Influences of Coherent Structures on Validity of the Constant Flux Layer Assumptions in the Unstable Atmospheric Surface Layer

Washington State University, Pullman WA

Investigators

Abstract

In many applications such as weather and climate modeling, it is assumed that the transfer rate for scalars (i.e., scalar fluxes), including heat, water vapor, CO2, and other greenhouse gases, is constant with respect to height in the atmospheric surface layer, the lowest layer of the atmosphere. This so-called constant flux layer assumption is also widely used in land-surface flux measurements to quantify scalar fluxes to and from land surfaces. However, the reported failure of this assumption implies that scalar fluxes measured and modeled in the atmospheric surface layer are not equivalent to the fluxes across the surface-atmosphere interface, leading to uncertainty in measured and modeled fluxes in these applications. Despite the abundant studies on the roles of large turbulent eddies (i.e., coherent structures) in contributing to scalar fluxes, it is not well understood as to how large turbulent eddies contribute to changes in fluxes with height in the atmospheric surface layer. The objective of this project is to study what physical mechanisms regulate the attributes of large eddies across height that lead to varying contributions to fluxes with height, contributing to the failure of the constant flux layer assumption across a wide range of atmospheric conditions. By leveraging the existing facilities and as guided by flux budget equations, a field experiment will be conducted over the large water body of Ross Barnett Reservoir in Ridgeland, Mississippi, with a flux tower equipped with five levels of eddy covariance systems and other instruments. The field experiment will provide a unique dataset that minimizes the influence of advective terms and enables more precise examination of the attributes of large turbulent eddies. A combined approach of several analysis methods, such as fast Fourier transform, wavelet transform, and ensemble empirical decomposition mode, will be used to characterize the attributes of large turbulent eddies and their variations with height across instability ranges. Quadrant analysis will be used to quantify asymmetric flux contributions from sweeps and ejections of large turbulent eddies, enabling an analysis of the underlying mechanisms that modulate changes in fluxes with height. Further, such analyses will allow the study of mechanisms that lead to different behaviors of height-varying fluxes for different scalars. The field experiment will generate a unique dataset valuable for studying a wide range of topics in micrometeorology, hydrometeorology, boundary-layer turbulence, lake evaporation, water-atmosphere interactions, carbon emissions from inland waters, water budget for freshwater management, and aquatic ecosystems. An involved Ph.D. student will gain first-hand experience in designing, preparing, and conducting micrometeorological experiments and learn eddy covariance techniques, data analysis theories and tools, and many other research skills. Mini-research projects using the datasets collected in this project will be developed and incorporated into the curriculum of one undergraduate course and two graduate courses being taught by the PI for students’ term projects. Short films and presentations about the field experiment and research findings will be disseminated in scientific conferences and seminars as well as the PI’s department websites for diverse audiences. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.