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 Ͻ1 to 10 mm) and sensible heat flux (from Ϫ50 to 400 W m Ϫ2 ). For each site, parameter kB Ϫ1 [ϭln(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 Ϫ1 increasing with z 0m , and thus the momentum transfer coefficient increases faster than the heat transfer coefficient with z 0m . The term kB Ϫ1 often becomes negative at night for relatively smooth surfaces (z 0m ϳ 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 Ϫ1 schemes are evaluated using the same datasets. The results indicate that a scheme that can reproduce the diurnal variation of kB Ϫ1 generally performs better than schemes that cannot.

Snow and frozen soil are important factors that influence terrestrial water and energy balances through snowpack accumulation and melt and soil freeze-thaw. In this study, a new land surface model (LSM) with coupled snow and frozen soil physics was developed based on a hydrologically improved LSM (HydroSiB2). First, an energy-balance-based three-layer snow model was incorporated into HydroSiB2 (hereafter HydroSiB2-S) to provide an improved description of the internal processes of the snow pack. Second, a universal and simplified soil model was coupled with HydroSiB2-S to depict soil water freezing and thawing (hereafter HydroSiB2-SF). In order to avoid the instability caused by the uncertainty in estimating water phase changes, enthalpy was adopted as a prognostic variable instead of snow/soil temperature in the energy balance equation of the snow/frozen soil module. The newly developed models were then carefully evaluated at two typical sites of the Tibetan Plateau (TP) (one snow covered and the other snow free, both with underlying frozen soil). At the snow-covered site in northeastern TP (DY), HydroSiB2-SF demonstrated significant improvements over HydroSiB2-F (same as HydroSiB2-SF but using the original single-layer snow module of HydroSiB2), showing the importance of snow internal processes in three-layer snow parameterization. At the snow-free site in southwestern TP (Ngari), HydroSiB2-SF reasonably simulated soil water phase changes while HydroSiB2-S did not, indicating the crucial role of frozen soil parameterization in depicting the soil thermal and water dynamics. Finally, HydroSiB2-SF proved to be capable of simulating upward moisture fluxes toward the freezing front from the underlying soil layers in winter.