Research

Our research group focuses on understanding the physics of wind-blown sand and dust (aeolian processes), and the interactions of mineral dust aerosols and climate. We predominantly use analytical and theoretical methods, as well as simple first-principles numerical models that run on desktop computers, and large-scale climate models that require supercomputers. My group also has limited involvement in field measurements.

Effects of dust aerosols on climate

One of the most pertinent questionsseawifs_canary_duststorm_large in attributing past and forecasting future climate changes is the forcing exerted by atmospheric “aerosols” on the climate system. These tiny particles suspended in the atmosphere affect Earth’s radiation budget by scattering and absorbing radiation, and by altering cloud properties such as reflectivity and lifetime. Among the several abundant species of aerosols, soil-derived mineral dust is particularly important to the Earth system because it interacts not only with radiation and clouds, but also partially controls the productivity and carbon cycling of both marine and terrestrial ecosystems by providing limiting nutrients such as iron and phosphorus. In addition, dust deposition can locally produce a substantial warming effect by lowering the reflectivity of snow and ice. Dust aerosols are also a hazard to human health, might suppress Atlantic hurricane activity, and affect the hydrological cycle.

Dust aerosols are thus one of the most ubiquitous aerosols in the atmosphere, yet their (radiative) effect on Earth’s climate is poorly known. In fact, we don’t even know whether dust cools or warms the planet, and whether future changes in dust emission induced by climate changes will enhance or oppose those climate changes. My research group strives to answer these fundamental questions using a combination of first-principles physics, parameterizations of small-scale processes (see below), and simple experiments with climate models (usually NCAR‘s Community Earth System Model).

The physics of dust aerosol emission and other aeolian processes

To be able to answer the fundamental questions of the interactions between dust and climate, an accurate representation of dust emission in climate models is a necessity. Unfortunately, dust emission is a very complex process that depends on interactions that span length scales ranging from micrometers to 1000s of kilometers, making it a very difficult process to represent accurately in weather and climate models. My group thus strives to develop analytical models for dust emission that can be readily implemented into large-scale climate and weather models. We do so using a combination of theory, numerical modeling constrained by measurements, and the occasional field measurements.

Part of the complexity of dust emission arises from the fact that dust aerosols are rarely lifted directly by wind, but are instead predominantly emitted through an intermediary process called SaltationFigure-JasperKoksaltation (Fig. 1). In saltation, larger sand-sized particles (~70 – 500 um), which are more easily lifted by wind because their cohesive forces are small compared to aerodynamic forces, move in ballistic trajectories. The resulting impacts of saltating particles can ‘crack’ the soil in a manner that is analogous to the fragmentation of a wine glass when dropped on the floor. In past work, I’ve shown that this analogy of dust emission with brittle fragmentation yields an accurate and scale-invariant expression for the size distribution of dust aerosols at emission. Most recently, I’ve developed a physical theory for how the flux of dust aerosols depends on wind and soil conditions. This theory shows improved agreement with small-scale (in situ) measurements of dust emission and seems to improve simulations of the dust cycle with climate models.

Since saltation is central to understanding dust emission, my group also works on the physics of wind-blown sand and, more generally, on the physics of aeolian processes. I developed COMSALT, a numerical model of wind-blown sand written in MATLAB, that is freely available to interested researchers, and can be downloaded here. In addition to contributing to understanding and describing sand transport and dust emission, COMSALT has also been used to understand mega(ripple) formation, and sand transport on Mars and Saturn’s moon Titan.

Effects of turbulence on fluxes of dust and sand

Saltation, and thus dust emission, is driven by turbulent winds close to the surface, which are characterized by strong spatial and temporal fluctuations in speed and direction. Consequently, saltation is highly intermittent, with pronounced variability on timescales of seconds to hours. In contrast, existing sand transport and dust emission models describe saltation as a process that is uniform in time and space, and driven by a constant downward flux of horizontal fluid momentum. This disconnect between theory and models on the one hand, and the reality in the field on the other, is likely one of the main causes of the often poor performance of parameterizations of sand transport and dust emission.

My group strives to overcome these deficiencies of ‘classical’ saltation theory by bridging the gap between the small scales on which dust emission takes place (centimeters to meters) and the large scales on which dust affects weather and climate (1 – 10,000 km). We do this using a combination of theory and numerical modeling of saltation, field campaigns, Large Eddy Simulation (LES) modeling (in collaboration with my colleague Marcelo Chamecki), and global modeling.

This project is also the main focus of my NSF postdoctoral scholar Raleigh Martin.

Fig2_8_AeolianStreamers

Pockets of sand transport on a beach, clearly showing the spatial and temporal variability of naturally-occurring sand transport (and thus dust emission). Photograph courtesy of Andreas Baas.

Extraterrestrial dust and sand transport

Although my main research interestsmars-globduststrm are in the Earth sciences, I’m also interested in solving extraterrestrial puzzles by applying terrestrial insights. In particular, aeolian processes are of great importance to our dusty planetary neighbor Mars. To better understand how all this aeolian activity is possible on a planet with an atmosphere less than 1% as dense as our own, I applied a combination of numerical modeling (COMSALT) and theory (papers here and here). I found that the low Martian gravity and vertical air drag combine to make bouncing Martian sand akin to playing golf on the Moon: particles travel much higher trajectories than on Earth, allowing them to gain substantial momentum even in light winds, so that on landing they splash up enough new particles to keep transport going at low wind speeds. This hysteresis effect – transport is difficult to initiate yet easy to sustain – could allows Martian sand and dust transport to occur for wind speeds an order of magnitude lower than previously thought ESP_022364_1890_low_respossible. This finding helps explains a long-standing contradiction (see NPR story here): that sand moves in many areas of Mars, even though wind speeds are only rarely sufficient to set the sand grains in motion. Subsequent observations of Martian sand transport rates, the properties of ripples and dunes, and the inferred threshold wind speed for sand transport, have provided limited experimental support for this hysteresis theory.

I am also part of a NASA team to investigate saltation on Saturn’s moon Titan.

Media interest

Since my work is funded by taxpayer dollars, and because the ultimate objective of my work is to benefit society, I strive to communicate interesting research findings to the public using the media. Interesting media stories on my research have appeared in the print magazines The Economist, PhysicsWorld, New Scientist, Science News, and the well-known scientific journal Nature. Some interesting online stories are at MSNBC and Wired.

I was also interviewed on National Public Radio for my 2010 PRL article, finding that even light winds can move sand on Mars. The audio fragment is here (note that the story’s title “Dunes On Mars: How Sand Shifts Without Wind” is misleading: it’s of course the wind that shifts the sand, it’s just that my research found that the wind is much more efficient at moving the sand than previously thought.)