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  • Author or Editor: Toshio Koike x
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Kun Yang
and
Toshio Koike

Abstract

A scheme was proposed by Zhang et al. to estimate soil water content from soil temperature measurements by using an adaptive Kalman filter. Their scheme is based on the fact that soil heat capacity and thermal conductivity are a monotonic function of soil water content. However, thermal diffusivity, a more critical thermal parameter in such an estimation, is not a monotonic function of soil water content in most cases. This could result in multiple solutions in some cases when deriving soil water content from soil temperatures.

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Kun Yang
,
Nobuyuki Tamai
, and
Toshio Koike

Abstract

Turbulent exchange between the surface and the atmosphere strongly depends on the stability of the surface layer. If surface radiometric temperature, rather than aerodynamic temperature, is used to parameterize the surface turbulent fluxes, the solution of the stability parameter is related to the thermal roughness length z T , which is generally not identical to the aerodynamic roughness length z 0. This note derives the exact solution of the stability parameter equation for a stable surface layer and proposes an approximate analytical solution for an unstable surface layer. The solution can improve the computational efficiency of flux parameterization and is applicable in a wide range of z/z 0 (50–104) and z 0/z T (from less than 1 to greater than 104).

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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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