The physics of wind-blown sand and dust

The transport of sand by wind is a potent erosion force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This article presents a short review of the physics of wind-driven sand. Specifically, we review the physics of saltation, the formation and development of sand dunes and ripples. We also discuss some classes of the governing equations which describe the physics of wind-driven sand and dune formation. We describe selected types of dunes and conditions under which they occur, and also some features of dunes as well as processes that they are involved in. We show that the normalized dunes height collapses using a simple product of the Froude and Reynolds numbers. This would obscure the effects of frictional dissipation, which clearly plays an important role in all mentioned upper process. Ignoring friction, one can construct a simple energy balance between the kinetic energy of the impacting and the potential energy of the dunes, where we assume the dunes thickness is proportional to ds. This produces the following scaling. In other words, was one to increase the grain diameter ds by a factor of 10 ~ i.e., reduce Re by 100! for the same impact conditions, then the frictionless flow would predict a 10-fold reduction in , whereas the experiments suggest a 100-fold reduction. This shows clearly that viscous forces play a role in the granular dunes formation (and their relevant dynamics), as well as gravity and inertia.
 These circumstances move us to conclude the vide range of (non-dissipative) hydrodynamic approaches to describe dunes formation and their dynamics just as a robust model approaches.

Some coastal sand dunes, such as the Himi, Uchinada, Katayamazu and Sanribama dunes, are distributed widely along the coast of the Japan Sea in the central Japan. I have made for about two decades research of the coastal sand dunes throughout the Japanese Islands, especially the Hokuriku district, from the view points of the age of formation of sand dunes, palaeoenvironment during the time of the formation of sand dunes and the relationship between the age of formation and change of sea-level.In this article I have proved the following six items in regard to the coastal sand dunes of the Hokuriku district:(1) Summary of some gists of the distribution, arrangement, basement and age of formation of the coastal sand dunes.(2) The age of formation of the coastal sand dunes which are distributed in the Hokuriku district is divided into seven periods as follows: First period-the Late Pleistocene Riss/Würm Interglacial age, Hiradoko stage; Second period-the late substage of the Earliest Jomonian age to the early substage of the Early Jomonian age; Third period-the late substage of the Early Jomonian age, the highest sea-level stage of the Flandrian transgression age; Fourth period-the earliest substage of the Middle Jomonian age; Fifth period-the Late Jomonian age to Yayoian age; Sixth period-the early substage of the Tumulusian age, the old tomb age, the Minor transgression age; Seventh period-the middle Muromachi age. Among them, the time when larger dunes were formed is the fourth, fifth and sixth periods.(3) Judging from the view point of relationship between the formation of the coastal sand dunes and the palaeoenvironment in the time of formation, both the palaeotemperature and palaeomoisture during the Holocene epoch were not in direct connection with the formation of the coastal sand dunes.(4) The necessary conditions for the formation of the coastal sand dunes are the following: (a) supply of sand, (b) migration of sand, and (c) sedimentation of sand. The conditions were prescribed by a position of strand line, it needs the fix of the position of the strand line for the formation of the coastal sand dunes.(5) From the view point of relationship between the change of sea-level and the formation of the coastal sand dunes, when the environment was changed topographically by the change of sea-level, a scale of sand dune may be changed by the fact that the above-mentioned three conditions are promoted or hindered under this changed environment. Roughly speaking, better conditions for the formation of sand dune may be just after the time when a sea-level changes from rising to lowering.(6) The relationship between the relative change of sea-level, the change of palaeoenvironment and the formation of the coastal sand dunes in every stage of the Holocene epoch is described in this article in the concrete.

Computer simulation of aeolian sand ripples and dunes

The morphology of sand dunes formed on the Maisaka and Hamamatsu coasts facing the Enshu-nada Sea was investigated by field observation. The development of sand dunes was numerically predicted using a model predicting the effect of both waves and windblown sand. In the field observation, the development of sand dunes with a rhythmic shape similar to a sand spit was observed in the backshore area. It was found that pine trees died in the area with a narrow sand dune owing to wave run-up and the dispersion of salinity during storm wave conditions. In the numerical simulation, the formation of sand dunes was successfully reproduced under the condition that the predominant wind blew at a large angle relative to the direction normal to the shoreline.

Three dimensional quantitative simulations on the formation and evolution of aeolian sand ripples and sand dunes are presented. The simulations successfully replayed the self-reparation behavior of sand ripples and the collision behavior of sand dunes. The simulation results revealed the dominant physical laws governing the self-reparation behavior of sand ripples and Y-junctions, and the phenomena that two aeolian barchan dunes collide with each other in a manner similar to solitons or solitary waves. The results indicate that the self-reparation time of sand ripple will reduce with the frictional wind speed increasing, and the existence of Y-junction and the increase of sand diameter will extend the self-reparation process of sand ripple. The collision’s consequence between sand dunes is related with the two sand dunes relative scales and the angle between the line lock of center of dunes and wind direction. When the scales of two sand dunes are close and the angle between the line lock of center of dunes and wind direction is small, the solitary waves behavior will appear.

Martian dust lifting is believed to occur through two primary mechanisms: dust devils and wind stress forced dust lifting. Gale Crater's varied terrain and meteorology provide a unique in situ perspective on Martian dust lifting, with the Mars Science Laboratory Curiosity rover passing through both conditions and locations detrimental to dust lifting (e.g., the crater floor) and those with active sand motion and frequent dust lifting (e.g., the Bagnold Dunes). Between Ls = 248° in Mars Year 33 and Ls = 51° in Mars Year 37, over ∼3.5 Mars years and 2,300 sols, the rover's Navigation Cameras took 1,260 dedicated image sequences to search for dust lifting. Approximately 42.7% of all sequences, and 9.5% of the total images have shown active dust lifting, both dust devils and linear/straight‐line wind stress dust lifting. 79% of dust lifting events are classified as dust devils, while ∼16% are linear wind stress dust lifting and the remainder are of an indeterminate type. We analyze this large catalog of dust lifting events to provide ground truth on theoretical and model expectations of dust lifting and show that dust lifting in Gale Crater occurs throughout the Martian year, is strongly peaked in frequency near solar noon (even after accounting for observational biases), and that dust lifting shows an affinity for sand‐covered surfaces which highlights the importance of saltating sand grains for Martian dust lifting in both dust devils and wind stress forced lifting.

The atmospheric circulation and dust activity in different orbital epochs on Mars

Much of the surface of Mars is covered by dunes, ripples, and other features formed by the blowing of sand by wind, known as saltation. In addition, saltation loads the atmosphere with dust aerosols, which dominate the Martian climate. We show here that saltation can be maintained on Mars by wind speeds an order of magnitude less than required to initiate it. We further show that this hysteresis effect causes saltation to occur for much lower wind speeds than previously thought. These findings have important implications for the formation of dust storms, sand dunes, and ripples on Mars.

Numerical experiments were completed examining the variability of key aspects of the Martian dust cycle and investigating their importance in predicting conditions for spacecraft atmospheric descent and landing. The dust cycle – lifting, transportation and deposition – is a significant Martian climate cycle. The geographical and temporal variation in dust lifting processes were investigated using a Martian Global Circulation Model. The geographical representation of Martian dust lifting by wind stress was used to explore the experimental impact of changes in model resolution. It was found that increasing the resolution improved the model's geographical representation of observed dust lifting regions, such as resolving important storm-forming regions in the northern hemisphere. This improvement was unanticipated in the case of changes in vertical resolution, and the horizontal resolution work identified an important length scale for dust lifting (of the order of 100 kilometres). The temporal variation of a dust lifting process was investigated through experiments focusing on the diurnal variability of Martian dust devils (small-scale convective vortices). This research compared results with published lander and rover observations and found that dust devils were more active during morning hours than anticipated, suggesting that the generally accepted description of dust devil behaviour on Mars is incomplete. Predictions were made of the atmospheric and near-surface environment encountered by the ESA ExoMars Schiaparelli landing module. The experiments produced a reasonable representation of atmospheric quantities along the descent trajectory and were able to generate similar low-altitude wind fields to those reported by the spacecraft. The global-scale model also out-performed a higher resolution mesoscale model. These findings are significant in the field of Martian climate modelling, are important for the planning of Martian dust devil observation campaigns and future missions to the planet`s surface, and will also be relevant to researchers operating atmospheric models for other planetary bodies.

: This final report describes activities performed as part of the ARO-funded project entitled Eolian Modeling System (EMS): Predicting Windblown Sand and Dust Hazards in Battlefield Environments. The objectives of the research were to 1) develop numerical models for the atmospheric transport of windblown sand and dust and validate those models using thicknesses of dust deposits measured downwind of dust sources, 2) develop numerical models for the relationship between surface moisture and vegetation and the transport of sand and dust from surfaces. These models will be validated using field measurements collected by the U.S. Geological Survey, and 3) understand the climatic and meteorological conditions responsible for triggering windblown sand and dust events in study areas of the southwestern U.S.

We employ the NASA Ames Mars general circulation model (GCM) to investigate the dust lifting mechanisms responsible for the observed Martian dust cycle and the net surface response to the combined influence of dust lifting and deposition. This GCM includes lifting, transport, and deposition of radiatively active dust. Two dust lifting mechanisms are accounted for: wind stress lifting and dust devil lifting. A “baseline” simulation is presented and shown to compare well to available spatial and temporal observations of atmospheric opacity, wind stress dust lifting events, and atmospheric temperatures recorded during a nonglobal dust storm year. Multiple simulations were conducted to explore the model's sensitivity to a wide range of dust lifting parameters (the functional dependence of surface dust flux on wind stress, the wind stress threshold for lifting, etc.) Model results robustly suggest that wind stress lifting produces the peak in atmospheric dust load during southern spring and summer and that dust devils maintain the background haze of atmospheric dust during northern spring and summer. These results are consistent with previously published conclusions. Dust devil and wind stress lifting contribute equally to the simulated total amount of dust lifted annually during nonglobal dust storm years. The simulated spatial pattern of annual net deflation/deposition suggests that the low thermal inertia regions (Tharsis, Arabia, and Elysium) are not currently net dust accumulation regions. This net deflation is the result of dust devil dust lifting, suggesting that dust devils could play an important role in the present‐day pattern of surface dust reservoirs.

This paper consists of a historical review on the formation of sand dunes and some considerations on the relationship between the dune formation and the intercalated humus layers in several districts. The following results were obtained;(1) Buried dunes beneath the surface of Tatebayashi upland in Tochigi Prefecture.Sand layers constituting the buried dunes are divided in two, both having been deposited under stable circumstances. And volcanic ash layers are found in this district, divided into three; upper, middle and lower. There is every reason to believe that these dunes were formed at the beginning of the fall of the middle ash, or at the end of that of the lower ash.(2) Sand dunes on Sanrihama and Kaetsu uplands.The formation of some of Sanrihama dunes were due to a relative rising of sea level in early Holocene epoch. At the bottom of shallow valleys in Kaetsu upland, humus accumulation occured in stagnant water prior to the formation of dunes.(3) Kashima dunes in Ibaragi Prefecture.In the Kashima peninsula are found sand dunes with three intercalated humus layers, which are thought to have been formed after the formation of sand bars caused by a rise of sea level during the Jomon culture period, early Holocene.(4) Genkai dunes in Fukuoka Prefecture.Sand dunes of this area were formed during the time of high sea level in Pleistocene epoch. Aeolian sand layers are thin in depth and cover uplifted sand bars of marine deposits forming coastal terrace topography. Furthermore, five buried humus layers called Kurosuna are found among sand beds of Holocene dunes, which indicate the ages of dune formation and the stable period.

Whirlwinds, familiar on Earth, can be informative probes of the Martian surface environment.

Coastal sand dunes dating to the latter part of the late Pleistocene were constructed at substantially lower sea levels on Kikaijima Island. They are preserved because of large-magnitude uplifts ascertained from occurrences of outstanding late Pleistocene and Holocene coral terraces. In addition, because Holocene sand dunes occur close to those of Marine Oxygen Isotope Stage 3 (MIS 3), the island is ideal for examining the environments of the formation of sand dunes, which were constructed under different climatic and sea-level conditions. Distributional and depositional features of sand dunes are clarified, and their chronology is constructed on the basis of tephrochronology, 14C dating, and chronological relationships of the sand dunes with marine terraces of known age. The environments of sand-dune formation on Kikaijima are examined in relation to climatic and sea-level records within regional and global contexts. Nine tephra layers are recognized. These include two widespread tephras, Kikai-Akahoya tephra (K-Ah) and Aira-Tn tephra (AT). The other seven tephras, consisting of fine ash layers, are newly recognized and named Kikaijima-1 tephra (Kj-1) to Kikaijima-7 tephra (Kj-7), of which the upper six, lying between K-Ah and AT, are aged from 13,000 to 30,000 cal BP. Holocene sand dunes began to form at c. 8,000 cal BP before the culmination of maximum Holocene sea-level rise. This is earlier than dune formation on other coasts of the Japanese Islands, where formation was generally after the culmination of maximum Holocene sea-level rise. The earlier formation on Kikaijima Island is probably due to coastal emergence caused by the conspicuous uplifts of Kikaijima and the high production of calcareous beach sands sourced from coral and foraminiferal and other materials. Late Pleistocene sand dunes, predominantly distributed at the southwestern part of Kikaijima Island, were formed in MIS 3. These dunes consist of longitudinal and parallel dunes. The largest longitudinal dunes on the lowest MIS 3 terraces, at the northern area of the southwestern part of Kikaijima Island, were formed in late MIS 3, c. 40 to 32 ka, while small and parallel sand dunes on the upper MIS 3 terraces were formed in early MIS 3. The occurrence and chronology of the Holocene and late Pleistocene sand dunes suggest that their formation on Kikaijima Island is mainly related to the occurrence of coastal sandy beaches during sea-level high stands. Although Holocene sand dunes are related to Holocene high stand, those of MIS 3 are related to high stands during cycles of sea-level fluctuations in MIS 3. The longitudinal landform of the sand dunes recognized in the late MIS 3 suggests that the prevailing winds in MIS 3 were stronger than in the Holocene. This study provides critical data for constructing a chronological framework that integrates various aspects of palaeoenvironments, as well as human interactions and responses in southern Kyushu.

No matter from the viewpoint of occupied area or from the complexity of dynamical behavior, sand dunes and sand dune fields are the most significant manifestation of aeolian geomorphology system. Small-scale sand ripples can not grow into large-scale sand dunes, and sand dunes have distinct characteristics compared with sand ripples. The spatial and temporal scale in the formation and evolution of a sand dune is much larger than that of a sand ripple, and field observation and simulation on sand dunes are more difficult than on sand ripples. For example, the fundamental task in simulating the formation and evolution of dune field is to solve the trans-scale problem, that is, the transition from the small scale physical processes (such as the transport of sand particles) to the large scale physical processes (such as the formation and evolution of dune fields). Obviously, such trans-scale transition is not a simple linear superposition. In other words, it is impossible to characterize the formation and evolution of a dune field by modeling the process of every sand particle, which brings an infinite number of degrees of freedom into the modeling. Apart from capability limitation of present computers, the characteristics and physical mechanism of the formation and evolution of dune field are inherently different from those for sand movement and sand ripples. However, the dynamical behaviors of large-scale dune fields are definitely related with sand movement. Therefore, ignorance or complete parameterization of sand movement would possibly cause mis-reflecting the real formation and evolution physical process of dune field.