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

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 (z0m; from <1 to 10 mm) and sensible heat flux (from ?50 to 400 W m?2). For each site, parameter kB?1 [=ln(z0m/z0h), where z0h is the thermal roughness length] exhibits clear diurnal variations with higher values during the day and lower values at night. Mean values of z0h for the individual sites do not change significantly with z0m, resulting in kB?1 increasing with z0m, and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z0m. The term kB?1 often becomes negative at night for relatively smooth surfaces (z0m ? 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.