front range the lidar recorded in detail 1 a low level leeside wind maximum 2 propagating wind gusts exhibiting two distinct periodicities and 3 the eastern edge of a mountain wave feature where a jumplike flow reversal occurred such structures have not previously been observed with comparable detail by conventional in situ and remote sensing instruments the observed phenomena were similar to results obtained from mountain wave numerical models the most notable of the structural similarities was between the observed and modeled wind gusts

terrain induced turbulence over lantau island 7 june 1994 tropical storm russ case study numerical simulations of terrain induced turbulence associated with airflow over lantau island of hong kong are presented lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level this research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of chek lap kok which is located to the lee of lantau intensive ground and aerial observations were taken from may through june 1994 during the lantau experiment lantex this paper focuses on flow associated with the passage of tropical storm russ on 7 june 1994 during which severe turbulence was observed the nature of the environmental and topographic forcing on 7 june 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction high resolution numerical simulations initialized using local sounding data were performed using the clark model the simulation results indicate that gravity wave dynamics played a very minor role in the flow distortion and generation of turbulence as a result of this flow regime relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with tropical storm russ results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62 5 m over local nested domains of about 13 24 km on a side the 125 m model resolution simulated highly distorted flow in the lee of lantau with streaks emanating downstream from regions of sharp orographic gradients at this resolution the streaks were nearly steady in time at the higher horizontal resolution of 62 5 m the streaks became unstable resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125 m resolution simulation the temporally averaged fields changed little with the increase in resolution however there was a three to fourfold increase in the temporal variability of the flow as indicated by the standard deviation of the wind from a 10 min temporal average overall the higher resolution simulations compared quite well with the observations whereas the lower resolution cases did not the high resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies the depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution a physical explanation using simple linear arguments based on the blocking effects of the eddies is presented the nature of the flow separation is analyzed using bernoulli s energy form to display the geometry of the separation bubbles the height of the 80 m2 s22 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125 m resolution are presented as well as one experiment testing the influence of static stability suppressing turbulence development all these tests showed no significant effect implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed

observations and numerical simulations of mountain waves in the presence of directional wind shear three research aircraft observed small amplitude gravity waves over the south western french alps on 13 november 1999 during the mesoscale alpine programme map radiosonde ascents and global positioning system dropsondes deployed by map research aircraft indicate that the upstream flow was characterized by topographic blocking below 2 km and low level directional wind shear associated with a synoptic scale depression that extended above the alpine crest the in situ vertical velocity data from the three research aircraft and backscatter from lidar onboard the low level and upper level aircraft exhibit gravity wave signatures in the lower troposphere that are characterized by relatively short horizontal wavelengths on a scale of 5 km or less the in situ flight level and backscatter data also suggest a rapid decrease in the wave amplitude with height due to a directional critical layer that partially absorbs wave energy in the lower to middle troposphere in concert with a decrease in the scorer parameter with height that traps the wave energy numerical simulation results obtained from the non hydrostatic coupled ocean atmosphere mesoscale prediction system model using a horizontal resolution of 556 m indicate the presence of low level wave breaking above the highest peaks and confirm the presence of trapped waves and a directional critical level results based on linear theory and nonlinear model simulations suggest that the absorption of wave energy associated with the directional critical layer has an important impact on the wave characteristics including the momentum flux copyright 2006 royal meteorological society

on the onset of bora and the formation of rotors and jumps near a mountain gap this study investigates the onset phase of a strong adriatic bora windstorm that occurred on 4 april 2002 the target area is a gap about 20 km wide embedded in the coastal mountain barrier of the dinaric alps that favours strong jet like winds airborne aerosol back scatter lidar measurements on board the dlr falcon research aircraft together with surface and upper air observations are used to verify high resolution numerical experiments conducted with the mesoscale atmospheric model rams and a single layer shallow water model swm especially during the breakthrough phase of the bora the flow at the gap exit exhibits a complex spatial structure and temporal evolution on a transect through the centre of the gap a hydraulic jump forms this is located close to the coast throughout the night and starts to propagate downstream in the early morning on a transect through the edge of the gap a lee wave induced rotor becomes established due to boundary layer separation it starts to propagate downstream about two hours after the jump this flow evolution implies that the onset of strong winds at the coast occurs several hours earlier downstream of the centre of the gap than downwind of the edge of the gap consequently the wind field in the vicinity of rijeka airport located downwind of the gap is strongly inhomogeneous and transient and represents a potential hazard to aviation measured bora winds at the surface exceed 20 ms 1 and the simulated wind speed in the gap wind layer exceeds 30 ms 1 the simulated turbulent kinetic energy exceeds 10 m2 s 2 rams indicates that wave breaking near a critical level is the dominant mechanism for the generation of the windstorm gap jets can be identified downstream of several mountain passes the simulated wave pattern above the dinaric alps the wave decay with height due to directional wind shear and the strong flow descent on the leeward side of the barrier are supported by measured back scatter intensities basic bora flow features including gap jets and jumps are remarkably well reproduced by swm simulations the rams reference run captures observed flow phenomena and the temporal flow evolution qualitatively well a cold low level bias an overestimated bora inversion strength and a slightly too early bora onset are probably related to insufficient turbulent mixing in the boundary layer the amplitude of trapped gravity waves the time of the bora breakthrough and the inversion strength are found to be quite sensitive to the turbulence parametrization copyright 2008 royal meteorological society

climatology of the sierra nevada mountain wave events abstract this note presents a satellite based climatology of the sierra nevada mountain wave events the data presented were obtained by detailed visual inspection of visible satellite imagery to detect mountain lee wave clouds based on their location shape and texture consequently this climatology includes only mountain wave events during which sufficient moisture was present in the incoming airstream and whose amplitude was large enough to lead to cloud formation atop mountain wave crests the climatology is based on data from two mountain wave seasons in the 1999 2001 period mountain wave events are classified in two types according to cloud type as lee wave trains and single wave clouds the frequency of occurrence of these two wave types is examined as a function of the month of occurrence october may and region of formation north middle south or the entire sierra nevada range results indicate that the maximum number of mountain wave events in the lee of the sierra nevada occurs in the m

understanding mountain wave phases in ers tandem dinsar interferogram using wrf model simulation repeat pass spaceborne differential synthetic aperture radar interferometry dinsar is commonly used to measure surface deformation however the phase delay due to the atmospheric water vapor has a significant influence on the accuracy of dinsar results on the other hand the signal delay in dinsar can give the spatial variation information of water vapor during the two image acquisitions there are some ripple like phase signals in dinsar which are probably due to the mountain wave in the study area in this paper the weather research and forecasting wrf model is used to simulate the atmospheric delay and compared to the phase delay derived from dinsar results to understand the mountain wave phases in interferograms the results indicate that the ripple like phase signal in the dinsar phase is due to different intensities of the mountain wave in two acquisitions the wrf model can be used to explain the mechanism of mountain wave phase in dinsar which may then hopefully be used for dinsar atmospheric correction

high resolution observations with mu radar of a kh instability triggered by an inertia gravity wave in the upper part of a jet stream kelvin helmholtz kh instability is likely one of the most important sources of clear air turbulence in the lower atmosphere it produces billows which mix and transport heat and materials vertically in the stably stratified atmosphere billows can also dissipate energy therefore they can affect the larger scale dynamics while only a few direct observations have been reported in the tropopause region in this work the authors report very detailed observations of billow structures around 16 km altitude in the upper part of the jet stream observations were made with very high frequency vhf band mid and upperatmosphere mu radar shigaraki japan 34 85 n 136 10 e whose height resolution was improved with a range imaging technique kh billow structures were observed for at least 2 h and were found to have horizontal wavelengths of about 5 3 km and vertical extents between 0 5 and 1 0 km analysis of wind and temperature profiles measured by radiosondes launched from nearby meteorological stations indicated the presence of nearly monochromatic disturbances likely due to a dominant inertia gravity wave igw superimposed on the background wind field the presence of the igw was also confirmed by analysis of wind profiles measured by the mu radar just before the kh billows were detected by the observations in range imaging mode the igw with vertical and horizontal wavelengths of about 3 5 and 600 km respectively may have been a direct radiation from the jet stream as suggested by recent works and likely played a major role in the onset of the observed kh instability

distal enhanced sedimentation from volcanic plumes insights from the secondary mass maxima in the 1992 mount spurr fallout deposits some tephra fallout deposits show an increase of mass and thickness at distances from the source 100 km areas of secondary mass maximum asmm which demonstrates distal enhanced sedimentation from volcanic plumes we explore development of the asmms during the 1992 august and september mount spurr eruptions usa by combining eld data on the spatial distribution of mass and grain size with 1 simulations of individual particle settling through a homogeneous and horizontally strati ed atmosphere and 2 mesoscale models of the three dimensional wind eld that include the effect of the underlying topography the crosswind and downwind variations of deposit characteristics indicate that the increase of sedimentation at the asmms is not formed solely because of preferential settling of small ash particles 125 m as commonly assumed in aggregation models instead asmm grain sizes correspond to the ne modes of the bimodal total grain size distributions there also appears to be a link between the asmm and the topography the mass local minima occur across the windward ank of 2 km high mountain ranges while the asmms spread on the leeward ank mesoscale models of the three dimensional wind eld show vertical oscillations in the wind over mountainous regions which may enhance mechanisms of en masse sedimentation aggregation hydrometeor formation and particle boundary layers as well as strong spatial variations of the horizontal wind eld in the lower troposphere our study demonstrates the importance of using grain size as well as mass data to constrain the complex processes responsible for particle sedimentation from volcanic plumes

lidar observations of a breaking mountain wave associated with extreme turbulence observations from a doppler lidar which are enhanced by stratospheric aerosols from mount pinatubo s eruption provide unique measurements of mountain waves over the rocky mountains including nearly instantaneous observations of wave breaking in the lower stratosphere the wave breaking is revealed by a flow reversal aloft that marks a localized critical layer created when the wave amplitude was great enough to cause overturning the altitude spatial scale and duration of this wave induced critical layer are documented the mountain waves led to turbulence affecting commercial aircraft including one accident in which a cargo jet lost an engine and part of a wing in extreme turbulence at the same time the lidar was making observations nearby although earlier observations of mountain wave drag assumed wave stationarity over 3 5 h significant changes over less than 2 h are documented here such nonstationarity could contribute to discrepancies remaining between observations and simulations of mountain wave drag

an investigation of turbulence generation mechanisms above deep convection abstract an investigation of the generation of turbulence above deep convection is presented this investigation is motivated by an encounter between a commercial passenger aircraft and severe turbulence above a developing thunderstorm near dickinson north dakota on 10 july 1997 very high resolution two and three dimensional numerical simulations are used to investigate the possible causes of the turbulence encounter these simulations explicitly resolve the convection and the turbulence causing instabilities the configurations of the models are consistent with the meteorological conditions surrounding the event the turbulence generated in the numerical simulations can be placed into two general categories the first category includes turbulence that remains local to the cloud top and the second category includes turbulence that propagates away from the convection and owes its existence to the breakdown of convectively generated gravity waves in both the two and three dimensional calculations th

climatology of high wind events in the owens valley california the climatology of high wind events in the owens valley california a deep valley located just east of the southern sierra nevada is described using data from six automated weather stations distributed along the valley axis in combination with the north american regional reanalysis dataset potential mechanisms for the development of strong winds in the valley are examined contrary to the common belief that strong winds in the owens valley are westerly downslope windstorms that develop on the eastern slope of the sierra nevada strong westerly winds are rare in the valley instead strong winds are highly bidirectional blowing either up northward or down southward the valley axis high wind events are most frequent in spring and early fall and they occur more often during daytime than during nighttime with a peak frequency in the afternoon unlike thermally driven valley winds that blow up valley during daytime and down valley during nighttime strong winds may blow in either direction regardless of the time of the day the southerly up valley winds appear most often in the afternoon a time when there is a weak minimum of northerly down valley winds indicating that strong wind events are modulated by local along valley thermal forcing several mechanisms including downward momentum transfer forced channeling and pressure driven channeling all play a role in the development of southerly high wind events these events are typically accompanied by strong south southwesterly synoptic winds ahead of an upper level trough off the california coast the northerly high wind events which typically occur when winds aloft are from the northwest ahead of an approaching upper level ridge are predominantly caused by the passage of a cold front when fast moving cold air behind the surface front undercuts and displaces the warmer air in the valley forced channeling by the sidewalls of the relatively narrow valley aligns the wind direction with the valley axis and enhances the wind speeds

severe downslope windstorm calculations in two and three spatial dimensions using anelastic interactive grid nesting a possible mechanism for gustiness abstract the clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model application of the conservation condition of kurihara and others is shown to result in significant improvements in the treatment of interactive nesting a significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow this result for the anelastic system is in agreement with the earlier results of phillips and shukla who considered the hydrostatic shallow water system of equations the interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 january 1972 in boulder using both two and three spatial dimensions the three dimensional simulation results in a gustiness signature in the

u s streamflow patterns in relation to the el nino southern oscillation the relationship between the el nino southern oscillation enso and unimpaired streamflow over the contiguous united states is studied the extreme phases of the southern oscillation have been linked to fairly persistent classes of atmospheric anomalies over the low and middle latitudes at regional and global scales of particular interest in this investigation is the identification of regions of land that appear to have strong and consistent enso related streamflow signals the first harmonic extracted from a 24 month enso composite at each station is assumed to be the enso related signal appearing in streamflow anomalies these regions were identified by the similarity in phase of the harmonic vectors the vectorial display of these harmonics over a map of the united states provides the areal extents of enso influence on streamflow coherent and significant streamflow responses to hypothesized enso forcing are found in four regions of the united states the gulf of mexico the northeast the north central and the pacific northwest once an enso event sets in a long range forecasting utility may be available for these regions the results of this analysis which are consistent with previous studies on precipitation and temperature demonstrate the mid latitude hydrologic response to the tropical enso phenomena

aircraft measurements of wave momentum flux over the colorado rocky mountains results are presented from a programme designed to measure the momentum flux associated with standing gravity waves over the central rocky mountains of north america the purpose was to determine momentum losses from the westerlies as they cross a mountainous section of the northern hemisphere where wave drag is believed to be substantial the data from one to three instrumented aircraft operating on 20 flight days mostly during january to march of 1973 are in reasonably good conformity with linear wave theory and also correlate well with surface measurements of downslope winds downward westerly momentum flux averaged 0 5 1 0 dynes cm 2 0 05 0 1 pa the mean westerly flow on the flight days was near the climatological mean for the period of the project although the mean flow over this period as a whole was much weaker than the climatological mean

two and three dimensional simulations of the 9 january 1989 severe boulder windstorm comparison with observations abstract simulations of the 9 january 1989 colorado front range windstorm using both realistic three dimensional 3d orography and a representative two dimensional 2d east west cross sectional orography are presented both coriolis forcing and surface friction drag law formulation were included for all experiments the model results were compared with analyses of doppler lidar scan data available from the surface to 4 km msl provided by the environmental technology laboratory of the national oceanic and atmospheric administration noaa the fully three dimensional simulations with realistic orography used time dependent inflow boundary conditions these experiments were designed in part to assess the ability of mesoscale models to predict the onset and general characteristics of downslope windstorms the present experiments highlight the sensitivity of wind storm onset and positioning of surface gusts to both model resolution and surface physics which is in agreement with previous findings these

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