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Title: Temperature regimes and turbulent heat fluxes across a heterogeneous canopy in an Alaskan boreal forest

Author
item STARKENBURG, D. - University Of Alaska
item FOCHESATTO, G.J. - University Of Alaska
item CRISTOBAL, J. - University Of Alaska
item PRAKASH, A. - University Of Alaska
item GENS, R. - University Of Alaska
item Alfieri, Joseph
item NAGANO, H. - University Of Alaska
item HARAZONO, Y. - University Of Alaska
item IWATA, H. - University Of Alaska
item KANE, D.L. - University Of Alaska

Submitted to: Journal of Geophysical Research Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2015
Publication Date: 3/16/2015
Publication URL: http://handle.nal.usda.gov/10113/61057
Citation: Starkenburg, D., Fochesatto, G., Cristobal, J., Prakash, A., Gens, R., Alfieri, J.G., Nagano, H., Harazono, Y., Iwata, H., Kane, D. 2015. Temperature regimes and turbulent heat fluxes across a heterogeneous canopy in an Alaskan boreal forest. Journal of Geophysical Research Atmospheres. 120(4):1348-1360. DOI: 10.1002/2014JD022338.

Interpretive Summary: Understanding the impacts of changing climate dynamics, on weather patterns and water availability is critical to managing water and other natural resources, identify agricultural best practices, and maintaining both ecosystem integrity and food security. At the same time, Boreal forests, which represent nearly one-third of the earth’s forested surface, play an important role in global water, carbon, and nutrient cycles. Unfortunately, the underlying processes controlling the role of Boreal forest in these natural cycles remain poorly understood. The current study focused on the influence local forest density on evaporative water loss. Using data collected at two adjacent sites, one in dense forest and the other in a sparse canopy, the partitioning of energy between heating the air and driving evaporation was investigated. The results showed that most of the available energy was consumed to heat the air regardless of forest density; nonetheless, the amount of energy consumed by heating the air is greater at the relatively sparse site while water loss was greater at the densely forested site. This clearly indicates that local variability in surface conditions strongly influence the magnitude of evaporation.

Technical Abstract: We evaluate local differences in thermal regimes and turbulent heat fluxes across the heterogeneous canopy of a black spruce boreal forest on discontinuous permafrost in interior Alaska. The data was taken during an intensive observing period in the summer of 2013 from two micrometeorological towers 600 m apart in a central section of boreal forest, one in a denser canopy (DC) and the other in a sparser canopy (SC), but under approximately similar atmospheric boundary layer (ABL) flow conditions. Results suggest that on average 34 % of the half-hour periods in a day are non-stationary, primarily during night and during ABL transitions. Also, thermal regimes differ between the two towers; specifically between midnight and 0500 53 AKST it is about 3 oC warmer at DC. On average, the sensible heat flux at DC was greater. For midday periods, the difference between those fluxes exceeded 30 % of the actual flux and over 30 W/m2 in magnitude more than 60 % of the time. These differences are due to higher mechanical mixing as a result of increased density in roughness elements at DC. Finally, the vertical distribution of turbulent heat fluxes verifies a maximum atop the canopy crown (2.6 h) when compared with the sub-canopy (0.6 h) and above canopy (5.1 h), where h is the mean canopy height. We argue that these spatial and vertical variations of sensible heat fluxes result from the complex scale aggregation of energy fluxes in a heterogeneous canopy.