The 'footprints' of eddy correlation sensible heat flux density, and other micrometeorological measurements
Academic Article
Overview
Additional Document Info
View All
Overview
abstract
Similar sensible heat flux density measurements were obtained during mainly unstable atmospheric conditions, using the eddy correlation (EC) technique, at heights of 0.50, 1.00 and 1.25 m above a short grassland canopy surface. The fetch-to-height ratio corresponding to the 1.0 m placement height was 190:1. Calculations based on fetch showed that the measurements below a height of 1.25 m were within the equilibrium layer whereas those at or above 1.25 m were above the equilibrium layer. The greater sensible heat measurements above a height of 1.25 m were probably from advected sensible heat from nearby tar roads and buildings. Measurements of atmospheric stability were obtained by calculating the ratio of height above surface to the Monin-Obukhov length. Most measurements were obtained under unstable conditions when mixed convection prevailed. Our measurements show that it is possible for EC sensors to be placed as low as 0.50 m above the surface, during unstable periods, without there being a significant difference from the measurements at a height of 1.00 m. Measurements were obtained with a pan filled with soil placed 0.27 m below the fine wire thermocouple of an EC system placed 1.00 m above the surface. These results demonstrated that the reduction in the sensible heat below 0.5 m was not from acoustic reflections from the surface. Possibly, the reduction was due to small eddies near the surface being contained within the sonic separation distance. An analysis of 'footprints' showed that, for our unstable conditions, at least 96% of the measured sensible heat at a height of 0.25 m was from our experimental site. This fraction decreased to less than 70% at the 2.0-m placement height. Calculations showed that the fetch requirement for micrometeorological measurements above a forest canopy was more stringent than for a grassland canopy.
