mesoscale anticyclonic circulations in the lee of the central rocky mountains composite analyses of terrain forced mesoscale anticyclonic circulations over southern wyoming and northern colorado are constructed these suggest two different types of circulations based on upstream flow direction the air within type 1 circulations originates to the west of the continental divide and contains little moisture type 2 circulations form in more moist northerly flow and are sometimes associated with snowbands both types tend to form during the afternoon and dissipate after sunset although type 2 events may follow frontal passages and occur at night case studies of one event of each type suggest that anticyclonic vorticity generation occurs within the lowest kilometer above ground level when that layer is nearly vertically mixed in both potential temperature and velocity an analogy with vorticity generation in mixed layer models is considered and it is shown that the conditions for generating negative vorticity in those models are satisfied in each observed case the mixed layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations and may therefore explain the observed late day snowfall maximum along the front range of colorado

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

climatology of strong intermountain cold fronts abstract motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern utah this paper presents a 25 yr climatology of strong cold frontal passages over the intermountain west and adjoining western united states using conventional surface observations and the north american regional reanalysis strong cold frontal passages are identified based on a temperature fall of 7 c or greater in a 2 3 h period a concurrent pressure rise of 3 hpa or greater and the presence of a large scale 700 hpa temperature gradient of at least 6 c 500 km 1 the number of strong cold frontal passages exhibits a strong continental signature with very few events 10 along the pacific coast and more than 200 events east of the continental divide the number of events increases dramatically from the cascade mountains and sierra nevada to northern utah indicating that the intermountain west is a frequent cold front breeding ground a composite of th

influences of the sierra nevada on intermountain cold front evolution abstractrecent studies indicate that strong cold fronts develop frequently downstream of the sierra nevada over the intermountain west to help ascertain why this paper examines the influence of the sierra nevada on the rapidly developing intermountain cold front of 25 march 2006 comparison of a weather research and forecasting wrf model control simulation with a simulation in which the height of the sierra nevada is restricted to 1500 m roughly the elevation of the valleys and basins of the intermountain west shows that the interaction of southwesterly prefrontal flow with the formidable southern high sierra produces a leeward orographic warm anomaly that enhances the cross front temperature contrast several processes generate this orographic warm anomaly including flow modification by the sierra nevada i e windward blocking of low level pacific air leeward subsidence and increased southerly flow from the mojave desert and lower colorado river basin into the intermountain west diabatic hea

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

evolution and structure of a cold front in an alpine valley as revealed by a doppler lidar the propagation of a cold front and its interaction with foehn winds is investigated in an alpine valley based on observations collected during the field campaign of the mesoscale alpine programme map on 6 november 1999 the key instrument of this study is a doppler lidar that had been operated in the wipp valley austria the cold front approached the european alps from the northwest became distorted at the mountain barrier and entered the east west aligned inn valley near the town of innsbruck primarily via two passes it continued to propagate towards innsbruck from both valley directions as two separate fronts that eventually collided east of innsbruck after part of the cold air had entered the adjacent north south aligned wipp valley a synthesis of doppler lidar measurements with conventional meteorological data including automatic weather stations and radiosondes leads to the conclusion that the cold front in the wipp valley was an atmospheric density current characterized by an elevated head a front relative feeder flow and a typical propagation speed of 7 m s 1 the foehn flow on top of the density current caused strong wind shear and triggered shear flow instability that led to the formation of a turbulent wake behind the head as the density current propagated towards the brenner pass it slowed down the shape of the frontal surface varied in time its inclination of about 10 20 is steeper than previously reported for the inn valley but is consistent with other observations of atmospheric density currents copyright 2010 royal meteorological society

the structure and evolution of a simulated rocky mountain lee trough abstract this paper describes the life cycle of a lee trough associated with the passage of a baroclinic wave over the rocky mountains based on two overlapping simulations by the pennsylvania state university national center for atmospheric research mesoscale model the evolution of the lee trough may be summarized as follows a surface cyclone and lee trough developed and intensified as an upper level short wave trough approached and moved over the rockies near the low center the lee trough was found to be frontogenetical with the frontogenesis being forced by 1 confluent deformation associated with the wind fields of the developing cyclone and lee trough and 2 differential vertical motion along the eastern slopes of the rocky mountains and the adjacent sloping terrain of the high plains in turn the frontogenesis drove an ageostrophic circulation that sharpened the lee trough the lee trough represented a break in trajectory origin with trajectories ending ahead of the lee trough originating over

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

ice sheet variability around the north atlantic ocean during the last deglaciation millennial scale variability in the flux of ice rafted detritus to north atlantic sediments during the last glacial period has been interpreted to reflect a climate forced increase in the discharge of icebergs from ice sheet margins surrounding the northern north atlantic ocean 1 but the relationship between ice sheet variability and climate change is not clear as both the sources of ice rafted detritus and the ice marginal processes are varied and complex 2 4 terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice sheet oscillations and whether the ice sheet or sector was advancing or retreating with respect to climate change here we constrain the age and anatomy of a prominent readvance of the british ice sheet in the northern irish sea region at similar to 14 14 c kyr bp similar to 16 4 calendar kyr bp the analysis indicates that the british ice sheet participated in an iceberg discharge episode known as heinrich event 1 comparison with other terrestrial and marine ice sheet records suggests that the dynamic collapse of the laurentide ice sheet beginning at 14 6 15 0 c 14 kyr bp1 4 similar to 17 2 17 6 calendar kyr bp 5 initiated varied responses from other ice sheet margins around the northern north atlantic region these observations support the argument that the release of icebergs and meltwater during heinrich event 1 disrupted the north atlantic thermohaline circulation 6 8 leading to a delay or reversal of deglaciation of the northern hemisphere and at least as far south as 40 degrees s for two to three thousand years 5 9 10 suggesting a climate forcing and response similar to that of the ensuing younger dryas cold snap 11 12

a case study of the interaction of a mesoscale gravity wave with a mesoscale convective system abstractthis study documents the high amplitude mesoscale gravity wave mgw event of 7 march 2008 in which two mgws strongly impacted the sensible weather over a large portion of the southeast united states these mgws exhibited starkly contrasting character despite propagating within similar environments the primary i e long lived mgw was manifest by a solitary wave of depression associated with rapid sinking motion and adiabatic warming while the secondary short lived mgw was manifest by a solitary wave of elevation mgwel dominated by rising motion and moist adiabatic cooling genesis of the primary mgw occurred as a strong cold front arrived at the foot of mexico s high terrain and perturbed the appreciable overriding flow the resulting gravity wave became ducted in the presence of a low level frontal stable layer and caused surface pressure falls up to 4 hpa the mgw later amplified as it became coupled with a stratiform precipitation system which led to its evolution into an intense

factors affecting cold air outbreaks east of the rocky mountains abstract a two layer isentropic model is used to examine the parameters controlling the severity of cold air outbreaks east of the rocky mountains and in particular the channeling effect of the mountain range the case that is scrutinized is the cold air outbreak of early february 1989 the parameters investigated include a the baro clinicity expressed as the ratio of upper to lower layer zonal scale velocities b the rossby number c the height of the topography d the initial longitudinal position of the cold air mass relative to the mountain ridge and e the presence of a wave in the upper level flow it is found that the rossby number determines the intensity of a cold air outbreak whereas the baroclinicity as well as the height of the orography and the initial position of the cold air mass control the extent to which the outbreak is channeled by the mountain range an upper level trough over the central united states favors the evolution of an outbreak

comparison of mount pinatubo and el chichon volcanic events lidar observations at 10 6 and 0 69 μm analyses of a dense set of lidar observations of the volcanic clouds from mount pinatubo and el chichon primarily over boulder colorado and at a wavelength of 10 59 m reveal notable similarities and differences the importance of tropopause folding events for purging stratospheric debris into the troposphere is confirmed as well as the role of gravitational settling to bring debris into the grasp of the folding mechanism seasonal influences appear more important than time after eruption in both poleward transport and in purging stratospheric material downward we discuss the lengthening of the decay process for the mount pinatubo clouds by either an equatorial reservoir of debris or by eruptions of mount spurr simultaneous observations at 0 69 and 10 59 m help monitor aerosol effective radii versus altitude with good temporal and vertical resolution

numerical simulations of an observed gravity current and gravity waves in an environment characterized by complex stratification and shear abstract numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic two dimensional dry primitive equation model data from one of the most complete documentations to date of gravity waves associated with a gravity current presented in an earlier study are used both to prescribe the gravity current s environment and for validation of the simulated gravity current and its associated gravity waves these comparisons indicate that the gravity current observed by a doppler wind profiler and sodars was well simulated in terms of depth density contrast and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations e g wavelengths were not observed the validated simulations are used to test these tentative hypotheses the simulatio

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

analysis and simulations of a troposphere stratosphere gravity wave model i abstract an analytical model is presented that accommodates nonhydrostatic shearing stratified flow over an obstacle and that can be modified to include a superposed stratosphere with constant wind and higher stability a simulation code is used in parallel with the analytic calculations to demonstrate a methodology for determining the wavelength and magnitude of gravity wave energy reflected and that transmitted by the tropopause

snowfall associated with a terrain generated convergence zone during the winter icing and storm project abstract the longmont anticyclone a region of low level anticyclonic turning and convergence during episodes of northerly winds along the front range of the rocky mountains is documented for a snow event that occurred during the winter icing and storms project the complex terrain in this region especially the barrier to the west and the sloping cheyenne ridge to the north is critical for the formation of this mesoscale feature upward motions related to this persistent convergent region downstream of the cheyenne ridge can strongly influence local snowfall distributions the particular event studied in this manuscript was weakly forced on the synoptic scale through close examination of doppler radar special sounding and surface mesonetwork data the effects of the longmont anticyclone on snowfall were determined the results of the analyses suggest that the convergence triggered convective snowbands in a region of delayed postfrontal cold advection at low levels a series of mesoscale model simulat

deep tropospheric gravity waves created by leeside cold fronts abstract the water vapor channel of the geostationary operational environmental satellite goes 8 reveals narrow 30 km wide elongated 500 1000 km bands that propagate southward on the eastern side of the rocky mountains from colorado to texas two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail and several other cases are summarized the wind profilers observe vertical motions exceeding 1 m s 1 in narrow plumes at the leading edge of the fronts in broader zones in the upper troposphere and in the lower stratosphere these cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images the bands occur when the rocky mountains block either arctic leeside cold fronts coming from the north or northeast or pacific cold fronts coming from the northwest the blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain modified fron

implications of the hydrostatic assumption on atmospheric gravity waves abstract the validity of the hydrostatic approximation is examined for use in predicting the dynamics of topographically generated atmospheric gravity waves lee waves propagating in an atmosphere with realistic wind shear to isolate nonhydrostatic effects linear analytic solutions derived both with and without the hydrostatic assumption are compared the atmospheric profiles of wind and stability are chosen both to render the governing equations analytically tractable and be representative of typical atmospheric conditions two atmospheric models are considered 1 a troposphere only model in which the wind increases linearly with height and the stability is constant and 2 a troposphere stratosphere model which retains the important effect of the vertical wind shear in the troposphere and adds the essential feature of a stability jump at the tropopause the nonhydrostatic trapping effect of wind shear on gravity wave modes is clearly illustrated in the troposphere only atmospheric model in the tro

mountain waves downslope winds and low level blocking forced by a midlatitude cyclone encountering an isolated ridge abstractthe interaction of a midlatitude cyclone with an isolated north south mountain barrier is examined using numerical simulation a prototypical cyclone develops from an isolated disturbance in a baroclinically unstable shear flow upstream of the ridge producing a cold front that interacts strongly with the topography the structure and evolution of the lee waves launched by the topography are analyzed including their temporal and their north south variation along the ridge typical mountain wave patterns are generated by a 500 m high mountain but these waves often exhibit significant differences from the waves produced in 2d or 3d simulations with steady large scale flow structures corresponding to the instantaneous conditions over the mountain in the evolving flow when the mountain height is 2 km substantial wave breaking occurs both at low levels in the lee and in the lower stratosphere despite the north south uniformity of the terrain profile large north south variations are apparent in w

initiation of a solitary wave family in the demise of a nocturnal thunderstorm density current abstract this paper describes the characteristics and evolving nature of a vigorous thunderstorm density current very early in the morning of 9 may 1981 in oklahoma because the ambient lower atmosphere was stratified interesting interactions between the outflow current and the ambient environment resulted the leading portion of the current was modulated by at least three gravity wavelike perturbations of horizontal spacing 12 km which initially coexisted with it however as the current evolved it initiated an undular borelike disturbance which propagated ahead into the stable boundary layer carrying cold outflow air in large amplitude rolls eventually the wave family left the decelerating outflow air in its wake this borelike disturbance resembles the australian morning glory phenomenon and appears to represent an early stage in the development of a solitary wave family the observations resemble other reported morning glories and solitary waves as well their laboratory and numerically simulated

multiscale analysis of a meso β frontal passage in the complex terrain of the colorado front range abstract data from a mesoscale observing network are used to describe the evolution of a complex boundary between a dry air mass near the foothills of the rocky mountains and a shallow moist air mass over the eastern plains synoptic scale analyses revealed that the origin of the moist air mass was associated with lee cyclogenesis mesoscale analyses provided a detailed picture of a localized anticyclonic circulation that developed within the larger scale flow mixing ratio data from the mesoscale observing network indicated the position of the boundary between the air masses it is shown that the cooler moister air on the plains advanced toward and retreated away from the foothills during the evening eventually downslope winds that were opposing the motion of the mesoscale boundary decreased and the anticyclonic circulation on the plains became more organized on an even smaller scale doppler lidar measurements revealed characteristics of the wind flow associated with the mesofront and the interacti

the frontal hydraulic head a micro α scale 1 km triggering mechanism for mesoconvective weather systems abstract measurements from the noaa boulder atmospheric observatory bao 300 m tower the national center for atmospheric research ncar sabreliner aircraft and the noaa goes 5 satellite give evidence for the cross front scale collapse of nonprecipitating surface cold frontal zones to horizontal distances of 1 km or less the leading edges of these frosts possess the characteristic structure of density current flows an elevated hydraulic head followed by a turbulent wake vertical motions at the frontal heads exceed 5 m s 1 at 300 m agl the ascent at the frontal head may act as a 1 km scale triggering mechanism for the release of potential instability and the formation of intense squall line mesoconvection

the role of an apparent mesoscale frontogenetic circulation in squall line initiation abstract a line of severe thunderstorms is observed in satellite imagery to develop explosively from a narrow line of shallow convection at the most rapidly intensifying part of a surface cold front concurrent evaporation of the leading edge of a large area of stratus and stratocumulus clouds behind the front results in the appearance of a mesoscale clear zone adjoining the line convection feature the clear zone enlarges to its maximum width of 65 km less than an hour prior to the genesis of the frontal squall line these observations suggest the possibility that a transverse circulation about the front generated the line convection and clear zone in the upward and downward branches of the circulation respectively and ultimately the squall line analysis of the synoptic surface data indicates the likely presence of a thermally direct frontogenetic circulation at the leading edge of the clear zone the implied frontogenetic process exhibits a rapid e folding time of 3 h corresponding to the develop

low level airflow in southern wyoming during wintertime abstract a number of low level flights were conducted with an instrumented aircraft to investigate wind characteristics in the planetary boundary layer over the low regions of the continental divide in southern wyoming the airflow upwind of the continental divide is stably stratified and as it converses through wyoming s wind corridor rdquo a strong horizontal thermal gradient or baroclinic zone develops the coldest air moves through the northern part of the wind corridor and is ovelain by a very stable layer trapped lee waves are prevalent in the planetary boundary layer of the wind corridor through the corridor the forces normal to the flow an in approximate balance and the flow is an anomalous anticyclonic flow around a low downwind of the wind corridor the airflow forms a convergence zone on its southern boundary with air from north central colorado the convergence zone appears to remain distinct for 100 km downwind of the medicine bow mountains the flow through smaller gaps was also invest

doppler sodar and radar wind profiler observations of gravity wave activity associated with a gravity current abstract observations from two doppler sodars and a radar wind profiler have been used in conjunction with data from a rawinsonde station and a mesoscale surface observation network to conduct a case study of a gravity current entering into an environment containing a nocturnal inversion and an elevated neutral layer on the basis of synoptic and mesoscale analyses it is concluded that the gravity current might have originated either as a scale contracted cold front or as a gust front resulting from thunderstorm outflows observed very near the leading edge of a cold front despite this ambiguity the detailed vertical structure of the gravity current itself is well resolved from the data moreover the vertical velocity measurements provided by the sodars and the radar wind profiler at high time resolution have given unique information about the height structure of gravity waves excited by the gravity current although only wave periods and not phase speeds or wavelengths are directly measured it is p

meteorological tower measurements of a surface cold front abstract measurements from the boulder atmospheric observatory meteorological research tower are used to describe the structure and physical processes of a strong surface cold front analysis reveals that the horizontal gradients in temperature and wind velocity at the front are concentrated within a 200 m distance and measured vertical velocities exceed 5 m s 1 at the leading edge of the front calculations with the miller frontogenesis equation show the magnitude and relative importance of cross frontal confluence versus differential vertical motion tilting in forcing frontogenesis

holocene sea levels last glacial maximum glaciomarine environments and geophysical models in the northern irish sea basin uk newly acquired geological records from the isle of man reveal intercalated intertidal and terrestrial sediments that record a relative sea level rise from ca 3 86 m in the early holocene up to a mid holocene maximum between ca 6 6 and 4 5 cal kyr bp at ca 2 25 m when compared with similar marine records from elsewhere in the northern irish sea basin the sea level data confirm a strong north south trend in glacioisostatic loading but little east to west difference in loading this pattern results from the central northern sectors of the british ice sheet over scotland strongly influencing the glacioisostatic downwarping throughout the northern irish sea basin differential ice thicknesses over the irish sea the lake district and ireland did not cause perturbations of local glacioisostatic signatures the holocene marine data in the central irish sea record a slow termination of global eustatic sea level rise during the early holocene geophysical predictions of relative sea level partially constrained by the holocene rsl observations but extrapolated to the last glacial maximum show the potential for shallow marine conditions in the central irish sea basin and in northeast ireland between ca 21 0 16 0 cal kyr bp however glaciomarine deposits in northeast ireland suggest the model relative sea level predictions are too low and require increased ice loading or a thinner lithosphere to generate extra glacioisostatic depression at the time of deglaciation furthermore predicted deglacial relative sea level for northeast ireland does not match the geological record highlighting complex ice margin ocean interaction during deglaciation on the isle of man where glacioisostatic loading is of a similar magnitude to that across northeast ireland the presence of grounded ice inhibited marine incursion during early deglaciation but the occurrence of glaciolacustrine conditions along the ice margin as it moved northwards supports model predictions that relative sea level fell below present during the late glacial early holocene due to rapid glacioisostatic rebound

the 2002 2003 submarine gas eruption at panarea volcano aeolian islands italy volcanology of the seafloor and implications for the hazard scenario abstract a submarine gas eruption started in november 2002 offshore of panarea volcano aeolian islands italy the intensity of the gas emission and the considerable anomalies of the geochemical and geophysical parameters have alarmed the volcanological community and the italian civil protection agency on the possibility that this activity may represent a volcanic unrest at panarea volcano we used a high resolution marine digital terrain model mdtm with 0 5 m accuracy and detailed underwater surveys by scuba diving to study the exhalation centres and the geological morphological and structural features of the seafloor and to clarify the relationships between gas vents distribution submarine volcanological structures and the recent deformation of this area the underwater surveys revealed an exhalative field characterised by 21 main active gas vents and craters from metres to tens of metres in diameter and a pervasive exhalation from the seafloor hundreds of fossil craters associated with gas pipes and hydrothermal alteration are also present on the seafoor craters pipes and conduits are not associated with juvenile deposits and relate to a long standing history of gas exhalation and eruptions we present the first geological sketch map of the seafloor surrounding the exhalative area and establish the role of ne and nw trending fractures as the main pathways for the gas exhalation

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

convection waves observations of gravity wave systems over convectively active boundary layers widespread gravity wave systems have been found to exist over convectively active boundary layers cbls in the presence of vertical wind shear in contrast to mountain waves these convection waves occur over flat terrain and are ubiquitous over fields of shallow fair weather cumuli or clear air thermals they extend vertically to at least 9km a m s l probably to the tropopause first discovered by glider pilots as thermal waves they have now been systematically investigated by research aircraft during the ncar convection wave project which also incorporates an effort in numerical simulation the flight results are reported in this paper while the numerical simulations are described in a companion paper typical wavelengths of convection waves were found to range from 5 to 15 km average 9 km and typical vertical motion amplitudes from 1 to 3 m s 1 in all cases vertical wind shear exceeded 3 10 3s 1 under these conditions cloud tops were measured to move with relative velocities of 8 to 10 m s 1 against their environment hence they present an obstacle to the surrounding airflow the analogy to mountainous obstacles launching gravity waves into the atmosphere is suggestive and borne out by the aforementioned numerical simulations which describe this process step by step the interaction between the cbl and the overlying stable layers is studied by spectral and cross spectral analysis of aircraft data including coherence and phase relations between wave motions and cloud population underneath consideration is given to other potential sources of the observed gravity waves at this stage the conclusions of our pilot study may be considered tentative some implications for further research are also discussed

a case study of lee waves over the lake district in northern england the structure and intensity of stationary orographically forced over the lake district and northern pennines on he afternoon of 26 november 1991 is determined through a combination of field observations and high resolution numerical modelling observations were provided by the meteorological office s c130 aircraft at four different levels and by five radisondes of differing ascent rates released in rapid succession from an observing station located on the cumbrian coast vertical velocity was inferred from both of these data sources and with the help of satellite imagery it was possible to build up a comprehensive picture of the lee wave structure in addition a non hydrostatic primitive equation model was run with a 2 km grid covering a 180 km square region of the northern pennines in an attempt to simulate the gravity wave motion observed the model was initialized with an upstream profile formed by merging a descent profile obtained from the aircraft with one from the sondes the model orography was defined using a terrain height data set with 500 m resolution and with some smoothing to the model grid after about four hours of integration the model achieves a quasi steady state with strong lower tropospheric lee waves verification on the model response was sought by comparing the vertical velocity in the model along the paths of the aircraft and sondes with that actually deduced from the observations the overall agreement of the simulated amplitude and phase of the lee waves with that observed is very encouraging as found in a different case study shutts 1992 the success of the model in predicting lee wave motion gives strong support for the use of these models for the development of gravity wave drag parametrization schemes the domain averaged surface frictional stress was found to be much larger than the wave drag and consistent with the high roughness lengths likely in the case of mountainous terrain

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