Precision of airborne wind measurement for atmospheric flight research

An aircraft-based platform was developed to complement tower-based flux measurements. The ability to measure vertical eddy fluxes of gases from aircraft platforms represents an important capability to obtain spatially resolved data, but represents a unique challenge, when attempting to calibrate the aircraft-instrument combination. The primary calibration challenges involve the lack of a known “calibration standard” for atmospheric wind and the effects of the aircraft itself upon the local airflow around the flow measurement sensors. The work documented herein consisted of three separate investigations to determine the airborne wind measurement precision of a particular aircraft-instrument combination. Rigorous tests of the particular airflow measurement probe in a wind tunnel were complemented with in-flight calibrations. The effect of airflow distortion around the aircraft’s body on the measured relative airflow was determined through a simple set of in-flight maneuvers and data analysis. A least-squares estimate of the horizontal wind and measurement bias components was developed. These data were compared with an independent model using an internet-based program (called the HYbrid Single Particle Lagrangian Integrated Trajectory model, abbreviated “HYSPLIT”) to compute the wind trajectories in the atmosphere (the path of an air parcel). Estimates of the variability of actual atmospheric wind during an evaluation flight were performed. The precision of the vertical wind measurement for spatial scales larger than approximately 2m is independent of aircraft flight speed over the range of airspeeds studied and the 1σ precision is approximately 0.03 ms -1. It was estimated, from flight test data, and from variations in aircraft attitude and airspeed in flux-measurement configuration, that the uncertainty in vertical wind measurement caused by varying lift-induced upwash is ∼0.05m s-1. The estimated variability of the actual atmospheric wind scaled inversely with airspeed