Comparison and Evaluation of Heat Flux Measurements at the Surface Using an Eddy-Covariance Tower and Small Unmanned Aircraft System

One common method for measuring vertical turbulent flux in the atmospheric boundary layer is the eddy-covariance technique. This method uses high frequency wind data along with data corresponding to some other atmospheric variable (usually gas concentration, water vapor, or temperature) to calculate the flux of that variable. The mathematical model used to calculate flux involves the separation of the mean and the fluctuating components; the flux is proportional to the covariance between the fluctuating component of the vertical wind and the fluctuating component of the other atmospheric component. While the eddy-covariance method of flux towers is accurate, the system is expensive, and it is unrealistic to include flux towers at a large number of weather stations. As an alternative, we propose to use small unmanned aerial systems (sUAS) equipped with an inertial measurement unit (used to derive winds) along with temperature, pressure, and humidity sensors, to determine vertical heat flux as a function of altitude. In Deardorff’s “Laboratory studies of the entrainment zone of a convectively mixed layer” (1980), a sensible heat flux equation is proposed, which requires measurements of the time derivative and vertical spatial derivative of potential temperature. Measurements of the potential temperature as well as measurements of the time derivative and vertical spatial derivative of specific humidity (for latent heat flux) can be determined from the data collected by the sUAS. A series of measurements were conducted at the University of Oklahoma Kessler Atmospheric and Ecological Field Station (KAEFS) with the intention of measuring heat flux from an eddy-covariance system and from sUAS flights. Here we provide an overview of the methods used to calculate heat flux from the two systems, the results from the calculations, and their respective accuracies.