Evaluating and bridging the flux-variance and surface renewal methods

Two micrometeorological methods that utilize high frequency sampling of air temperature were tested against eddy covariance (EC) sensible heat flux (H) measurements at three sites representing agricultural, agro-forestry and forestry systems. The two methods encompass conventional and newly proposed forms of the flux-variance (FV) and surface renewal (SR) schemes.  In terms of measurement setup, the sites represent surface, roughness and roughness to surface transitional layers, respectively. After the selection of the most reliable approaches, regression analyses against EC showed that both methods can estimate H with slopes within ±10 % from unity, and coefficient of determination R² >0.9 across all three sites. The best performance, of both FV and SR, was at the agricultural field, where the measurements were within the surface layer.  The worst performance occurred in the tall, relatively heterogeneous forest, where the measurements were taken in the roughness sublayer, the depth of which (with its inherent uncertainty) needs to be taken into account in the calculations. In addition to the evaluation of the FV and SR forms, an alternative perspective relating ramp-like structures to the vertical temperature gradients in the surface boundary layer is introduced here. Ramp-like structures carry much of the heat flux and temperature variance, representing opportunities to constrain the coefficients of the two methods. As a corollary, we introduce a novel approach emerging from bridging FV and SR methods that combines information about the coherent structures with the overall variance to obtain heat fluxes in a turbulent atmosphere. The proposed approach yields reliable H estimates without the need for site-specific calibration and instrumentation other than a single fast thermocouple.

 

Acknowledgement: This study was conducted with support of SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797) and USDA NIFA-AFRI Sustainable Bioenergy Program, 2011-67009-20089, Loblolly pine-switch grass intercropping for sustainable timber and biofuels production in the Southeastern United States.  Funding for AmeriFlux core site US-NC4 (natural forested wetland) was provided by the USDA NIFA (Multi-agency A.5 Carbon Cycle Science Program) award 2014-67003-22068. Additional funding was provided by the DOE NICCR award 08-SC-NICCR-1072, the USDA Forest Service award 13-JV-11330110-081, and the DOE LBNL award DE-AC02-05CH11231.