Boundary layer theory for canopies covering complex terrain: Going from eddies in motion to biosphere-atmosphere exchanges and their representation in climate models

Abstract

The key processes that affect the Earth’s radiative balance and the hydrological cycle are intricately linked to land cover, topography, and exchanges of mass, energy, and momentum between the land cover and the overlying atmospheric boundary layer (ABL). Forests, which now cover some 30% of the land area, are mainly situated on complex terrain as farming has displaced much of the forests on flat terrain. Net carbon dioxide sinks and water vapor sources from forests, the subject of this project, are measured 24 hours a day, 7 days a week, 52 weeks a year across multiple years on towers using micro-meteorological methods appropriate for flat terrain. These measurements are now integrated within a global network (FLUXNET), where such data are publicly available.

This proposal seeks to develop a framework for addressing the effects of gentle topography on such measurements and the mathematical modeling of gas exchanges between the atmosphere and forests situated on complex terrain. Over the last three decades, the measuring capabilities and numerical simulations of air flow have significantly expanded to allow experiments and simulations of the ABL under increasingly realistic conditions to be conducted. Building on these developments, the project focuses on the Amazon forest where micrometeorological measurements from the ARM deployment during the GOAMAZON field campaign and from the long record of FLUXNET sites in the region will be leveraged.

The conjecture to be evaluated is that even gentle topography produces pressure variations that are approximately out of phase with topography and can be leveraged in developing corrections in space to “flat-world” conditions. The conjecture will be evaluated across a wide range of air density strati cation regimes encountered diurnally within the PBL over the Amazon forest. More specifically, high-fidelity canopy-resolving Large Eddy Simulations (LES) are proposed to build a public database of high-resolution flow over complex topography for several target locations in central Amazonia. Analysis of observational data and numerical model investigations will be conducted to evaluate the following research questions:

1. What is the three-dimensional structure of the PBL over forests on complex topography under different atmospheric conditions (characterized by wind speed and direction, density gradients, etc.)?
2. What is the impact of gentle topography on the interpretation of short- and long-term tower observations of evapotranspiration (ET) and net ecosystem carbon dioxide exchange (NEE)?
3. What is the significance of secondary flows originating from the small-scale topographic features on surface-atmosphere gas exchange, and can these contributions be represented in regional and climate models through approximate interactive effects between topography and pressure variations?

This research will contribute to the Department of Energy’s Atmospheric System Research goals of understanding key processes that affect the Earth’s radiative balance and hydrological cycle, especially processes that limit the predictive ability of regional and global models. The investigators have synergistic experiences in turbulence simulations, theories, and experiments for flow over complex terrain cover by vegetation, and field experiments in Amazonia. They also have a history of collaborative work on peer-reviewed manuscripts and student advising.