Uncertainty Analysis of Vertical Wind Motion Measurement for Airborne Flux Measurements

Measurement of atmosphere-surface exchange is a fundamental part of a more quantitative understanding of climate change. Among the impacts are changes in the fluxes of Biogenic Volatile Organic Compounds (BVOCs) and net ecosystem exchange of carbon, in forest ecosystems. Flux measurements are most frequently conducted on a local scale using flux measurement towers, where the measurement of vertical air motion is correlated with the concentration of the scalar to be studied. To expand the understanding of fluxes to a regional scale, and enable the scaling-up of fluxes to ecosystem levels, we need complementary approaches to the flux-tower model. We are thus engaged in the development of a flexible and low cost aircraft platform for flux measurements, using both eddy covariance, for water vapor and CO2, and Disjunct Eddy Accumulation (DEA), for volatile organic compounds (VOCs). The most challenging aspect of airborne flux measurements is the measurement of vertical scale turbulence. The uncertainties inherent in the measurement of vertical air motion depend on the ability to sense the rotational and translation motion of the aircraft, and the sensed wind, on a 10Hz time scale. We approach this using a combination of a pressure sphere probe for wind measurements, and an integrated Inertial Navigation/Global Positioning System (INS/GPS) to measure the aircraft translation and rotation. This paper summarizes the results of a series of low-speed wind tunnel tests and in-flight calibration maneuvers to determine the uncertainties in vertical wind measurement. The paper summarizes the airspeeds and flight regimes at which different error sources are dominant or negligible. Finally, an error propagation is developed and discussed. This process will lead us to the ability to conduct reliable flux measurements from a low cost aircraft, for a variety of studies of air-surface exchange of gases.