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 (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.

Turbulent flux observations and modelling over a shallow lake and a wet grassland in the Nam Co basin, Tibetan Plateau

“….36 (Yang et al 2005) Thermal roughness length z 0h =70ν⋅u * −1 ⋅exp (−βu * 0.5 |T * | 0.25 ) β=7.2 s 0.5 m −0.5 K −0.25 (Yang et al 2008) Bare soil evaporation the model output with a sensitivity analysis (Fourier Amplitude Sensitivity Test by Cukier et al 1978). Assuming typical measurement errors for the initial parameter distribution they estimated that water temperature contributed up to 50 % of the overall error while the influence of wind velocity, air temperature and humidity are comparatively small, each contributing 10-20 % to the error.…”

“…To account for these conditions, SEWAB has been adapted for the Tibetan Plateau by (1) a revised calculation of the soil thermal conductivity as used in Yang et al (2005), (2) a different formulation of the thermal roughness length after Yang et al (2008) and (3) by changing the parameterisation of bare soil evaporation according to Mihailović et al (1993). The formulations can be seen in Table 3.…”

On the Relevance of ln(z0/z0T) = kB-1

“…In subsequent work, it has become apparent that nighttime data yield much more scatter than daytime, as must be expected because at night both w T and |T 0 − T z | are small and their ratio is therefore poorly determined. This issue was emphasized by, e.g., Stewart et al (1994), Verhoef et al (1997), andYang et al (2008), whose experiments revealed representative values of kB −1 that differed greatly from the depictions illustrated in Fig. 1.…”

“…In comparison, the original analysis by Owen and Thomson (as mentioned above) yielded a = 0.6 and b = 0.45. Many of the alternative expressions listed by Yang et al (2008) share the assumption that b = 0.5. The black line in Fig.…”

Uncertainty in carbon exchange modelling in a forest canopy due to kB−1 parametrizations

“…Thus, to date, various relationships have been proposed to relate this dissimilarity with other more readily measurable variables such as momentum roughness length, friction velocity, canopy height, and heat flux (e.g. Owen and Thomson, 1963;Garratt and Hicks, 1973;Thom, 1975;Brutsaert, 1982;Verma, 1989;Jensen and Hummelshøj, 1995;Zilitinkevich, 1995;Chen et al, 1997;Verhoef et al, 1997;Massman, 1999;Yang et al, 2008;Chen and Zhang, 2009;Hong and Kim, 2010). Some of these, and others, are subsequently abbreviated as shown in Table I.…”