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Kun Yang, Toshio Koike, Hirohiko Ishikawa, Joon Kim, Xin Li, Huizhi Liu, Shaomin Liu, Yaoming Ma, and Jieming Wang

Abstract

Parameterization of turbulent flux from bare-soil and undercanopy surfaces is imperative for modeling land–atmosphere interactions in arid and semiarid regions, where flux from the ground is dominant or comparable to canopy-sourced flux. This paper presents the major characteristics of turbulent flux transfers over seven bare-soil surfaces. These sites are located in arid, semiarid, and semihumid regions in Asia and represent a variety of conditions for aerodynamic roughness length (z 0 m; from <1 to 10 mm) and sensible heat flux (from −50 to 400 W m−2). For each site, parameter kB −1 [=ln(z 0 m/z 0 h), where z 0 h is the thermal roughness length] exhibits clear diurnal variations with higher values during the day and lower values at night. Mean values of z 0 h for the individual sites do not change significantly with z 0 m, resulting in kB −1 increasing with z 0 m, and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z 0 m. The term kB −1 often becomes negative at night for relatively smooth surfaces (z 0 m ∼ 1 mm), indicating that the widely accepted excess resistance for heat transfer can be negative, which cannot be explained by current theories for aerodynamically rough surfaces. Last, several kB −1 schemes are evaluated using the same datasets. The results indicate that a scheme that can reproduce the diurnal variation of kB −1 generally performs better than schemes that cannot.

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