Flow distortion errors caused by rigid obstacles in dry deposition measurement systems

Abstract

Flow distortion errors on wind and friction velocity induced by a box simulating the housing of a gas analyzer used in dry deposition eddy correlation measurements were determined in a field experiment. ‘Undisturbed’ and ‘disturbed’ wind and friction velocities, measured with two dry deposition monitoring systems run simultaneously, were compared, one to the other. In the ‘disturbed’ case the box was mounted below the 3-component probe of the sonic anemometer of one of these systems, while in the ‘undisturbed’ case the box was removed. When the probe was located on the upstream side of the box, the results showed satisfactory agreement with theoretical estimates using Wyngaard’s potential-flow approach and a spherical model for the box. This model can be applied to obtain first-order corrections for flow distortion errors induced by cubic-like (or spherical) obstacles such as a gas analyzer housing used in dry deposition research systems, or to determine the optimal location of this housing relative to the sonic probe in such systems. When the probe was located halfway downstream and halfway to the side of the box, the experimental flow distortion errors did not exceed those for the upstream case. This implies that to keep flow distortion errors in dry deposition systems as small as possible the sonic probe can be placed upstream but also to the side of the gas analyzer housing. The results of our experiments also confirmed that correcting for flow distortion with the commonly used tilt equations yields underestimated values.