Research

The main current focus of the Earth, Wind & Particles group is to facilitate more accurate predictions of future climate changes by understanding and quantifying the climate impact of aerosols, and in particular of desert dust aerosols. Desert dust accounts for about two thirds of the total aerosol mass and have approximately doubled since preindustrial times. Dust impacts the climate system in many ways, most prominently by scattering and absorbing radiation and seeding clouds, and also by darkening snowpacks and fertilizing ecosystems upon deposition. The approximate doubling of dust since pre-industrial times might thus have substantially affected the Earth’s climate, yet we currently don’t even know whether dust net cools or warms the planet. Work in our group is partially focused on figuring out this net climate impact of dust, and thereby determining the radiative forcing that dust has exerted on the climate system since pre-industrial times. Since the radiative forcing exerted by dust and other aerosols is the main uncertainty in estimates of the climate sensitivity that determines the climate’s response to future greenhouse gas emissions, this work is critical to produce more accurate forecasts of future climate changes. Moreover, quantifying the climate impacts of dust helps determine whether possibly substantial future changes in dust, for instance due to desertification, would oppose or enhance future anthropogenic climate changes. See here for a semi-recent keynote talk on these topics.

Research in the Earth, Wind & Particles group can be roughly divided in two categories: work on answering big picture questions, such as “what is the effect of desert dust on Earth’s radiation budget”, and work on understanding small-scale processes, such as “how does the deposition of dust aerosols depend on the turbulence intensity”. These top-down and bottom-up research approaches fertilize each other: answering the big picture questions usually requires a detailed understanding of the underlying small-scale processes. Vice versa, determining which small-scale processes are worthy of detailed study is informed by an understanding of where the main uncertainties lie in answering the big picture questions.

We use a variety of analytical and theoretical methods, simple first-principles numerical models that run on desktop computers, and large-scale climate models that require supercomputers. Our group also occasionally conducts targeted field and laboratory measurements.

Work on big picture questions in the Earth, Wind & Particles group

Our group strives to answer fundamental questions related to the impacts of desert dust on climate and society. How much dust is actually in the atmosphere? Does dust warm or cool the climate, and by how much? What is the radiative forcing that dust has exerted on the climate system? Will future climate-induced changes in desert dust oppose or enhance anthropogenic climate change? What are the impacts of dust on the hydrological cycle? What are the global health impacts of the inhalation of dust aerosols?

Climate models have long been the main tool in answering such big picture questions. However, these models suffer from a key weakness: they need to prescribe poorly known attributes of dust, and aerosols in general, such as the sizes and optical properties of these particles. Climate models thus cannot represent the experimental uncertainties in those attributes. To make matters worse, many climate models prescribe dust attributes that are inconsistent with current experimental and observational results. The consequent biases in model simulations have made it difficult to determine the impacts of dust on climate and society.

My group has recently developed a new methodology to determine (dust) aerosol effects on climate and society. Specifically, we developed a “meta modeling” framework that combines climate model simulations with constraints on key determinants of dust effects on the Earth system, such as the amount of dust in the atmosphere, its sizes, and its optical properties. This joint analytical-modeling approach is more accurate than conventional ensembles of climate model results because it directly integrates observational and experimental constraints on the properties and abundance of (dust) aerosols. We have used this methodology to constrain the effect of dust on the Earth’s energy balance (paper here). We found that current models have a clear bias towards fine dust. Since fine dust cools by scattering solar radiation, whereas coarse dust warms by also absorbing solar and terrestrial radiation, we found that the effect of dust on the climate by “directly” scattering and absorbing radiation is actually substantially less cooling than models have thus far calculated (see here for an interesting commentary). Ongoing work in our group is focused on determine dust effects on human health, the hydrological cycle, and the contribution of dust loading changes to 20th century climate change.

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Work on small-scale processes: microscopic dust physics 

Answering the fundamental questions of the interactions between dust and climate requires an accurate understanding of dust interactions with the atmosphere and radiation. In particular, it requires an understanding of how dust is emitted and deposited, aged by atmospheric chemistry during transport, and how it interacts with radiation and clouds. Work in our group thus focuses on understanding these fundamental processes, and then using that understanding to develop parameterizations that are simple enough that they can be readily implement into large-scale climate and weather models. We do so using a combination of theory, careful analysis of existing measurements, first-principles numerical modeling, and the occasional field or laboratory measurements.

Representative current work on understanding the physics of dust and its impacts on the Earth system include: understanding the optical properties of aspherical dust for implementation in satellite retrievals and climate models, accounting for the effects of turbulence and soil properties on dust emission, understanding dust emission from sand dunes as a possible contributor to the global dust cycle (here), and combining measurements and climate model simulations to develop an accurate database of 3D dust properties (here) that can be used to calculate dust impacts on various aspects of the Earth system. Noteworthy past work includes showing that the process of dust emission is analogous to the fragmentation of glass (here). That is, dust emitted by the blowing of (much larger) sand grains by the wind, the impacts of which on the soil can ‘crack’ the soil in a manner that is analogous to the fragmentation of a wine glass when dropped on the floor. This insight yielded an accurate and scale-invariant expression for the size distribution of dust at emission that is now in widespread use in climate models.

Extraterrestrial dust and sand transport

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Although our group’s main research interests are in the Earth sciences, we’re 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, our group used a combination of numerical modeling and theory (papers here, here, and here). We 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 allow Martian sand and dust transport to occur for wind speeds an order of magnitude lower than previously thought possible. In fact, rather than occurring continuously for high wind speed as on Earth, we think that sand and dust transport on Mars occurs in “clusters” that are initiated by rare lifting events, and sustained by relatively moderate winds (see video below). These findings help 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. 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.

This movie from the Curiosity rover shows a dust devil racing across the landscape. If you look closely, you’ll also see two white streaks, which we think are saltation clusters. This video was prepared by Mark Lemmon (TAMU & JPL), and the link to the original JPL/NASA page is here.