cycles and propagation of deep convection over equatorial africa abstractlong term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales satellite observations are used to understand the timing duration and frequency of deep convection in equatorial africa a region with some of the most intense thunderstorms yet little has been published about the propagation characteristics of mesoscale convection in that region diurnal subseasonal and seasonal cycles of cold cloud proxy for convective precipitation are examined on a continental scale organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions convection is triggered by heating of elevated terrain sea land breezes and lake breezes coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward they have an average 17 6 h duration and 673 km span most have zonal phase speeds of 8 16 m s 1 p

potential sea level rise from antarctic ice sheet instability constrained by observations large parts of the antarctic ice sheet lying on bedrock below sea level may be vulnerable to marine ice sheet instability misi a self sustaining retreat of the grounding line triggered by oceanic or atmospheric changes there is growing evidence that misi may be underway throughout the amundsen sea embayment ase which contains ice equivalent to more than a metre of global sea level rise if triggered in other regions the centennial to millennial contribution could be several metres physically plausible projections are challenging numerical models with sufficient spatial resolution to simulate grounding line processes have been too computationally expensive to generate large ensembles for uncertainty assessment and lower resolution model projections rely on parameterizations that are only loosely constrained by present day changes here we project that the antarctic ice sheet will contribute up to 30 cm sea level equivalent by 2100 and 72 cm by 2200 95 quantiles where the ase dominates our process based statistical approach gives skewed and complex probability distributions single mode 10 cm at 2100 two modes 49 cm and 6 cm at 2200 the dependence of sliding on basal friction is a key unknown nonlinear relationships favour higher contributions results are conditional on assessments of misi risk on the basis of projected triggers under the climate scenario a1b ref 9 although sensitivity to these is limited by theoretical and topographical constraints on the rate and extent of ice loss we find that contributions are restricted by a combination of these constraints calibration with success in simulating observed ase losses and low assessed risk in some basins our assessment suggests that upper bound estimates from low resolution models and physical arguments up to a metre by 2100 and around one and a half by 2200 are implausible under current understanding of physical mechanisms and potential triggers

evidence of multimodal structure of the baroclinic tide in the strait of gibraltar 1 the multimodal structure of the baroclinic tides in the strait of gibraltar is studied using observations and numerical simulations observational data and model results revealed the presence of two types of tidal internal waves generated over camarinal sill cs one propagates toward the mediterranean and disintegrates into series of nonlinear short internal waves with amplitudes of 50 m and more the second type behind the first propagates slower and has a longer wavelength the vertical structure with both upward and downward displacements of isopycnals can be identified as a manifestation of higher baroclinic modes analysis of the empirical orthogonal functions of the adcp measurements performed over cs and model time series has shown that the second baroclinic mode predominates in the second type of internal waves its amplitude can reach one third that of the first baroclinic mode of the leading waves of depression

origin and propagation of a disturbance associated with an african easterly wave as a precursor of hurricane alberto 2000 abstract in this study it is proposed that mesoscale convective complexes mccs and a mesovortex mv were embedded within a wavelike disturbance over north africa that led to the genesis of hurricane alberto 2000 the wavelike disturbance observed may be classified as an african easterly wave aew based on the cloud top area and brightness values observed from infrared satellite data four genesis and three lysis stages are identified within a cycle of moist convection associated with the pre alberto disturbance the availability of water vapor is the most essential factor controlling the convective cycle of the pre alberto disturbance over the african continent the presence of significant topography also contributes to the generation or decay of the associated mccs through regulation of the water vapor supply further analysis of meteosat satellite imagery reveals that the incipient disturbances for 23 of 34 eastern atlantic tropical cyclones originated from the ethiopian highlands eh region du

african easterly wave variability and its relationship to atlantic tropical cyclone activity abstract automatic tracking of vorticity centers in european centre for medium range weather forecasts analyses has been used to develop a 20 yr climatology of african easterly wave activity the tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the african continent the rainy zone equatorward of 15 n is dominated by 600 mb activity and the much drier saharan region poleward of 15 n is more dominated by 850 mb activity over the atlantic ocean there is just one storm track with the 600 and 850 mb wave activity collocated based on growth decay and genesis statistics it appears that the 850 mb waves poleward of 15 n over land generally do not get involved with the equatorward storm track over the ocean instead there appears to be significant development of 850 mb activity at the west african coast in the rainy zone around 10 n 10 w which it is proposed is associated with latent heat release based on the tracking statistics it has been shown th

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

generation of african easterly wave disturbances relationship to the african easterly jet abstract the relationship between african easterly waves and the background climatology in which they form is studied using a regional climate model the surface and lateral boundary conditions in the model are manipulated to modify the background climatology especially the african easterly jet and the itcz and the behavior of the waves in these different settings is evaluated three climate simulations are presented with monthly mean lateral and surface boundary conditions one has a strong jet and a strong itcz the second has a strong jet and a weak itcz and the third has a weak jet and a strong itcz in these simulations the presence of wave activity is more closely associated with the concentration of the itcz than the strength of the african easterly jet in particular the simulation with a strong jet accompanied by a weak itcz does not produce significant wave activity but a weak jet with a strong itcz has realistic wave disturbances both the charney stern and the fjortoft necessary conditi

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

a study of the energetics of african easterly waves using a regional climate model abstract the evolution and spatial distribution of the energetics of african waves are studied complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation it is found that baroclinic overturning is the dominant energy source although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet the generation of active waves usually results from the nearly in phase evolution of baroclinic and barotropic conversions which are associated with significant rainfall over africa significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet

the role of convectively coupled atmospheric kelvin waves on african easterly wave activity abstractthe role of convectively coupled atmospheric kelvin waves cckws on african easterly wave aew activity is explored over tropical africa during boreal summer examination of the pre alberto aew in 2000 highlights the observation that the convective trigger for the initiation of the aew was generated by a strong cckw and that the subsequent intensification of the aew at the west african coast was associated with a second cckw composite analysis shows that generally aew activity increases during and after the passage of the convectively active phase of strong cckws the increase in aew activity is consistent with convective triggering at the leading edge of the convective phase of the cckw this convective triggering occurs in a region where the background low level easterly vertical wind shear is increased by the cckw as the aew propagates westward through the convectively active phase of the cckw it can develop in an environment favorable for convection it is also shown that this phase of

intense convective systems in west africa and their relationship to the african easterly jet for may september 1998 convective systems in west africa were identified from observations by the tropical rainfall measuring mission satellite microwave imager at 85 ghz using re analysis data the 10 day average position of the african easterly jet aej was diagnosed from the 700 hpa zonal winds the distance from each convective system s centroid to the axis of the aej was calculated each convective system s minimum brightness temperatures were ranked so that intense convective systems were defined as those in the 10th percentile or lower the weak 10th percentile and intense convective systems were represented statistically as two separate populations the weak by the skewed gumbel distribution and the intense by the normal distribution from may to august the peak in activity of weak convective systems remained south of 10 n but shifted east of 10 e the peak in activity of the intense convective systems followed the seasonal migration of the aej northwards and became increasingly separate from the peak in activity of weak convective systems the majority of both weak and intense convective systems occurred within 0 50 of high terrain the high convective available potential energy high shear aej environment in the vicinity of high terrain appeared to have the greatest probability of generating intense convective systems in the study area copyright 2006 royal meteorological society

coupling between a glacier and a soft bed ii model results the relation between the local effective pressure and shear stress on till beneath storglaciaren sweden discussed in iverson and others 1999 provides an empirical basis for studying the processes that control the strength of the ice bed coupling particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms model calculations based on studies of cone penetration through fine grained sediment and sliding theory agree with the observed relation between shear stress and effective pressure if the water layer at the ice bed interface is assumed to thicken rapidly as the effective pressure approaches zero studies of the hydraulics of linked cavities provide support for this assumption if the mean thickness of the water layer reflects the extent of microcavity development at the interface comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath storglaciaren only at intermediate effective pressures at very low effective pressures like those inferred at the site of the tiltmeter discussed in iverson and others 1999 and at sufficiently high effective pressures ploughing and sliding should focus motion near the glacier sole a calculation using parameter values appropriate for ice stream b west antarctica suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth this conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down glacier from ploughing particles thereby weakening the bed near the glacier sole however given the apparent sensitivity of the ice bed coupling to basal conditions that may be highly variable any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism

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

three dimensional structure and dynamics of african easterly waves part i observations the mean structure of african easterly waves aews over west africa and the adjacent atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered satellite derived outgoing longwave radiation these results are compared with previous studies and an idealized modeling study in a companion paper which provides evidence that the waves bear a close structural resemblance to the fastest growing linear normal mode of the summertime basic state flow over africa there is a significant evolution in the three dimensional structure of aews as they propagate along 10 n across west africa at this latitude convection occurs in northerly flow to the east of the greenwich meridian then shifts into the wave trough and finally into southerly flow as the waves propagate offshore into the atlantic itcz in contrast to the north of the african easterly jet along 15 n convection remains in southerly flow throughout the waves trajectory along 10 n over west africa the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction as in the companion paper offshore and along 15 n the relationship between the convection and dynamics is more complex and not as easily explained in terms of dynamical forcing alone

marine ice sheet profiles and stability under coulomb basal conditions the behavior of marine terminating ice sheets such as the west antarctic ice sheet is of interest due to the possibility of rapid grounding line retreat and consequent catastrophic loss of ice critical to modeling this behavior is a choice of basal rheology where the most popular approach is to relate the ice sheet velocity to a power law function of basal stress recent experiments however suggest that near grounding line tills exhibit coulomb friction behavior here we address how coulomb conditions modify ice sheet profiles and stability criteria the basal rheology necessarily transitions to coulomb friction near the grounding line due to low effective stresses leading to changes in ice sheet properties within a narrow boundary layer ice sheet profiles taper off towards a flatter upper surface compared with the power law case and basal stresses vanish at the grounding line consistent with observations in the coulomb case the grounding line ice flux also depends more strongly on flotation ice thickness which implies that ice sheets are more sensitive to climate perturbations furthermore with coulomb friction the ice sheet grounds stably in shallower water than with a power law rheology this implies that smaller perturbations are required to push the grounding line into regions of negative bed slope where it would become unstable these results have important implications for ice sheet stability in a warming climate

the genesis of african easterly waves by upstream development abstracta genesis mechanism for african easterly waves aews is proposed in the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development it is proposed that new aews can be generated upstream of existing aews a local eddy kinetic energy budget of the aew that ultimately became hurricane alberto 2000 demonstrates that upstream development explains its genesis more convincingly than previous theories of aew genesis the energetics and ageostrophic secondary circulation of a composite aew are consistent with a new aew forming as a result of this mechanism some strengths and weaknesses of upstream development as a paradigm for aew genesis are discussed with respect to other potential mechanisms

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

three dimensional structure and dynamics of african easterly waves part iii genesis this paper promotes the view that african easterly waves aews are triggered by localized forcing most likely associated with latent heating upstream of the region of observed aew growth a primitive equation model is used to show that aews can be triggered by finite amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable heating close to the entrance region of the african easterly jet aej is shown to initiate aews downstream the heating leads to an initial trough that reaches the west african coast about 5 7 days later depending on the nature of the heating profile after this a structure that projects strongly onto the leading linear normal mode of the basic state becomes established characterized by a number of westward propagating disturbances that strongly resemble aews the sensitivity of the forced aews to the nature and location of the heating profile is examined aews are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the aej in the present study this was best achieved by lower tropospheric heating from shallow convection or upper level heating and lower level cooling from a stratiform precipitation profile both profiles have significant heating gradients in the vertical in the mid to lower troposphere this triggering paradigm for the genesis of aews has consequences for the variability and predictability of aews at weather and climate time scales in addition to the nature of the aej often emphasized it is crucial to consider the nature and variability of upstream heating triggers

the propagation and diurnal cycles of deep convection in northern tropical africa the propagation and diurnal cycle of organized convection in northern tropical africa are examined using five years 1999 2003 of digital infrared imagery for may august reduced dimension techniques are used to document the properties of cold clouds proxies for deep convection and precipitation large scale environments are diagnosed from global analyses organized convection in africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h a substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration episodes with 36 h duration and 1472 km span recur at a one per day interval most episodes have phase speed of 10 20 m s 1 which is faster than most african easterly waves convective episodes tend to initiate in the lee of high terrain consistent with thermal forcing from elevated heat sources average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed phase arrival of systems propagating from the east propagation occurs with moderate low to mid tropospheric shear which varies with the african easterly jet migration and west african monsoon phases frequent deep convection occurs with local shear maxima near high terrain for the peak monsoon period and for 10 w 10 e where easterly waves and convective systems are frequent 35 of cold cloud episodes occur east of the wave trough compared with about 24 to the west based on the coherent behaviour of organized propagating convection inferences may be made regarding the prediction of precipitation beyond one or two days copyright 2008 royal meteorological society

estimating the sliding velocity of a pleistocene ice sheet from plowing structures in the geologic record 1 as an ice sheet slides over its sediment bed some clasts partly embedded in the glacier sole plow through the bed surface the size distribution of such clasts if it can be characterized from structures in the geologic record can be used to estimate the sliding velocity of a past ice sheet by combining a theory of glacier sliding with a geotechnical theory of cone penetration sliding velocity can be calculated in terms of clast size parameters a fluidity parameter for ice and the thermodynamic properties of ice and clasts if frictional properties of the bed are measured the effective normal stress on the bed and bed shear strength during glaciation can also be calculated we used this approach to estimate the sliding velocity of an illinoian ice sheet that left plowing structures in cemented outwash near peoria illinois fluidity parameters for normal and basal temperate ice yielded sliding velocities of 140 168 m yr 1 and 60 72 m yr 1 respectively these are overestimates if solutes impeded regelation of ice past clasts or if friction between debris laden ice and clasts retarded slip preconsolidation stresses determined in laboratory tests on silt from the bed agree with effective normal stresses calculated using clast size parameters the high shear strength of the bed 145 kpa and primary structures preserved within it indicate that additional movement due to pervasive shear of the bed was unlikely application of this method elsewhere would provide basal velocity data that are otherwise unavailable for testing and tuning of ice sheet models

the interaction between deep convection and easterly wave activity over africa convective transitions and mechanisms abstractrecent work using observational data from the international satellite cloud climatology project isccp and reanalysis products suggests that african easterly waves aews form in associati

analysis of convection and its association with african easterly waves abstract the association between convection and african easterly wave aew activity over tropical africa and the tropical atlantic during the boreal summer is examined using satellite brightness temperature tb and ecmwf reanalysis datasets spectral analysis using 18 yr of tb data shows significant variance in the 2 6 day range across most of the region within the regions of deep convection this time scale accounts for about 25 35 of the total variance the 2 6 day convective variance has similar amplitudes over western and eastern africa while dynamic measures of aew activity show stronger amplitudes in the west this study suggests that weak aew activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern africa near darfur western sudan and ethiopia has a role in initiating aews westward the subsequent development and growth of aews in west africa is associated with stronger cohe

a slip law for glaciers on deformable beds slip of marine terminating ice streams over beds of deformable till is responsible for most of the contribution of the west antarctic ice sheet to sea level rise flow models of the ice sheet and till bedded glaciers elsewhere require a law that relates slip resistance slip velocity and water pressure at the bed we present results of experiments in which pressurized ice at its melting temperature is slid over a water saturated till bed steady state slip resistance increases with slip velocity owing to sliding of ice across the bed but above a threshold velocity till shears at its rate independent coulomb strength these results motivate a generalized slip law for glacier flow models that combines processes of hard bedded sliding and bed deformation

response of atmospheric convection to vertical wind shear cloud system resolving simulations with parameterized large scale circulation part i specified radiative cooling abstractit is well known that vertical wind shear can organize deep convective systems and greatly extend their lifetimes much less is known about the influence of shear on the bulk properties of tropical convection in statistical equilibrium to address the latter question the authors present a series of cloud resolving simulations on a doubly periodic domain with parameterized large scale dynamics based on the weak temperature gradient wtg approximation the horizontal mean horizontal wind is relaxed strongly in these simulations toward a simple unidirectional linear vertical shear profile in the troposphere the strength and depth of the shear layer are varied as control parameters surface enthalpy fluxes are prescribed the results fall in two distinct regimes for weak wind shear time averaged rainfall decreases with shear and convection remains disorganized for larger wind shear rainfall increases with shear as convection becomes organized into linear mesoscale systems this nonmonotonic dep

the influence of african easterly waves on convection over tropical africa and the east atlantic abstractusing data from the tropical rainfall measuring mission trmm the modulation of convection by african easterly waves aews is investigated over regions of the east atlantic and tropical africa to explain the modulation of convection the large scale environment lift moisture conditional instability and shear is also examined as a function of aew phase in each region over semiarid portions of tropical africa unconditional rain rates are greatest in the northerly phase of aews due to the strong adiabatic forcing for ascent along the guinea coast the western coast of africa and over the east atlantic where forcing for ascent is weaker rainfall is shifted toward the trough where the air is moist significant contrasts in the characteristics of convection as a function of aew phase comparable in magnitude to regional contrasts are also observed in all regions large and high echo top convective systems are more sensitive to aew phase than small and low echo top systems in semiarid region

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate