The Characteristics and Parameterization of Aerodynamic Roughness Length over Heterogeneous Surfaces

Aerodynamic roughness length （Z0m） is a key factor in surface flux estimations with remote sensing algorithms and/or land surface models. This paper calculates ZOrn over several land surfaces, with 3 years of experimental data from Xiaotangshan. The results show that Z0m is direction-dependent, mainly due to the heterogeneity of the size and spatial distribution of the roughness elements inside the source area along different wind directions. Furthermore, a heuristic parameterization of the aerodynamic roughness length for heterogeneous surfaces is proposed. Individual Z0m over each surface component （patch） is calculated firstly with the characteristic parameters of the roughness elements （vegetation height, leaf area index, etc.）, then Z0m over the whole experimental field is aggregated, using the footprint weighting method.

The Influences of Thermodynamic Characteristics on Aerodynamic Roughness Length over Land Surface

It has previously been shown that aerodynamic roughness length changes significantly along with near- surface atmospheric thermodynamic state; however, at present, this phenomenon remains poorly understood, and very little research concerning this topic has been conducted. In this paper, by using the data of different underlying surfaces provided by the Experimental Co-observation and Integral Research in Semi-arid and Arid Regions over North China, aerodynamic roughness length （z0） values in stable, neutral, and unstable atmospheric stratifications are compared with one another, and the relationship between z0 and atmospheric thermodynamic stability （（） is analyzed. It is found that z0 shows great differences among the stable, neutral, and unstable atmospheric thermodynamic states, with the difference in z0 values between the fully thermodynamic stable condition and the neutral condition reaching 60% of the mean z0. F^trthermore, for the wind speed range in which the wind data are less sensitive to z0, the surface z0 changes more significantly with （, and is highly correlated with both the Monin-Obukhov stability （4o） and the overall Richardson number （Rib）, with both of their correlation coefficients greater than 0.71 and 0.47 in the stable and unstable atmospheric stratification, respectively. The empirical relation fitted with the experimental observations is quite consistent with the Zilitinkevich theoretical relation in the stable atmosphere, but the two are quite distinct and even show opposite variation tendencies in the unstable atmosphere. In application, however, verification of the empirical fitted relations by using the experimental data finds that the fitted relation is slightly more applicable than the Zilitinkevich theoretical relation in stable atmospheric stratification, but it is much more suitable than the Zilitinkevich relation in unstable atmospheric stratification.

Development and test of a multifactorial parameterization scheme of land surface aerodynamic roughness length for flat land surfaces with short vegetation

Aerodynamic roughness length is an important physical parameter in atmospheric numerical models and microme- teorological calculations, the accuracy of which can affect numerical model performance and the level of micrometeorological computations. Many factors influence the aerodynamic roughness length, but formulas for its parameterization often only con- sider the action of a single factor. This limits their adaptive capacity and often introduces considerable errors in the estimation of land surface momentum flux （friction velocity）. In this study, based on research into the parameterization relations between aerodynamic roughness length and influencing factors such as windrow conditions, thermodynamic characteristics of the sur- face layer, natural rhythm of vegetation growth, ecological effects of interannual fluctuations of precipitation, and vegetation type, an aerodynamic roughness length parameterization scheme was established. This considers almost all the factors that af- fect aerodynamic roughness length on flat land surfaces with short vegetation. Furthermore, using many years＇ data recorded at the Semi-Arid Climate and Environment Observatory of Lanzhou University, a comparative analysis of the application of the proposed parameterization scheme and other experimental schemes was performed. It was found that the error in the friction velocity estimated by the proposed parameterization scheme was considerably less than that estimated using a constant aero- dynamic roughness length and by the other parameterization schemes. Compared with the friction velocity estimated using a constant aerodynamic roughness length, the correlation coefficient with the observed friction velocity increased from 0.752 to 0.937, and the standard deviation and deviation decreased by about 20% and 80%, respectively. Its mean value differed from the observed value by only 0.004 m s-l and the relative error was only about 1.6%, which indicates a significant decrease in the estimation error of surface-layer momentum flux. The test results show that the multifactorial universal parameterization scheme of aerodynamic roughness length for flat land surfaces with short vegetation can offer a more scientific parameteriza- tion scheme for numerical atmospheric models.

On the surface fluxes characteristics and roughness lengths at Zhongshan station,Antarctica

Over Antarctica,surface fluxes play an important role in the local atmospheric dynamical processes.To reveal the surface fluxes characteristics and aerodynamic and thermal roughness lengths over Zhongshan station,Antarctica,this paper analyzes the data observed at the station during 3 March 2008 through 15 February 2009.It is found that easterlies dominated this site throughout the whole year,with a maximum(average)speed of 25(5.6)m s−1 at 3.9 m height,and the annual maximum(minimum)surface temperature reached 291.05(230.05)K,while the annual maximum(minimum)air-specific humidity was 4.1(0.05)g/kg at 3.9 m height.The maximum(minimum)values of seasonal mean temperature,humidity,each radiation components,sensible and latent heat flux occurred in summer(winter),while for the seasonal averaged wind speed andτthe minimums(maximums)appeared in summer(autumn).After comparing with a partially linear regression method for aerodynamic roughness length and four previous equations that derive thermal roughness length from surface Reynolds number,constant values of aerodynamic roughness length as 3.6×10^(−3)m and thermal roughness length as 1.2×10^(−4)m at this site were validated by using the other three level observations and suggested for future studies.