FLUX SAMPLING ERRORS FOR AIRCRAFT AND TOWERS

Various errors and influences leading to differences between tower-and aircraft-measured fluxes are surveyed. This survey is motivated by re ports in the literature that aircraft fluxes are sometimes smaller tha n tower-measured fluxes. Both tower and aircraft flux errors are large r with surface heterogeneity due to several independent effects. Surfa ce heterogeneity may cause tower flux errors to increase with decreasi ng wind speed. Techniques to assess flux sampling errors are reviewed. Such error estimates suffer various degrees of inapplicability in rea l geophysical time series due to nonstationarity of tower time series (or inhomogeneity of aircraft data). A new measure for nonstationarity is developed that eliminates assumptions on the form of the nonstatio narity inherent in previous methods. When this nonstationarity measure becomes large, the surface energy imbalance increases sharply. Finall y, strategies for obtaining adequate flux sampling using repeated airc raft passes and grid patterns are outlined.