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

bed erosion during fast ice streaming regulated the retreat dynamics of the irish sea ice stream abstract marine terminating ice stream behaviour often defines the stability of ice sheets and is driven by a complex interplay of climatic oceanic topographic and glaciological factors here we use new integrated high resolution extensive 2100 km2 and continuous geophysical sedimentological and geotechnical data to reconstruct past glacial environments during the last glacial maximum from a well preserved palaeo landscape the data is from the axial centre of the irish sea ice stream isis which drained 17 of the former british irish ice sheet recent geochronological data of the palaeo isis show a build up and advance of ice to marine terminating maximum limits in the southern celtic sea 27 25 ka bp followed by rapid ice margin retreat into the northern irish sea basin isb by 20 8 0 7 ka bp however the flow dynamics in the central and axial bed of the isis through this timeframe are not well understood here we use our new glacial landscape reconstruction to identify the spatial and temporal patterns of flow re organisation and re activations for the marine terminating isis from this we infer how ice streaming was driven by a variety of factors through advance deglaciation and towards a temporary lift off of ice from its bed and an ultimate demise overprinted subglacial bedforms with differing ice flow directions indicate an on off behaviour to the ice streaming an increasing topographical influence and substantial realignment of ice flows subsurface geophysical data reveal the erosive capability of the ice stream through time with a first erosive component in the formation of mega scale glacial lineations leaving bedrock exposed at the ice stream bed the depositional component of msgl crest building occurred in the same ice flow phase whilst the ice stream was laterally constricted in two locations likely contributing to changes in ice margin retreat rates we also propose that changes in basal drag associated with exposed bedrock at the ice bed interface influenced the retreat dynamics particularly when this exposure was near the grounding zone the wider implications of this work are that episodic and highly erosive ice streaming during ice advance and early retreat can change ice bed conditions radically and in turn influence glacial dynamics during later retreat episode thus constituting a feedback process to be considered in modelling the dynamics of marine terminating ice streams

diatom blooms and associated vegetation shifts in a subarctic peatland responses to distant volcanic eruptions we test the hypothesis that rich occurrences of diatoms observed at transitions between major peat units representing different vegetation communities in a peat sequence from subarctic northern sweden reflect responses to acid deposition from the samalas ad 1257 and laki ad 1783 1784 eruptions we observe sudden changes in the mire ecosystem and thereby in the trophic status and biogeochemical cycling of the peatland both the eruptions are known to have been associated with significant acid deposition events and climatic anomalies as recorded in polar ice cores to test the hypothesis new chronological analyses and age modelling were applied to existing biogeochemical and biological records from the peat sequence this approach yielded modelled age ranges of ad 1239 1284 1 ad 1210 1303 2 median ad 1260 and ad 1674 1795 1 ad 1665 1875 2 median ad 1743 respectively for the stratigraphic transitions hence the modelled age ranges bracket the ages of the eruptions in question and the hypothesis could therefore not be rejected impacts of acid deposition from the eruptions are assumed to have caused instant acidification vegetation damage increased nutrient cycling and blooms of opportunistic epiphytic diatoms in addition cooling may have contributed to vegetation changes through permafrost inception frost heave and thereby altered hydrological conditions

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

a comparative study of mesospheric solar tides observed at adelaide and kyoto observations of winds in the upper mesosphere and lower thermosphere 80 110 km measured at the geographically conjugate locations of adelaide 35 s 138 e and kyoto 35 n 136 e in the years 1983 1985 are used to study hemispheric differences in the behavior of the solar diurnal and semidiurnal tides comparisons of the tidal amplitude and phase parameters show that there are distinct hemispheric asymmetries the diurnal tide is usually much larger in amplitude at adelaide and it exhibits a height structure which indicates a relatively larger contribution from the propagating 1 1 mode at this location comparisons with recent theoretical work suggest that while some of these differences may be due to hemispheric differences in forcing and or to interactions between the tide and the background winds in the middle atmosphere it is also possible that they indicate weaker turbulent induced damping of the 1 1 mode in the southern hemisphere the characteristics of the semidiurnal tide agree well at the two locations it is found that the zonal wind components are about 6 hours out of phase and the meridional components are in phase in all seasons this behavior taken together with the inferred vertical wavelengths which are greater than 100 km in summer and 45 80 km in winter indicates that there are strong contributions from the antisymmetric 2 3 mode probably forced by wind induced mode coupling from the symmetric 2 2 mode the phases near 90 km exhibit a bimodal or two state behavior with the phases being almost constant in one state near the june solstice and then making a rapid transition of between 4 and 6 hours to another phase stable state near the december solstice the duration of the state near the june solstice 6 8 months is longer than the duration of the state at the december solstice although the agreement between recent models and our observations is relatively good better agreement may be expected when more realistic wind distributions especially for the southern hemisphere are used in the models

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

a new approach to inferring basal drag and ice rheology in ice streams with applications to west antarctic ice streams drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers simultaneously inferring these parame ters is challenging since basal drag and ice viscosity are coupled in the momen tum balance which governs ice flow here we test the ability of adjoint based inverse methods to infer the slipperiness coefficient in a power law sliding law and the flow rate parameter in the constitutive relation we modify existing inversions by including surface strain rates into the regularization of the in version using synthetic data generated with physically motivated variations in basal drag and ice rheology we show that this allows for more accurate in ferences we apply this method to bindschadler and macayeal ice streams in west antarctica our results show relatively soft ice in the shear margins and spatially varying basal drag with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag we interpret the former to reflect a combination of heating through viscous dissipation and changes in the crystalline structure these results suggest that adjoint based inverse methods can provide inferences of basal drag and ice rheology when the regularization is informed by strain rates

a reconnaissance of the major holocene tephra deposits in the upper cook inlet region alaska abstract the upper cook inlet region of southcentral alaska would be significantly impacted by a major tephrafall owing to a widespread population and heavily travelled transportation corridors to evaluate the likelihood of such an occurrence the tephra deposits of the region have been inventoried approximately 90 deposits of holocene age are sufficiently thick to have been preserved for sampling the frequency of such major tephrafalls ranges from 1 every 200 years near sources on the west side of upper cook inlet to 1 every 1000 years on the more populated east side the volcanoes located on the west side of upper cook inlet are from north to south hayes spurr redoubt and iliamna hayes volcano produced the most extensive set of 6 to perhaps 8 tephra layers in the region about 3650 yr b p and produced one other less extensive tephra layer during holocene time spurr and redoubt volcanoes have produced respectively approximately 35 and 30 holocene layers which were dispersed eastward toward population centers no holocene tephra layers of iliamna have been recognized with certainty consequently several tephra layers which originated to the south of the region must have a source at augustine volcano or some more distant volcano tephra layers of hayes volcano are calc alkaline dacites most of the spurr deposits are tholeiitic basaltic andesites whereas those of redoubt volcano are calc alkaline andesites and dacites

age and impacts of the caldera forming aniakchak ii eruption in western alaska the mid holocene eruption of aniakchak volcano aniakchak ii in southwest alaska was among the largest eruptions globally in the last 10 000 years vei 6 despite evidence for possible impacts on global climate the precise age of the eruption is not well constrained and little is known about regional environmental impacts a closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90 120 yr following tephra deposition during this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a cyperaceae dominated assemblage to a poaceae dominated vegetation cover suggesting a drier and or more nutrient rich ecosystem oribatid mites are extremely abundant in the peat at the depth of the ash and show a longer term increasingly wet peat surface across the tephra layer the radiocarbon sample immediately below the tephra gave a date of 1636 1446 cal yr bc suggesting that the eruption might be younger than previously thought our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long lasting

a 500 year long record of tephra falls from redoubt volcano and other volcanoes in upper cook inlet alaska abstract volcanic ash layers preserved in glacial lacustrine sediments at skilak lake on the kenai peninsula of southcentral alaska constitute a record of eruptions at redoubt volcano and other alaskan volcanoes which affected the upper cook inlet area during the last 500 years high resolution magnetic susceptibility profiling delineates similar sequences of tephra layers in several 1 m long lake sediment cores electron microprobe analyses of glass shards from the tephras indicate correlation of some ash layers with known reference tephras from the source volcanoes while other ash layers record previously unknown prehistoric eruptions skilak lake cores contain ash from the historic 1912 katmai eruption the 1902 redoubt eruption and the 1883 mount st augustine eruption as well as prehistoric ash layers erupted from crater peak at mt spurr ca 250 350 years ago from redoubt volcano at ca 300 400 years ago and again at ca 350 450 years ago and a 500 year old ash from mount st augustine still older tephras from redoubt volcano and crater peak at mt spurr are found lower in the cores the cores indicate that volcanoes in the cook inlet area have erupted every 10 35 years during the 20th century and ash falls have occurred at skilak lake at least once every 50 100 years for the last 500 years with redoubt spurr and augustine volcanoes being the most important sources of tephra

holocene tephrostratigraphy of varved sediment records from lakes tiefer see ne germany and czechowskie n poland a detailed holocene tephrostratigraphic framework has been developed for two predominately varved lake sediment sequences from ne germany lake tiefer see and central n poland lake czechowskie a total of thirteen tephras and cryptotephras of icelandic provenance were detected and chemically fingerprinted in order to define correlatives and to integrate known tephra ages into the sediment chronologies out of these three cryptotephras askja ad1875 askja s and h asseldalen were identified in both records thus allowing a detailed synchronization of developing high resolution palaeoenvironmental proxy data the early holocene saksunarvatn ash layer and the middle holocene lairg b and hekla 4 cryptotephras in lake tiefer see are further important anchor points for the comparison with other high resolution palaeoclimate records in central and northern europe tentative correlations of cryptotephras have been made with a historical basaltic grimsv otn eruption ad890 e ad856 and three late holocene rhyolitic eruptions including the 2 1 ka glen garry and two unknown high silicic cryptotephras of probably icelandic provenance 1 9 cal ka bp

lidar observations of atmospheric dynamics in the troposphere and lower stratosphere over arecibo a periodic wave structure with a vertical wave length typically 2 km between 14 and 23 km altitudes has been identified in lidar measurements of relative density profiles at 589 nm that were made by the university of illinois group during the march april aida 1989 campaign effort the equivalent temperature fluctuations are several degrees in magnitude also seen are stratified aerosol layers at altitudes near 25 km with a vertical extent of 1 km both of these phenomena show a downward phase speed between 1 and 3 cm s 1 the lower periodic structure is believed to be the same phenomenon seen previously in 430 mhz measurements of winds obtained at arecibo observatory the source of this structure remains uncertain cornish and larsen 1989 and sato and woodman 1982a considered this structure to be a quasi inertial long period wave hines 1989 proposed instead the identification of this structure with a stationary wave system generated by orographic waves created through the interaction of the subtropical trade winds with the mountainous terrain of puerto rico the higher altitude feature seen in the aida lidar observations is believed to be an aerosol layer with a scattering ratio of about 1 15 an alternative explanation is clear air turbulence but the signature of the 10 k fluctuation deduced from the lidar observations was not found in the san juan balloon measurement of the temperature profile for the lower stratosphere

application of a polythermal three dimensional ice sheet model to the greenland ice sheet response to steady state and transient climate scenarios abstract steady state and transient climate change computations are performed with the author s three dimensional polythermal ice sheet model simulation code for polythermal ice sheets for the greenland ice sheet the distinctive feature of this model is the detailed consideration of the basal temperate ice layer in which the water content and its impact on the ice viscosity are computed its transition surface to the cold ice region is accounted for by continuum mechanical jump conditions on this interface the simulations presented include steady states subject to a range of physical parameters and two different climates present and glacial conditions as well as three types of transient scenarios namely i sinusoidal milankovic period forcing ii paleoclimatic forcing from the greenland ice core project core reconstruction and iii future greenhouse warming forcing

the impact of the laacher see tephra on terrestrial and aquatic ecosystems in the black forest southern germany late glacial lake sediments containing the laacher see tephra lst 11 000 yr bp have been analysed for pollen and diatoms at two sites hirschenmoor rotmeer in the montane belt of the black forest southern germany in order to detect the possible impact of this major volcanic eruption on terrestrial and aquatic biota the pollen assemblages at both sites show a minor increase in grasses and sedges following the tephra deposition whereas the pollen of aquatic macrophytes do not show any consistent pattern partial redundancy analysis and monte carlo permutation tests suggest however that the lst had no statistically significant effect either on the terrestrial or aquatic pollen assemblages at either site when the effects of time and climatic change are allowed for statistically the diatom assemblages at both sites changed after the deposition of the lst and diatom inferred ph estimates suggest a short but non significant excursion towards lower values multivariate analyses of the diatom data indicate a statistically significant impact of the deposition of the lst and associated changes in sediment lithology on the diatom assemblages when the effects of climatic change and time are allowed for statistically however owing to the interaction between tephra and lithology it is not possible to conclude if the diatom changes were caused by the deposition of the lst or by lithological changes

volcanic impacts on peatland microbial communities a tephropalaeoecological hypothesis test abstract volcanic eruptions affect peatlands around the world depositing volcanic ash tephra and a variety of chemicals including compounds of sulphur these volcanic impacts may be important for many reasons in particular sulphur deposition has been shown to suppress peatland methane flux potentially reinforcing climatic cooling experiments have shown that sulphur deposition also forces changes in testate amoeba communities potentially relating to the reduced methane flux large volcanic eruptions in regions with extensive peatlands are relatively rare so it is difficult to assess the extent to which volcanic eruptions affect peatland microbial communities palaeoecological analyses across tephra layers provide a means to resolve this uncertainty in this study testate amoebae were analysed across multiple monoliths from a peatland in southern alaska containing two tephras probably representing the 1883 eruption of augustine volcano and a 20th century eruption of redoubt volcano results showed relatively distinct and often statistically significant changes in testate amoeba community coincident with tephra layers which largely matched the response found in experimental studies of sulphur deposition the results suggest volcanic impacts on peatland microbial communities which might relate to changes in methane flux

simulation of lidar measurements of gravity waves in the mesosphere lidar measurements of mesospheric gravity wave characteristics are simulated using a simple numerical model simple theory is used to superimpose gravity wave fluctuations upon typical background densities and sodium abundances in the height range 80 100 km which we then assume are perfectly observed by a rayleigh and na lidar respectively these lidar data are then analyzed for their gravity wave content using the standard reduction and analysis methods which are routinely applied to these data sets our goal is to assess whether this analysis of lidar data faithfully recovers the underlying wave field in both quasi monochromatic and spectral wave studies the height range over which na number density can be measured and the shape of the background na profile impose a limit on the gravity wave information which can be extracted from na lidar data the limitations due to background layer shape do not exist for rayleigh lidar measurements the simulations reveal that quasi monochromatic gravity waves with vertical wavelengths larger than approximately 10 km may not be reliably retrieved from na lidar data it is also shown that there are limitations on the accurate extraction of spectral parameters from na lidar data due to the limited vertical extent of the mesospheric na layer

assessing the impact of volcanic activity on mid holocene climate in ireland the need for replicate data analyses of pollen tephra mineral input and degree of peat humification from three neighbouring raised peat profiles at corlea central ireland covering the period of the deposition of a tephra

seven tephra isochrones in scotland this paper reports the presence and geochemical characteristics of seven separate tephras in scotland that fell in ad 1510 and about 450 bp 2100 bp 3600 bp 3830 bp 5600 bp and around 6000 bp the results come from 14 peat bogs spread throughout the highlands and islands where 26 tephra deposits have been characterized using over 400 grain discrete electron probe micro analysis firstly tephras are identified and defined by major element characteristics at a reference site secondly tephras are linked between sites on the basis of geochemistry and dated informal names based on british sites are suggested for tephras not yet linked to precise sources in iceland despite some apparent variation in colour all analysed grains in these seven deposits are silicic or intermediate in composition sio2 57 this phenomenon does not appear to be an artifact of preservation for two reasons first small 40 m basic glass shards in peat bogs in northern iceland are found to have undergone minimal alte

the impact of high tephra loading on late holocene carbon accumulation and vegetation succession in peatland communities abstract peatlands are major terrestrial stores of carbon c of importance to the global climate system recent studies have made progress in understanding the climatic controls on the c cycle however important interactions between volcanic deposition and peatland c stores remain to be addressed this study uses a 3000 year peatland record from northern japan to examine the interactions between carbon accumulation vegetation community succession and volcanic ash deposition plant macrofossil and testate amoebae records are presented alongside records of total organic carbon nitrogen and phosphorous age depth models are developed using a bayesian approach with seven ams radiocarbon dates and two identified historical tephras from baitoushan ad 969 981 cal bp and hokkaido komagatake ad 1640 310 cal bp volcanoes results show that moderate to high tephra loading can shift peatland plant communities from sphagnum to monocotyledon domination this vegetation change is associated with increased peat humification and reduced carbon accumulation where tephra deposition and reworking has occurred the apparent rate of carbon accumulation can be halved while high tephra loading of the mire surface is sustained sphagnum species vary in their tolerance to tephra deposition after each ash fall sphagnum magellanicum disappeared from the plant macrofossil record whereas sphagnum papillosum showed apparent continuity of development through the 1856 94 cal bp ko c1 tephra high rates of carbon accumulation peaking at 100 g m 2 yr 1 2 3 times faster than the average for northern peatlands were recorded in the sphagnum communities that established after the cessation of tephra deposition and reworking from the ad 969 baitoushan ash fall b tm tephra this peak in c accumulation was coincident with a radical shift in mire nutrient cycling most probably caused by the interaction of s magellanicum with leachates from the underlying tephras the phase of high c accumulation continued for over 300 years offsetting the initial negative impact of the b tm tephra on peatland c accumulation these results suggest that management for ash tolerant sphagnum species could be a highly effective strategy for minimising volcanic disruption to peatland carbon accumulation the study also shows that consideration of volcanic impacts on peatlands is essential for development of more realistic terrestrial carbon balance models in volcanically active regions

mathematical modeling and numerical simulation of polythermal glaciers 1 a mathematical model for polythermal glaciers and ice sheets is presented the enthalpy balance equation is solved in cold and temperate ice together using an enthalpy gradient method to obtain a relationship between enthalpy temperature and water content we apply a brine pocket parameterization scheme known from sea ice modeling the proposed enthalpy formulation offers two advantages 1 the discontinuity at the cold temperate transition surface is avoided and 2 no treatment of the transition as an internal free boundary is required fourier s law and fick type diffusion are assumed for sensible heat flux in cold ice and latent heat flux in temperate ice respectively the method is tested on storglaciaren northern sweden numerical simulations are carried out with a commercial finite element code a sensitivity study reveals a wide range of applicability and defines the limits of the method realistic temperature and moisture fields are obtained over a large range of parameters

deformation induced melting in the margins of the west antarctic ice streams flow of glacial ice in the west antarctic ice sheet localizes in narrow bands of fast flowing ice streams bordered by ridges of nearly stagnant ice but our understanding of the physical processes that generate this morphology is incomplete here we study the thermal and mechanical properties of ice stream margins where flow transitions from rapid to stagnant over a few kilometers our goal is to explore under which conditions the intense shear deformation in the margin may lead to deformation induced melting we propose a 2 d model that represents a cross section through the ice stream margin perpendicular to the downstream flow direction we limit temperature to the melting point to estimate melt rates based on latent heat using rheology parameters as constrained by laboratory data and observations we conclude that a zone of temperate ice is likely to form in active shear margins

temperate ice in the shear margins of the antarctic ice sheet controlling processes and preliminary locations abstract the flux of grounded ice from the antarctic ice sheet ais primarily occurs through narrow fast flowing outlet glaciers and ice streams shearing generates heat in the lateral glacier margins where there is a transition from fast flow to near stagnant ice or rock this heat warms the ice and can form zones of water saturated porous ice at the melting temperature known as temperate ice here we derive a one dimensional thermomechanical model to infer which ais shear margins may contain temperate ice our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone both of these quantities are described by two nondimensional numbers the brinkman and peclet numbers that are functions of observable parameters such as shear strain rate surface temperature surface accumulation rate and ice thickness the development of temperate zones is incited by shear strain rates brinkman number and suppressed by the advection of cold ice from the surface peclet number which scales with surface accumulation rate we also include a parametrization for lateral advection which can suppress the formation of temperate ice applying recent observations and outputs from a regional climate model we show that many active glacier shear margins could contain temperate ice the spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates

distal volcanic impacts on peatlands palaeoecological evidence from alaska abstract despite the fact that volcanic ash tephra layers are found preserved in peat deposits around the world comparatively little research has investigated the impacts of distal volcanic emissions on peatlands this study investigates the impacts of several late holocene volcanic eruptions on five peatlands in southern alaska using a palaeoecological approach testate amoebae analysis peat humification analysis and a basic analysis of plant macrofossil components were applied across 11 tephra layers changes in macrofossil and testate amoebae assemblages occur across several of the tephra layers the humification results were considered unreliable because of a methodological problem a finding which may have implications for other studies using this technique redundancy analyses on testate amoebae data show statistically significant changes associated with two tephras the most likely causes of the impacts are volcanic gases acidic precipitation or tephra derived leachates the finding that some tephras are associated with impacts whereas others are not may relate to the season of the eruption or meteorological conditions at the time of ash fall these results suggest the sensitivity of peatlands and peatland microbial communities to distal volcanic products and imply that changes in key palaeoclimatic proxies may be caused by a mechanism independent of climate change implications of the results for peat based palaeoclimatic studies are discussed as are possible directions for future research

the environmental impact of the minoan eruption of santorini thera statistical analysis of palaeoecological data from golhisar southwest turkey eastwood w j tibby j roberts n birks h j b lamb h f 2002 the environmental impact of the minoan eruption of santorini thera statistical analysis of palaeoecological data from golhisar southwest turkey holocene 12 4 431 444

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

thin tephra layers in peat revealed by x radiography x radiography has been found to provide a rapid economical non destructive method of locating very thin fine grained tephra of icelandic origin in scottish peats tephra layers form time parallel marker horizons or isochrones that are of great archaeological significance because they cover large areas offer precise chronological control for associated material and occur at times of significant cultural and environmental changes not all of the discrete deposits of recent volcanic fallout in scotland are sufficiently dense to show clearly on x radiographs but those which do provide particularly clear easily located marker horizons with many applications in palaeoenvironmental studies the tephra layer in caithness produced by fallout from a c 4000 by eruption of hekla provides a particularly good example of the use of x radiography to locate volcanic ash the method also reveals other structures of archaeological and palaeoenvironmental significance butler 1992 journal of archaeological science 19 151 161

the sensitivity of a tanzanian crater lake to catastrophic tephra input and four millennia of climate change diatom genera in many large east african lakes change little throughout the holocene period suggesting relatively stable ecological conditions and some resilience to environmental change ecosystem stability is less common in smaller more sensitive lakes such as those within volcanic craters where external impacts can cause abrupt and rapid fluctuations a 4100 year diatom and cyanobacteria pigment record from lake massoko a volcanic crater lake in southern tanzania is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago it is hypothesized that the tephra reduced the rate of p diffusion from the sediments and increased the si p ratio in the lake a period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c 110 years the magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall the statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate of change analysis multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago the lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition

volcanic ash from the 1362 a d oraefajokull eruption iceland in the greenland ice sheet a continuous record of electrical conductivity measurements ecm was made on site during the drilling of a 200 m ice core at summit greenland and was used to identify horizons in the ice that might be linked to volcanic eruptions in one detailed section that we studied a large peak in the number of particles two orders of magnitude above the background was measured the particle peak was not associated with an ecm peak however the particles were identified as volcanic ash on the basis of both particle morphology and chemical composition the ash composition suggests an explosive rhyolitic eruption and is believed to have originatedfrom oraefajokull in iceland in 1362 a d

a 9000 year old ash bed on the faroe islands abstract radiocarbon datings of the saksunarvatn ash bed on the faroe islands indicate an age of 9000 9100 yr b p the saksunarvatn ash bed differs geochemically from both ash zone 1 in the north atlantic and the vedde ash bed of norway and the norwegian sea all mentioned ashes are assumed to originate from iceland the vedde ash has been dated at 10 600 60 yr b p consequently the saksunarvatn and vedde ash beds provide an opportunity for precise dating of events around the pleistocene holocene boundary in marine cores especially from the region where the two plumes overlap

towards a tephrochronology for the holocene of the north of ireland discrete layers of tephra have been found in both raised and blanket bogs in the north of ireland some of the layers have been identified to an icelandic source and in two cases specific eruptions of known radiocarbon age are suggested nine layers spanning the holocene are replicated in three lowland raised bogs one of the major layers has also been identified at a number of sites in two separate upland blanket peat systems and in a lake sediment

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

a model for polythermal ice incorporating gravity driven moisture transport the flow of ice sheets and glaciers dissipates significant amounts of heat which can result in the formation of temperate ice a binary mixture of ice and small amounts of melt water that exists at the melting point many ice masses are polythermal in the sense that they contain cold ice below the melting point as well as temperate ice temperature and melt water or moisture content conversely affect the flow of these ice masses through their effect on ice viscosity and sliding behaviour ice flow models therefore require a component that can solve for temperature and moisture content and determine the free boundary between the cold and temperate subdomains we present such a model based on the theory of compacting partial melts by contrast with other models we describe gravity and pressure gradient driven drainage of moisture while maintaining a divergence free ice flow at leading order we also derive the relevant boundary conditions at the free cold temperate boundary and find that the boundary behaves differently depending on whether ice enters or exits the temperate region the paper also describes a number of test cases used to compare with a numerical solution and investigates asymptotic solutions applicable to the limit of small compaction pressure gradients in the temperate ice regions a simplified enthalpy gradient model is finally proposed which captures most of the behaviour of the full model in this limit

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

an investigation of measured temperature profiles and vhf mesosphere summer echoes at midlatitudes during the summers of 1992 and 1994 experiments were conducted in germany 52 n to investigate the region of the summer mesopause in connection with mesospheric summer echoes mse mse form in layers and are associated with dramatically enhanced radar scattering cross sections furthermore since the summer mesopause is characterized by very low temperatures it has been proposed that mse layers are related to the presence of subvisible ice particles parallel measurements were made using the sounding system sousy vhf radar and rayleigh lidar radar observations showing mse signatures are presented together with accompanying temperature profiles obtained from the lidar data generally the ambient temperatures in the summer mesosphere at midlatitudes are above that required to create ice crystals however the presence of waves can reduce the temperature our observations show this to be the case in particular during one of the mse events two long period inertia gravity waves were observed these waves had estimated local periods of 6 and 8 hours with accompanying vertical wavelengths of 10 and 15 km respectively we find a good correlation between the locations of the calculated temperature minima and the observed mse layers furthermore the temperature minima were generally below the saturation temperature for water vapor lending support to the supposition that the backscatter is connected with the presence of ice particles

flotation and free surface flow in a model for subglacial drainage part 1 distributed drainage we present a continuum model for melt water drainage through a spatially distributed system of connected subglacial cavities and consider in this context the complications introduced when effective pressure or water pressure drops to zero instead of unphysically allowing water pressure to become negative we model the formation of a partially vapour or air filled space between ice and bed likewise instead of allowing sustained negative effective pressures we allow ice to separate from the bed at zero effective pressure the resulting model is a free boundary problem in which an elliptic obstacle problem determines hydraulic potential and therefore also determines regions of zero effective pressure and zero water pressure this is coupled with a transport problem for stored water and the coupled system bears some similarities with hele shaw and squeeze film models we present a numerical method for computing time dependent solutions and find close agreement with semi analytical travelling wave and steady state solutions as may be expected we find that ice bed separation is favoured by high fluxes and low ice surface slopes and low bed slopes while partially filled cavities are favoured by low fluxes and high slopes at the boundaries of regions with zero water or effective pressure discontinuities in water level are frequently present either in the form of propagating shocks or as stationary hydraulic jumps accompanied by discontinuities in potential gradient

long term mobilisation of chemical elements in tephra rich peat ne iceland this paper presents geochemical profiles of a tephra bearing minerotrophic peat column from ne iceland obtained using various elemental analyses of the solid phase and the pore water the influence of tephra grain size thickness and composition of each tephra on the peat geochemistry was investigated interpretations are supported by a statistical approach in particular by autocorrelation and by microscopy observations minerotrophic peat geochemistry may be strongly dependent upon post depositional mobilization and possible leaching of elements as demonstrated by fe and trace metal concentration profiles chemical elements and more specifically potentially harmful metals can be slowly leached out of volcanic falls during their weathering and re accumulate downwards it is emphasised that a tephra deposit can act as an active geochemical barrier blocking downward elemental movements and leading to the formation of enriched layers in this study the formation of poorly amorphous fe phases above the hekla 3 tephra is shown these poorly crystalline fe phases scavenged ni

tephra geochemistry a discussion of some persistent analytical problems distal tephrochronological studies are reliant upon precise and accurate geochemical quantitation this quantitation can involve difficulties not often encountered in more conventional petrology solving these problems is of particular importance in the holocene and late quaternary of northwest europe where numerous tephra layers of icelandic origin have been detected in peat bogs and lake and marine sediments electron probe microanalysis epma has proved the most applicable analytical technique when insufficient quantities of tephra are available for bulk analysis standard epma techniques require fine tuning to the tephra problem as the geochemical instability of glass is a common feature in many previous studies geochemical data of avoidably low quality have been produced and little attention appears to have been directed to modification of epma methods it is suggested that the black box approach to geochemical analysis should be avoided by the consideration of several factors these include

tephra analysis in ombrotrophic peatlands a geochemical comparison of acid digestion and density separation techniques ombrotrophic peatlands often lack any substantial local mineral component and therefore present exceptional opportunities for the extraction and identification of tephra horizons following concerns of shard geochemistry alteration during isolation using acid digestion an alternative methodology using heavy liquids has been proposed however harsh basic and acidic digestions are rarely required in organic rich ombrotrophic peatlands consequently this study tested for geochemical differences between tephra shards from a known rhyolitic eruption isolated from the same stratigraphic depth of a peat sequence using density separation and an acid digestion methodology typically applied to ombrotrophic peat samples and analysed using electron probe microanalysis findings suggest that the two samples could not be geochemically distinguished using visual and statistical methods and that in the context of rhyolitic tephras in ombrotrophic peats acid digestion can be used without significant effects on the interpretation of results density separation may still be required in peatlands with high mineral input and further work is required to examine the effect of the acid digestion technique on tephra of non rhyolitic compositions copyright 2015 john wiley sons ltd

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

a full stokes flow thermo mechanical model for firn and ice applied to the gorshkov crater glacier kamchatka the gorshkov crater glacier at ushkovsky volcano kamchatka is characterized by a large aspect ratio and special thermodynamic conditions at the bedrock caused by a locally enhanced and spatially varying geothermal heat flux furthermore large parts of this glacier consist of firn rather than pure ice which alters the rheological properties such as viscosity and compressibility of the glacier we present a newly developed thermo mechanically coupled three dimensional flow model based on the finite element fe modeling software elmer and apply it to the gorshkov crater glacier by assuming steady state conditions the present day velocity field temperature field basal melting rate and age distribution are simulated we find that flow velocities are generally small 10 s of centimeters per year horizontal and vertical velocities are of comparable magnitude which shows that the shallow ice approximation is not applicable owing to the spatially variable volcanic heat flux the thermal regime at the ice base is cold in the deeper parts of the glacier and temperate in the shallower parts the measured temperature profile and age horizons at the k2 borehole are reproduced quite well and remaining discrepancies may be attributed to transient non steady state conditions firn compressibility is identified as a crucial element for the modeling approach

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

correlation of pliocene and pleistocene tephra layers between the turkana basin of east africa and the gulf of aden abstract electron microprobe analyses of glass shards from volcanic ash in pliocene and pleistocene deep sea sediments in the gulf of aden and the somali basin demonstrate that most of the tephra layers correlate with tephra layers known on land in the turkana basin of northern kenya and southern ethiopia previous correlations are reviewed and new correlations proposed together these data provide correlations between the deep sea cores and to the land based sections at eight levels ranging in age from about 4 to 0 7 ma specifically we correlate the moiti tuff 4 1 ma with a tephra layer at 188 6 m depth in dsdp hole 231 and with a tephra layer at 150 m depth in dsdp hole 241 the wargolo tuff with a tephra layer at 179 7 m in dsdp hole 231 and with a tephra layer at 155 3 m depth in dsdp hole 232 the lomogol tuff defined here with a tephra layer at 165 m in dsdp hole 232a the lokochot tuff with a tephra layer at 140 1 m depth in dsdp hole 232 the tulu bor tuff with a tephra layer at 160 8 m depth in dsdp hole 231 the kokiselei tuff with a tephra layer at 120 m depth in dsdp hole 231 and with a tephra layer at 90 3 m depth in dsdp hole 232 the silbo tuff 0 74 ma with a tephra layer at 35 5 m depth in dsdp hole 231 and possibly with a tephra layer at 10 9 m depth in dsdp hole 241 we also present analyses of other tephra from the deep sea cores for which correlative units on land are not yet known the correlated tephra layers provide eight chronostratigraphic horizons that make it possible to temporally correlate paleoecological and paleoclimatic data between the terrestrial and deep sea sites such correlations may make it possible to interpret faunal evolution in the lake turkana basin and other sites in east africa within a broader regional or global paleoclimatic context

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

rayleigh lidar observations of gravity wave activity in the upper stratosphere at urbana illinois abstract during 13 nights of rayleigh lidar measurements at urbana illinois in 1984 86 thirty six quasi monochromatic gravity waves were observed in the 35 50 km altitude region of the stratosphere the characteristics of the waves are compared with other lidar and radar measurements of gravity waves and with theoretical models of wave saturation and dissipation phenomena the measured vertical wavelengths 2 ranged from 2 to 11 5 km and the measured vertical phase velocities cz ranged from 10 to 85 cm s 1 the vertical wavelengths and vertical phase velocities were used to infer observed wave periods tob which ranged from 100 to 1000 min and horizontal wavelengths x which ranged firm 70 to 2000 km there may be errors in the inferred values of the horizontal wavelengths because they were calculated by assuming that the observed period inferred the intrinsic period dominant wave activity was found at vertical wavelengths between 2 4 km and 7 10 km no significant seasonal variations were evide

double peaked hydroxyl airglow profiles observed from windii uars the windii instrument onboard the uars satellite made many thousands of measurements of the apparent brightness of the hydroxyl airglow as a function of tangent height during the night the measurements were mainly at latitudes between 42 s and 42 n most of the profiles of volume emission rate indicate a simple layer with a peak between 85 and 90 km however from 5 to 25 of the measured profiles have a more complex structure for the layer profile most frequently with two peaks separated by 4 to 6 km the horizontal extent varies from 200 to several thousand kilometers and the duration may be as long as 1 hours here the windii data from november 1991 to april 1995 are used to study the frequency of occurrence of the double peaked layers as a function of latitude season and local time and to explore possible causes of the unusual profiles early in the night the double peaked layers occur preferentially at low to medium latitudes migrating toward the equator by midnight where they remain for the rest of the night there is a pronounced asymmetry between the hemispheres which reverses between the march april equinox and the september october equinox the solstice behavior is similar though not so pronounced through simulation three possible causes to the observed oh double peaked layers were identified gravity waves are a likely source of isolated small scale events but the large horizontal scale long duration structures are more likely to result from temperature inversion layers or mixing events or a combination of the two

environmental impact of the laacher see eruption at a large distance from the volcano integrated palaeoecological studies from vorpommern ne germany abstract in order to investigate the environmental impacts of the laacher see eruption 12 900 calendar years b p and the deposition of the laacher see tephra at a large distance from the volcano fine resolution micromorphological diatomological and palynological investigations were carried out on closely spaced areas in the reinberg basin vorpommern ne germany the lower part of the lst originates from atmospheric input whereas the upper part was washed in from the surrounding basin slopes diatom populations expanded as a consequence of the input of silica while simultaneously dissolution of dead diatoms decreased the tephra provided a favourable habitat for many epilithic and epipelic diatom taxa the diatoms show a slight acidification and an increase in specific conductivity of the water whereas no eutrophication is indicated these effects disappeared after lake sediments covered the tephra the pollen diagrams from the reinberg basin as well as many other pollen diagrams from ne germany indicate that the vegetation reacted to rising temperatures after the cooler gerzensee fluctuation to a short lived water level rise connected with higher precipitation after the volcanic eruption and to minor openings in the forest vegetation due to increased fire probably connected with severe thunderstorms

chemical alteration of tephra in the depositional environment theoretical stability modelling the study of the chemical stability of vitreous material in aqueous media is well established there has to date been little consideration of the implications of variations in the chemical durability of tephra in quaternary tephrochronology chemical alteration can take the form of cationic leaching from the matrix or complete destruction of the silica network either of which could constrain the ability to chemically identify distal tephra here we apply established models of vitreous durability to the published chemical analyses of a large number of icelandic tephras in order to predict their relative durabilities under equivalent conditions this suggests that some important tephras have relatively poor chemical stability and that rhyolitic tephras are in general more stable than basaltic we conclude that tephras should be expected to show predictable differential chemical stability in the post depositional environment copyright 2003 john wiley sons ltd

site specific sediment responses to climate change during the last 140 years in three varved lakes in northern poland accurate dating and unambiguous chronological correlation using cryptotephras provide a powerful tool to compare the varved sediment records of the lakes gleboczek jg czechowskie jc and jelone

simulating the impacts of distal volcanic products upon peatlands in northern britain an experimental study on the moss of achnacree scotland the potential impact of prehistoric volcanism on distal areas has been the subject of some discussion in both ecology and archaeology an experimental approach is used here to examine the potential effects of distant volcanism on a peatland environment simulations of different tephra and acidity loading designed to approximately replicate the prehistoric hekla 4 ashfall were monitored over 2 years impacts on the peatland ecosystem were assessed by qualitative observations of plant health and abundance semi quantitative observations of flowering measurements of peat ph humification and testate amoebae community composition plots with higher acid loading showed immediate and lasting impacts on plants although other treated plots were less affected changes in testate amoebae and peat humification were inconsistent both within and between plots the experiment demonstrates the potentially severe effects of high acid loading on peatland plants although some responses remain unclear

a younger dryas ash bed in western norway and its possible correlations with tephra in cores from the norwegian sea and the north atlantic abstract a bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western norway it is formally mamed the vedde ash bed and its age is approximately 10 600 yr b p i e mid younger dryas the bed consits of pure glass having a bimodal basaltic and rhyolitic somposition the geochemistry of the glass shards suggests an icelandic source by means of stratigraphic position and geochemistry the ash is correlated with ash zones found in cores from the continental shelf the norwegian sea and the north atlatic

can rapidly accumulating holocene peat profiles provide sub decadal resolution proxy climate data to test the potential for developing sub decadal resolution records of ombrotrophic bog surface wetness bsw selected abrupt climate events were identified in cores from four sites in north west europe and time slices over each event were investigated at 2 5 mm resolution using peat humification testate amoeba and plant macrofossil analyses age depth models based on radiocarbon spheroidal carbonaceous particle and tephra dates were applied and the error in these models was used to determine the effective temporal sampling resolution which in most cases was sub decadal fine resolution results revealed some inconsistencies in the co variability of individual proxies and there was a lack of correspondence between some results from adjacent cores however we found evidence of abrupt shifts in bsw occurring over ca 5 20 years indicating a more rapid proxy response than identified in previous research to maximize potential temporal resolution and produce reliable sub decadal bsw data in future research we recommended that peat based studies should obtain the best possible chronological control focus on sites with high species diversity and rapid accumulation rates and perform replicate coring or use open peat sections where the stratigraphy can be clearly seen to assess intra site differences in microtopography site specific factors should be a primary consideration in the selection of sites for future fine resolution research

snapshots in time precise correlations of peat based proxy climate records in scotland using mid holocene tephras mid holocene tephra layers have been located and geochemically analysed from seven ombrotrophic bogs in scotland the tephras found in these scottish peats occur as stratigraphically discrete horizons not visible to the naked eye and originate from volcanic activity in iceland identifying and geochemically typing the tephra layers can enable precise correlations between sites on regional scales depending on the spatial extent of the airfall events dispersal of both the glen garry and hekla 4 tephras is excellent over scotland enabling snapshot views of the past at the time of these tephra depositions palaeoecological analyses have also been undertaken on these bogs providing detailed reconstructions of past changes in bog vegetation and surface wetness a proxy for past climates correlations between the ombrotrophic bogs at the time of the glen garry tephra revealed significant differences between the proxy climate records in the north and the south of scotland suggesting asynchronous changes between northern scotland and the rest of great britain

processes controlling the downstream evolution of ice rheology in glacier shear margins case study on rutford ice stream west antarctica ice rheology governs how glaciers flow and respond to environmental change the rheology of glacier ice evolves in response to a variety of mechanisms including damage heating melting and the development of crystalline fabric the relative contributions of these rheological mechanisms are not well understood using remotely sensed data and physical models we decouple the influence of each of the aforementioned mechanisms along the margins of rutford ice stream a laterally confined outlet glacier in west antarctica we show that fabric is an important control on ice rheology in the shear margins with an inferred softening effect consistent with a single maximum fabric fabric evolves to steady state near the onset of streaming flow and ice progressively softens downstream almost exclusively due to shear heating the rate of heating is sensitive to local shear strain rates which respond to local changes in bed topography as ice is squeezed through the basal trough the impact of shear heating on the downstream evolution of ice rheology in a laterally confined glacier suggests that the thermoviscous feedback wherein faster ice flow leads to higher rates of shear heating further softening the ice is a fundamental control on glacier dynamics

icelandic volcanic ash and the mid holocene scots pine pinus sylvestris pollen decline in northern scotland a volcanic ash layer in peat from northern scotland has been identified and coincides exactly with an abrupt decline in pinus sylvestris l scots pine pollen frequencies this provides an isochrone time equivalent marker horizon with which to investigate the timing of the holocene pine decline furthermore two possible causes of the southward shift of the range of pinus in scotland c 4000 bp are suggested a direct effect of acid pollution by chemicals produced by the eruption of hekla h 4 or a volcanically induced climatic perturbation these possibilities have wider implications for the influence of volcanism on postglacial environmental change

na doppler temperature lidar initial mesopause region observations and comparison with the urbana medium frequency radar meteoric ablation in the upper atmosphere is the source of a dense layer of atomic na between about 80 and 105 km altitude peak densities are typically several thousand atoms cm3 and the total abundance varies seasonally at mid latitudes with the maximum in winter the na layer is a sensitive tracer of the thermal and density structure of this region of the atmosphere and can be probed with exceptional accuracy and resolution using resonance fluorescence lidar techniques we report the first observations of vertically resolved na density temperature and wind profiles in the mesopause region using a new four frequency doppler temperature lidar the initial zonal wind and temperature observations were obtained with a resolution of 30 min and 1 5 km and rms accuracies of 12 m s and 2 k these measurements were conducted simultaneously with the medium frequency mf radar at the urbana atmospheric observatory 40 n 88 w during the night of march 14 15 1991 the mean temperature profile exhibits a well defined mesopause at 95 5 km with a temperature of 181 k the mean zonal wind profile exhibits a strong 58 m s westward jet at 97 km which appears to be related to the mesopause during the 6 hour and 45 min observation period the na doppler winds averaged between 88 and 98 km exhibit the same general trends as the mf radar apparent and true winds in this same region however the lidar and radar velocity estimates differ by as much as 40 m s at some times during the night

icelandic volcanic ash and the mid holocene scots pine pinus sylvestris decline in the north of ireland no correlation tephra linked pollen and dendrochronological studies were performed on mid holocene subfossil peats and pines and two lowland raised bogs in the north of ireland the results demonstrated that at these sites contrary to recent findings in the north of scotland there was no temporal link between the decline in pine pollen and tephra from the eruption of the icelandic volcano hekla h4

observations of horizontal and vertical wave number spectra of gravity wave motions in the stratosphere and mesosphere over the mid pacific we present vertical and horizontal wave number spectra of density perturbations in the upper stratosphere 25 40 km and the upper mesosphere 80 105 km inferred from 45 hours of airborne na rayleigh lidar observations in the vicinity of hawaii the mean density variance is 0 92 2 for the upper stratosphere and 6 1 2 and for the upper mesosphere the mean vertical shear variance and richardson s number for the upper mesosphere are 6 1 km 2 and 0 69 respectively the observed vertical wave number m and horizontal wave number k spectra exhibit power law dependencies the mean slopes of the vertical wave number spectra are 2 5 for the upper stratosphere and 3 1 for the upper mesosphere the mean slope of the horizontal wave number spectra is 1 9 for the upper mesosphere the mean characteristic vertical and horizontal wavelengths inferred from the spectra are 13 4 and 5810 km respectively in all cases the magnitudes of the upper stratosphere m spectra are more than an order of magnitude smaller than those of the upper mesosphere at all observable scales in the stratosphere the m spectra exhibit significant energy at low wave numbers that are less than the values expected for the characteristic wave numbers this suggests the presence of gravity wave sources capable of generating significant energy at long vertical scales the source is believed to be geostrophic adjustment of the jet stream a large amplitude highly coherent quasi monochromatic short horizontal scale h 16 km oscillation in the na layer was observed on the march 22 flight the oscillation resembled short horizontal scale structures often observed in oh airglow images and was found to be consistent with a gravity wave interpretation