the role of gravity wave breaking in a case of upper level near cloud turbulence abstractan observed turbulence encounter that occurred outside a mesoscale convective system over the central united states on 3 june 2005 is investigated using observations and high resolution num

the mesoscale forcing of a midlatitude upper tropospheric jet streak by a simulated convective system part i mass circulation and ageostrophic processes abstract the mutual forcing of a midlatitude upper tropospheric jet streak by organized mesoscale adiabatic and diabatic processes within a simulated convective system scs is investigated using isentropic diagnostics results from a three dimensional numerical simulation of an scs are examined to study the isallobaric flow field modes of dominant ageostrophic motion and stability changes in relation to the mutual interdependence of adiabatic processes and latent heat release isentropic analysis affords an explicit isolation of a component of isallobaric flow associated with diabatic processes within the scs prior to convective development within the simulation atmospheric destabilization occurs through adiabatic ageostrophic mass adjustment and low level convergence in association with the preexisting synoptic scale upper tropospheric jet streak the scs develops in a baroclinic zone and quickly initiates a vigorous mass circulation by the mature stage a pronounced vertical couplet of low level

mesoscale air motions associated with a tropical squall line abstract composites of radar and wind observations in a coordinate system attached to a moving tropical squall line confirm that such a squall system is composed of two separate circulation features a convective squall line region and a stratiform anvil region the squall line region is characterized by mesoscale boundary layer convergence which feeds deep convective updrafts and mid to upper level divergence associated with outflow from the cells the anvil region is characterized by mid level convergence which feeds both a mesoscale downdraft below the anvil and a mesoscale updraft within the anvil cloud before this study the mesoscale updraft in the anvil cloud of the tropical squall system had been somewhat speculative and both the anvil updraft and downdraft had been inferred only qualitatively the occurrence of the anvil updraft is now proven and quantitative profiles of the mesoscale anvil updraft and downdraft have been obtained

ice speed of a calving glacier modulated by small fluctuations in basal water pressure ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska greenland and antarctica high frequency measurements of ice speed and basal water temperatures from a calving glacier in patagonia show that changes in basal water pressure by a few per cent can significantly affect ice flow speed ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska1 2 greenland and antarctica3 4 glaciers that calve in water flow much faster than those that terminate on land as a result of enhanced basal ice motion where basal water pressure is high5 however a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding here we present high frequency measurements of ice speed and basal water pressures from glaciar perito moreno a fast flowing calving glacier in patagonia we measured water pressure in boreholes drilled at a site where the glacier is 515 5 m thick and where more than 60 of the ice is below the level of proglacial lakes we found that the mean basal water pressure was about 95 of the pressure imposed by the weight of the overlying ice moreover changes in basal water pressure by a few per cent drove nearly 40 of the variations in ice flow speed the ice speed was strongly correlated to air temperature suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system we conclude that basal water pressure in calving glaciers is important for glacier dynamics and closely connected to climate conditions

a compact lightweight multipurpose ground penetrating radar for glaciological applications we describe a compact lightweight impulse radar for radio echo sounding of subsurface structures designed specifically for glaciological applications the radar operates at frequencies between 10 and 75 mhz its main advantages are that it has a high signal to noise ratio and a corresponding wide dynamic range of 132 db due mainly to its ability to perform real time stacking up to 4096 traces as well as to the high transmitted power peak voltage 2800 v the maximum recording time window 40 s at 100 mhz sampling frequency results in possible radar returns from as deep as 3300 m it is a versatile radar suitable for different geophysical measurements common offset profiling common midpoint transillumination etc and for different profiling set ups such as a snowmobile and sledge convoy or carried in a backpack and operated by a single person its low power consumption 6 6 w for the transmitter and 7 5 w for the receiver allows the system to operate under battery power for mayor que7 hours with a total weight of menor que9 kg for all equipment antennas and batteries

water pressure and basal sliding on storglaciaren northern sweden the subglacial hydrology of the ablation area of storglaciaren a small valley glacier in northern sweden is dramatically affected by a subglacial ridge or riegel water pressures above this riegel are relatively constant while down glacier from it they vary significantly the lower part of the glacier accelerates in response to peaks in basal water pressure the upper part may be weakly coupled to the lower part during these peaks a power law fit of observed basal water pressures and measured surface velocities yields where u s is the surface velocity and p e is the effective water pressure ice overburden pressure minus subglacial water pressure data from findelengletscher reported by iken and bindschadler 1986 yield an identical exponent and a coefficient one order of magnitude larger the similar exponent implies that the process producing the velocity variations on both glaciers is similar the variations in velocity are inferred to be due to hydraulic jacking on both glaciers

low richardson number in the tropical cyclone outflow layer abstractdropsondes from the noaa g iv aircraft were used to examine the presence of low bulk richardson numbers rb in tropical cyclones at least one 400 m layer above z 7 5 km exhibited rb 1 in 96 of the sondes and rb 0 25 in 35 of the sondes the latter represent almost certain turbulence sondes from major hurricane ivan 2004 were examined in detail turbulent layers fell into three broad groups the first was found below cloud base near the edge of the central dense overcast cdo where relative humidity fell below 40 near zero static stability existed within the turbulent layer with stability and shear maxima above it this structure strongly resembled that seen previously from sublimation of precipitation beneath cloud base the second type of turbulent layer was located within cdo clouds in the upper troposphere and was due almost entirely to near zero static stability this most likely arose as a result of cooling via longwave flux divergence below cdo top the third type of turbulent

intensity of thunderstorm generated turbulence revealed by large eddy simulation thunderstorms are characterized by turbulent processes that constitute an important aviation hazard and cause vertical transport of atmospheric constituents turbulence occurs within cloud and in the surrounding clear air but despite its importance the characteristics of thunderstorm generated turbulence and its spatial distribution are poorly understood especially outside of cloud here we use large eddy simulation to characterize turbulence generated by a canonical thunderstorm the simulation identifies regions of notable three dimensional anisotropic turbulence more than 5 km above the storm in a shallow layer above the storm s anvil and a horizontally asymmetric pattern of weaker turbulence that extends more than 50 km horizontally away from the cloud our results provide the first continuous estimate of turbulence intensity in and around thunderstorms and represent a major step toward improved turbulence avoidance methods the results have broader implications for understanding the fundamental aspects of how thunderstorms affect their environment through vertical exchange processes

ocean forcing of glacier retreat in the western antarctic peninsula in recent decades hundreds of glaciers draining the antarctic peninsula 63 to 70 s have undergone systematic and progressive change these changes are widely attributed to rapid increases in regional surface air temperature but it is now clear that this cannot be the sole driver here we identify a strong correspondence between mid depth ocean temperatures and glacier front changes along the 1000 kilometer western coastline in the south glaciers that terminate in warm circumpolar deep water have undergone considerable retreat whereas those in the far northwest which terminate in cooler waters have not furthermore a mid ocean warming since the 1990s in the south is coincident with widespread acceleration of glacier retreat we conclude that changes in ocean induced melting are the primary cause of retreat for glaciers in this region

organization and environmental properties of extreme rain producing mesoscale convective systems this study examines the radar indicated structures and other features of extreme rain events in the united states over a 3 yr period a rainfall event is defined as extreme when the 24 h precipitation total at one or more stations surpasses the 50 yr recurrence interval amount for that location this definition yields 116 such cases from 1999 to 2001 in the area east of the rocky mountains excluding florida two kilometer national composite radar reflectivity data are then used to examine the structure and evolution of each extreme rain event sixty five percent of the total number of events are associated with mesoscale convective systems mcss while a wide variety of organizational structures as indicated by radar reflectivity data are seen among the mcs cases two patterns of organization are observed most frequently the first type has a line often oriented east west with training convective elements it also has a region of adjoining stratiform rain that is displaced to the north of the line the second type has a back building or quasi stationary area of convection that produces a region of stratiform rain downstream surface observations and composite analysis of rapid update cycle version 2 ruc 2 model data reveal that training line adjoining stratiform tl as systems typically form in a very moist unstable environment on the cool side of a preexisting slow moving surface boundary on the other hand back building quasistationary bb mcss are more dependent on mesoscale and storm scale processes particularly lifting provided by storm generated cold pools than on preexisting synoptic boundaries

kinematic and precipitation structure of the 10 11 june 1985 squall line abstract high frequency 90 min rawinsonde data from a special mesoscale network 26 sites have been combined with wind profiler dense automated surface network data 80 stations spaced 50 km apart and a series of high resolution dual doppler radar analyses in a common framework attached to a moving squall line system to form a comprehensive dataset describing the mature phase of the 10 11 june 1985 squall line observed during pre storm the dual doppler radar analyses covered a 200 300 km2 area from the leading edge of the convective line to the back edge of the trailing stratiform precipitation region thus providing high resolution wind information over a very broad portion of the storm system the comprehensive analysis is used to resolve several aspects of the trailing stratiform region that had remained unclear from previous studies first a difference in the horizontal scale was found between the mesoscale updraft which at upper levels was on the scale of the trailing stratiform cloud a

subglacial water pressure and ice speed variations at johnsons glacier livingston island antarctic peninsula to study subglacial hydrological condition and its influence on the glacier dynamics we drilled johnsons glacier on livingston island in the antarctic peninsula region subglacial water pressure was recorded in boreholes at two locations over 2 years accompanied by high frequency ice speed measurements during two summer melt seasons water pressure showed two different regimes namely high frequency and large amplitude variations during the melt season january april and small fluctuations near the overburden pressure the rest of the year speed up events were observed several times in each summer measurement period ice motion during these events substantially contributed to total glacier motion for example fast ice flow over 1 week accounted for 70 of the total displacement over a 25 day long measurement period we did not find a clear relationship between subglacial water pressure and ice speed this was probably because subglacial hydraulic conditions were spatially inhomogeneous and thus our borehole data did not always represent a large scale subglacial condition ice temperature measurements in the boreholes confirmed the existence of a cold ice layer near the glacier surface our data provide a basis to better understand the dynamic and hydrological conditions of relatively unstudied glaciers in the antarctic peninsula region

possible mechanisms of clear air turbulence in strongly anticyclonic flows abstract the forecasting of clear air turbulence cat continues to be a challenging problem despite progress made in the understanding of vertical shear kelvin helmholtz instabilities the possible connections between horizontal anticyclonic flows and cat are addressed analytical expressions are derived to show that current cat diagnostics do not correctly account for the dynamics of strongly anticyclonic situations in gradient balanced anticyclonic flows nonfrontogenetical enhancement of vertical shear may lead to cat a review of observations theory and modeling is presented to support the claim that strong anticyclonic relative vorticity can also lead to cat through the generation of gravity wave activity by geostrophic adjustment and inertial instability cat diagnostics are then discussed in light of these claims observational work is in progress to investigate the possibility of inertial instability triggered cat

influences of gravity waves on convectively induced turbulence cit a review thunderstorms are known to produce turbulence such turbulence is commonly referred to as convectively induced turbulence or cit and can be hazardous to aviation although this turbulence can occur both within and outside the convection out of cloud cit is particularly hazardous since it occurs in clear air and cannot be seen by eye or onboard radar furthermore due to its small scale and its ties to the underlying convection it is very difficult to forecast guidelines for out of cloud cit avoidance are available but they are oversimplified and can be misleading in the search for more appropriate and physically based avoidance guidelines considerable research has been conducted in recent years on the nature of the phenomenon and in particular its connection to gravity waves generated by the convection this paper reviews the advances in our understanding of out of cloud cit and its relation to convective gravity waves and provides several detailed examples of observed cases to elucidate some of the underlying dynamics

two modes of accelerated glacier sliding related to water 1 we present the first glacier wide detailed measurement of basal effective pressure and related observations including bed separation to elucidate the role of water in sliding the hard bedded glacier instrumented in our study exhibited two phases of accelerated sliding motion apparently driven by separate mechanisms the first acceleration phase up to 5 fold increase in speed was closely tied to an increase in bed separation the faster second phase up to 9 fold increase in speed was related to an unusually high level of connectivity of subglacial waters we infer the first mode was related to cavity opening and the second mode was related to reduced ice contact with the bed glacier sliding over a hard bed is typically represented by sliding laws that include the effective basal pressure but neither sliding phase was accompanied by a simultaneous decrease in local or regional effective pressure

short term variations in glacier flow controlled by subglacial water pressure at lauteraargletscher bernese alps switzerland short term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in lauteraargletscher bernese alps switzerland horizontal surface flow speed oscillated diurnally showing a correlation with the water level in a borehole flow speed increased as a function of the water level with an asymptote at the ice overburden level this observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions detailed examination of the diurnal variations however showed that the speed was larger when the pressure was increasing than when it was decreasing greater speed with increasing pressure was interpreted by subglacial watercavity opening and or longitudinal stress coupling with the upper reaches of the glacier upward surface movements were observed when the glacier flow speed increased simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources vertical straining of ice and volume increase of subglacial water cavities the vertical surface movement was largely affected by the vertical strain and the uplift events could not be simply attributed to cavity opening

analysis of a small vigorous mesoscale convective system in a low shear environment part ii evolution of the stratiform precipitation and mesoscale flows abstract the evolution of the mesoscale flow and precipitation distribution are investigated for a small mesoscale convective system mcs that evolved in a nearly barotropic environment exhibiting moderate instability and weak wind shear observations primarily from a single doppler radar detail the growth of the mcs from the merger of several clusters and lines of vigorous convective cells into a mature state consisting of a weaker convective line trailed by an expanding stratiform precipitation region analysis of radar reflectivity reveals that this stratiform region formed in situ in the presence of weak mesoscale updraft as decaying convective cores coalesced rather than through rearward advection of ice particles directly from the convective region in the absence of sufficient low level shear the mcs collapsed rapidly as it assumed the structure of the archetypal convective line and trailing stratiform precipitation region velocity azimuth displays reveal mesoscale updrafts of about 70 cm s 1 d

vertical motion structure in maritime continent mesoscale convective systems results from a 50 mhz profiler abstract wind profiler data wore used to determine the vertical motion structure in four tropical mcsoscale convective systems mcss which occurred during the down under doppler and electricity experiment dundee near darwin northern territory australia three of the mcss occurred during the monsoon break convective regime and one occurred during the monsoon regime in the break regime cases each with a leading convective and trailing stratiform region structure the wind profiler sampled low level convective cells on the leading edge of the convective region trailed by deeper updrafts of comparable magnitude surface rainfall measurements from a network of raingauges showed two comparable peaks in rainfall intensity that roughly corresponded to the passage of low level and deep convective updraft 71 80 of the system total rainfall was associated with the passage of the convective line in the stratiform region the profiler data showed generally weak vertical drafts 1 m s 1 and the presenc

diurnal fluctuations in borehole water levels configuration of the drainage system beneath bench glacier alaska usa water levels were measured in boreholes spaced along the entire length of bench glacier alaska usa for a period in excess of 2 years instrumented boreholes were arranged as nine pairs along the center line of the glacier and an orthogonal grid of 16 boreholes in a 3600 m 2 region at the center of the ablation area diurnal fluctuations of the water levels were found to be restricted to the late melt season pairs of boreholes spaced along the length of the ablation area often exhibited similar fluctuations and diurnal changes in water levels three distinct and independent types of diurnal fluctuations in water level were observed in clusters of boreholes within the grid of boreholes head gradients suggest water did not flow between clusters and a single tunnel connecting the boreholes could not explain the observed pattern of diurnal water level fluctuations inter borehole and borehole cluster connectivity suggests the cross glacier width of influence of a segment of the drainage system connected to a borehole was limited to tens of meters a drainage configuration whereby boreholes are connected to a somewhat distant tunnel by drainage pipes of differing lengths often hundreds of meters is shown with a numerical test to be a plausible explanation for the observed borehole behavior

snowfall in coastal west antarctica much greater than previously assumed 1 a new antarctic accumulation distribution based on regional model output calibrated with 1900 in situ observations is used to re assess accumulation in 24 antarctic ice drainage basins when compared to the previous compilation good agreement is found for 19 of the 24 basins representing 93 of the ice sheet that is reasonably well covered with observations in contrast the amundsen sea sector of west antarctica and the western antarctic peninsula both data sparse regions are found to receive 80 96 more accumulation than previously assumed for the pine island and thwaites glaciers west antarctica which have recently undergone rapid acceleration and thinning this means a downward adjustment of their contribution to global sea level rise from 0 24 to 0 14 mm per year model time series do not show a significant change in antarctic accumulation over the period 1980 2004

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

the oklahoma kansas mesoscale convective system of 10 11 june 1985 precipitation structure and single doppler radar analysis abstract the 10 11 june mesoscale convective system observed in kansas during pre storm is studied using a variety of observations including conventional radar satellite and single doppler radar this storm at maturity consisted of a strong line of convection trailed by a broad region of stratiform rain the pre storm doppler radar observations show that the general airflow pattern is similar to that seen in previously analyzed cases however since the doppler observations were quite extensive in time and space they permit several details of the airflow to be revealed for the first time a rear inflow jet front to rear flow aloft and a mesoscale updraft and downdraft were all present the mesoscale downdraft commenced at the top of the slanted rear inflow jet sublimation and evaporation of hydrometeors in this flow apparently generated the necessary cooling to drive the mesoscale downdraft circulation the intensity and slope of the rear inflow jet varied with location in the storm which apparen

instruments and methods direct measurement of sliding at the glacier bed sliding at the base of trapridge glacier yukon territory canada was measured using a drag spool we describe this simple and inexpensive instrument as well as its installation and operation from 1990 to 1992 seven sites were instrumented with drag spools at six of the sites basal sliding during the period of observation accounted for 50 70 of the total flow observed at the glacier surface the contribution from ice creep is known to be small so most of the remaining surface motion must be attributed to subglacial sediment deformation for the seventh site the observed sliding rate was 90 of the total flow an indication that the sliding contribution varies spatially across the bed diurnal variations in the response of one of our instruments appear to be correlated to subglacial water pressure fluctuations and are interpreted in terms of changes in sliding velocity rather than the opening and closing of basal cavities

retreating glacier fronts on the antarctic peninsula over the past half century the continued retreat of ice shelves on the antarctic peninsula has been widely attributed to recent atmospheric warming but there is little published work describing changes in glacier margin positions we present trends in 244 marine glacier fronts on the peninsula and associated islands over the past 61 years of these glaciers 87 have retreated and a clear boundary between mean advance and retreat has migrated progressively southward the pattern is broadly compatible with retreat driven by atmospheric warming but the rapidity of the migration suggests that this may not be the sole driver of glacier retreat in this region

a three dimensional calving model numerical experiments on johnsons glacier livingston island antarctica calving from tidewater glaciers and ice shelves accounts for around half the mass loss from both polar ice sheets yet the process is not well represented in prognostic models of ice dynamics benn and others proposed a calving criterion appropriate for both grounded and floating glacier tongues or ice shelves based on the penetration depth of transverse crevasses near the calving front computed using nye s formula the criterion is readily incorporated into glacier and ice sheet models but has not been fully validated with observations we apply a three dimensional extension of benn and others criterion incorporated into a full stokes model of glacier dynamics to estimate the current position of the calving front of johnsons glacier antarctica we find that two improvements to the original model are necessary to accurately reproduce the observed calving front 1 computation of the tensile deviatoric stress opening the crevasse using the full stress solution and 2 consideration of such a tensile stress as a function of depth our modelling results also suggest that johnsons glacier has a polythermal structure rather than the temperate structure suggested by earlier studies

decelerated mass loss of hurd and johnsons glaciers livingston island antarctic peninsula a new 10 year surface mass balance smb record of hurd and johnsons glaciers livingston island antarctica is presented and compared with earlier estimates on the basis of local and regional meteorological conditions and trends since johnsons is a tidewater glacier we also include a calving flux calculation to estimate its total mass balance the average annual smb over the 10 year observation period 2002 11 is 0 15 0 10 m w e for hurd glacier and 0 05 0 10 m w e for johnsons glacier adding the calving losses to the latter results in a total mass balance of 0 09 0 10 m w e there has been a deceleration of the mass losses of these glaciers from 1957 2000 to 2002 11 which have nearly halved for both glaciers we attribute this decrease in the mass losses to a combination of increased accumulation in the region and decreased melt the increased accumulation is attributed to larger precipitation associated with the recent deepening of the circumpolar pressure trough while the melt decrease is associated with lower summer surface temperatures during the past decade

the influence of convectively generated thermal forcing on the mesoscale circulation around squall lines abstract the dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around 2d squall lines are examined using a series of numerical simulations the features that develop in a realistic reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a dry simulation with no microphysical parameterization the thermal forcing in the dry simulation is a scaled and smoothed time average of the latent heat released and absorbed in and near the leading convective line in the reference simulation the mesoscale circulation in the dry simulation resembles the mesoscale circulation in the reference simulation and around real squall lines it includes an ascending front to rear flow a midlevel rear inflow a mesoscale up and downdraft an upper level rear to front flow ahead of the thermal forcing and an upper level cold anomaly to the rear of the thermal forcing it is also shown t

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

dynamic thinning of glaciers on the southern antarctic peninsula growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice especially if it is resting on bedrock below sea level much of the southern antarctic peninsula satisfies this condition and also possesses a bed slope that deepens inland such ice sheet geometry is potentially unstable we use satellite altimetry and gravity observations to show that a major portion of the region has since 2009 destabilized ice mass loss of the marine terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of 56 8 gigatons per year constituting a major fraction of antarctica s contribution to rising sea level the widespread simultaneous nature of the acceleration in the absence of a persistent atmospheric forcing points to an oceanic driving mechanism

trapped gravity waves and their association with turbulence in a large thunderstorm anvil during pecan abstractgeostationary operational environmental satellite 14 goes 14 1 km visible satellite data with 1 min frequency revealed horizontally propagating internal gravity waves emanating from tropo

numerical simulation of radial cloud bands within the upper level outflow of an observed mesoscale convective system abstract turbulence affecting aircraft is frequently reported within bands of cirrus anvil cloud extending radially outward from upstream deep convection in mesoscale convective systems mcss a high resolution convection permitting model is used to simulate bands of this type observed on 17 june 2005 the timing location and orientation of these simulated bands are similar to those in satellite imagery for this case the 10 20 km horizontal spacing between the bands is also similar to typical spacing found in a recent satellite based climatology of mcs induced radial outflow bands the simulated bands result from shallow convection in the near neutral to weakly unstable mcs outer anvil the weak stratification of the anvil the ratio of band horizontal wavelength to the depth of the near neutral anvil layer 5 1 to 10 1 and band orientation approximately parallel to the vertical shear within the same layer are similar to corresponding aspects of horizontal convective rolls in the atmospheric boundar

hurd peninsula glaciers livingston island antarctica as indicators of regional warming ice volume changes during the period 1956 2000 johnsons and hurd glaciers are the two main glacier units of hurd peninsula ice cap livingston island south shetland islands antarctica they presently cover an area of about 10 km 2 johnsons is a tidewater glacier while hurd glacier ends on emerged land in this paper we estimate the changes in ice volume during the period 1956 2000 and compare them with the regional meteorological records the volume change estimates are based on the comparison of digital terrain models for the glacier surface constructed from aerial photographs taken by the british antarctic survey in 1956 and from our geodetic measurements in 1999 2000 the total volume estimates are based on an ice thickness map constructed from radio echo sounding profiles 18 25 mhz done in 1999 2001 showing maximum ice thickness of about 200 m we estimate the changes in ice volume during the period 1956 2000 to be 0 108 0 048 km 3 which represents a 10 0 4 5 decrease from the 1956 total volume of 1 076 0 055 km 3 and is equivalent to an average annual mass balance of 0 23 0 10 m w e during 1956 2000 ice thickness changes range from 40 to 20 m averaging 5 5 4 4 m most areas show ice thinning the thickening is limited to a small area within johnsons glacier all glacier fronts except johnsons calving front show retreat these changes are consistent with the regional meteorological records for mean summer temperature which show a trend of 0 023 0 0058 ca 1 during the period 1956 2000

glacier surge after ice shelf collapse the possibility that the west antarctic ice sheet will collapse as a consequence of ice shelf disintegration has been debated for many years this matter is of concern because such an event would imply a sudden increase in sea level evidence is presented here showing drastic dynamic perturbations on former tributary glaciers that fed sections of the larsen ice shelf on the antarctic peninsula before its collapse in 1995 satellite images and airborne surveys allowed unambiguous identification of active surging phases of boydell sjogren edgeworth bombardier and drygalski glaciers this discovery calls for a reconsideration of former hypotheses about the stabilizing role of ice shelves

large scale meteorological conditions associated with midlatitude mesoscale convective complexes abstract objective analyses of composited meteorological conditions attending ten mesoscale couvective complexes mcc reveal a number of distinctive characteristics and important interactions with the large scale environment the systems appear to be linked to eastward progression of a weak middle tropospheric short wave trough initial thunderstorms develop within a region of mesoscale convergence and lifting that is apparently forced primarily by low level warm advection the mcc system acquires mesoscale organization while it moves eastward ahead of the short wave trough diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in upper levels the mature mcc exhibits many similarities to tropical convective systems although it occurs within a considerably different large scale setting inflow within the lower half of the troposphere feeds convection within a region characterized by significant net upward mass flux and widespread precipitation thickne

measured basal water pressure variability of the western greenland ice sheet implications for hydraulic potential the gradient of the hydraulic potential field at the ice bedrock interface beneath the greenland ice sheet gris dictates the routing and energetics of subglacial water thereby influencing drainage system characteristics and sliding dynamics in the ablation zone of the gris variable water pressure due to an active subglacial drainage system and basal topography with high relief potentially interact to drive unknown spatial patterns and temporal changes in the hydraulic potential field here we present a suite of water pressure measurements collected in 13 boreholes along a 46 km transect on the western gris to investigate the role of spatial and temporal basal water pressure adjustments in hydraulic potential gradient dynamics all borehole sites show pressures with similar seasonality having relatively steady and high values during winter variable and irregular behavior during spring and fall and diurnal cycles that can persist for multiple weeks during the peak melt season despite much higher variability during the melt season the median pressure of the summer period is nearly the same as the median pressure of the winter period however time variability of water pressure due to basal drainage processes can force changes in the magnitude and orientation of the hydraulic potential field over diurnal periods we find that the basal water pressure across the transect generally mimics the ice thickness field but with superimposed large pressure gradients that develop at shorter scales within the basal drainage system this leads to a complex hydraulic potential field across regions of similar ice thickness

mountain glaciers and ice caps around antarctica make a large sea level rise contribution the intergovernmental panel on climate change ipcc estimates that the sum of all contributions to sea level rise for the period 1961 2004 was 1 1 0 5 mm a 1 leaving 0 7

the role of vertical shear on aviation turbulence within cirrus bands of a simulated western pacific cyclone abstractat 0300 utc 9 september 2010 commercial aircraft traveling between tokyo and hawaii encountered regions of moderate and severe intensity turbulence at about 12 km elevation in or just above banded structures in the cirrus anvil associated with an oceanic cyclone located off the east coast of japan the generation mechanisms of the cirrus bands and turbulence are investigated using the advanced research weather research and forecasting model with five nested domains having a finest horizontal grid spacing of 370 m the simulation reproduces the satellite observed patterns of cloud brightness including the bands and suggests that synoptic scale vertical shear within the anvil cloud layer and radiative effects including longwave cooling at cloud top and warming at cloud base act together to produce banded structures within the southern edge of the cirrus cloud shield the character of the bands within the nearly neutral or convectively unstable layer of the cirrus shield is similar to boundary l

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

tropopause evolution in a rapidly intensifying tropical cyclone a static stability budget analysis in an idealized axisymmetric framework abstractupper level static stability n2 variations can influence the evolution of the transverse circulation and potential vorticity in intensifying tropical cyclones tcs this paper examines t

convection permitting simulations of the environment supporting widespread turbulence within the upper level outflow of a mesoscale convective system abstract widespread moderate turbulence was recorded on three specially equipped commercial airline flights over northern kansas near the northern edge of the extensive cirrus anvil of a nocturnal mesoscale convective system mcs on 17 june 2005 a noteworthy aspect of the turbulence was its location several hundred kilometers from the active deep convection i e large reflectivity regions of the mcs herein the mcs life cycle and the turbulence environment in its upper level outflow are studied using rapid update cycle ruc analyses and cloud permitting simulations with the weather research and forecast model wrf it is demonstrated that strong vertical shear beneath the mcs outflow jet is critical to providing an environment that could support dynamic e g shearing type instabilities conducive to turbulence comparison of a control simulation to one in which the temperature tendency due to latent heating was eliminated indicates that strong vertical shear and corresponding reductions in the l

on the formation of the vein type uranium yellow ores from the schwarzach area ne bavaria germany and on the behaviour of p as v and se during supergene processes most of the u occurrences situated at the western edge of the bohemian massif show in their upper parts secondary u minerals the immigration p v as and se from the country rocks and the ore body via saprolite and paleosoils into the secondary ore minerals is investigated the p content is suggested to be closely related to hydromorphic paleosoils from those intermediate stages it may have been brought into the upper parts of the vein structures no laterogenic impoverishment in the wall rocks with respect to phosphorus or apatite could be determined the arsenic distributions is also governed as at other sulfide deposits by underlying sulfides and reducing conditions of a primary hypogene ore zone whereas vanadium content in u secondary minerals is closely related to the enclosing country rocks granites on account of their low content in mafic minerals have low v contents in u secondary minerals

widespread acceleration of tidewater glaciers on the antarctic peninsula over the last half century the antarctic peninsula ap has been among the most rapidly warming regions on earth this has led to increased summer snowmelt loss of ice shelves and retreat of 87 of marine and tidewater glacier fronts tidewater glacier flow is sensitive to changes in basal water supply and to thinning of the terminus and faster flow leads directly to sea level rise the flow rates of most ap tidewater glaciers have never been measured however and hence their dynamic response to the recent changes is unknown we present repeated flow rate measurements from over 300 glaciers on the ap west coast through nine summers from 1992 to 2005 we show that the flow rate increased by similar to 12 on average and that this trend is greater than the seasonal variability in flow rate we attribute this widespread acceleration trend not to meltwater enhanced lubrication or increased snowfall but to a dynamic response to frontal thinning we estimate that as a result the annual sea level contribution from this region has increased by 0 047 0 011 mm between 1993 and 2003 this contribution together with previous studies that assessed increased runoff from the area and acceleration of glaciers resulting from the removal of ice shelves implies a combined ap contribution of 0 16 0 06 mm yr 1 this is comparable to the contribution from alaskan glaciers and combined with estimated mass loss from west antarctica is probably large enough to outweigh mass gains in east antarctica and to make the total antarctic sea level contribution positive

a composite model of mesoscale convective complexes abstract a composite analysis technique is used to investigate the evolution of mesoscale features of mesoscale convective complexes mccs the early stage of the mcc lifecycle is characterized by convergence vertical motion and heating being centered in the lower troposphere as the mcc matures the level of peak upward motion and heating shifts to the upper troposphere the system achieves and maintains its maximum divergence upward motion and anticyclonic vorticity in the upper troposphere during the latter half of the life cycle this is in contrast to gate tropical clusters where the maximum divergence upward motion and anticyclonic vorticity occurred at the mature stage of the cluster and then weakened this difference might be explained by an mcc being an inertially stable form of mesoscale convective system whose radius exceeds the rossby radius of deformation the mcc is shown to be an efficient rain producer exhibiting a precipitation efficiency exceeding 100 at the mature stage due to th

surface ice motion deviating toward the margins during speed up events at gornergletscher switzerland 1 high frequency ice flow measurements during speed up events in gornergletscher switzerland revealed intriguing ice motion which has never been reported in detail before during the summer 2005 more than a 100 flow speed increase was observed three times at four gps stations installed across gornergletscher the speed ups were accompanied by a decimeter scale surface uplift two of the events were triggered by intensive surface melt and rainfall while the third one was due to the outburst of gornersee a glacier dammed lake located 2 km upglacier an interesting observation was ice motion deviating toward the side margins during the events as the glacier accelerated a transverse cross glacier velocity component was generated turning the flow direction away from the central flow line toward the margins when the glacier decelerated the transverse velocity component reversed so that the ice flowed back to the azimuth of the initial flow direction in the most significant case the trajectory of the survey stake deviated from the original track by 0 2 m in the transverse direction we hypothesize that the observed lateral ice motion was caused by locally elevated subglacial water pressure when the ice sole decoupled from the bed at a part of the glacier a point source of vertical displacement was transmitted to the surface through viscous ice this caused the transverse as well as vertical surface motion as observed in ground motion during magma intrusion the hypothesis was tested with a two dimensional ice flow model applied to the transverse glacier cross section the model confirmed that the surface ice would move toward the margins as observed in gornergletscher if subglacial water pressure exceeded the ice overburden pressure over a limited part of the bed

mass loss of larsen b tributary glaciers antarctic peninsula unabated since 2002 ice mass loss continues at a high rate among the large glacier tributaries of the larsen b ice shelf following its disintegration in 2002 we evaluate recent mass loss by mapping elevation changes between 2006 and 2010 11 using differencing of digital elevation models dems the measurement accuracy of these elevation changes is confirmed by a null test subtracting dems acquired within a few weeks the overall 2006 2010 11 mass loss rate 9 0 2 1 gt a 1 is similar to the 2001 02 2006 rate 8 8 1 6 gt a 1 derived using dem differencing and laser altimetry this unchanged overall loss masks a varying pattern of thinning and ice loss for individual glacier basins on crane glacier the thinning pulse initially greatest near the calving front is now broadening and migrating upstream the largest losses are now observed for the hektoria green glacier basin having increased by 33 since 2006 our method has enabled us to resolve large residual uncertainties in the larsen b sector and confirm its state of ongoing rapid mass loss

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

radioglaciological studies on hurd peninsula glaciers livingston island antarctica we present the results of several radio echo sounding surveys carried out on johnsons and hurd glaciers livingston island antarctica between the 1999 2000 and 2004 05 austral summer campaigns which included both radar profiling and common midpoint measurements with low 20 25 mhz and high 200 mhz frequency radars the latter have allowed us to estimate the radio wave velocity in ice and firn and the corresponding water contents in temperate ice which vary between 0 and 1 6 depending on the zone maximum ice thickness is 200 m with a mean value of 93 6 2 5 m total ice volume is 0 968 0 026 km 3 for an area of 10 34 0 03 km 2 the subglacial relief of johnsons glacier is quite smooth while that of hurd glacier shows numerous overdeepenings and peaks the radar records suggest that hurd glacier has a polythermal structure contrary to the usual assumption that glaciers in livingston island are temperate this is also supported by other dynamical and geomorphological evidence

exploring glacier dynamics with subglacial water pressure pulses evidence for self organized criticality 1 in order to determine whether brief excursions or pulses in subglacial water pressure inferred by kavanaugh and clarke 2000 2001 occur water pressures at the bed of trapridge glacier yukon canada were recorded using an interface board that continuously monitored a pressure transducer during the 231 day period between 16 july 2005 and 4 march 2006 more than 7000 pressure pulses were recorded with magnitudes reaching nearly 3 times the flotation value comparison of the pressure pulse record with those from a number of other instruments installed in this soft bedded glacier indicates that these pulses are generated by stress transients that compress the water within the borehole calculations suggest that these transients are as large as 75 times the nominal driving stress both the magnitudes and interevent times for these pulses are well fitted by power law distributions that are remarkably similar to those exhibited by earthquakes these similarities suggest that the ice bed interface of a soft bedded glacier behaves much like an earthquake fault and raises the possibility that such glaciers self organize to a critical state further evidence for self organized criticality soc of soft bedded glaciers is suggested by an examination of well known ice dynamical properties and the rheological properties of subglacial sediments which suggests that soc might be a natural consequence of the rate independent behavior of subglacial sediments

polythermal glacier hydrology a review 1 the manner by which meltwater drains through a glacier is critical to ice dynamics runoff characteristics and water quality however much of the contemporary knowledge relating to glacier hydrology has been based upon and conditioned by understanding gleaned from temperate valley glaciers globally a significant proportion of glaciers and ice sheets exhibit nontemperate thermal regimes the recent growing concern over the future response of polar glaciers and ice sheets to forecasts of a warming climate and lengthening summer melt season necessitates recognition of the hydrological processes in these nontemperate ice masses it is therefore timely to present an accessible review of the scientific progress in glacial hydrology where nontemperate conditions are dominant this review provides an appraisal of the glaciological literature from nontemperate glaciers examining supraglacial englacial and subglacial environments in sequence and their role in hydrological processes within glacierized catchments in particular the variability and complexity in glacier thermal regimes are discussed illustrating how a unified model of drainage architecture is likely to remain elusive due to structural controls on the presence of water cold ice near glacier surfaces may reduce meltwater flux into the glacier interior but observations suggest that the transient thermal layer of near surface ice holds a hydrological role as a depth limited aquifer englacial flowpaths may arise from the deep incision of supraglacial streams or the propagation of hydrofractures forms which are readily able to handle varied meltwater discharge or act as locations for water storage and result in spatially discrete delivery of water to the subglacial environment the influence of such drainage routes on seasonal meltwater release is explored with reference to summer season upwellings and winter icing formation moreover clear analogies emerge between nontemperate valley glacier and ice sheet hydrology the discussion of which indicates how persistent reassessment of our conceptualization of glacier drainage systems is required there is a clear emphasis that continued integrated endeavors focused on process glaciology at nontemperate glaciers are a scientific imperative to augmenting the existing body of research centered on ice mass hydrology

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

mechanisms supporting long lived episodes of propagating nocturnal convection within a 7 day wrf model simulation abstract a large domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one week period of successive zonally propagating heavy precipitation episodes occurring over the central united states similar to climatological studies of phase coherent warm season convection the longest lived precipitation episodes initiate during the late afternoon over the western great plains 105 100 w reach their greatest intensity at night over the central great plains 100 95 w and typically weaken around or slightly after sunrise over the midwest 95 85 w the longest lived episodes exhibit average zonal phase speeds of 20 m s 1 consistent with radar observations during the period composite analysis of the life cycle of five long lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east west oriented lower tropospheric frontal zone an elevated 2 km deep layer of high e air helps sustain convect

numerical study of convection observed during the winter monsoon experiment using a mesoscale two dimensional model abstract a two dimensional version of the pennsylvania state university mesoscale model has been applied to winter monsoon experiment data in order to simulate the diurnally occurring convection observed over the south china sea the domain includes a representation of part of borneo as well as the sea so that the model can simulate the initiation of convection also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle this allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck which typically occupies several hundred kilometers of the domain frank and cohen s cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area the major conclusions are ice phase processes are important in determining the level of maximum large scale heating and vertical motion because there is a strong anvil componen

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

spatial pattern and stability of the cold surface layer of storglaciaren sweden the mechanisms controlling the spatial distribution and temporal fluctuations of the thermal structure in polythermal glaciers have to date been poorly investigated and are not fully understood