Land-surface Atmosphere Research Group

Nebraska Sand Hills

The Sand Hills of Nebraska represent a unique natural laboratory in which to investigate landatmosphere interactions. This vast area of sand sheets and sand dunes, now stabilized by sparse grass and shrubs, is interspersed by interdunal meadows covered with much denser grass cover irrigated from below by groundwater supplied by the rapid infiltration of precipitation into the dunes. Because of this, the Sand Hills presents multiple, contrasting land surfaces – dry dunal uplands and moist interdunal valleys – to the overlying atmosphere and, therefore, the fluxes of heat and moisture into the atmosphere can vary over relatively small spatial scales. The major questions addressed by my research are how these spatial inhomogeneities affect the atmospheric thermodynamics and how this might feed back onto the landscape through variations in atmospheric temperature and precipitation, with consequent impacts on groundwater and streamflow.

My research on land surface-atmosphere interactions in the Sand Hills began following a twosemester seminar on mesoscale modeling I co-taught with Mark Anderson during the 1998-99 academic year. In this class, students used MM5 for projects investigating various mesoscale processes and phenomena. Several of the students focused on processes over Nebraska and identified atmospheric effects that seemed to be localized over the Sand Hills. Additional modeling led to the development of a successful proposal to NOAA, involving atmospheric scientists and hydrologists to investigate the interconnections between the atmospheric hydrologic cycle over the Sand Hills and groundwater flows, and the variations between the differing land surface characteristics of the dunes and valleys (Chen et al. 2003). This successful collaboration led to further interdisciplinary research as part of an NSF Biocomplexity grant involving more than a dozen researchers, including ecologists, hydrologists, geologists and atmospheric scientists (see http://sandhills-biocomplexity.unl.edu/). My primary role in this project involved investigating the variations in the fluxes of heat and moisture from different land covers into the atmosphere (Radell and Rowe 2008, 2009) as well as collaborating with the other investigators on integrative studies of the unique Sand Hills’ ecosystems (Sridhar et al. 2006). Although the funding for this project ended in 2007, collaboration continues today (Feng et al. 2008).

Water resources in the Great Plains, the United States and the world are coming under increasing pressure from both population growth and climate change. Research that adds to our scientific understanding of the hydrologic cycle has the potential to lead to the development better ways to manage this essential resource.

Atmospheric circulation in the Jurassic

This work represents the first and longest collaboration between an atmospheric scientist and a geologist since the formation of the Department of Geosciences in 1996. Shortly after the merger Dave Loope and I began to investigate the climate of the Jurassic in relation to the vast desert regions of what is now the southwestern United States. We were successful in obtaining NSF funding for the early stages of this research, which would include computer simulations of Jurassic climate(s). This meant developing an entirely new research track for me – paleoclimate modeling – including learning a completely new modeling system, along with developing new datasets to provide the appropriate boundary conditions needed for the Jurassic environment. While I was learning to use the National Center for Atmospheric Research (NCAR) Community Climate Simulation Model, version 3 (CCSM3) to simulate the climate of 200 million years ago, we attempted to reconcile our theoretical understanding of the atmospheric general circulation; previous, coarse-resolution simulations of the early Jurasssic; and the predominant winds that were preserved by the ancient dune deposits of the Colorado Plateau (Loope et al. 2001; Loope and Rowe 2003; Loope et al. 2004). This met with limited success – some aspects of the climate based on theory and previous simulations meshed well with our interpretation of the climate and atmospheric circulation we could infer from the geologic record; but not every aspect fit as well as others. When we were able to run our own simulations, we found that we were unable to simulate an atmospheric circulation that matched the geologic record of the Colorado Plateau, regardless of how we altered atmospheric composition (e.g., by changing CO2 concentration), land surface boundary conditions (e.g., by changing vegetation cover or topography), or how we simulated sea-surface temperatures (e.g., by prescribing SST or by using a simple, slab ocean model). Regardless of our efforts, the atmospheric circulation was almost completely constrained by the fact that we live on a rotating planet which is tilted slightly relative to its orbital plane and which is largely covered with ocean. That the continents were arranged into a single large land mass during this period had only relatively minor impacts on the general circulation of the atmosphere, altering only some of the regional circulations from what we recognize today with the modern configuration of continents. That is, climate theory – and the atmospheric general circulation that can be derived from it – is quite robust.

By this time, our colleague Bob Oglesby, with whom we had been working, had joined the faculty at UNL. We decided that if we couldn’t alter the pattern of the atmospheric circulation over the Colorado Plateau through reasonable changes to boundary conditions or atmospheric composition, we should move the Colorado Plateau to where its geologic record of winds would match the atmospheric circulation produced by the model. When we shifted the entire Pangean land mass southward by 30° latitude, our simulations still were constrained by basic climate theory and the general circulation matched well with the eolian geologic record of the Jurassic over Colorado Plateau. However, this was accomplished only by doing great harm to the reconstructions of paleogeography constructed largely from paleomagnetic research and data (Rowe et al. 2007). This conundrum remains unsolved and we are currently seeking additional funding to continue our investigations. Reconciling this conundrum would represent a significant achievement in both paleoclimate modeling and paleomagnetics.