Research in our group focuses on understanding the fluid dynamics of the turbulent lowest mile of the atmosphere, known as the atmospheric boundary layer (ABL). Turbulent transport in the ABL controls the exchanges of momentum, heat, water vapor, and trace gases between Earth’s surface and the atmosphere and has wide-ranging implications for weather and climate, hydrology, air quality, ecology, renewable energy, greenhouse gas cycling, and many other fields. Due to the complexity of the equations that describe the motion of a turbulent fluid, an exact analytical description of turbulence has eluded researchers for nearly two centuries. In order to make progress in research, we use a combination of in situ observations, high-performance numerical simulations, and analytical techniques. Current research themes in our group include studying the basic physics of the atmospheric boundary layer, turbulence over complex terrain (e.g. complex surface topography, land cover variability, and urban areas) where classical theories break down, as well as the turbulent transport of heavy particles (e.g. dust, snow, aerosols, and biological particles) in environmental flows. We also develop new research tools to study these flows using the large eddy simulation technique. More information on each of these topics can be found below.
Atmospheric Boundary Layers (ABLs) and Turbulence
Research on turbulent flows has a long history in both the engineering and geophysical communities. While there are many similarities between atmospheric turbulence and flows in the laboratory (e.g. over an airfoil or in a pipe), atmospheric boundary layers have the added complications of complex surfaces, thermal stratification, and diurnal variability. During the day, solar heating of the surface creates unstable stratification which causes buoyantly-driven motions that increase turbulent mixing through the ABL. At night, longwave radiative cooling of the ground leads to stable stratification, which suppresses vertical motions and turbulent transport. Thus the structure of the ABL and properties of turbulence change dramatically throughout a single diurnal cycle. We have worked on a variety of topics in atmospheric turbulence, including errors in turbulence measurements and their implications for our understanding of ABL transport, theoretical work to connect empirical scaling laws with observed properties of turbulence, and large-scale organization in the convective boundary layer and how it influences turbulent fluxes. A main area of emphasis in our current work is the structure and dynamics of the daytime convective boundary layer.
ABLs in Complex Terrain
Over flat, horizontally homogeneous terrain, a number of aspects of turbulent transport are fairly well understood. Under these idealized conditions, scaling laws relating turbulent transport to gradients of mean quantities (e.g. wind speed or temperature) collapse to a set of universal curves as a function of atmospheric stability. These scaling laws serve as the basis for interpreting experimental measurements and modeling turbulent transport in weather, climate, and hydrological prediction models.
However, over complex terrain (e.g. urban areas, complex surface topography, land cover variability, or sloping surfaces) the underlying assumptions in these classical scaling laws break down. Consequently, there is a need to both understand the basic physics of ABLs in complex terrain and to develop parameterizations for use in weather, climate, and hydrological models. We investigate these questions using both observational data and numerical simulations. Current areas of emphasis include turbulent transport in realistic urban areas and over complex surface topography. More broadly, we are interested in understanding the basic physics of turbulence transport in complex terrain, developing new scaling laws for these flows, improving the fidelity of numerical simulations, and developing parameterizations suitable for use in larger-scale models.
Large Eddy Simulations
While ground-truth observations will always play a critical role in advancing our understanding of atmospheric turbulence, in situ measurements are typically taken at only a few locations in many experiments and therefore cannot fully characterize the spatial variability that occurs in real atmospheric boundary layers. A promising approach for studying these flows is large eddy simulation (LES), a computational technique where the largest turbulent motions (responsible for a large fraction of the transport) are resolved explicitly, and the effects of the smaller scales are modeled.
We use large eddy simulation to investigate the physics of idealized ABLs such as the nocturnal stable boundary layer or daytime convective boundary layer over flat, horizontally homogeneous terrain. We also are able to perform simulations of more complex ABLs, such as those over complex topography or urban areas, by using simulations that are able to represent the effects of complex topography on the ABL. Furthermore, we also work on representing additional physics in large eddy simulation in order to perform accurate, physically-based simulations of more complex ABLs. Recent work has focused on developing a model for heavy particle and passive scalar transport over complex surfaces, relevant for aeolian processes, snow transport, and urban air quality.
Another line of research we pursue is particle-laden flows in the atmosphere. Heavy particles of interest in the atmosphere include dust and sand, snow, biological particles (such as pollen, seeds, and spores), as well as cloud droplets and hydrometeors. The behavior of heavy particles differs significantly from the transport of trace gases due to gravitational settling and inertia, which leads to relative velocities between the particles and the fluid. A number of studies over the past several decades have revealed that heavy particles in a turbulent flow exhibit preferential concentration, that is, they tend to cluster in low vorticity regions, thereby increasing both particle collisions and aggregation.
In our current research, we are studying the deposition of heavy particles over complex topography, relevant for snow transport and aeolian processes in the atmospheric surface layer. We are developing new numerical techniques to study these problems using LES. More broadly, we are interested in the fluid dynamics of particle-laden flows and their implications for problems of relevance for ecology, human health, hydrology, and cloud processes.