Date of Degree

2-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor(s)

Chuixiang Yi

Subject Categories

Atmospheric Sciences | Environmental Sciences

Keywords

Advection flux, Canopy flow, Complex terrain, Numerical modeling, Recirculation, Stable stratification

Abstract

Canopy flow plays a substantial role in regulating atmosphere-biosphere exchanges of mass and energy. The worldwide FLUXNET has been developed to quantify the net ecosystem exchange of mass and energy through fluid dynamics in and above vegetation canopy using tower-based eddy covariance (EC) technique. However, EC measurements are subject to advection errors in complex terrain, particularly during nights when atmospheric stability is strong. Because EC measurements are one-dimensional (1D), three-dimensional (3D) air movement, CO2 transport, and temperature variation around the instrumented tower are unknown. We employ a Computational Fluid Dynamics (CFD) model to investigate the impact on CO2 transport of 2D and 3D characteristics of canopy flow resulting from interactions between large-scale synoptic flows and local topography, vegetation and thermal conditions.

Under neutral conditions, flow distortion over a forested hill is asymmetric, with recirculation on the lee slope. The presence of vegetation and steepening slope intensifies recirculation depth and extension. The recirculation regions are responsible for CO2 build-up behind the hills. The contribution of advection to the CO2 budget is significant and topographic-dependent. Gentle slopes can cause larger advection error than steep slopes. However, the relative importance of advection to CO2 budget is slope-independent. Under calm and stable conditions, canopy flow is thermally stratified: super-stable layers above and in the deep canopy and an unstable layer inbetween. Vertical exchanges of mass, momentum, and energy are limited by the stabilities of these layers. The pattern of two drainage flows are significantly modified by the interaction between thermal stratification and slope, and are better understood with the distribution of vortices, and the sources and sinks of turbulent kinetic energy. The numerical method is applied to the alpine forest at Renon, Italy to investigate how thermo-topographic and synoptic flows interact to govern canopy flow dynamics and CO2 transport. We found that recirculation with high CO2 concentration developed only when local slope wind is enhanced by synoptic wind. There's no recirculation formed as synoptic wind direction is opposite to the local wind direction and CO2 is quite well mixed. No recirculation appears without synoptic condition under which CO2 builds up mainly at downwind locations.

 
 

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