Turbulence Spectra and Cospectra Under the Influence of Large Eddies in the Energy Balance EXperiment (EBEX) |
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Authors: | Yu Zhang Heping Liu Thomas Foken Quinton L Williams Shuhua Liu Matthias Mauder Claudia Liebethal |
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Institution: | (1) Department of Physics, Atmospheric Sciences and Geoscience, Jackson State University, Jackson, MS 39217, USA;(2) Department of Micrometeorology, University of Bayreuth, Bayreuth, Germany;(3) Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Garmisch-Partenkirchen, Germany;(4) College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA; |
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Abstract: | During the Energy Balance EXperiment, the patch-by-patch, flood irrigation in a flat cotton field created an underlying surface
with heterogeneous soil moisture, leading to a dry (warm)-to-wet (cool) transition within the cotton field under northerly
winds. Moreover, the existence of an extremely dry, large bare soil area upstream beyond the cotton field created an even
larger step transition from the bare soil region to the cotton field. We investigated the turbulence spectra and cospectra
in the atmospheric surface layer (ASL) that was disturbed by large eddies generated over regions upstream and also influenced
by horizontal advection. In the morning, the ASL was unstable while in the afternoon a stable internal boundary layer was
observed at the site. Therefore, the turbulence data at 2.7 and 8.7 m are interpreted and compared in terms of interactions
between large eddies and locally generated turbulence under two atmospheric conditions: the unstable ASL beneath the convective
boundary layer (CBL) (hereafter the unstable condition) and the stable ASL beneath the CBL (hereafter the stable condition).
We identified the influences of multiple sizes of large eddies on ASL turbulence under both stratifications; these large eddies
with multiple sizes were produced over the dry patches and dry, large bare soil areas upstream. As a consequence of the disturbance
of large eddies, the broadening, erratic variability, and deviation of spectra and cospectra, relative to those described
by Monin–Obukhov similarity theory, are evident in the low- to mid-frequencies. Transfer of momentum, heat, and water vapour
by large eddies is distinctly observed from the turbulence cospectra and leads to significant run-to-run variations of residuals
of the surface energy balance closure. Our results indicate that these large eddies have greater influences on turbulence
at higher levels compared to lower levels, and in the unstable ASL compared to the stable ASL. |
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