A simple unified theory for flow in the canopy and roughness sublayer

The mean flow profile within and above a tall canopy is well known to violate the standard boundary-layer flux–gradient relationships. Here we present a theory for the flow profile that is comprised of a canopy model coupled to a modified surface-layer model. The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing layer analogy for the flow at a canopy top. This analogy provides an additional length scale—the vorticity thickness—upon which the flow just above the canopy, within the so-called roughness sublayer, depends. A natural form for the vertical profiles within the roughness sublayer follows that overcomes problems with many earlier forms in the literature. Predictions of the mean flow profiles are shown to match observations over a range of canopy types and stabilities. The unified theory predicts that key parameters, such as the displacement height and roughness length, have a significant dependence on the boundary-layer stability. Assuming one of these parameters a priori leads to the incorrect variation with stability of the others and incorrect predictions of the mean wind speed profile. The roughness sublayer has a greater impact on the mean wind speed in stable than unstable conditions. The presence of a roughness sublayer also allows the surface to exert a greater drag on the boundary layer for an equivalent value of the near-surface wind speed than would otherwise occur. This characteristic would alter predictions of the evolution of the boundary layer and surface states if included within numerical weather prediction models.     

Parametric exponentially correlated surface emission model for L-band passive microwave soil moisture retrieval

Surface soil moisture is an important parameter in hydrology and climate investigations. Current and future satellite missions with L-band passive microwave radiometers can provide valuable information for monitoring the global soil moisture. A factor that can play a significant role in the modeling and inversion of microwave emission from land surfaces is the surface roughness. In this study, an L-band parametric emission model for exponentially correlated surfaces was developed and implemented in a soil moisture retrieval algorithm. The approach was based on the parameterization of an effective roughness parameter of H_p in relation with the geometric roughness variables (root mean square height s and correlation length l) and incidence angle. The parameterization was developed based on a large set of simulations using an analytical approach incorporated in the advanced integral equation model (AIEM) over a wide range of geophysical properties. It was found that the effective roughness parameter decreases as surface roughness increases, but increases as incidence angle increases. In contrast to previous research, H_p was found to be expressed as a function of a defined slope parameter m = s²/l, and coefficients of the function could be well described by a quadratic equation. The parametric model was then tested with L-band satellite data in soil moisture retrieval algorithm over the Little Washita watershed, which resulted in an unbiased root mean square error of about 0.03 m³/m³ and 0.04 m³/m³ for ascending and descending orbits, respectively.     

Field And Wind-Tunnel Studies Of Aerodynamic Roughness Length

The aerodynamic roughness length (z₀) values of three Gobi desert surfaces were obtained by measurement of the boundary-layer wind profile in the field. To clarify the factors affecting the Gobi surface aerodynamic roughness length, a wind-tunnel experiment was conducted. The wind-tunnel simulation shows that z₀ values increase with increasingsize and coverage of roughness elements. Especially, the shape and height of roughnesselements are more important than other factors in affecting roughness length. The roughness length increases with decreasing values of the geometric parameter (the ratio of element horizontal surface area to height, ) of roughness elements. But at a higher free stream velocity, the height is more important than the shape in affecting roughness length.     

黄河源高寒湿地-大气间暖季水热交换特征及关键影响参数研究

湿地是由陆地和水体形成的自然综合体,具有重要的生态、水文和生物地球化学功能,黄河源高寒湿地作为黄河重要的水源涵养区,对其下垫面水热交换特征及关键影响参数的研究具有非常重要的意义。本文利用中国科学院西北生态环境资源研究院麻多黄河源气候与环境变化观测站2014年6~8月观测资料,分析了黄河源区高寒湿地-大气间暖季水热交换特征,并利用公用陆面模式（Community Land Model,简称CLM）模拟了热通量变化,提出针对高寒湿地的粗糙度优化方案。主要结果如下:（1）暖季向上、向下短波与净辐射的平均日变化规律一致,向上、向下长波平均日变化平缓,地表温度升高相对于向下短波具有滞后性,潜热通量始终为正值并大于感热通量;（2）温度变化显著层结为20 cm以上土壤浅层,存在明显的日循环规律,土壤中热量09:00（北京时,下同）下传至5 cm深度,温度升高,11:00至10 cm深度,13:00至20 cm深度,18:00后开始上传,温度降低,40 cm及以下深度受此影响较小,热量在土壤中整体由浅层向深层输送;（3）土壤湿度平均日变化小,5 cm深度为土壤湿度最小层,10 cm深度为最大层;（4）麻多高寒湿地动力学粗糙度Z_0m在暖季变化稳定,可作为常数,Z_0m=0.0143 m;（5）提出更加适合高寒湿地下垫面暖季附加阻尼kB-1参数化方案,使得热通量模拟效果较CLM原始方案有所提高。以上结果对于研究湿地下垫面陆面过程具有重要意义。     

Statistics and spectra of turbulence under different roughness in the near-neutral atmospheric surface layer

Accurate knowledge of the contacts between surface roughness and the resultant wind speed are important for climatic models, wind power meteorology, agriculture and erosion hazards especially on sand saltation in arid and semi-arid environments, where vegetation cover is scarce. In this study, synchronous measurements of three-dimensional wind speed below 5 m are carried out in three different surface roughness conditions in Minqin, China, and the difference in the turbulence statistics and the structure of the very large-scale motions (VLSMs) were revealed. The results show that the slope of the mean wind profile (MVP), the turbulent kinetic energy (TKE) and Reynolds stress increase with the surface roughness. The roughness seems to suppress the ejection events and the surface roughness will not only weaken the energy of the VLSMs, but also reduce the scale values of VLSMs near the wall. These influences may cause some changes regarding the dust transportation in streamwise and vertical directions during the sand and dust storm (SDS). That is, the decrease of the mean velocity near the ground will reduce the dust transportation in the streamwise direction and influence of the roughness on the ejection and sweep events will change the dust transportation in the vertical direction. Furthermore, the increase of roughness will weaken the scale and energy of VLSMs, which will lead to the decrease of the capacity of dust transportation. © 2019 John Wiley & Sons, Ltd.     

Comparison Of Upwind Land Use And Roughness Length Measured In The Urban Boundary Layer

The results of wind measurements taken in two studies at an urban sitein Birmingham (UK) are presented. Displacement heights and roughnesslengths are calculated that compare well with previous measurements inthe urban canopy. Dimensionless windshear and vertical-velocity standarddeviation are also shown to obey Monin–Obukhov similarity theory. Themean roughness lengths measured at the highest measuring position (45 m)increase with the fraction of upwind urban cover calculated using asource-area model.     

Boundary-Layer Stress Instabilities in Neutral,Rotating Turbulent Flows

Boundary-layer instabilities are studied by analyzing the results of laboratory simulations of wall turbulence in a shear-driven rotating flow. The experiments were carried out in the Turin University Laboratory rotating water tank, where a circular flow was generated by either increasing (spin-up) or decreasing (spin-down) the rotation speed of the platform. The flow was measured using a Particle Image Velocimetry technique and the developed turbulence analyzed. Two cases were accounted for, in the former the measurements were performed over a smooth surface (bottom of the tank), while in the latter a rough-to-smooth transition was considered. The turbulent boundary layer developed inside the tank is analyzed by means of vertical profiles of mean and turbulent quantities and on the basis of drag coefficients. Then turbulent structures developed in the different cases are shown and discussed in terms of the vorticity fields. Finally, an analysis based on the concept of swirling strength was carried out to select among the vortex extremes those associated with a coherent structure.     

Parameterization of exchange processes over a barley field

Wind and temperature profiles and surface radiation temperature were measured over barley during two observation periods in Estonia. The calculated displacement height (d) could be taken as 3/4 and 2/3 of the canopy height (h) for a dense and a moderate canopy respectively. The roughness length for wind speed was best approximated with the relation z_0u = 0.3(h - d). The excess resistance to heat exchange kB^-1 was the most doubtful parameter, but it was concluded that kB^-1 = 2 is a good estimate for the time being.     

Notes on an Internal Boundary-Layer Height Formula

The derivation of the Panofsky–Dutton internal boundary-layer(IBL) height formula has been revisited. We propose that the upwindroughness length (rather than downwind) should be used in theformula and that a turbulent vertical velocity (_w) ratherthan the surface friction velocity (u_*) should be considered asthe appropriate scaling for the rate of propagation ofdisturbances into the turbulent flow. A published set ofwind-tunnel and atmospheric data for neutral stratification hasbeen used to investigate the influence of the magnitude ofroughness change on the IBL height.