Andrew Gunn

Andrew Gunn

Geomorphology, Geophysical Fluid Dynamics

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publications

Macroscopic flow disequilibrium over aeolian dune fields

Geophysical Research Letters, 2020

Aeolian dune fields are self-organized patterns formed by wind-blown sand. Dunes are topographic roughness elements that impose drag on the atmospheric boundary layer (ABL), creating a natural coupling between form and flow. While the steady-state influence of drag on the ABL is well studied, non-equilibrium effects due to roughness transitions are less understood. Here we examine the large-scale coupling between the ABL and an entire dune field. Field observations at White Sands, New Mexico, reveal a concomitant decline in wind speed and sand flux downwind of the transition from smooth playa to rough dunes at the upwind dune-field margin, that affects the entire ∼10-km long dune field. Using a simple theory for the system that accounts for the observations, we generalize to other roughness scenarios. We find that, via transitional ABL dynamics, aeolian sediment aggradation can be influenced by roughness both inside and outside dune fields.

Recommended citation: Gunn, A., et al. (2020). "Macroscopic flow disequilibrium over aeolian dune fields." doi:10.1029/2020GL088773. https://doi.org/10.1029/2020GL088773

Spatial and Temporal Development of Incipient Dunes

Geophysical Research Letters, 2020

In zones of loose sand, wind‐blown sand dunes emerge due the linear instability of a flat sedimentary bed. This instability has been studied in experiments and numerical models but rarely in the field, due to the large time and length scales involved. We examine dune formation at the upwind margin of the White Sands Dune Field in New Mexico (USA), using 4 years of lidar topographic data to follow the spatial and temporal development of incipient dunes. Data quantify dune wavelength, growth rate, and propagation velocity and also the characteristic length scale associated with the growth process. We show that all these measurements are in quantitative agreement with predictions from linear stability analysis. This validation makes it possible to use the theory to reliably interpret dune‐pattern characteristics and provide quantitative constraints on associated wind regimes and sediment properties, where direct local measurements are not available or feasible.

Recommended citation: Gadal, C., et al. (2020). "Spatial and Temporal Development of Incipient Dunes." doi:10.1029/2020GL088919. https://doi.org/10.1029/2020GL088919

Conditions for aeolian transport in the Solar System

(IN REVIEW) Nature Astronomy, 2020

Sand dunes arise wherever loose sediment is mobilized by winds that exceed threshold speeds, and grains are sufficiently strong to survive collisions. The ubiquity of dunes in our solar system is remarkable and confounding; their occurrence under conditions of thin atmospheres, and/or friable materials, challenges our understanding of sediment transport mechanics. Current threshold theories lose meaning and diverge from one another when extrapolated to some planetary bodies, because they neglect physical processes that become relevant under such exotic conditions. Here we draw on results in contact, rarified gas, statistical and adhesion mechanics to present more complete theories for the fluid and impact thresholds of aeolian transport. Our theoretical predictions compare well with all available experimental threshold observations, and shed light on the contentious issues of sediment mineralogy on Titan and the high threshold for dune activity on Mars. This work will aid in interpreting planetary atmospheric dynamics from observed dunes, and determining what observations are required for future space missions.

Recommended citation: Gunn, A., Jerolmack, D. J. (2020). "Conditions for aeolian transport in the Solar System." eartharxiv:1872. https://doi.org/10.31223/X5SC70

Circadian rhythm of dune-field activity

Geophysical Research Letters, 2021

Wind-blown sand dunes are both a consequence and a driver of climate dynamics; they arise under persistently dry and windy conditions, and are sometimes a source for airborne dust. Dune fields experience extreme daily changes in temperature, yet the role of atmospheric stability in driving sand transport and dust emission has not been established. Here we report on an unprecedented multi-scale field experiment at the White Sands Dune Field (New Mexico, USA), where we demonstrate that a daily rhythm of sand and dust transport arises from non-equilibrium atmospheric boundary layer convection. A global analysis of 45 dune fields confirms the connection between surface wind speed and diurnal temperature cycles, revealing an unrecognized climate feedback that may contribute to the growth of deserts on Earth and dune activity on Mars.

Recommended citation: Gunn, A., et al. (2021). "Circadian rhythm of dune-field activity." doi:10.1029/2020GL090924. https://doi.org/10.1029/2020GL090924

What sets aeolian dune height?

(IN REVIEW) Nature Communications, 2021

Wherever a loose bed of sand is subject to sufficiently strong winds, aeolian dunes form at wavelengths and growth rates that are well predicted by linear stability theory. As dunes mature and coarsen, however, their growth trajectories become more idiosyncratic; nonlinear effects, sediment supply, wind variability and geologic constraints become increasingly relevant, resulting in complex and history-dependent dune amalgamations. Here we examine a fundamental question: do aeolian dunes stop growing and, if so, what determines their ultimate size? Earth’s major sand seas are populated by giant sand dunes, evolved over tens of thousands of years. We perform a global analysis of the topography of these giant dunes, and their associated atmospheric forcings and geologic constraints, and we perform numerical experiments to gain insight on temporal evolution of dune growth. We find no evidence of a previously proposed limit to dune size by atmospheric boundary layer height. Rather, our findings indicate that dunes may grow indefinitely in principle; but growth slows with increasing size, and may ultimately be limited by sand supply. We also demonstrate that giant dune size depends on both wind climate and sand supply through their control on dune morphology, revealing a topographic signature of geologic and climatic forcing in Earth’s sand seas.

Recommended citation: Gunn, A., et al. (2021). "What sets aeolian dune height?". https://doi.org/10.31223/X5QG8S

21st-century stagnation of sand-sea activity

(IN REVIEW) Nature Communications, 2021

Sand seas are vast expanses of Earth’s surface that are covered in dunes—topographic patterns manifest from above-threshold winds and a supply of loose sand. Transitions in dune morphology and associated vegetation state are threshold phenomena that can switch states on kilometer or decadal scales due to small changes in climate. Predictions of the role of future climate change for sand-sea activity are sparse and contradictory. Here we examine the impact of climate on all of Earth’s presently-unvegetated sand seas, using ensemble runs of an Earth System Model for historical and future Shared Socioeconomic Pathway (SSP) scenarios. We find that almost all of the sand seas decrease in activity relative to present-day and industrial-onset for all future SSP scenarios, largely due to more intermittent sand-transport events. An increase in event wait-times and decrease in sand transport—in most cases linked to reduced off-season transport—is conducive to the rise of vegetation. We expect dune-forming winds will become more unimodal, and produce larger incipient wavelengths, due to weaker and more seasonal winds. Our results indicate that these qualitative changes in Earth’s desert landscapes can not be mitigated.

Recommended citation: Gunn, A., et al. (2021). "21st-century stagnation of sand-sea activity". TBD

Tuning sedimentation through surface charge and particle shape

Geophysical Research Letters, 2021

Mud forms the foundation of many coastal and tidal environments. Clay suspensions carried downstream from rivers encounter saline waters, which encourages aggregation and sedimentation by reducing electrostatic repulsion among particles. We perform experiments to examine the effects of surface charge on both the rate and style of sedimentation, using kaolinite particles as a model mud suspension and silica spheres with equivalent hydrodynamic radius as a control. Classic hindered settling theory reasonably describes sedimentation rate for repulsive clay particles and silica spheres, which form a highly concentrated jamming front. The hindered settling description breaks down for attractive clay particles, which aggregate to form clay gels that consolidate like a soft solid. Water flow form fracture-like channels in the bulk of the gel, which disappear as gel enters a creep regime. Results may help toward understanding the effect of surface charge and particle shape on the sedimentation and erodibility of natural mud.

Recommended citation: Seiphoori, A., et al. (2021). "Tuning sedimentation through surface charge and particle shape." eartharxiv:1738. https://doi.org/10.1029/2020GL091251

teaching

Oceanography

Undergraduate, Teaching Assistant, Earth & Environmental Science, University of Pennsylvania, 2017

Oceanography

Undergraduate, Teaching Assistant, Earth & Environmental Science, University of Pennsylvania, 2018

Earth Surface Processes

Undergraduate & Graduate, Field Teaching Assistant, Earth & Environmental Science, University of Pennsylvania, 2019

Oceanography

Undergraduate, Head Teaching Assistant, Earth & Environmental Science, University of Pennsylvania, 2020

Earth Surface Processes

Undergraduate & Graduate, Head Field Teaching Assistant, Earth & Environmental Science, University of Pennsylvania, 2020