on temperature inversions and the mesospheric surf zone 1 mesospheric thermal inversions are investigated in a numerical simulation with the whole atmosphere community climate model an upward extension of the national center for atmospheric research s community climate model the seasonal character spatial extent and magnitude of the inversion layers are realistic during winter in the model the occurrence of wintertime inversions is a direct consequence of the rapid decay with height of vertically propagating planetary waves which induces large temperature perturbations in the upper mesosphere to maintain hydrostatic equilibrium the magnitude of the inversions is highly correlated with planetary wave amplitude so that large inversions develop during episodes of planetary wave amplification gravity waves do not play a major direct role in the formation of the inversions because the largest thermal tendencies associated with gravity wave breaking occur well above the range of altitudes where inversions are found however gravity waves play an essential indirect role because they set up a critical line in the upper mesosphere where rossby waves break in the mesospheric surf zone

measurements of gravity wave activity within and around the arctic stratospheric vortex lidar measurements of gravity wave activity have been conducted at eureka in the high arctic since 1993 the waves are detected by the fluctuations they induce in temperature it has been found that the amount of wave energy in the upper stratosphere is related to the position of the stratospheric polar vortex in each of the four winters reported here the wave activity was a maximum within the westerly jet at the edge of the vortex a minimum inside the vortex near its centre and intermediate outside the vortex the spectra of wave induced fluctuations show that it is at the longest resolved vertical wavelengths 8 to 15 km that wave energy is being influenced by the background meteorological conditions these findings are interpreted in terms of the doppler shifting and critical level filtering that is imposed by the background wind profile

a study of a self generated stratospheric sudden warming and its mesospheric lower thermospheric impacts using the coupled time gcm ccm3 1 a stratospheric sudden warming episode was self consistently generated in the coupled national center for atmospheric research thermosphere ionosphere mesosphere and electrodynamics general circulation model climate community model version 3 time gcm ccm3 taking advantage of the unique vertical range of the coupled model ground to 500 km we were able to study the coupling of the lower and upper atmosphere in this warming episode planetary wave 1 is the dominant wave component in this warming event analysis of the wave phase structure and the wave amplitude indicates that the wave may experience resonant amplification prior to the peak warming the mean wind in the high latitude winter stratopause and mesosphere decelerates and reverses to westward due to planetary wave forcing and forms a critical layer near the zero wind lines the wind deceleration and reversal also change the filtering of gravity waves by allowing more eastward gravity waves to propagate into the mesosphere and lower thermosphere mlt which causes eastward forcing and reverses the westward jet in the mlt this also changes the meridional circulation in the upper mesosphere from poleward downward to equatorward upward causing a depletion of the peak atomic oxygen layer at 97 km and significant reduction of green line airglow emission at high latitudes and midlatitudes planetary waves forced in situ by filtered gravity waves in the mlt grow in the warming episode their growth and interaction with tides create diurnal and semidiurnal variabilities in the zonal mean zonal wind

interpretations of observed climatological patterns in stratospheric gravity wave variance observational analyses of gravity waves in the stratosphere have revealed various climatological patterns in gravity wave activity seasonal geographical and vertical variations have all been observed in this work a linear model of gravity wave propagation is applied to investigate the underlying causes of some of the observed patterns a collection of monochromatic gravity waves that represent a broad spectrum of wavenumbers and frequencies is input at 6 km altitude in the model propagation of the waves through realistic background atmospheric wind and stability fields is treated with linear ray theory and a simple saturation condition to limit amplitudes to stable values the wave spectrum at the 6 km source height is specified to be constant at all latitudes longitudes and times so the variability that appears at higher altitudes is due entirely to background atmosphere variations before the model results are compared to the observations the spectrum of waves is filtered in a way that mimics the limitations of each of the observation techniques the filtering is described in terms of vertical wavelength and is referred to as the observational filter in a vertically varying background wind gravity waves are doppler shifted in intrinsic frequency and refracted to different vertical wavelengths as they propagate vertically through the atmosphere the observational filter and the wave refraction effects can thus couple in interesting ways that have not been explicitly considered in previous work the model shows that this coupling can give rise to geographical seasonal and vertical variations in gravity wave observations without any variations in the spectrum or amplitude of gravity wave sources in the troposphere thus careful consideration of both the background wind profile and observational filter can greatly affect the interpretation of the observed climatological patterns in gravity wave activity

a shear instability seeding mechanism for quasiperiodic radar echoes the quasiperiodic echoes with periods in the range of 1 10 minutes originally discovered at the middle and upper atmosphere mu radar in japan in the early 1990s are now known to be a common feature of midlatitude sporadic e a possible role of shear instabilities and the associated kelvin helmholtz structure in seeding the plasma instabilities that produce the radar echoes is suggested in this paper the proposed mechanism has the advantage that the seed structure is generated in situ as part of the same neutral wind structure that serves to initiate the generation of the sporadic e layer in addition the range of observed periods can easily be explained since the kelvin helmholtz structure is embedded in the shear flow and moves with the mean flow in the shear layer the period observed in the ground based frame is simply a product of the horizontal wavelength and the mean flow velocity at the center of the disturbance finally a typical feature of the radar observations has been that the quasiperiodic structure often ebbs and wanes in intensity over a period of 30 to perhaps 60 min recent simulations of unstable neutral shears show that the timescale for the build up and eventual restabilization of the shear flow is 30 40 min comparable to the timescale found in the observations

a review of the mesosphere inversion layer phenomenon an active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere mlt from 60 to 130 km especially in regard to the influences that govern variability the physical processes of this region are diverse and complex with strong coupling between the mlt and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves gws from sources above and below the measurements of mlt winds and temperatures required for such studies represent daunting technical challenges at low and midlatitudes the mesosphere inversion layer mil phenomenon a 10 km wide region of enhanced temperatures t 15 50k is observed with great regularity in both the upper mesosphere 60 70 km and the mesopause 90 100 km observations are largely based upon rayleigh and na temperature lidar systems but coherent radar observations have shown that the mil phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions gw activity is believed to play an important role in the development of a linkage between the mil and the tidal structure through gw coupling that results in an amplification of the tidal thermal structure this linkage is readily evident for the upper mil but is seen only occasionally for the lower mil further study of mil properties should emphasize continual 24 hour temperature observations especially for the lower mil region to confirm the linkage of the development of the mil to the mlt tidal structure

downward control of the mean meridional circulation and temperature distribution of the polar winter stratosphere abstract according to the downward control principle the extratropical mean vertical velocity on a given pressure level is approximately proportional to the meridional gradient of the vertically integrated zonal force per unit mass exerted by waves above that level in this paper a simple numerical model that includes parameterizations of both planetary and gravity wave breaking is used to explore the influence of gravity wave breaking in the mesosphere on the mean meridional circulation and temperature distribution at lower levels in the polar winter stratosphere the results of these calculations suggest that gravity wave drag in the mesosphere can affect the state of the polar winter stratosphere down to altitudes below 30 km the effect is most important when planetary wave driving is relatively weak that is during southern winter and in early northern winter in southern winter downwelling weakens by a factor of 2 near the stratopause and by 20 at 30 km when gravity wave drag is not included

mineralization of an intra oceanic arc in an accretionary orogen insights from the early silurian honghai volcanogenic massive sulfide cu zn deposit and associated adakites of the eastern tianshan nw china the honghai volcanogenic massive sulfide vms deposit occurs in the kalatage inlier of the dananhu haerlik arc eastern tianshan in the southern altaids the deformed deposit hosted in a suite of early paleozoic basic to intermediate volcanic volcaniclastic and sedimentary rocks consists of lenticular massive to semimassive and sulfide silicate stockwork orebodies the orebodies are characterized by zoning of metallogenic elements and sulfide minerals outward from a central stockwork zone as follows fe cu fe cu zn au ag cu zn fe zn cu au ag zn cu and fe cu zn a typical vms hydrothermal alteration zone that formed around the foot of the massive orebodies is several times larger than the lenticular massive orebody the hydrothermally altered rocks contain sericite chlorite and epidote zones from the core outward re os and argon dating of pyrite and sericite shows that the honghai vms deposit formed in the early silurian 436 2 ma and was deformed in the early devonian 410 4 ma this is the first documented early paleozoic vms deposit in the eastern tianshan our multidisciplinary data indicate that the honghai cu zn deposit formed in an immature nascent island arc where early tholeiitic lavas evolved into transitional basic andesitic volcanic rocks and calc alkaline intrusions with relatively high epsilon nd t values 6 2 8 4 and low sr 87 sr 86 i ratios 0 70412 0 70541 the sr nd pb s isotopic and geochemical data suggest that the ore forming melts started in mantle derived volcanic rocks and deep subvolcanic intrusions and some alteration was influenced by seawater the deposit formed in a high level subvolcanic intrusion driven by sub seafloor co2 rich nacl cacl2 h2o hydrothermal fluids temperatures 99 377 degrees c salinities 6 18 nacl equivalent circulating in active extensional faults in a submarine caldera extension of the immature nascent intra oceanic arc was one of the important factors that controlled the development and preservation of the vms deposit future work will benefit from this improved understanding of the vms deposits formed in immature nascent intra oceanic arcs especially in early paleozoic island arcs in the eastern tianshan in the southern altaids

gravity wave interactions with the diurnal propagating tide observations of the diurnal propagating solar tide in the mesosphere and lower thermosphere 50 150 km indicate substantial annual variations in tidal amplitudes observations and modeling efforts suggest that interactions with internal gravity waves can result in significant modifications of the diurnal tide herein a two dimensional 2 d linear steady state tidal model is used in conjunction with hybrid lindzen matsuno gravity wave parameterization to examine the role of gravity wave interactions in the seasonal variability of the diurnal tide in the mlt the analysis indicates that annual variations of the background atmosphere result in significant annual variations in the eddy diffusion and the diurnal harmonic of the momentum flux divergence wave stress produced by breaking gravity waves the seasonal variability in these two mechanisms can explain the observed semi annual variability in the diurnal tide amplitude minima at solstice and maxima at equinox the relative roles of wave stress and diffusion are examined and it is shown that both play important roles in tidal modification by damping its amplitude the effects of varying inputs to the gravity wave parameterization are explored

vhf radar observations of gravity wave production by cold fronts over southern australia abstract four extended observational campaigns were conducted during august and november 1988 with an st stratosphere troposphere radar in southern australia during the passage of cold fronts over the system giving around 30 days of three dimensional wind measurements with 15 min time and 0 5 km height resolution over the 2 11 5 km height range order of magnitude increases in the variance of time fluctuating wind velocities were measured during frontal passages which are definitively ascribed to gravity waves the time height morphology of the horizontal and vertical velocity fluctuations differed bursts of horizontal velocity variance u 2 2 10 100 m2 s 2 arose at upper levels about a day before the frontal boundary arrived and this activity gradually extended to lower heights as the front neared the arrival of the frontal boundary marked a sudden reduction in this activity after the frontal boundary passed reduced activity persisted for 12 hours after which bursts in u 2 2 returne

hf radar observations of quasiperiodic e layer echoes over north america midlatitude e region plasma irregularities have been observed with a new low power hf coherent scatter radar at clemson south carolina the observations reveal a high incidence of quasiperiodic striations like those first observed at the middle and upper atmosphere mu radar in shigaraki japan quasiperiodic echoes were seen on approximately 70 of the evenings in august 1998 on which observations were made they occurred mainly at altitudes above 100 km had range rates usually between 100 and 150 m s implying propagation toward the radar downward and southward but typically exhibited doppler velocities indicating rapid phase propagation away from the radar upward and northward the quasiperiodic echoes sometimes had periods comparable to the brunt vaisala period but more often had significantly shorter periods closer to 1 min the doppler shifts of the strongest echoes oscillated with a period of just under 1 hour and the phase of this oscillation seemed to control the occurrence of the radar echoes

a multidiagnostic investigation of the mesospheric bore phenomenon 1 imaging measurements of a bright wave event in the nighttime mesosphere were made on 14 november 1999 at two sites separated by over 500 km in the southwestern united states the event was characterized by a sharp onset of a series of extensive wavefronts that propagated across the entire sky the waves were easily visible to the naked eye and the entire event was observed for at least 5 hours the event was observed using three wide angle imaging systems located at the boston university field station at mcdonald observatory mdo fort davis texas and the starfire optical range sor albuquerque new mexico the spaced imaging measurements provided a unique opportunity to estimate the physical extent and time history of the disturbance simultaneous radar neutral wind measurements in the 82 to 98 km altitude region were also made at the sor which indicated that a strong vertical wind shear of 19 5 ms 1km 1 occurred between 80 and 95 km just prior to the appearance of the disturbance simultaneous lidar temperature and density measurements made at fort collins colorado 1100 km north of mdo show the presence of a large 50 k temperature inversion layer at the time of the wave event the observations indicated that the event was most probably due to an undular mesospheric bore a relatively uncommon disturbance which has only recently been reported taylor et al 1995a evidence is also shown to suggest that a large east west tropospheric frontal system lying over the northern united states was the origin of the disturbance

lidar observations of gravity wave activity and arctic stratospheric vortex core warming measurements of stratospheric thermal structure and gravity wave activity have been obtained with a rayleigh lidar in the canadian high arctic at eureka 80 n 86 w during five recent winters the observations reveal that an annual late december warming of the upper stratosphere occurred in the polar vortex core and was sustained through the winter increased gravity wave activity was detected in the vortex jet during the warming that these two phenomena developed in parallel suggests they are related it is proposed that increased gravity wave momentum deposition above the jet maximum forced flow into the vortex core where it descended and warmed adiabatically

simulations of the february 1979 stratospheric sudden warming model comparisons and three dimensional evolution abstract the evolution of the stratospheric flow during the major stratospheric sudden warming of february 1979 is studied using two primitive equation models of the stratosphere and mesosphere the united kingdom meteorological office stratosphere mesosphere model smm uses log pressure as a vertical coordinate a spectral entropy coordinate version of the smm entropy coordinate model or ecm that has recently been developed is also used both models produce similar successful simulations through the peak of the warming capturing the splitting of the vortex and the development of small scale structures such as narrow baroclinic zones the ecm produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming due mainly to better conservation properties for rossby ertel potential vorticity in this model another advantage of the ecm is the automatic increase in vertical resolution near baroclinic zones comparison of smm simulations with forecasts perf

rayleigh lidar observations of mesosphere temperature structure ground based observations of atmospheric density profiles to 92 km were obtained for four successive seasons between summer 1989 and spring 1990 these results were obtained with a powerful rayleigh lidar facility located at wright patterson air force base dayton ohio this instrument combined a 14 w xef laser transmitter with a 2 54 m receiver mirror to observe returns from altitudes between 40 and 95 km analysis of the scale height dependence of the density profiles produced temperatures with a measurement error of about 5 k 2 5 at 90 km when the lidar data was averaged for 20 min and smoothed in height over 2 7 km examination of these profiles for the total of 18 nights showed that there often existed in the mesophere a layer of enhanced temperatures when compared with the u s standard profile the layer centroid height was about 85 km for summer and 70 to 75 km for winter data obtained for the equinoctial periods showed the amplitude of these layers to be weak the winter temperature profiles showed evidence forlong period waves passing through the region of the thermal anomaly while the equinox profiles revealed more sporadic wave activity with shorter vertical wavelengths both the winter and summer temperature data displayed regions where the observed lapse rate approached the adiabatic lapse rate in the summer the wave activity near the inversion layer was weak

gravity wave structure between 60 and 90 km inferred from space shuttle reentry data abstract density fluctuations obtained along seven space shuttle reentry tracks are used in this paper to examine the horizontal structure and the vertical distribution of density variance in the mesosphere and lower thermosphere the tracks lie primarily over open ocean at middle and low latitudes and represent the only measurements of horizontal atmospheric structure at these heights available to date the density fluctuations are interpreted in terms of gravity wave motions and reveal significant density and velocity variance at horizontal scales ranging from 10 to 1000 km fluctuation amplitudes are used to infer corresponding velocity perturbations and characteristic vertical scales and frequencies of the wave spectrum results suggest that the mean velocity variance is smaller over the pacific ocean than observed over major land masses and that the variance increases with height in a manner consistent with that expected in the presence of wave saturation processes

semidiurnal and diurnal tidal effects in the middle atmosphere as seen by rayleigh lidar the rayleigh lidar at the centre d essais des landes has been adjusted to provide continuous day and nighttime measurements when clear weather permits with high resolution in both time and space these measurements are processed to show tidal effects in the 30 to 80 km height range the observed temperature variations in two 11 day series of data from november 1988 and january 1989 are treated by spectral analysis and by least squares fitting sinusoids for 12 and 24 hour periods the observed tidal amplitudes correspond with experimental results from rocket studies and are generally consistent though somewhat larger than predicted in theoretical models observed lidar phases are roughly corroborated by radar observations and also follow the same general trends as model predicitions

observed coupling of the mesosphere inversion layer to the thermal tidal structure rayleigh lidar observations of mesosphere temperature profiles obtained from 40 to 100 km from logan utah 41 7 111 8 w altitude 1 9 km over 10 nights in late february 1995 revealed an interesting development between 60 to 75 km of a winter mesosphere inversion layer with an amplitude of 20 30 k and a downward phase progression of 1 km hr the data also showed two altitude regions exhibiting significant cooling of 10 30 k in extent these were located below and above the peak of the inversion layer respectively at altitudes of 50 55 km and 70 80 km when these results were compared with the predictions of a global wave scale model gswm the observed thermal mesosphere structure is similar to the computed composite tidal structure based upon the semi diurnal and diurnal tides with the exception that observed amplitudes of heating and cooling are 10x larger than predicted gswm values we suggest that these events over utah are caused through a localized mechanism involving the coupling of gravity waves to the mesopause tidal structure

the development of narrow baroclinic zones and other small scale structure in the stratosphere during simulated major warmings numerical simulations of stratospheric major warmings with a multi level primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small scale structure in the simulated flow for a simulation of the warming of 1984 85 the following examples are cited 1 the maintenance of the aleutian high by injections of air with low values of potential vorticity 2 the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity 3 the merger of a pair of anticyclones at the height of the warming 4 differential advection of potential vorticity in a vertically sheared flow and 5 the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical for a simulation of the warming of 1979 the example given is of the in situ generation in the stratosphere of inertio gravity waves near a developing baroclinic zone the dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail it is shown that the ageostrophic flow mainly the vertical motion provides a positive feedback on the frontogenetic effect of the geostrophic large scale flow similarities and differences with frontogenesis at the ground and in the upper troposphere are enumerated

synoptic scale study of the arctic polar vortex s influence on the middle atmosphere 1 observations 1 we present nightly rayleigh lidar temperature measurements of the high latitude middle atmosphere taken at three arctic sites over similar time periods in midwinter the four reported case studies depict changes in the thermal structure of the stratosphere and lower mesosphere over a period of days to weeks that can be attributed to movement and interaction of the polar vortex the aleutian high and planetary waves as evidenced from the national center for environmental prediction tropospheric and stratospheric analyses in cases where substantial i e large horizontal scale movement of the vortex is observed it is noted that regional middle atmospheric arctic temperatures can change by tens of degrees in other cases we show that even very subtle i e small horizontal scale movement of the vortex system over similar timescales can result in notable temperature changes in the regional middle atmosphere it appears that the observed events are dependant on the location strength and structure of the polar vortex and aleutian high all of these synoptic scale measurements indicate a change in the local dynamical structure of the middle atmosphere and are discussed in regards to this issue

propagation of the arctic oscillation from the stratosphere to the troposphere geopotential anomalies ranging from the earth s surface to the middle stratosphere in the northern hemisphere are dominated by a mode of variability known as the arctic oscillation ao the ao is represented herein by the leading mode the first empirical orthogonal function of low frequency variability of wintertime geopotential between 1000 and 10 hpa in the middle stratosphere the signature of the ao is a nearly zonally symmetric pattern representing a strong or weak polar vortex at 1000 hpa the ao is similar to the north atlantic oscillation but with more zonal symmetry especially at high latitudes in zonal mean zonal wind the ao is seen as a north south dipole centered on 40 45 n in zonal mean temperature it is seen as a deep warm or cold polar anomaly from the upper troposphere to 10 hpa the association of the ao pattern in the troposphere with modulation of the strength of the stratospheric polar vortex provides perhaps the best measure of coupling between the stratosphere and the troposphere by examining separately time series of ao signatures at tropospheric and stratospheric levels it is shown that ao anomalies typically appear first in the stratosphere and propagate downward the midwinter correlation between the 90 day low pass filtered 10 hpa anomaly and the 1000 hpa anomaly exceeds 0 65 when the surface anomaly time series is lagged by about three weeks the tropospheric signature of the ao anomaly is characterized by substantial changes to the storm tracks and strength of the midtropospheric flow especially over the north atlantic and europe the implications of large stratospheric anomalies as precursors to changes in tropospheric weather patterns are discussed

mesospheric inversions and their relationship to planetary wave structure 1 mesospheric inversions are studied in vertical soundings from the french lidar at observatoire de haute provence and in synoptic global structure that was observed simultaneously by the uars satellite the latter provides the instantaneous three dimensional 3 d structure of the circulation that accompanies mesospheric inversions a numerical simulation with a 3 d primitive equation model is then shown to reproduce major features of the observed behavior both reveal an extensive pattern of inverted thermal structure the behavior is closely related to planetary waves which in the model experience strong absorption in the upper mesosphere and lower thermosphere inverted thermal structure mirrors the synoptic pattern of potential vorticity which marks the polar night vortex both are strongly distorted during stratospheric warmings when inverted thermal structure at midlatitudes is favored the region of negative lapse rate coincides with those altitudes where planetary wave temperature undergoes an abrupt phase shift wave temperature is then driven out of phase with wave geopotential this alters the vertical structure of planetary waves from westward tilt and upward amplification below the inversion to nearly barotropic structure and upward decay above the inversion accompanying the upward decay of planetary waves above the inversion is a decay of flow distortion as the disturbed vortex is gradually restored toward polar symmetry

all sky imaging observations of mesospheric fronts in oi 557 7 nm and broadband oh airglow emissions analysis of frontal structure atmospheric background conditions and potential sourcing mechanisms 1 all sky imaging observations of distinct large horizontal transient mesospheric structures with a spatial scale of 100 km detected in images of broadband oh and oi 557 7 nm airglow emissions were made over clemson south carolina on the night of 14 15 october 2001 we designate these structures as mesospheric fronts and present a detailed summary of this night series of observations paying particular attention to the details of the different frontal structures the wave like activity seen throughout the night and the background atmospheric conditions these data are compared to other observations of similar mesospheric fronts found in the literature and we seek to understand them in relation to mesospheric bores ducted gravity waves mesospheric wall events and nonlinear gravity wave interactions we find that the observed frontal characteristics and the atmospheric background structure exhibit a close resemblance to previous observations of mesospheric bores owing to this similarity and supported by gravity wave ray tracing experiments we propose a sequence of events that generated the mesospheric fronts observed in the airglow emission furthermore we note this similarity in atmospheric structure suggests a potential means of predicting the occurrence of such mesospheric phenomena

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

mesospheric temperature inversions with overlying nearly adiabatic lapse rate an indication of a well mixed turbulent layer the rayleigh lidar technique was applied to study the thermal structure of the middle atmosphere observations were carried out on a routine basis for one year 130 clear nights at the main campus of york university near toronto 44 n 80 w mesospheric temperature inversions were generally found to occur below a height of 70 km during winter and above during summer the most interesting aspect of our observations was that the inversions were often associated with an overlying nearly adiabatic lapse rate which extended for several kilometres we interpret this as being an indication or signature of a well mixed turbulent layer a one dimensional numerical model was applied to demonstrate that a well defined turbulent layer within the mesosphere can bring about a thermal structure quite similar to that which was commonly observed an inversion with overlying nearly adiabatic lapse

warm stratopause in the antarctic winter abstract the fact that the temperature near the stratopause in the polar night is remarkably higher in the antarctic than in the arctic is described the reason why the stratopause can exist in the polar night and be warmer in the antarctic is considered in terms of gravity waves and planetary waves

seasonal variations of the diurnal tide induced by gravity wave filtering wind measurements with the uars spacecraft show that between 90 and 100 km altitude the fundamental diurnal tide is modulated semi annually with peak amplitudes of almost 100 m s near equinox more than a factor of two larger than during solstice applying hines doppler spread parameterization dsp for small scale gravity waves gw we present results from a numerical spectral model nsm to describe the seasonal variations in the mean zonal circulation interacting self consistently with the diurnal tide with the dsp the gw momentum source increases the amplitude of the tide at altitudes above 70 km in contrast to the rayleigh friction parameterization that causes damping since the tide absorbs gw momentum it reduces the wave generated semi annual oscillations sao in the mean zonal winds at equatorial latitudes outside the tropics gw momentum is absorbed by the mean zonal winds that peak near the summer and winter solstices as a result the gw reach the upper mesosphere with more momentum during equinox periods and thus cause a seasonal modulation of the tide that is predominately semi annual

mesospheric temperature inversion and gravity wave breaking more than 500 nightly mean temperature profiles have been obtained since 1981 by two rayleigh lidars located in south of france 550 km apart in the mesosphere a temperature inversion a feature persisting for several days is frequently observed simultaneously at both sites from a statistical study of the inversion characteristics it is shown that its altitude ranges from 55 72 km in winter to 70 83 km in summer while its probability of occurrence presents a semi annual variation with a maximum higher in winter than in summer and a minimum lower in may than in september the seasonal variation of the temperature inversion is shown to be very similar to that of the mst radar echoes in the mesosphere associated to turbulent layers generated by the breaking of gravity waves a crude estimate of the amplitude growth with height of a gravity wave in the presence of an inversion layer indicates that the wave will break preferably inside and above this layer the persistence of a temperature inversion during several days with amplitude as large as 40 k is tentatively explained by the heating of the turbulent layers generated by the continuous breaking of gravity waves above the secondary minimum of temperature

rayleigh lidar observations of a mesospheric inversion layer during night and day a narrow field of view rayleigh lidar has been constructed at millstone hill mit haystack observatory 42 6 n 71 5 w for observations of middle atmospheric temperatures throughout the diurnal cycle during a 31 5 h measurement on 19 21 march 2001 a mesospheric thermal inversion layer was observed in both the night and day it developed near 60 km in altitude progressed downward at 0 40 0 06 km h and had an overlying adiabatic lapse rate the inversion amplitude correlated with the evolution of stratospheric gravity wave activity although the mesospheric perturbations were too large to be due to conservative gravity wave growth alone the 24 h mean temperature profile shows no evidence of a residual inversion layer

mesosphere inversion layers and stratosphere temperature enhancements 1 it has been known for at least 30 years that vertically narrow thermal layers form within the middle atmosphere two types of temperature enhancements the low latitude to midlatitude mesosphere inversion layer mil and the high latitude winter stratosphere temperature enhancement ste have both received much attention within the atmospheric science community because of their unexplained formation mechanisms and potential impacts on the middle atmosphere global circulation numerous experimental numerical and theoretical studies have attempted to explain certain aspects of these respective thermal layers but no one theory consistently and satisfactorily describes all the features observed we present a review of the literature and explicitly propose a classification scheme based on the different formation mechanisms suspected to cause these events for the mil we demonstrate that there are two subtypes the first one is tidally driven and tends to occur above 85 km this mil originates from large amplitude tidal waves propagating into the mesosphere and their subsequent nonlinear interactions with gravity waves which can often create the appearance of a double mil separated by approximately one vertical tidal wavelength 25 km the other subtype of mil is formed by a climatological planetary wave dissipation mechanism that occurs at a zero wind line the dissipation of the planetary wave tends to generate a mesoscale 1000 km inversion layer in the range of 65 80 km these two formation mechanisms explain a host of observed characteristics including the reason behind the downward progression of some mils and not others the different climatological nature of the two forms of mil events and the relative scarcity of mil observations at high latitudes the ste is believed to be generated by an altogether different process namely the nonlinear interaction between the polar vortex and planetary waves aleutian high the induced temperatures typically peak around 40 km and often exceed 300 k generating what appears to be a low hot stratopause when vertical temperature profiles are combined with synoptic analyses one observes that the ste is the consequence of high latitude vortex interactions creating a baroclinic atmosphere i e a downward adiabatic compression induced by an ageostropic flow we summarize the details of the relationship between this feature and sudden stratospheric warmings as well as the potential for in situ gravity wave generation we close with a review of currently unexplained mil ste features and offer new directions for future middle atmosphere thermal layer research

the gravity wave response above deep convection in a squall line simulation abstract high frequency gravity waves generated by convective storms likely play an important role in the general circulation of the middle atmosphere yet little is known about waves from this source this work utilizes a fully compressible nonlinear numerical two dimensional simulation of a midlatitude squall line to study vertically propagating waves generated by deep convection the model includes a deep stratosphere layer with high enough resolution to characterize the wave motions at these altitudes a spectral analysis of the stratospheric waves provides an understanding of the necessary characteristics of the spectrum for future studies of their effects on the middle atmosphere in realistic mean wind scenarios the wave spectrum also displays specific characteristics that point to low physical mechanisms within the storm responsible for their forcing understanding these forcing mechanisms and the properties of the storm and atmosphere that control them are crucial first steps toward developing

on the origin of mesospheric bores abstract a dramatic front of airglow radiance and wave structure reported by taylor et al 1995 was attributed in a previous paper of ours dewan and picard 1998 to a manifestation of an internal undular bore in the mesosphere in the current paper we address the question of what physical process could be responsible for generating such a bore at that altitude while it is relatively easy to find sources of internal tropospheric bores including the dramatic morning glory the same cannot be said for mesospheric bores it will be argued here that a likely candidate for the generator of such bores is the interaction of gravity waves with the mean flow at a critical layer this interaction could take place within an already existing inversion layer the latter playing the role of the channel in which the bore propagates as huang et al 1998 have shown a similar wave critical level interaction may be responsible for the inversion layer in question hence we are proposing that the physical process producing the channel is the same as the one responsible for subsequently generating the bore

vhf echoes from the midlatitude mesosphere and the thermal structure observed by lidar observations with a 46 5 mhz radar system at aberystwyth wales 52 n 4 w over the period february 1990 to may 1994 have shown a pronounced seasonal variation in the height range of mesospheric echoes these were observed over the range 64 84 km in all seasons but those at still greater heights were confined to summer months coordinated radar and lidar observations at the same site during 1993 have shown that these intense echoes in summer analogous to polar mesosphere summer echoes pmse are associated with low mesopause temperatures the weaker echoes observed in winter seem to be associated with regions of superadiabatic lapse rates and echoes are observed both above and below the maxima in temperature inversions commonly observed below the mesopause the types of instability implied in these associations of echo height and temperature structure are considered

rayleigh lidar observations of mesospheric inversion layers at poker flat alaska 65 n 147 w rayleigh lidar measurements of the stratosphere and mesosphere have been made on an ongoing basis over a three year period at poker flat alaska 65 n 147 w these observations have yielded 27 nightly measurements of the middle atmosphere temperature profile 40 80 km these nighttime measurements are distributed between august and april mesospheric inversion layers have been observed on five occasions the average altitude of the inversion layer peak is 60 km with average amplitude of 18 k the temperature gradients on the topside of the inversion layers approach the adiabatic lapse rate the inversion layers do not exhibit the apparent downward phase velocities that are commonly observed at lower latitudes furthermore the inversion layers appear significantly less frequently than at lower latitudes the observations are discussed in terms of current models and observations at other sites

observations of strong wind shears and temperature enhancements during several sporadic na layer events above haleakala we describe two sporadic na nas layer events observed on 26 september and 6 october 1993 by the na wind temperature w t lidar above haleakala maui 20 8 n 156 2 w during aloha 93 both events occurred between 90 and 95 km altitude and appeared to be associated with large amplitude gravity waves both were accompanied by temperature enhancements of more than 40 k the temperature and na density variations at the peak of the nas as well as the vertical motions of the nas layers and regions of elevated temperature were highly correlated the vertical wind structure exhibited no unusual characteristics in the vicinities of the nas the zonal wind profile for the 6 october event exhibited a very strong vertical shear approximately 1 km below the nas layer that approached the dynamic instability limit of 42 m s km however the polarity of the shear was opposite that predicted by the classical wind shear theory of sporadic e layers es formation

gravity wave momentum flux in the lower stratosphere over convection this work describes a method for estimating vertical fluxes of horizontal momentum carried by short horizontal scale gravity waves lambda sub x 10 100 km using aircraft measured winds in the lower stratosphere we utilize in situ wind vector and pressure altitude measurements provided by the meteorological measurement system mms on board the er 2 aircraft to compute the momentum flux vectors at the flight level above deep convection during the tropical experiment of the stratosphere troposphere exchange project step tropical data from flight 9 are presented here for illustration the vertical flux of horizontal momentum these observations points in opposite directions on either side of the location of a strong convective updraft in the cloud shield this property of internal gravity waves propagating from a central source compares favorably with previously described model results

p wave teleseismic traveltime tomography of the north american midcontinent the remains of the 1 1 ga midcontinent rift mcr lie in the middle of the tectonically stable portion of north america previous and ongoing studies have imaged strong heterogeneity associated with the mcr in the crust but have not imaged such within the mantle it is unclear whether this is due to the absence of rift related mantle structures or these studies had insufficient resolution to image them to address this issue we measured 46 374 teleseismic p wave delay times from seismograms recorded by the usarray transportable array superior province rifting earthscope experiment and surrounding permanent stations we included these and 54 866 delay times from prior studies in our tomographic inversion we find that high velocity anomalies are widespread in our study area but there are also prominent low velocity anomalies two of these are coincident with high bouguer gravity anomalies associated with the mcr in iowa and the minnesota wisconsin border at 50 to 150 km depth extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift related material that underplated the crust although we cannot exclude that the subcrustal mantle lithosphere beneath the mcr is anomalously enriched hydrated or warm other anomalies occur at syntaxes of the penokean orogen one with the superior province and marshfield terrane in southern minnesota and another with the yavapai and mazatzal terranes both at 100 to 250 km depth in the midmantle we image two linear high velocity anomalies interpreted as subducted fragments of the farallon and kula plates