Wind-Blown Sand: Threshold of Motion

The fluid threshold for wind-blown sand is the minimum shear velocity required to initiate grain movement by the force of the wind alone, and is used to predict dust emission and landform change in sandy environments. R.A. Bagnold derived the most commonly cited model of the threshold from a set of wind tunnel experiments. He visually observed the fluid threshold by measuring flow conditions corresponding to the initiation of bedload movement, a mode of transport that occurs prior to saltation. His model was developed using unimodal grain size populations and requires only the average size to predict the threshold. However, field environments often exhibit non-unimodal surface populations. The fluid threshold for mixed size surfaces in fluvial environments corresponds to the coarsest grain size, not the average, resulting in a larger threshold shear velocity to initiate movement. Larger thresholds yield smaller transport rates and could explain the consistent over-prediction of aeolian transport models. Yet, the fluid threshold of mixed size sands has not been tested in an aeolian field environment. This is due to the previous inability to separate the bedload from saltation. The purpose of this research is to test Bagnold’s model of the fluid threshold in a field environment composed of dry, naturally mixed grain sizes in Jericoacoara, Brazil. A bedload trap was designed to separate bedload from saltation, and the initiation of bedload and near surface flow conditions were measured simultaneously. Field observations were compared to Bagnold’s model as well as other models of the fluid threshold. Observed fluid thresholds did not vary with average grain size for the mixed size population. The thresholds for finer and coarser bedload samples were approximately equal to the Bagnold-predicted threshold for coarser grains. All models tested under predicted the threshold for finer grains. These results suggest the fluid threshold for mixed size sands corresponds to the coarsest grain size fraction, similar to the results found in fluvia

The Inner-Shelf Dynamics Experiment

17 USC 105 interim-entered record; under review.The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-D-19-0281.1The inner shelf, the transition zone between the surfzone and the midshelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from September–October 2017, conducted from the midshelf, through the inner shelf, and into the surfzone near Point Sal, California. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves, and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the midshelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.U.S. Office of Naval Research (ONR)ONR Departmental Research Initiative (DRI)Inner-Shelf Dynamics Experiment (ISDE

Toward More Realistic Simulation and Prediction of Dust Storms on Mars

Global dust storms have major implications for the past and present climate, geologic history, habitability, and future exploration of Mars. Yet their mysterious origins mean we remain unable to realistically simulate or predict them. We identify four key Knowledge Gaps and make four Recommendations to make progress in the next decade

Contemporary research in coastal dunes and aeolian processes

Coastal dunes are found along the sandy coasts of oceans, seas, and large lakes all around the world. They are dynamic landforms that evolve along complex morphological and biological continua in response to a range on controls linked to climate, sea level, sediment movement, vegetation cover, and land use. By collating research across the full spectrum of processes shaping different types and sizes of dunes and smaller aeolian bedforms, special issues can aid researchers to identify new research directions and methods emerging from the discipline. This editorial summarizes the 25 contributions to the special issue Coastal dunes: links between aeolian processes and landform dynamics. We grouped the contributions into four broad themes: (1) long-term dune evolution, (2) short-term aeolian transport, (3) research methods, and (4) coastal dune management. Contributions to the special issue demonstrate that research interest in coastal dunes, and particularly the impacts of human interventions on dunes, continues to grow. It also shows how aeolian research on coastal dunes covers a range of temporal and spatial scales, from ripple dynamics and instantaneous airflow-transport processes to dune field evolution with rising sea levels and large-scale dune stage shifts. We highlight potential avenues for future research including vegetation roughness characteristics and their effect on wind flow and sediment transport, the challenges of upscaling short- and small-scale results to larger and longer spatiotemporal scales, and the study of both natural and managed dune landscapes.9 página

Title: Modern Mars' geomorphological activity, driven by wind, frost, and gravity

International audienceExtensive evidence of landform-scale martian geomorphic changes has been acquired in the last decade, and the number and range of examples of surface activity have increased as more high-resolution imagery has been acquired. Within the present-day Mars climate, wind and frost/ice are the dominant drivers, resulting in large avalanches of material down icy, rocky, or sandy slopes; sediment transport leading to many scales of aeolian bedforms and erosion; pits of various forms and patterned ground; and substrate material carved out from under subliming ice slabs. Due to the ability to collect correlated observations of surface activity and new landforms with relevant environmental conditions with spacecraft on or around Mars, studies of martian geomorphologic activity are uniquely positioned to directly test surface-atmosphere interaction and landform formation/evolution models outside of Earth. In this paper, we outline currently observed and interpreted surface activity occurring within the modern Mars environment, and tie this activity to wind, seasonal surface CO2 frost/ice, sublimation of subsurface water ice, and/or gravity drivers. Open questions regarding these processes are outlined, and then measurements needed for answering these questions are identified. In the final sections, we discuss how many of these martian processes and landforms may provide useful analogs for conditions and processes active on other planetary surfaces, with an emphasis on those that stretch the bounds of terrestrial-based models or that lack terrestrial analogs. In these ways, modern Mars presents a natural and powerful comparative planetology base case for studies of Solar System surface processes, beyond or instead of Earth

It\u27s Time for Focused in situ Studies of Planetary Surface-Atmosphere Interactions

A critical gap in planetary observations has been in situ characterization of extra-terrestrial, present-day atmospheric and surface environments and activity. While some surface activity has been observed and some in situ meteorological measurements have been collected by auxiliary instruments on Mars, existing information is insufficient to conclusively characterize the natural processes via concurrent and high-resolution measurement of environmental drivers and activity. Thus, many atmospheric, aeolian, and other surface processes models - which are used to generate key constraints on science and exploration in many areas of planetary investigation-such as surface exposure/erosion estimates, landscape interpretation, and modeling dust storm development-remain untested under non-Earth conditions. Analogous terrestrial processes are often studied intensively via numerical modeling that integrates empirical results from laboratory and/or field studies of process-response interactions between the atmosphere and relevant surface landforms. Incorporation of such in situ measurements into model development has significantly advanced our understanding of atmosphere-surface interactions and related geomorphic processes on Earth, and is poised to do so on other planets. However, to date, such testing and refinement have not been possible in other planetary environments, partially because investigations of this sort require new technologies, mission architectures, and operations designs (e.g., different from large rovers focused on geochemical investigations) to fully address the key gaps in our understanding while keeping cost and risk low. Fortunately, technological developments in the areas of surface access, instrumentation, and onboard processing/memory now enable small spacecraft to accommodate meteorological and aeolian instrumentation that could collect the needed measurements to fill this critical gap while remaining within typical small spacecraft resource budgets. Furthermore, maturity of our understanding of the broader geologic and atmospheric context on Mars provides a ready framework for ingestion of discrete ground truth measurements into our understanding of the broader and multi-scale martian natural systems and processes. These advancements make addressing key science questions with novel mission concepts feasible, promising results that would significantly advance our understanding of extraterrestrial surface-atmosphere interactions. This summary follows from a community-generated white paper for the ongoing Planetary Science/Astrobiology Decadal Survey, small spacecraft concept development at JPL, and numerous JPL and community discussions