the climate memory of an arctic polythermal glacier knowledge of glacier equilibrium line altitude ela changes and trends in time is essentialfor future predictions of glacier volumes we present a novel method for determining trends in elachange at mccall glacier alaska usa over the last 50 years based on mapping of the cold temperatetransition surface cts marking the limit between cold and temperate ice of a polythermal glacier latent heat release from percolating meltwater and precipitation keeps the ice column temperate in theaccumulation area a change from accumulation to ablation zone reduces this heat release leadinglocally to glacier ice cooling by mapping the cts along the whole glacier length using radio echosounding and employing a thermodynamic model the timing of the cooling was determined fromwhich past elas were constructed these are in accord with mass balance measurements carried out onmccall glacier since the 1950s we show that with a warming climate mccall glacier tends to cool ina counter intuitive way

forced gravity waves and the tropospheric response to convection we present theoretical work directed toward improving our understanding of the mesoscale influence of deep convection on its tropospheric environment through forced gravity waves from the linear hydrostatic non rotating incompressible equations we find a two dimensional analytical solution to prescribed heating in a stratified atmosphere which is upwardly radiating from the troposphere when the domain lid is sufficiently high we interrogate the spatial and temporal sensitivity of both the vertical velocity and potential temperature to different heating functions considering both the near field and remote responses to steady and pulsed heating we find that the mesoscale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves which are in turn dependent upon the temporal and spatial structure of the source and the assumed stratification we find a 50 reduction in tropospherically averaged vertical velocity when moving from a trapped i e low lid to upwardly radiating i e high lid solution but even with maximal upward radiation we still observe significant tropospheric vertical velocities in the far field 4 hours after heating ends we quantify the errors associated with coarsening a 10 km wide heating to a 100 km grid in the way a general circulation model gcm would observing a 20 reduction in vertical velocity the implications of these results for the parameterisation of convection in low resolution numerical models are quantified and it is shown that the smoothing of heating over a grid box leads to significant in grid box tendencies due to the erroneous rate of transfer of compensating subsidence to neighbouring regions further we explore a simple time dependent heating parameterisation that minimises error in a parent gcm grid box albeit at the expense of increased error in the neighbourhood

the west african squall line observed on 23 june 1981 during copt 81 mesoscale structure and transports abstract durirg the night of 23 24 june 1981 new korhogo ivory coast a squall line passed over the instrumented area of the copt 81 experiment observations were obtained with a dual doppler radar system a sounding station and 22 automatic meteorological surface stations data from these instruments and from satellite pictures were analyzed to depict the kinematic and thermodynamic structure of the squall line composite analysis techniques were used to obtain a vertical cross section of the reflectivity structure and of the wind field relative to the line the redistributions of air moisture and thermodynamic energy by the convection wet calculated through averaged two dimensional wind fields from a dual doppler radar system the method also allowed the evaluation of the exchanges that were occurring between the convective and the stratiform regions this squall line had many similarities with tropical squall lines previously described by others the leading convective part composed of intense updr

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

a three dimensional simulation of a tropical squall line convective organization and thermodynamic vertical transport abstract three dimensional convective scale simulations of an african squall line observed during the french copt 81 experiment are presented three simulations with different representations of large scale forcing are performed on a domain of 50 km along the line by 80 km across the line they exhibit a similar circulation pattern characteristic of a squall line but differ in intensity the first simulation supposes an unperturbed environment and produces a slow moving squall line 7 m s 1 with weaker total precipitation rate then observed 25 the second one includes a representation of observed thermodynamic and dynamic environment modifications and produces a fast moving squall line 10 m s 1 still weaker than observations 50 or the rain rate the third simulation takes into account the forcing induced by the rear inflow jet as depicted by smull and houze and observed on that day it allows the system to reach an intensity in agreement with observations the convective region 30 km wid

bed properties and hydrological conditions underneath mccall glacier alaska usa during three summer field seasons 2003 2005 and 2006 we carried out radio echo sound ing measurements with a 5 mhz central frequency ice penetrating radar on mccall glacier arctic alaska usa along the central flowline and 17 cross profiles two way travel time was after migration converted to ice thickness which in combination with a recent digital elevation model of the surface of the glaciated area resulted in a detailed map of the bed topography this reveals a complex basal topography in the confluence area of the different glacial cirques the pattern of subglacial water flow following the hydraulic potential gradient was calculated for the whole glacier area and shows a confluence of subglacial water downstream from the confluence of the glacier cirques from the ice thickness map the total ice volume was estimated as slightly less than 0 5 km 3 bed reflection power brp was determined for the glacier after correction for ice thickness dependence results reveal a clear relationship between the brp pattern and basal sliding anomalies along the central flowline

numerical study of an observed orogenic mesoscale convective system part 2 analysis of governing dynamics abstract a detailed analysis of the dynamics and thermodynamics responsible for the structure growth and propagation of an orogenic mesoscale convective system simulated in two dimensions is made the process of scale interaction is addressed through fourier analysis and reynolds averaging analysis of representative predicted variables diabatic forcing and momentum acceleration terms additional dynamical analysis is accomplished through sensitivity experiments in which coriolis diabatic heating and ambient airflow are varied the general conclusion is that the simulated orogenic development is a geostrophic adjustment process to convective heating which is itself modulated and maintained by orographically induced flow systems the heating scales range over a nearly continuous spectrum ranging from 10 250 km the heating occurs in response to primary advective gravity modes the larger scale gravity wave disturbances modulate the smaller scales by organizing mean upward vertical motion patterns the la

elevation and volume changes on the harding icefield alaska airborne surface elevation profiles of the harding icefield south central alaska were made in 1994 and 1996 thirteen glaciers were profiled along with the upper region of the icefield the profiles were compared to u s geological survey topographic maps made in the 1950s to obtain elevation and volume changes comparison of the changes for the different glaciers shows no significant correlation between volume change and the type of glacier or characteristics such as location aspect size slope or terminus changes estimated total volume change for this 43 year period is about 34 km 3 which corresponds to an area average elevation change of 21 m the estimated error in this elevation change of 5 m is mainly due to errors in the maps at higher elevations our measurements provide an accurate baseline against which future determinations of volume change can be made

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

climatic rhythm of the last interglacial in northern eurasia abstract comparison of palaeoclimatic and palaeontological data derived from last interglacial mikulino deposits and holocene deposits has revealed similar patterns of climatic succession and alteration the holocene epoch therefore can be used as a model for events during the last interglacial use of both holocene analogues and data from the interglacial deposits has enabled reconstruction of seasonal temperature and precipitation distribution patterns for interglacial time across northern eurasia and tentatively for parts of north america optimum interglacial climates were marked by increases in both summer and winter temperatures and by generally increased precipitation the thermal inertia of the ocean basins delayed temperature changes in continental regions to extraterrestrial changes causing the positions of seasonal maxima and minima to shift temporally during interglacial time the course of the climatic successions within the mikulino interglacial was similar thourghout euraia allowing for variations due to continental climatic effects changes in precipitation followed similar patterns but in many regions the proportional increases in precipitation during the interglacial are considered to be more significant than the absolute precipitation values the trigerring mechanism for these changes has not as yet been definitively identified but the role of carbon dioxide and methane gases could be of primary importance the pattern of temperature changes in particular the lack of synchroneity of shifts in the winter and summer maxima are not compatible with results predicted from application of milankovich forcing models as the sole agents of climatic change the distribution of air masses during the last interglacial differed significantly from present patterns arctic air masses were confined to more northerly locations and the icelandic mininum increased in significance the siberian anticyclonic circulation was reduced considerably in importance and monoonal activity increased

potential vorticity anomalies associated with squall lines abstract this study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines using data from the oklahoma kansas pre storm experiment we analyze potential vorticity fields near two squall lines one with and one without a trailing stratiform region from this observational analysis we suggest that squall lines with trailing stratiform regions can leave large positive midtropospheric potential vorticity anomalies in their wake to further interpret these observations we consider a two dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates which results in a simple potential pseudodensity inverse potential vorticity equation using apparent heat source fields that model those computed diagnostically from pre storm data we find that theory does indeed predict large midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines t

increase of 10 m ice temperature climate warming or glacier thinning in 1972 and 1995 shallow ice temperatures were measured at identical locations and depths on polythermal mccall glacier brooks range alaska u s a mean annual ice temperatures at 10 m depth t 10 have systematically increased by 1 k for the ablation area 1400 1900 m while closer to the firn area where meltwater percolation and refreezing play a role they remained approximately unchanged interpreting these findings in terms of climate change requires careful consideration of the observed thinning of the glacier which causes lowering of the surface through an existing vertical temperature gradient such temperature gradients can be particularly large in the ablation areas of polythermal glaciers on mccall glacier they are on the order of 0 2 k m 1 we also study the evolution of a 75 m deep temperature profile measured at one location in 1972 using a one dimensional heat diffusion model we find that this profile was in approximate equilibrium with the mean surface temperature extrapolated from the 1972 data using the observed rate of surface lowering and measured rates of vertical advection we find that both the measured temperature change and the vertical temperature gradient at 10 m depth can be reproduced only if the mean annual surface temperature on mccall glacier has increased by 1 1 0 3 k between 1972 and 1995 this result is consistent with the observed trend toward more negative mass balances on the glacier in the 1990s

volume change of mccall glacier arctic alaska usa 1956 2003 a long history of research documents that mccall glacier arctic alaska usa continues to lose mass at a rate that is likely increasing with time we present a photo comparison 1958 2003 that visually documents these volume changes along with survey measurements that quantify these losses measurements of longitudinal profiles initially acquired from airborne laser altimetry and repeated by ground based surveys indicate that the areally averaged rate of thinning increased between 1956 93 and 1993 2002 from 0 35 0 07 m a 1 to 0 47 0 03 m a 1 respectively total volume loss was 8 3 107 1 7 107 m3 and 2 7 107 0 2 107 m3 all in water equivalent for these two time periods these profiles also indicate that a 1 km stretch of the mid ablation area is behaving differently from this trend with a rate of thinning that is not changing with time lastly we present a comparison of several methods for calculating volume change and assess their relative errors

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

the flow of a polythermal glacier mccall glacier alaska u s a we have analyzed the flow of polythermal mccall glacier in arctic alaska using measurements of surface velocity from the 1970s and 1990s together with measurements of ice thickness and surface slope we have investigated both the present flow and seasonal and long term flow variations our analysis of the present flow reveals that i longitudinal stress coupling is important along the entire length of the glacier and ii there is significant basal sliding beneath a 2 km long section of the lower glacier this sliding exists year round and it accounts for more than 70 of the total motion there we have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up glacier from it both effects are reflected in the observed surface profile of mccall glacier the longitudinal stress coupling length of mccall glacier is three times the ice thickness almost twice that typical of temperate glaciers this is a direct effect of lower strain rates which themselves are associated with the smaller mass balance gradients of arctic and continental glaciers long term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution this means that long term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small seasonally velocities reach their annual minimum in spring and increase during the short summer melt season by up to 75 above mean winter values however the extra motion associated with the period of elevated velocities is only about 5 of the total annual motion the speed up is due to an increase in basal sliding this implies that most of the glacier bed is at the melting point the zone affected by the melt season speed up extends well up glacier of any moulins or other obvious sources for meltwater at the bed

mass balance in the glacier bay area of alaska usa and british columbia canada 1995 2011 using airborne laser altimetry the glacier bay region of southeast alaska usa and british columbia canada has undergone major glacier retreat since the little ice age lia we used airborne laser altimetry elevation data acquired between 1995 and 2011 to estimate the mass loss of the glacier bay region over four time periods 1995 2000 2000 05 2005 09 2009 11 for each glacier we extrapolated from center line profiles to the entire glacier to estimate glacier wide mass balance and then averaged these results over the entire region using three difference methods normalized elevation area weighted method and simple average we found that there was large interannual variability of the mass loss since 1995 compared with the long term post lia average for the full period 1995 2011 the average mass loss was 3 93 0 89 gt a 1 0 6 0 1 m w e a 1 compared with 17 8 gt a 1 for the post lia 1770 1948 rate our mass loss rate is consistent with grace gravity signal changes for the 2003 10 period our results also show that there is a lower bias due to center line profiling than was previously found by a digital elevation model difference method

contribution of alaskan glaciers to sea level rise derived from satellite imagery over the last 50 years retreating glaciers and ice caps gic contributed 0 5 mm yr to sea level rises slr and one third is believed to originate from ice masses bordering the gulf of alaska however these estimates of ice wastage in alaska are based on methods that measure a limited number of glaciers and extrapolate the results to estimate ice loss for the many thousands of others how these methods capture the complex pattern of decadal elevation changes at the scale of individual glacier and mountain range is unclear here combining a comprehensive glacier inventory with elevation changes derived from sequential digital elevation models dems we found that between 1962 and 2006 alaskan glaciers lost 41 9 8 6 km 3 yr water equivalent w e and contributed 0 12 0 02 mm yr to slr our ice loss is 34 lower than previous estimates reasons for our lower values include the higher spatial resolution of our glacier inventory and the reduction of ice thinning under debris and at the glacier margins which were not resolved in earlier work estimates of mass loss from gic in other mountain regions could be subject to similar revisions

the mass balance of mccall glacier brooks range alaska u s a its regional relevance and implications for climate change in the arctic mccall glacier has the only long term mass balance record in arctic alaska average annual balances over the periods 1958 72 and 1972 93 were 15 and 33cm respectively recent annual balances 1993 96 are about 60 cm and the mass balance gradient has increased for an arctic glacier with its low mass exchange rate this marks a significant negative trend recently acquired elevation profiles of mccall glacier and ten other glaciers within a 30 km radius were compared with topographic maps made in 1956 or 1973 most of these glaciers had average annual mass balances between 25 and 33 cm while mccall glacier averaged 28 cm for 1956 93 indicating that it is representative of the region in contrast changes in terminus position for the different glaciers vary markedly thus mass balance trends in this region cannot be estimated from fractional length changes at time scales of a few decades we developed a simple degree day accumulation mass balance model for mccall glacier the model was tested using precipitation and radiosonde temperatures from weather stations at inuvik canada and barrow kaktovik and fairbanks alaska and was calibrated with the measured balances the inuvik data reproduce all measured mass balances of mccall glacier well and also reproduce the long term trend towards more negative balances data from the other stations do not produce satisfactory model results we speculate that the arctic front oriented east west in this region causes the differences in model results

the development of organized convection in a simplified squall line model squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration precipitation output and area of influence time dependent models have identified surface outflow boundaries and low level environmental wind shear as the crucial elements in squall line formation and or maintenance steady state models have shown how properties of the downdraught reservoir or cold pool must be matched to the far field wind and thermodynamic profiles to sustain the convection however the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain for help with these issues a time dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far field environments the convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by storm fronts forced or neutral ascent occurs at a surface gust front on the upwind side of the cold pool we distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts it is found that subsidence over the cold pool disorganizes the updraughts whereas a deep overturning circulation downshear from the gust front has an organizing effect hence factors which allow the initial subsidence to occur ahead of the gust front namely contrasting lower and upper level winds and large potential buoyancy favour squall line development it is argued that in cases with large convective potential energy the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught

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

svalbard glacier elevation changes and contribution to sea level rise 1 we compare satellite altimetry from the ice cloud and land elevation satellite icesat 2003 2007 to older topographic maps and digital elevation models 1965 1990 to calculate long term elevation changes of glaciers on the svalbard archipelago results indicate significant thinning at most glacier fronts with either slight thinning or thickening in the accumulation areas except for glaciers that surged which show thickening in the ablation area and thinning in the accumulation areas the most negative geodetic balances occur in the south and on glaciers that have surged while the least negative balances occur in the northeast and on glaciers in the quiescent phase of a surge cycle geodetic balances are related to latitude and to the dynamical behavior of the glacier the average volume change rate over the past 40 years for svalbard excluding austfonna and kvitoya is estimated to be 9 71 0 55 km3 yr 1 or 0 36 0 02 m yr 1 w equivalent for an annual contribution to global sea level rise of 0 026 mm yr 1 sea level equivalent

airborne surface profiling of glaciers a case study in alaska a relatively lightweight and simple airborne system for surface elevation profiling of glaciers in narrow mountain valleys has been developed and tested the aircraft position is determined by kinematic global positioning system gps methods the distance to the glacier surface is determined with a laser ranger the accuracy is about 0 3 m sufficient to permit future changes to be observed over short time intervals long term changes can be estimated by comparison of profiles with existing maps elevation profiles obtained in 1993 94 from three glaciers in central and south central alaska are compared with maps made about 1950 the resulting area averaged seasonally corrected thickness changes during the interval are gulkana glacier central alaska range 11 m worthington glacier central chugach mountains 7 m and bear lake glacier kenai mountains 12 m all three glaciers retreated during the interval of comparison the estimated uncertainty in the average thickness change is 5 m which is mainly due to errors in the existing maps constraints on the accuracy of the maps are obtained by profiling in proglacial areas

mccall glacier record of arctic climate change interpreting a northern alaska ice core with regional water isotopes abstract we explored modern precipitation and ice core isotope ratios to better understand both modern and paleo climate in the arctic paleoclimate reconstructions require an understanding of how modern synoptic climate influences proxies used in those reconstructions such as water isotopes therefore we measured periodic precipitation samples at toolik lake field station toolik in the northern foothills of the brooks range in the alaskan arctic to determine 18 o and 2 h we applied this multi decadal local precipitation 18 o temperature regression to 65 years of mccall glacier also in the brooks range ice core isotope measurements and found an increase in reconstructed temperatures over the late 20th and early 21st centuries we also show that the mccall glacier 18 o isotope record is negatively correlated with the winter bidecadal north pacific index npi climate oscillation mccall glacier deuterium excess d excess 2 h 8 18 o values display a bidecadal periodicity coherent with the npi and suggest shifts from more southwestern bering sea moisture sources with less sea ice lower d excess values to more northern arctic ocean moisture sources with more sea ice higher d excess values northern ice covered arctic ocean mccall glacier moisture sources are associated with weak aleutian low al circulation patterns and the southern moisture sources with strong al patterns ice core d excess values significantly decrease over the record coincident with warmer temperatures and a significant reduction in alaska sea ice concentration which suggests that ice free northern ocean waters are increasingly serving as terrestrial precipitation moisture sources a concept recently proposed by modeling studies and also present in greenland ice core d excess values during previous transitions to warm periods this study also shows the efficacy and importance of using ice cores from arctic valley glaciers in paleoclimate reconstructions

gravity waves compensating subsidence and detrainment around cumulus clouds abstract gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field we explore this role in a two dimensional numerical experiment on a simple moist convecting system consisting of an isolated long lasting nonprecipitating cloud in a calm atmosphere with no surface forcing the cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic the buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence unlike the circulations inside clouds this adjustment takes place without turbulent mixing hence the buoyancy adjustment time t1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the mixing time t2 which it takes a tracer initially concentrated at s

validation of high resolution grace mascon estimates of glacier mass changes in the st elias mountains alaska usa using aircraft laser altimetry we acquired center line surface elevations from glaciers in the st elias mountains of alaska northwestern canada using aircraft laser altimetry during 2000 05 and compared these with repeat measurements acquired in 2007 the resulting elevation changes were used to estimate the mass balance of 32 900 km 2 of glaciers in the st elias mountains during september 2003 to august 2007 yielding a value of 21 2 3 8gta 1 equivalent to an area averaged mass balance of 0 64 0 12 m a 1 water equivalent w e high resolution 2 arc degrees spatial and 10 day temporal gravity recovery and climate experiment grace mass balance estimates during this time period were scaled to glaciers of the st elias mountains yielding a value of 20 6 3 0 gt a 1 or an area averaged mass balance of 0 63 0 09 m a 1 w e the difference in balance estimates altimetry minus grace was 0 6 4 8 gta 1 well within the estimated errors differences likely resulted from uncertainties in subgrid sampling of the grace mass concentration mascon solutions and from errors in assigning an appropriate near surface density in the altimetry estimates the good correspondence between grace and aircraft altimetry data suggests that high resolution grace mascon solutions can be used to accurately assess mass balance trends of mountain glacier regions that are undergoing large changes

elevation volume and terminus changes of nine glaciers in north america nine glaciers in alaska and washington u s a originally mapped as part of the international geophysical year igy in 1957 58 were re mapped between 1993 and 1996 eight using airborne surface elevation profiling and the ninth using ground based kinematic global positioning system methods elevation volume and terminus changes were determined for the approximately 38 year period between the igy mapping and the profiling all nine glaciers showed substantial thinning at lower elevations seven of the nine thickened at higher elevations none of the glaciers had a significant net volume increase two had close to zero change and the others had a decrease for the eight glaciers for which we could obtain quantitative information the mean thickness change was 10 m with a large scatter 8 m standard deviation the volume and terminus changes had no clear geographic pattern and no simple relationship between volume change and terminus advance or retreat was identified the largest error in the estimated volume changes is due to map errors

localized basal motion of a polythermal arctic glacier mccall glacier alaska usa we analyzed the ice flow of mccall glacier alaska usa by numerical glacier modeling and radio echo sounding res model experiments were carried out with a higher order numerical ice flow model and results were validated with measurements of annual ice velocities and compared with previous estimates of ice flow dynamics during the 2003 summer campaign detailed res measure ments were carried out along the central flowline of the ablation area with a 5 mhz central frequency ice penetrating radar where 10 m ice temperatures are approximately 7 5 c the bed reflection power brp beneath this central flowline abruptly increases at one location area followed by a slow decrease down glacier the model experiments show that basal sliding 50 is necessary to match the observed annual mean surface velocities in the area that is characterized by high brp values however when thermomechanical effects are taken into account a temperate basal ice layer is apparent in the ablation area which locally softens the ice and can explain to a certain extent the anomalous flow field the model results confirm that the present temperature field is a remnant of a larger glacier geometry that was near steady state before the onset of enhanced surface thinning in the 1970s

two dimensional convection in non constant shear a model of mid latitude squall lines numerical simulations of two dimensional deep convection are analysed using analytical models extended to include shallow downdraughts and non constant shear the cumulonimbus are initiated by low level convergence created by a finite amplitude downdraught these experiments have constant low level shear and differ only in the profile of mid and upper level winds quasi steady convenction is produced if the mid and upper level flow has small shear and the low level shear is large the surface precipitation ismaximized for no intial relative relative flow aloft if stationary this storm p o can give prodigious locilized rainfall p o is the two dimentisonal equivalent of the supercell these results are placed in context with previous two dimensional simulations attention is drawn to the similiarity with previous two dimensional simulations attention is drawn to the similarity with squall lines in central and eastern u s a storm p o is analysed by construction of time averaged fields of streamfunction vorticity teperature and height deviation the smoothness of these fields suggests a conceptual model of the storm dynamics which involves cooperation between distinct charcteristic flows an overturning updraught a jump type updraught a shallow downdraught a low level rotor and a boundary layer an idealized analytical model is described by solution of the equations for steady convection these solutions for the remote flow are derived from energy conversation mass continuity and a momentum budget and they give relationships between the non dimensional parameteres of the problem it is apparent that the convection is a high froude or low richardson number flow demanding the existence of a cross storm pressure gradient inherent in this idealized model is a vortex sheet between updraught and down draught and it is considered that the dynamical instability of this sheet is related to complexities in the numerical simulation furthermore these results show that in two dimensions both non constant shear and a shallow downdraught are necessary to maintain steady convection

late holocene glaciation and twentieth century retreat northeastern brooks range alaska lichenometric dating of moraines deposited by eight glaciers in the northeastern brooks range suggests major glacial advances or stillstands at about 2600 1000 450 and 60 lichenometric l years bp the latter two advances of the early middle and latest part of the little ice age respectively formed promi nent moraines at all glaciers studied in response to overall twentieth century warming these glaciers have retreated at average rates of 2 to 19 m yr 1 from the distinct ridges formed in ad 1890 glacier thinning at rates of 0 7 to 0 9 m yr 1 accounts for over 90 of the ice loss

diabatic divergence profiles in western pacific mesoscale convective systems abstract heating in the atmosphere can be expressed as diabatic divergence d which is nearly equal to the actual horizontal divergence in tropical convection high quality profile measurements from airborne doppler radar purls in ten mesoscale convective systems mcs observed during toga coare are examined and the mean profile is compared with rawinsonde array measurements young convective features have strong near surface convergence while older cells with better developed downdrafts and stratiform precipitation areas have their peak convergence aloft in the mean then surface flow is only weakly convergent or oven divergent so that the main convergence into mcss is deep and peaked aloft with a sharp melting convergence at 0 c divergence prevails above 10 km altitude but was undersampled by the radar unusual but well sampled observations in the purl dataset include a persistent widespread profile feature in one well sampled mcs a cyclone rainband oscillatory reverberations c

interactions between upper and lower tropospheric gravity waves on squall line structure and maintenance abstract using a simplified thermodynamic sounding and variable vertical wind shear we investigate the role of gravity waves on the structure and propagation of a simulated two dimensional squall line based on an observed squall line environment the modeled troposphere has been divided into three distinct thermodynamic layers these consist of an absolutely stable atmospheric boundary layer abl an elevated well mixed layer and an upper tropospheric layer of intermediate stability we find the mixed layer to have a dual role it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts and it provides an ideal layer to trap meso scale 20 200 km wave energy generated in the stable layers the generated waves thus have a significant and lasting impact on the simulation we also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long lived squall lines experiments that vary the vertical wind shear profile demonstrate

the two dimensional response of a tropical jet to propagating lines of convection this paper uses simple one and two dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet representative of the african easterly jet the results are used to infer changes in the environment of the forcing region which would in reality tend to influence the evolution of the storm through convective mechanisms which are not resolved here from linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing this result is confirmed in two dimensional nonlinear simulations when linear wave modes are computed in a basic state that includes the jet structure small sensitivities to the basic state jet are found the two dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing principally it is found that the modes of response to the forcing may be associated with characteristic changes in the basic state shear convective available potential energy cape and convective inhibition cin which would be expected to have significant influence on the convective system itself

rapid wastage of alaska glaciers and their contribution to rising sea level we have used airborne laser altimetry to estimate volume changes of 67 glaciers in alaska from the mid 1950s to the mid 1990s the average rate of thickness change of these glaciers was 0 52 m year extrapolation to all glaciers in alaska yields an estimated total annual volume change of 52 15 km 3 year water equivalent equivalent to a rise in sea level sle of 0 14 0 04 mm year repeat measurements of 28 glaciers from the mid 1990s to 2000 2001 suggest an increased average rate of thinning 1 8 m year this leads to an extrapolated annual volume loss from alaska glaciers equal to 96 35 km 3 year or 0 27 0 10 mm year sle during the past decade these recent losses are nearly double the estimated annual loss from the entire greenland ice sheet during the same time period and are much higher than previously published loss estimates for alaska glaciers they form the largest glaciological contribution to rising sea level yet measured

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

analysis of the african easterly jet using aircraft observations from the jet2000 experiment analyses of the african easterly jet aej are presented which are based on meridional transects of high resolution dropsonde observations made during jet2000 an aircraft campaign conducted in the last week of august 2000 the observations have confirmed that the aej is closely defined by geostrophic balance the baroclinicity between the extreme northern and southern profiles accurately determines the altitude of the jet core while the location and morphology of the jet core correspond to a locally defined geostrophic wind measure the potential vorticity pv structure has also been found to accord with theoretical expectations with distinctive positive and negative pv anomalies equatorward and poleward of the jet core respectively the thermodynamic structure of the aej environment can be categorized into coherent layers the monsoon layer is a humid zone connected to the land surface extending northwards into the sahel and increasing in depth towards the south this layer is affected by the land surface on diurnal time scales through the growing convective mixed layer and through shallow cumulus clouds above the monsoon layer is the saharan air layer sal which can be identified as a layer of low static stability and low pv the sal is deep where it merges with the saharan boundary layer in the north and becomes thinner toward the south it has been shown that the boundaries of the sal can be approximated to good accuracy as adiabatic surfaces meaning that the sal comprises air which is adiabatically connected to the land surface via the saharan boundary layer the upper region of the sal is identified as a layer of high relative humidity where altocumulus and stratocumulus layers are observed finally the troposphere above the sal is again almost pseudoadiabatic with small baroclinicity which determines the closure of the aej core aloft through inspection of thermodynamic tracers evidence of convective and lateral transport and exchange between these layers is also presented copyright 2005 royal meteorological society