Spectral similarity between scalars at very low frequencies in the unstable atmospheric surface layer over the Tibetan plateau |
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Authors: | Jun Asanuma Ichiro Tamagawa Hirohiko Ishikawa Yaoming Ma Taiichi Hayashi Yongqiang Qi Jiemin Wang |
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Institution: | (1) Department of Environmental Sciences, Peking University, Beijing, 100871, China;(2) Lab for Environmental Physics, The University of Georgia, Griffin, GA 30223, USA |
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Abstract: | Similarity between temperature and water vapour was investigated in the Fourier domain by using their coherency spectra, defined
as the correlation coefficient between their Fourier modes, and the relative efficiency of their vertical transport. The class-averaged
values of these indices were computed from the turbulence measurements over sparse grasslands on the Tibetan plateau during
the intensive observations of GEWEX (Global Energy and Water Experiment) Asian Monsoon Experiment (GAME). It was found that
the energy-containing eddies at scales up to 100z (z being height) are characterised by well-maintained similarity between these scalars. The scalars are highly correlated with
each other, and their transport efficiencies are almost equal within this scale range. In contrast, similarity was not always
maintained at scales larger than 1000z. Detailed analyses showed that this breakdown of similarity occurs occasionally or sporadically, suggesting that it is caused
by events whose average return period is not much smaller than the current averaging time, i.e. 30 min. We speculate that
entrainment of drier and warmer air at the top of the atmospheric boundary layer caused the scalar dissimilarity at this scale
range. |
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Keywords: | Energy-containing range Entrainment Evaporation Land-atmosphere interaction Sensible heat flux |
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