The contribution of coherent structures to the aggregation of large-scale heat fluxes in the Alaskan boreal forest
Abstract
Understanding the sensitive interactions of high latitude boreal forest ecosystems to local and global climate requires the estimation of large-scale area-averaged heat fluxes, which result from the aggregation of local turbulent fluxes. This aggregation is conditioned by the thermodynamic properties of the flow, the aerodynamic characteristics of the canopy, and the frequency range of the turbulent flow interactions. In particular, the flow of the Atmospheric Boundary Layer (ABL) over the complex roughness of the Alaskan boreal forest results in coherent but complex three-dimensional vortex structures that vertically exchange heat and gases between the forest and the atmosphere. Such coherent structures, manifested as ramp-shapes in a temperature time series, are the lower frequency components of the otherwise stochastic high frequency turbulent flow. Such structures have been evaluated in a variety of forest types and at varying latitudes for their important contribution to energy fluxes and for their role in affecting, or being affected by, dynamic stability. We investigate the relationship between local turbulent heat fluxes and large-scale area-averaged turbulent heat fluxes in a boreal forest from the winter of 2011 to the summer of 2012. The data is based on two sonic anemometers installed on a tower in the center of a black spruce forest with a mean tree height of 4.7 m over permafrost near the University of Alaska Fairbanks. In order to determine the flux aggregation and its dependence on turbulence and flow conditions, the local anemometer data was combined with a Large Aperture Scintillometer (LAS) measuring along a transect over the forest and across the tower. Wavelet transform and conditional sampling were applied to the temporal series of turbulent flow speeds and temperature measurements to evaluate the presence of coherent structures. To understand the nature of coherent structures in the Alaskan boreal forest, the frequency of occurrence of coherent structures and their presence in one or both of the anemometer levels (sub-canopy and/or above canopy) was evaluated, as well as their connection to the dynamic stability of the flow. Heat flux contributions from coherent structures were compared to the total canopy heat flux. Statistical analyses of heat fluxes from the LAS were used in conjunction with the values from the anemometers to determine the role of coherent structures in contributing to the vertical aggregation of larger area-averaged heat fluxes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A13D0251S
- Keywords:
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- 3307 ATMOSPHERIC PROCESSES / Boundary layer processes;
- 3322 ATMOSPHERIC PROCESSES / Land/atmosphere interactions;
- 3379 ATMOSPHERIC PROCESSES / Turbulence;
- 3323 ATMOSPHERIC PROCESSES / Large eddy simulation