Comparison of the spatial and temporal distribution of fluxes of sensible heat, latent heat and CO2 from grid flights in BOREAS 1994 and 1996

Abstract

Analysis of airborne eddy correlation flux measurements of heat (H), moisture (LE) and CO2 (C) over two 16 km × 16 km heterogeneous grid sites in BOREAS 1994 (IFC‐2) and 1996 are compared in order to examine persistence and variability in the distributions of surface characteristics and fluxes between the two years. The data used were obtained in grid patterns flown at 30 m above ground level, under generally clear sky and thermally unstable conditions. Maps of fluxes and surface characteristics were constructed by block averaging over 2 km windows along the flight lines, analyzed for similarities, and used to quantify spatial variability of the fluxes. Sensitivity analysis suggested minor effects of boundary layer variability and window size on the main features of the source/sink distributions. Incident radiation was more highly correlated with grid‐averaged values of C than with H and LE. The dominant role of surface inhomogeneity, as opposed to local variations in solar energy input, on spatial variation of flux distributions was confirmed, and mesoscale motion was found negligible, probably because of the small sizes of homogeneous subareas with sufficient surface contrast to induce thermally generated motion. CO2 flux and greenness index were highly correlated, but correlation was site‐ and time‐specific. The previously observed low correlation between sensible heat flux and surface minus air temperature difference (Ts‐Ta), primarily over old black spruce, was confirmed. The high Bowen ratio over the forest contributed to the growth and development of the observed deep boundary layers over the sites, but no clear correlation emerged between boundary layer depth and observed near‐surface fluxes.