Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Chuixiang Yi

Committee Members

Timothy Eaton

Gary Hemming

Nir Krakauer

Subject Categories

Atmospheric Sciences | Climate | Environmental Sciences

Keywords

eddy covariance, eddy flux, recirculation, advection, microclimate

Abstract

Understanding the exchange of energy, moisture, and trace gases between the terrestrial biosphere and the atmosphere over complex terrain is a fundamental goal in achieving a complete model of global or regional climate. Net ecosystem exchange (NEE) of carbon dioxide is often a crucial input into climate models, and is also used as a means of validating regional model outputs. Calculations obtained from eddy flux tower data provide some of the best quality sources of NEE values; however, the standard formulation of the eddy covariance method is incomplete in terrain that includes common features such as hills, forests, cities, or other large obstructions.

A field experiment using two towers with eddy covariance instrumentation at multiple heights on a forested hillside was conducted to examine the physical phenomena behind the eddy covariance problems. Specifically, both recirculation and advective flux were measured and analyzed.

Recirculation was caused leeward of an obstruction in an unstable atmosphere, characterized by a negative vertical potential temperature gradient. A weak recirculation effect was also seen during conditions when the potential temperature was nearly constant with height (near-neutral). Lee waves appeared under stable conditions, affecting the measured direction of carbon dioxide flux depending on coincidences such as wind speed and air temperature.

A full investigation of NEE in complex terrain must include both horizontal and vertical advective flux. Sensors placed at multiple heights on a single tower yielded vertical advection calculations. Vertical advective flux was non-trivial in any atmosphere, and must be measured to fully investigate the NEE at sites in complex terrain. The horizontal advection analysis was incomplete, because the distance between the two towers in this experiment was similar to the size of the hillside, undermining the horizontal gradient measurement. Estimated horizontal advective flux was typically less than vertical advection, though not an order of magnitude less. Any horizontal advective flux measurement over complex terrain must be undertaken using a separate but nearby tower with sensors at multiple heights. The separation distance must be much less than the scale of the obstruction, in order to minimize the chances that complex flow patterns established by the terrain cause mismatches between air parcels measured at the first tower and those at the second.

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