The Contribution of Variability of Lift-induced Upwash to the Uncertainty in Vertical Winds Determined from an Aircraft Platform |
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| Authors: | Karl E Garman Phil Wyss Mark Carlsen James R Zimmerman Brian H Stirm Thomas Q Carney Robert Santini Paul B Shepson |
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| Institution: | (1) Department of Earth and Atmospheric Science, Purdue University, West Lafayette, IN, USA;(2) Department of Chemistry, Purdue University, West Lafayette, IN, USA;(3) Jonathan Amy Facility for Chemical Instrumentation, Purdue University, West Lafayette, IN, USA;(4) Department of Aviation Technology, Purdue University, West Lafayette, IN, USA;(5) Purdue Climate Change Research Center, Purdue University, West Lafayette, IN, USA |
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| Abstract: | Aircraft-based vertical flux measurements fill a gap in the spatial domain for studies of biosphere–atmosphere exchange. To
acquire valid flux data, a determination of the deviation from the mean vertical wind, w′, is essential. When using aircraft platforms, flux measurements are subject to systematic and random errors from airflow
distortion caused by the lift-induced upwash ahead of the aircraft. Although upwash is typically considered to be a constant
quantity over periods used for calculating fluxes, it can vary significantly over short (and longer) periods due to changes
in aircraft lift. The characterization of such variations in upwash are of undeniable importance to flux measurements, especially
when real-time computations of w′ are required. In this paper, the variability in upwash was compared to the calculated upwash from the model of Crawford
et al. (Boundary-Layer Meteorol, 80:79–94, 1996) using data taken during a long-period (phugoid mode) free oscillation of
the aircraft. The cyclic variation of lift during the free oscillation offers an ideal scenario in which to acquire in-flight
data on the upwash that is present, as well as to test the capability of upwash correction models. Our results indicate that
while this model corrects for much of the mean upwash, there can be significant variations in upwash on a time scale that
is important to flux measurements. Our results suggest that use of the measured load factor could be an easily implemented
operational constraint to minimize uncertainty in w′ due to changing upwash from changing aircraft lift. We estimate, using the phugoid data, and from variations in aircraft
attitude and airspeed in flux-measurement configuration, that the uncertainty in w caused by variable upwash is approximately ± 0.05 m s−1. |
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| Keywords: | Aircraft measurements Airflow Flow distortion Lift-induced upwash Phugoid Upwash Vertical flux |
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