Effects of canopy micrometeorology on the turbulent transport processes above and within an urban forest.

Abstract

Biogenic hydrocarbons contribute a significant fraction of the midday hydrocarbon burden in forested cities in the southeastern United States. Efforts to model the impact of these trace gases on urban photochemistry in the southeastern United States will require a thorough understanding of the micrometeorology and turbulent transport processes associated with urban forests, from which the bulk of these trace gases are emitted. In an effort to achieve such an understanding, two exploratory studies were conducted in the 65-acre Fernbank Forest, located in Atlanta/Decatur, Georgia. This dissertation represents the first detailed analysis of the turbulent exchange processes associated with an urban forest. Forest-atmosphere turbulent exchange is dominated by extreme, intermittent events, which are responsible for a large percentage of the average momentum and scalar fluxes, despite occurring only a small percentage of the time. Conditional analysis was performed on turbulent flux data collected above and within the Fernbank Forest. Results indicate that on average, these extreme events are relatively more important at night, when they account for a greater percentage of the average momentum flux than during the daytime. This is believed to be related to the increase in atmospheric stability above the canopy at night. However, due to the large hour-to-hour variability noted in the calculated stress and time fractions, it cannot be definitively stated that there is a significant difference in day/night contributions from these events. The interception of solar radiation by forest canopy elements results in the formation of a thermal inversion within the canopy, enhancing the local daytime stability of this layer. A conditional analysis of daytime hours (0900-1600 EDT) indicates that this increase in local atmospheric stability does not produce a corresponding increase in the relative importance of extreme transport events within the canopy. Finally, turbulence statistics above and within the forest were influenced by changes in over-forest fetch (upwind of measurements) as a function of prevailing wind direction. This suggests that forest-atmosphere exchange theories based on the study of extensive rural forests may not be applicable to urban forests, which typically have smaller homogeneous fetches.