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

Analysis of airborne eddy correlation flux measurements of heat (H), moistu re (LE) and CO2 (C) over two 16 km x 16 km heterogeneous grid sites in BORE AS 1994 (IFC-2) and 1996 are compared in order to examine persistence and v ariability in the distributions of surface characteristics and fluxes betwe en 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 cond itions. Maps of fluxes and surface characteristics were constructed by bloc k averaging over 2 km windows along the flight lines, analyzed for similari ties, and used to quantify spatial variability of the fluxes. Sensitivity a nalysis suggested minor effects of boundary layer variability and window si ze on the main features of the source/sink distributions. Incident radiatio n 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 variat ions in solar energy input, on spatial variation of flux distributions was confirmed, and mesoscale motion was found negligible, probably because of t he 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 ob served low correlation between sensible heat flux and surface minus air tem perature difference (T-s-T-a), primarily over old black spruce, was confirm ed. The high Bowen ratio over the forest contributed to the growth and deve lopment of the observed deep boundary layers over the sites, but no clear c orrelation emerged between boundary layer depth and observed near-surface f luxes.