An Airborne and Wind Tunnel Evaluation of a Wind Turbulence Measurement System for Aircraft-Based Flux Measurements*

Abstract

Although the ability to measure vertical eddy fluxes of gases from aircraft platforms represents an important capability to obtain spatially resolved data, accurate and reliable determination of the turbulent vertical velocity presents a great challenge. A nine-hole hemispherical probe known as the “Best Air Turbulence Probe” (often abbreviated as the “BAT Probe”) is frequently used in aircraft-based flux studies to sense the airflow angles and velocity relative to the aircraft. Instruments such as inertial navigation and global positioning systems allow the measured airflow to be converted into the three-dimensional wind velocity relative to the earth’s surface by taking into account the aircraft’s velocity and orientation. Calibration of the aircraft system has previously been performed primarily through in-flight experiments, where calibration coefficients were determined by performing various flight maneuvers. However, a rigorous test of the BAT Probe in a wind tunnel has not been previously undertaken. The authors summarize the results of a complement of low-speed wind tunnel tests and in-flight calibrations for the aircraft–BAT Probe combination. Two key factors are addressed in this paper: The first is the correction of systematic error arising from airflow measurements with a noncalibrated BAT Probe. The second is the instrumental precision in measuring the vertical component of wind from the integrated aircraft-based wind measurement system. The wind tunnel calibration allows one to ascertain the extent to which the BAT Probe airflow measurements depart from a commonly used theoretical potential flow model and to correct for systematic errors that would be present if only the potential flow model were used. The precision in the determined vertical winds was estimated by propagating the precision of the BAT Probe data (determined from the wind tunnel study) and the inertial measurement precision (determined from in-flight tests). The precision of the vertical wind measurement for spatial scales larger than approximately 2 m is independent of aircraft flight speed over the range of airspeeds studied, and the 1σ precision is approximately 0.03 m s−1.

Observations of surface momentum exchange over the marginal ice zone and recommendations for its parametrisation

“…To estimate surface momentum flux from the aircraft requires high-frequency measurements of wind velocity and altitude, along with an estimate of atmospheric stability. To measure 3-D winds the MASIN Twin Otter uses a nine-port Best Aircraft Turbulence (BAT) probe (Garman et al, 2006) mounted on the end of a boom above the cockpit and extending forward of the aircraft's nose; while the BAE146 uses a five-port radome probe on the nose of the aircraft. To measure altitude at low levels both aircraft use radar altimeters.…”

Constraining the CO<sub>2</sub> budget of the corn belt: exploring uncertainties from the assumptions in a mesoscale inverse system

“…For the quantification of vertical transport errors, we used aircraft observations, mainly vertical profiles of CO 2 concentrations, that were measured using a twin-engine Beechcraft Duchess (Garman et al, 2006) during summer 2007 over Iowa (Martins et al, 2009). The vertical profiles ranged from the surface to the lower free troposphere (∼3 km a.g.l.)…”

Eddy-covariance flux measurements with a weight-shift microlight aircraft

“…For this purpose, we follow Garman et al (2006) and define measurement precision as 1 σ repeatability. The precision of all variables entering the EC flux calculation is presented in Table 1.…”

“…The software package TK3 (Mauder and Foken, 2011) was used to process the tower EC data, applying the raw data treatments and flux corrections as put forward in Foken et al (2012). (i) The raw data were screened for spikes using the algorithm of Hojstrup (1993).…”