An introduction to flux measurements in difficult conditions

The origins of the aerodynamic techniques now widely used at sites around the world to measure continuous biosphere-atmosphere exchange of carbon and energy are briefly reviewed. A survey of the current state of this approach concludes that the technique often fails when standard analysis routines are applied to data from single towers in complex flows. In the daytime, problems are signaled by failure to close the surface energy balance because turbulent energy fluxes are routinely underestimated. Complex flows are more prevalent at night when they lead to failure to measure all the respired CO2. At such times, the aerodynamic methodology is commonly supplemented by biological models. A set of papers from a workshop on "Flux Measurements in Difficult Conditions" held at the National Center for Atmospheric Research (NCAR) in January 2006 are introduced next. Two papers review the causes and magnitude of these flow-based problems. Four papers describe intensive field experiments that detail the mechanisms that cause problematic complex flows. These experiments show, inter alia, that the technique of replacing nighttime eddy flux measurements by biological models can also be systematically biased. Finally, two model studies are used both to illustrate the physics behind these complex flows and to motivate an approach to systematic correction of single-tower results.