Turbulent Flux Transfer over Bare-Soil Surfaces: Characteristics and Parameterization.

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_0m; from <1 to 10 mm) and sensible heat flux (from -50 to 400 W m^-2). For each site, parameter kB^-1 [=ln(z_0m/z_0h), where z_0h is the thermal roughness length] exhibits clear diurnal variations with higher values during the day and lower values at night. Mean values of z_0h for the individual sites do not change significantly with z_0m, resulting in kB^-1 increasing with z_0m, and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z_0m. The term kB^-1 often becomes negative at night for relatively smooth surfaces (z_0m ∼ 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. [ABSTRACT FROM AUTHOR]