Flux estimates of latent and sensible heat, carbon dioxide, and ozone using an aircraft-tower combination

Several recent large-scale experiments in land-surface climatology have used the combination of aircraft- and tower-based flux measurements. Within these experiments, much effort has been made to assess the potential of aircraft to serve as extended observation platforms for the scaling up from local (tower-based) to regional estimates of surface-atmosphere exchange. Data collected as part of the California Ozone Deposition Experiment (CODE), during a one-month period of consistent day-to-day wind and radiation conditions, were particularly conducive to such a study. Aircraft-based flux estimates obtained at 30 and 60 m above two vineyard sites of approximately 9 km2 (site A, 6 km x 1.5 km and site B, 4.5 km x 2 krn) were compared to tower-based measurements made at 9.4 m above a section of site A. This paper shows that over the measuring period the sum of sensible and latent heat flux at 30 m, as measured with the aircraft-based system, was about 11 % less than that measured with a tower-based system at 9.4 m. It also shows that fluxes at 30 m were on the average about 10% larger than those at 60 m. It documents the flux variability of CO2, sensible and latent heat and ozone observed over relatively homogeneous surfaces. Although aircraft-tower comparisons must be interpreted with caution, due to the dissimilar surface areas sampled by the stationary and moving sensors located at different elevations, the consistent relationship obtained between these two sampling systems demonstrates the complementary nature of such measurements. It confirms that very repeatable measurements can be obtained with aircraft-based systems over distances as short as a few kilometers. It also demonstrates that compositing data for several runs and for several days !From aircraft- and tower-based measurements can provide diurnal patterns of fluxes of sensible and latent heat as well as CO2 and O3 on a regional scale.