spatial and temporal distribution of cyanobacterial soil crusts in the kalahari implications for soil surface properties localised patterns of erosion and deposition in vegetated semi arid rangelands have been shown to influence ecological change and biogeochemical cycles in the flat vegetated kalahari rangelands of southern africa the factors regulating erodibility of the fine sand soils and the erosivity of wind regimes require further investigation this paper reports on the spatial and temporal patterns of cyanobacterial soil crust cover from ten sites at five sampling locations in the semi arid kalahari and discusses the likely impact on factors regulating surface erodibility and erosivity cyanobacterial soil crust cover on kalahari sand varied between 11 and 95 of the ground surface and was higher than previously reported cover was inversely related to grazing with the lowest crust cover found close to boreholes and the highest in the game reserve and wildlife management zone in grazed areas crusts form under the protective canopies of the thorny shrub acacia mellifera fenced plot data showed that crusts recover quickly from disturbance with a near complete surface crust cover forming within 15 months of disturbance crust development is restricted by burial by wind blown sediment and by raindrop impact crusts had significantly greater organic matter and total nitrogen compared to unconsolidated surfaces crusts also significantly increased the compressive strength of the surface and thus decreased erodibility and changed the surface roughness establishing exactly how these changes affect aeolian erosion requires further process based studies the proportion of shear velocity acting on the surface in this complex mixed bush grass crust environment will be the key to understanding how crusts affect erodibility

wind tunnel evaluation of a wedge shaped aeolian sediment trap abstract a detailed set of tests in a wind tunnel investigated the characteristics of flow and sampling efficiency of a low cost sediment trap that can be used in wind tunnels and field applications the trap is constructed of thin sheet aluminum and has a wedge shape 32 internal inlet angle with a 20 500 mm rectangular inlet orifice that extends 40 mm beyond the wedge form relatively unimpeded airflow through the trap is achieved by the large exit area 130 500 mm which is covered with fine stainless steel mesh 62 5 m openings 60 porosity and collected all but the finer silt sediment sizes despite its angular shape the trap has good characteristics of flow and a very high sampling efficiency 90 over a wide range of wind velocities because of its near isokinetic characteristics of flow results also indicate that this trap and others that have wide side profiles are very sensitive to incident wind angle incident angles 5 significantly reduce the sampling efficiency and promote scour around the trap inlet

detecting induced in situ erodibility of a dust producing playa in australia using a bi directional soil spectral reflectance model abstract two sites on a playa claypan in south western queensland australia were chosen to represent the variability in soil surface conditions known to control wind erosion and dust production in the region the material was very fine and comprised of large amounts and different types of salts high electrical conductivity a propensity for slaking and the presence of cyanobacteria the surfaces of 4 plots at each site were modified using in situ rainfall simulation and wind tunnel abrasion the changes in the surface conditions were recorded using digital images and multi angular spectral measurements of reflectance and inverted against a soil bi directional spectral reflectance model optimised values of the model parameters produced the single scattering albedo ssa and a description of the scattering behaviour of the soil surfaces that included an estimate of microscopic roughness the model parameters removed the effect of the measurement conditions illumination and viewing geometry on the spectral reflectance abrasion of the untreated surfaces demonstrated that the variability in the antecedent surface conditions played an important role in their response rainfall drying and subsequent abrasion appeared to reset the soil surface conditions and controlled the spatial and temporal variation in the soil surface conditions redundancy analyses conducted separately for ssa of selected wavebands model parameter values and treatments elucidated the subtle and interactive effects of the treatments and surface responses the results showed that high intensity rainfall and abrasion was associated with wavebands in the visible region and reduced roughness low intensity rainfall was associated with wavebands in the near infrared and short wave infrared regions and increased roughness the analysis was interpreted as the result of eluviation due to rainsplash impact which caused size segregation and the preferential translocation of iron oxides to the surface and the creation of a thick surface seal and caused a thin crust which enabled the movement of salts and or mineral clays to the surface because of the strong evaporation gradient a poor relationship between aerodynamic resistance and microscopic roughness suggested that features of soil surface erodibility created by natural environmental processes such as rainsplash and wind erosion required a correction for macroscopic roughness the values of the model parameters appeared to be insensitive to the directional nature of the abrasion process the findings are significant for the development of this work towards an airborne or satellite based assessment of soil surface erodibility and erosion

subgrid parameterization for snow depth over mountainous terrain from flat field snow depth snow depth is an important variable for a variety of models including land surface meteorological and climate models various measurement networks were therefore developed to measure snow depth on the ground measurement stations are generally located in gentle terrain flat field measurements most often at lower or mid elevation while these sites have provided a wealth of information various studies have questioned the representativity of such flat field measurements for the surrounding topography especially in alpine regions using highly resolved snow depth maps at the peak of winter from two distinct climatic regions in switzerland and in the spanish pyrenees we developed two parameterizations to estimate domain averaged snow depth in coarse scale model applications over complex topography using easy to derive topographic parameters the first parameterization uses a commonly applied linear lapse rate removing the dominant elevation gradient in mean snow depth revealed remaining underlying correlations with other topographic parameters in particular the sky view factor the second parameterization combines a power law elevation trend scaled with the subgrid parameterized sky view factor using a variety of statistic measures showed that the more complex parameterization performs better when using mean high resolution flat field snow depth the performances slightly decreased when formulating the parameterizations for a single flat field snow depth measurement nevertheless the more complex parameterization still outperformed the linear lapse rate model as the parameterization was developed independently of a specific geographic region we suggest it could be used to assimilate flat field snow depth or snowfall into coarse scale snow model frameworks this article is protected by copyright all rights reserved

an experimental study on the susceptibility of crusted surfaces to wind erosion a comparison of the strength properties of biotic and salt crusts abstract penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems despite the early success of this procedure questions remain concerning the effect of the angle of penetration the role of fatigue and the relative importance of the ultimate strength as compared to the elasticity of the crust this paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts the results demonstrate that the angle of penetration is not important but the density of penetrations is the decreased strength observed with high densities of penetration repeated loading and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events the elasticity of the surface appears to be at least as important to consider as the ultimate strength although much stronger the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies the notable elasticity of biotic crusts appears to afford them some protection against abrasion

substantial glacier mass loss in the tien shan over the past 50 years long term changes of the mountain glaciers in the tien shan central asia are not well constrained remote sensing data and glaciological models reveal a 27 decline of glacier mass from 1961 to 2012 linked to increased summer melting

effects of soil moisture and dried raindroplet crust on saltation and dust emission 1 field experiments using a sand particle counter and an optical particle counter clarified the links between saltation and dust emission under wet and weakly crusted conditions in a fallow field previously cultivated with wheat in australia a crust was formed by the impact of raindroplets after small precipitation events a little soil moisture enhanced the strengths of crust and aggregation even though the soil was dried dust concentration was proportional to friction wind velocity but the proportionality was dependent on ground surface conditions such as the minimally dispersed particle size distribution of parent soil and the presence or absence of crust

tensile test of natural microbiotic crust abstract the mechanical properties of microbiotic crust are important to stabilize sandy surfaces in arid and semi arid regions we developed a new experimental approach to perform the tensile test of natural microbiotic crust the modulus of elasticity ultimate stress and one dimensional stress strain relation was obtained the microbiotic crust we measured exhibits linear elastic behavior firstly and then breaks this mechanical character was applied to the study of the rupture of microbiotic crust under the impact of one saltating grain a very simple dimensionless variable was suggested to quantitatively assess surface stability

aeolian processes and the biosphere ravi s d odorico p breshears d d field j p goudie a s huxman t e li j okin g s swap r j thomas a d pelt s v whicker j j zobeck t m 2011 aeolian processes and the biosphere reviews of geophysics 49 3

a wind tunnel study of particle kinematics during crust rupture and erosion abstract sediments with protective crusts of varying type were subjected to particles fed from an upwind source during wind tunnel experiments carried out to compare their ability to resist erosion and alter the kinematics of the saltation cloud a laser doppler anemometer measured the distribution of particle velocity for saltators impacting each crusted surface and for particles ejected from this same surface inclusive of ricochets biotic crusts grown on sand in an environmental chamber were able to withstand erosion over several hours of continuous particle impact as compared to brittle salt crusts which regardless of wind speed or sodium chloride concentration eroded fully within one half hour despite the appearance of deep pits and grooves on the ruptured crusts loss of mass dominated over particle trapping on these rough surfaces while decreasing salt concentration between 320 g kg 1 and 80 g kg 1 generally was found to be associated with an increase in the mass flux of particles ejected from the surface little to no correlation with either wind speed or particle impact speed was observed the range over which varying salt concentration affects the momentum of ejected particles is rather narrow within 160 g kg 1 temporal changes in the velocity distribution of ejected particles with crust rupture and deflation are complicated by variations in crust strength with depth this is especially true of biotic crusts and weak salt crusts fast moving particles associated with the upper 40 of the cumulative velocity distribution generally demonstrate little variation from the control surfaces and probably represent ricochets in comparison those within the lower 60 of the distribution are significantly affected by crust rupture and erosion and may include low energy rebounds as well as the entrainment of new particles loosened from within the crust while such measurements are exceedingly rare they are needed for the validation of physically based models of crust erosion

quasi analytical treatment of spatially averaged radiation transfer in complex terrain 1 we provide a new quasi analytical method to compute the subgrid topographic influences on the shortwave radiation fluxes and the effective albedo in complex terrain as required for large scale meteorological land surface or climate models we investigate radiative transfer in complex terrain via the radiosity equation on isotropic gaussian random fields under controlled approximations we derive expressions for domain averaged fluxes of direct diffuse and terrain radiation and the sky view factor domain averaged quantities can be related to a type of level crossing probability of the random field which is approximated by long standing results developed for acoustic scattering at ocean boundaries this allows us to express all nonlocal horizon effects in terms of a local terrain parameter namely the mean square slope emerging integrals are computed numerically and fit formulas are given for practical purposes as an implication of our approach we provide an expression for the effective albedo of complex terrain in terms of the sun elevation angle mean square slope the area averaged surface albedo and the ratio of atmospheric direct beam to diffuse radiation for demonstration we compute the decrease of the effective albedo relative to the area averaged albedo in switzerland for idealized snow covered and clear sky conditions at noon in winter we find an average decrease of 5 8 and spatial patterns which originate from characteristics of the underlying relief limitations and possible generalizations of the method are discussed

a wind tunnel study of the resilience of three fungal crusts to particle abrasion during aeolian sediment transport abstract this study is a comparison of the strength stability and resistance to abrasion of monospecific sand crusts formed by three species of free living fungi aureobasidium pullulans trichoderma harzianum and absidia corymbifera compared to three photoautotrophs examined in a previous study the fungal crusts were thicker and stronger in flexure by a factor of two or more in addition to the morphological properties of these fungi the organic substrate required for growth contributes to the total crust strength wind tunnel tests in which the crusts were subjected to particle impact from an upwind sediment source nominally 0 014 kg m 1 s 1 and varied levels of freestream velocity u showed that crusts comprised of trichoderma and absidia were stable at wind velocities under 10 m s 1 crusts of aureobasidium generally ruptured at velocities well below this level and to a much greater extent the distinctive morphologies and metabolic rates of the three fungi appear to explain these differences as compared to the wind tunnel experiments independent tests of crust strength in flexure which were intended to simulate crust flaking were found to be less sensitive indicators of relative crust stability in aeolian settings analysis of the time rate of expansion of the perimeter of crust loss indicates that this relation is approximately exponential with the exception of low wind velocities near the threshold for grain ejection where the momentum of the saltators at impact appears to be of prime importance

a scale of greatness and causal classification of mass extinctions implications for mechanisms a quantitative scale for measuring greatness g of mass extinctions is proposed on the basis of rate of biodiversity diminution expressed as the product of the loss of biodiversity called magnitude m and the inverse of time in which that loss occurs designated as intensity i on this scale the catastrophic cretaceous tertiary k t extinction appears as the greatest since the ordovician and the only one with a probable extraterrestrial cause the end permian extinction was less great but with a large magnitude m and smaller intensity i only some of its individual episodes involved some semblance of catastrophe other extinctions during the phanerozoic with the possible exception of the end silurian diversity plunge were parts of a forced oscillatory phenomenon and seem caused by marine and land habitat destruction during continental assemblies that led to elimination of shelves and after the devonian rain forests and enlargement of deserts glaciations and orogenies that shortened and thickened the continental crust only exacerbated these effects during the mesozoic and cainozoic the evolution of life was linearly progressive interrupted catastrophically only at the k t boundary the end triassic extinction was more like the paleozoic extinctions in nature and probably also in its cause by contrast the current extinction resembles none of the earlier ones and may end up being the greatest of all

threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air desert soils having clay crusts mostly from the mojave desert were tested for threshold friction velocity the friction velocity at which soil erosion begins with an open bottomed wind tunnel the soils were also tested for content of clay water soluble material calcium carbonate organic material mineralogy of clay and of salts soil moisture modulus of rupture and crust thickness if no loose material existed on the soil surface crusts having modulus of rupture greater than 0 7 bar and crust thickness of 0 7 cm to 0 3 cm were effective in protecting against wind erosion disturbed clay crusts having modulus of rupture before disturbance greater than 2 bar with thickness less than 1 9 cm did not experience significant wind erosion modulus of rupture was related to composition of soil but was shown to depend mostly on clay content soil composition is related to modulus of rupture in an empirical equation

the microstructure of microbiotic crust and its influence on wind erosion for a sandy soil surface in the gurbantunggut desert of northwestern china abstract microscopic examination of microbiotic soil crust shows that the profiles of soil with a dense cyanobacterial cover had two different layers a surface thin layer composed of aeolian born materials and an organic layer formed by filamentous cyanobacteria associated with sand particles the results indicate that microbiotic cover is an important determinant of sand fixation in the gurbantunggut desert northern part of xinjiang northwestern china microscopic examination of microbiotic crusts in this study revealed an intricate network of filamentous cyanobacteria and exopolysaccharides which binds and entraps sand grains and conglutinate fine particles with each other resistance to wind erosion paralleled the different disturbance levels on microbiotic soil crust sandy soil surface disturbances resulted in greatly decreased soil resistance to wind erosion maximum wind tunnel velocity in this test 25 m s 1 did not lead to any wind erosion on the surface of undisturbed microbiotic soil crust i e 100 covered by microbiotic soil crust as for different disturbance levels the highest threshold friction velocity was seen in the sand surface with 10 disturbance of microbiotic crust the surface microbiotic soil crusts have great effects on wind erosion rates wind erosion rates for sandy soil with 0 crust cover was about 46 21 17 times the soil with 90 crust cover at wind velocities of 18 22 25 m s 1 respectively this study confirms that the planners and managers of nature reserves in this area should understand the important ecological roles of microbiotic crust in desert ecosystems the reduction of trampling on the soil will eventually result in the re establishment of biological crusts and their associated organisms and ultimately lead to lower levels of wind erosion additionally strategies should be developed to manage livestock and oil exploration in order to avoid concentrated zones of impact

snow avalanche formation 1 snow avalanches are a major natural hazard endangering human life and infrastructure in mountainous areas throughout the world in many countries with seasonally snow covered mountains avalanche forecasting services reliably warn the public by issuing occurrence probabilities for a certain region however at present a single avalanche event cannot be predicted in time and space much about the release process remains unknown mainly because of the highly variable layered character of the snowpack a highly porous material that exists close to its melting point the complex interaction between terrain snowpack and meteorological conditions leading to avalanche release is commonly described as avalanche formation it is relevant to hazard mapping and essential to short term forecasting which involves weighting many contributory factors alternatively the release process can be studied and modeled this approach relies heavily on snow mechanics and snow properties including texture while the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner the knowledge of the quantitative relation between snow texture and mechanical properties is limited but promising developments are under way fracture mechanical models have been applied to explain the fracture propagation and micromechanical models including the two competing processes damage and sintering have been applied to explain snow failure there are knowledge gaps between the sequence of processes that lead to the release of the snow slab snow deformation and failure damage accumulation fracture initiation and fracture propagation simultaneously the spatial variability that affects damage fracture initiation and fracture propagation has to be considered this review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale from the size of a bond between snow grains to the size of a mountain slope will continue to be very challenging

temporal aspects of the abrasion of microphytic crusts under grain impact wind tunnel simulations of the response of two moss crusts to grain impact indicate that given sufficient time these surfaces will deteriorate under very low wind velocities only slightly above u t for the loose saltating grains in parallel with these experiments the frequency distributions of ultimate strength and penetration energy were determined for each of the two crust types via penetrometry pohlia was found to be stronger than tortula but even so both of these crusts had ultimate strengths 20 350 times higher than the force delivered by a single grain impacting each surface at a velocity of 1 ms 1 in comparison the modulus of deformation and penetration energy data were very similar for the two surface types especially for the weakest areas of crust development this observation is in accord with the wind tunnel simulations that also found no consistent difference in the response of these two crust types to impact in comparison with crusts formed by clay and salt fibrous microphytic crusts are morphologically complex and typically weak the notable elasticity of these surfaces does reduce the force of grain impact and thereby provides some protection against rupture one of the central conclusions of this study suggests that not only is the particle kinetic energy at impact important in crust breakdown but also tiny fractures at points of localized stress concentration contribute to a progressive reduction in the integrity of the filament net in some of the experiments conducted as part of this study up to 50 or more minutes of constant bombardment was required to produce small abrasion marks on selected areas of the microphytic crust this study prepares a foundation for future experiments needed to examine the breakdown of complex crusts formed in nature copyright 2002 john wiley sons ltd

effect of soil crusting on the emission and transport of wind eroded sediment field measurements on loamy sandy soil field data are reported for the horizontal and vertical flux of wind eroded sediment on an agricultural field in northern germany measurements were made during a windstorm that hit the region on 18 may 1999 the magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field measuring the physical crust strength at 45 locations with a torvane the relationships between crust strength and the horizontal fh and vertical fv sediment fluxes were investigated both fluxes decreased as the surface crust became stronger the decay behaved as an exponential function for both types of flux the horizontal sediment flux over a crusted surface can be accurately predicted by completing marticorena and bergametti s journal of geophysical research 100 1995 16415 erosion model with a crust function the vertical particle flux over crusted soil can be calculated by adding a similar function to alfaro and gomes s journal of geophysical research 106d 2001 18075 dust production model the study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust s protective effect provided that the crust has a minimum thickness however once the crust becomes perforated its protective effect disappears very quickly leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil

wind erosion of crusted soil sediments saltating particles increase the rate of dust release from sediments in arid and semi arid areas they also break interparticle bonds in aggregated and crusted soils thereby increasing the number of particles available for entrainment this pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths dislodgement of surface particles decreases with increasing crust strength as measured by a cylindrical flat ended penetrometer in addition initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator the volume of material removed depends linearly on the kinetic energy of the abraders

mountain snow distribution governed by an altitudinal gradient and terrain roughness 1 the heterogeneous mountain snow cover is challenging the eye and the analytical mind of the observer the snow distribution affects water resources natural hazards such as avalanches and ecology while a lot of recent research has helped to better understand this snow distribution and the processes that cause the heterogeneity it has not yet been possible to predict snow distribution satisfactorily on the basis of terrain parameters alone we present a model of the mean snow depth in topographic control units as a function of two terrain parameters the conventional elevation plus a fractal roughness parameter for this we used a unique data set of high resolution measurements of snow depth from an airborne laser scanner the model captures the heterogeneous snow distribution by merely analysing the terrain and the mean precipitation this unusually simple relationship holds for clusters of the snow depths of small topographical units by applying fractal analysis we describe the roughness of the terrain and use this parameter for the prediction of snow deposition rougher terrain holds less snow than smoother terrain this finding is important not only for avalanche warning or eco hydrological applications but also for reliably predicting how snow water storage may change in the light of the pronounced climate change already ongoing in mountain regions

parameterization of the spatially averaged sky view factor in complex topography large scale meteorological land surface or climate models require precise subgrid parameterizations to reproduce unresolved subgrid processes this is particularly important in mountainous terrain where subgrid surface properties e g glacier area extent are used to illustrate the impact of global climate change the sky view factor is one of the most frequently used terrain characteristics to parameterize the influence of topography on the surface radiation balance in complex terrain a precise subgrid parameterization of sky view factors is therefore essential for accurate parameterizations of radiative fluxes such as shortwave diffuse sky and terrain reflected radiation while its computation is straightforward for a particular site a systematic characterization of the spatially averaged sky view factor is still missing we use gaussian random fields with a gaussian covariance as topography model and based on an analytical approximation we derive a parameterization for the spatially averaged sky view factor the sky view factor parameterization is solely based on computationally cheap terrain parameters namely the correlation length of subgrid topographic features and the mean squared slope in the grid cell of the large scale model for validation we compare parameterized to numerically exact sky view factors computed for real terrain of the swiss alps from a 25 m digital elevation model with subsequent averaging to varying target domain sizes we obtain a very good agreement when approaching the resolutions of regional climate models the parameterization also compares well for various real topographies from the u s

shortwave radiation parameterization scheme for subgrid topography 1 topography is well known to alter the shortwave radiation balance at the surface a detailed radiation balance is therefore required in mountainous terrain in order to maintain the computational performance of large scale models while at the same time increasing grid resolutions subgrid parameterizations are gaining more importance a complete radiation parameterization scheme for subgrid topography accounting for shading limited sky view and terrain reflections is presented each radiative flux is parameterized individually as a function of sky view factor slope and sun elevation angle and albedo we validated the parameterization with domain averaged values computed from a distributed radiation model which includes a detailed shortwave radiation balance furthermore we quantify the individual topographic impacts on the shortwave radiation balance rather than using a limited set of real topographies we used a large ensemble of simulated topographies with a wide range of typical terrain characteristics to study all topographic influences on the radiation balance to this end slopes and partial derivatives of seven real topographies from switzerland and the united states were analyzed and gaussian statistics were found to best approximate real topographies parameterized direct beam radiation presented previously compared well with modeled values over the entire range of slope angles the approximation of multiple anisotropic terrain reflections with single isotropic terrain reflections was confirmed as long as domain averaged values are considered the validation of all parameterized radiative fluxes showed that it is indeed not necessary to compute subgrid fluxes in order to account for all topographic influences in large grid sizes

are flat field snow depth measurements representative a comparison of selected index sites with areal snow depth measurements at the small catchment scale single or a limited number of point observations such as from index stations are commonly assumed to be representative for the snow cover of larger areas in many applications this study presents a systematic investigation of the relationship between point observations and areal mean snow depths ranging from the scale of tens of metres to entire catchments we analyse aerial snow depth information from four mountain regions in the european alps one in the spanish pyrenees and one in the canadian rocky mountains obtained from airborne laser scanning surveys this rich data set allowed to compare point values with snow cover statistics reflecting the real snow depth distribution of the investigation areas we present two contrasting approaches in order to assess the representativeness of typical flat field snow depth measurements in the first approach we define potential index stations based on topographic characteristics as commonly applied for snow cover monitoring stations the point observations of these index stations are then compared with the mean values in their vicinities we show that most of the index stations strongly overestimate the snow depth of the catchment and of their surrounding area at distances of several hundreds of metres results confirm the expectation that the larger the support area the smaller the difference to the mean of the complete catchment the second approach was to analyse topographic characteristics of all cells with snow depths that deviated less than 10 from the catchment mean it appears that these representative cells are rather randomly distributed and cannot be identified a priori in summary our results show large potential biases of index stations with respect to snow distribution and therefore also snow water equivalent copyright c 2014 john wiley sons ltd

wind transport of sand surfaces crusted with photoautotrophic microorganisms abstract wind tunnel experiments show that while surfaces inoculated with photoautotrophic organisms are stable in winds up to 19 m s 1 only filamentous cyanobacteria especially nostoc commune afford considerable protection against abrasion from windborne particles crusts formed by the polysaccharide secreting alga chlamydomonas acidophila break down easily under impact the entrainment of large low density aggregates possibly contributing to higher transport rates than would otherwise be expected for untreated sediment surface disintegration encompasses a suite of complex scale dependent processes which vary with the structure thickness and strength of the crust independent measurements of crust strength based on flexure testing are in good agreement with wind tunnel evaluation of crust stability the accommodation of a relatively large amount of deformation without rupture is an important property of fibrous photoautotrophic mats perhaps outweighing consideration of their peak strength

crust strength a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces a wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength the sediments consisted of varying proportions of large sand sized particles with a fine particle cement the energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface it is important to consider distributions of parameters rather than mean values only since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities at the impact velocities used in this study mean velocity 4 4 m s 1 with standard deviation of 0 51 surfaces containing less than 12 per cent fine material were easily eroded but insignificant erosion occurred when the fine particle content exceeded 60 per cent small amounts of cementing material were easily ruptured allowing the large sand grains to be moved largely in creep by the bombarding particles a significant amount of energy was lost to the bed as the percentage of fine material increased the surface became more difficult to break up and less energy was lost to the bed the probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles copyright 2001 john wiley sons ltd

the effect of wind and precipitation on vegetation and biogenic crust covers in the sde hallamish sand dunes vegetation and biogenic crust covers play an important role in sand dune stabilization yet there is a lack of high temporal and spatial resolution data on sand dune cover a field experiment aimed at measuring the dynamics of biogenic crust and vegetation in sand dunes was conducted at the sde hallamish sand dunes in the northwestern negev desert israel from july 2008 to august 2010 the climate of the sde hallamish sand dunes is arid the mean annual precipitation over the past 13 years is 61 mm and the dunes are linear and partially stable mainly due to the presence of biogenic crust and partially due to the presence of vegetation in july 2008 10 10 m plots on the four dune habitats crest interdune north slope and south slope were treated as follows i removal of vegetation and biogenic crust ii removal of biogenic crust only iii removal of vegetation only iv partial removal of biogenic crust and vegetation and v control plot the surface coverage of sand biogenic crust and vegetation was monitored on a monthly basis using a remote sensing technique especially developed for the sde hallamish sand dunes it was found that strong wind events with durations of several days accounted for the coverage changes in biogenic crust and vegetation the response to precipitation was much slower in addition no rehabilitation of biogenic crust and vegetation was observed within the experiment time period the changes in biogenic crust cover were not necessarily related to changes in dune dynamics since often an increase in biogenic crust cover is a result of wind erosion that exposes old crust that was buried under the sand wind hardly erodes biogenic crust at all due to its high durability to wind action the sde hallamish dunes seem to have become more active as a result of a prolonged drought during the past several years the field experiment reported here indicates that biogenic crust cover exhibits large seasonal variations that are not necessarily related to the growth of new crust but rather to the exposure of old buried crust

radiosity approach for the shortwave surface radiation balance in complex terrain the influence of topography on the radiation balance in complex terrain has so far been investigated either with very simple or very sophisticated approaches that are limited respectively by an uncontrolled spatial representation of radiative fluxes or heavy computational efforts to bridge this gap in complexity this paper proposes the radiosity approach well known in computer graphics to study anisotropic reflections of radiation in complex terrain to this end the radiosity equation is rederived in the context of three dimensional radiative transfer the discretized equation is solved by means of an adapted version of progressive refinement iteration to systematically study terrain effects the geometrical disorder provided by the topography is considered in its simplest approximation by gaussian random fields these model topographies capture the most important length scales of complex terrain namely a typical elevation and a typical valley width via the variance and the correlation length of the field respectively the mean reflected radiation is computed as a function of these length scales and sun elevation thereby explicitly addressing finite system sizes and grid resolutions a comparison with an isotropic parameterization of terrain reflections reveals that mean values are similar whereas spatial distributions vary remarkably it is also shown that the mean reflected radiation in real topography is reasonably well characterized by the gaussian approximation as a final application of the method the effective albedo of a topography is shown to vary with sun elevation and domain averaged albedo leading to albedo differences up to 0 025

radiative transfer over resolved topographic features for high resolution weather prediction regional numerical weather prediction models are now routinely run at near kilometre scale resolutions where surface features are well resolved and mean grid box slopes are highly significant the resolved topography has an important effect on the radiative transfer at the surface which is not generally represented using fast two stream radiation codes this paper presents a parametrisation of surface radiative transfer for resolved topography suitable for use with a two stream code for the short wave bands the first order effects are the interaction of direct solar radiation with surface slopes and shading by surrounding terrain in the infrared long wave bands the first order effect is the transfer of radiation between visible points on the terrain making use of the sky view factor for a sloping surface and the enhanced area of the slope energy is conserved over an extended region where the mean flux into the surface will be independent of resolution these effects are included in the radiation scheme of the met office unified model and case studies are run over the uk using horizontal grid resolutions from 100 m to 1 5 km the scheme adds fine detail to the forecast which is shown to have a significant impact on forecast skill under clear sky conditions short wave surface effects can lead to temperature differences of up to 2 5 k and long wave effects to differences of up to 1 k the magnitude of these effects are expected to be much greater for more complex terrain a consistent extension to the scheme is outlined to take account of second order effects using short wave diffuse albedo and long wave surface emissivity these lead to a net change in the surface flux due to multiple surface reflections and should be considered in conjunction with a treatment of sub grid terrain complexity copyright 2011 british crown copyright the met office published by john wiley sons ltd