Collaborative Research: The Role of Coherent Structures in Scalar Transport over Heterogeneous Landscapes

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Scalar fluxes including sensible heat, latent heat, and trace gases such as CO2, methane, ammonia fluxes, from or to land surfaces have direct impacts on weather and climate, air quality, agricultural management, and ecosystem services. A better understanding of turbulence in the lower atmosphere is the prerequisite for better quantification and modeling of these scalar fluxes. Over the past few decades, there has been active research on the topology and transporting capacity of coherent structures in canonical turbulent boundary layers and convective systems. However, problems of practical relevance in micrometeorology have not profited from these studies. This project aims to fill the research gap by investigating the role of coherent structures in scalar transport in atmospheric boundary layer flows over heterogeneous landscapes. The project focuses on the interactions between land surface heterogeneity and coherent structures at the point- and landscape- scales, and their implications for the breakdown of Monin-Obukhov Similarity Theory (MOST) in describing scalar fluxes. At the point-scale, the breakdown of MOST arises from advective terms communicating the signatures of land surface heterogeneity and vertical transport terms bearing the signatures of coherent structures. At the landscape-scale where a spatial averaging is conducted, dispersive fluxes arise from the interactions between land surface heterogeneity and coherent structures. This work combines field experiments and large-eddy simulations (LES) under a unifying framework via flux budget equations and their spatially-averaged version. The field experiment will be conducted over a relatively flat but heterogeneous terrain in Idaho Falls, Idaho, with a 62-m tower and several short towers to be equipped with eddy covariance systems, a sodar, a wind profiler with RASS, 35 mesonet stations, and other instruments. The field work will provide a unique characterization of coherent structures and scalar turbulence in three dimensions and at high resolutions. It will allow quantification of horizontal and vertical terms in the scalar flux budget simultaneously. A series of LES runs will be used to extend the insights afforded by the field experiment from point to landscape scales. A novel approach based on quadrant analysis in time and space will be employed to model dispersive fluxes. The new datasets generated by the field experiment will provide opportunities for exploring other topics related to atmospheric boundary layer and land surface processes. Beyond the tutoring of two graduate students and two undergraduate students, the project will involve several educational and outreach components. It will generate mini-projects for students in several graduate and undergraduate courses taught by the PIs on boundary layer meteorology, applied meteorology, and fluid dynamics. Short films and videos will be created to present the significance of the findings to researchers and the general public.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.