Advances in the Measurements of the Structure Function Parameter for Temperature Using a Small Unmanned Aircraft System

Atmospheric turbulence can generate significant spatio-temporal fluctuations in the state parameters that describe the thermodynamic and kinematic structure of the planetary boundary layer. The spatial variability of these state variables can be characterized through the calculation of the structure function, which is related to the spatial variance of the atmospheric parameter of interest. Moreover, when the turbulence associated with the atmosphere can be assumed to be isotropic and locally homogeneous and to occur at spatial length scales that lie within the inertial subrange, then a scaled version of the structure function known as the structure function parameter can be calculated. In this study, we consider the structure function parameter for temperature C_T² corresponding to a variety of atmospheric conditions. In particular, we examine and compare three different methods for evaluating C_T² using atmospheric data from a small unmanned aircraft system (sUAS), 3-D sonic anemometer, and sodar. Data reported in this presentation were collected at the University of Oklahoma Kessler Atmospheric and Ecological Field Station at various times of the year and on the island of Hailuoto, Finland in Februay 2018 as part of the ISOBAR (Innovative Strategies for Observations in the Arctic Atmospheric Boundary Layer) field campaign. When calculating C_T² with sUAS, values are obtained by sampling the atmospheric temperature field across a range of spatial separations at a fixed height and then directly calculating the structure function from these observations. By repeating the method with altitude, values of C_T²(z) are found as a function of height. The observations are validated by comparing the C_T² values from the sUAS with those from a tower-mounted 3-D sonic anemometer. Finally, for the data set collected during ISOBAR, C_T²(z) from the sUAS were compared with values of C_T²(z) extracted from data using a Finnish Meteorological Institute sodar. The sodar method makes use of the sodar equation, which relates echo power from the atmosphere to the ratio C_T²/T². The sodar return power was used to derive a proxy for C_T²(z). The C_T² values derived from each method are compared to evaluate performance of the different techniques under various conditions.