Flux association in coherent structures transporting CO 2, H 2O, heat and ozone over the code grid site

Abstract

Aircraft-based eddy correlation flux data obtained at a height of 30 m above irrigated and non-irrigated agricultural land in southern California have been analyzed in terms of the coherent structures that dominate the turbulent exchange of energy and gases during daytime conditions. The analysis focused on transport of sensible heat, moisture, carbon dioxide and ozone in the gradient modes, i.e. excess up or deficit down for heat and moisture, and deficit up or excess down for carbon dioxide and ozone. Results are presented for composition and size of the dominant structures, over water-stressed and non-water-stressed surfaces, and on the relative frequency with which structures carrying only a single scalar, or given combinations of scalars, were encountered along the flight paths. Interpretation of results provides further evidence for the existence of a second (non-physiological) sink for ozone. The relative preponderance of structures that carry moisture, carbon dioxide and ozone simultaneously, particularly in the gradient up mode, reflects the importance of vegetation as co-located source/sink for these scalars. Surface characteristics resulting in thermal buoyancy and water vapour density gradients appears to be responsible for about 85% of gradient up transport. Finally, the detrending procedures described here may help to define more effective separation between local and mesoscale events in biosphere-atmosphere interactions.