Surface Aerodynamic Roughness Length Research Articles

Investigating the Aerodynamic Surface Roughness Length over Baghdad City Utilizing Remote Sensing and GIS Techniques

This study calculated the surface roughness length (Zo), zero-displacement length (Zd) and height of the roughness elements (ZH) using GIS applications. The practical benefit of this study is to classify the development of Baghdad, choose the appropriate places for installing wind turbines, improve urban planning, find rates of turbulence, pollution and others. The surface roughness length (Zo) of Baghdad city was estimated based on the data of the wind speed obtained from an automatic weather station installed at Al-Mustansiriyah University, the data of the satellite images digital elevation model (DEM), and the digital surface model (DSM), utilizing Remote Sensing Techniques. The study area was divided into 15 municipalities (Rasheed, Mansour, Shulaa, Karrada, Shaab, Adhamiyah, Sadre 2, Sadre 1, Rusafa, Alghadeer, Baghdad Aljadeedah, Karkh, Kadhumiya, Green zone, and Dora). The results indicated that the variations in Zo depend strongly on zero-displacement length (Zd) and the roughness element height (ZH) and wind speed. The research results demonstrated that Baghdad Aljadeedah has the largest (Zo) with 0.43 m and Rasheed has the lowest value of (Zo) with 0.19 m.; the average (Zo) of Baghdad city was 0.32 m.

Evaluation of Methods for Aerodynamic Roughness Length Retrieval from Very High-Resolution Imaging LIDAR Observations over the Heihe Basin in China

The parameterization of heat transfer based on remote sensing data, and the Surface Energy Balance System (SEBS) scheme to retrieve turbulent heat fluxes, already proved to be very appropriate for estimating evapotranspiration (ET) over homogeneous land surfaces. However, the use of such a method over heterogeneous landscapes (e.g., semi-arid regions or agricultural land) becomes more difficult, since the principle of similarity theory is compromised by the presence of different heat sources at various heights. This study aims to propose and evaluate some models based on vegetation geometry partly developed by Colin and Faivre, to retrieve the surface aerodynamic roughness length for momentum transfer ( z 0 m ), which is a key parameter in the characterization of heat transfer. A new approach proposed by the authors consisted in the use of a Digital Surface Model (DSM) as boundary condition for experiments with a Computational Fluid Dynamics (CFD) model to reproduce 3D wind fields, and to invert them to retrieve a spatialized roughness parameter. Colin and Faivre also applied the geometrical Raupach’s approach for the same purpose. These two methods were evaluated against two empirical ones, widely used in Surface Energy Balance Index (SEBI) based algorithms (Moran; Brutsaert), and also against an alternate geometrical model proposed by Menenti and Ritchie. The investigation was carried out in the Yingke oasis (China) using very-high resolution remote sensing data (VNIR, TIR & LIDAR), for a precise description of the land surface, and a fine evaluation of estimated heat fluxes based on in-situ measurements. A set of five numerical experiments was carried out to evaluate each roughness model. It appears that methods used in experiments 2 (based on Brutsaert) and 4 (based on Colin and Faivre) are the most accurate to estimate the aerodynamic roughness length, according to the estimated heat fluxes. However, the formulation used in experiment 2 allows to minimize errors in both latent and sensible heat flux, and to preserve a good partitioning. An additional evaluation of these two methods based on another k B − 1 parameterization could be necessary, given that the latter is not always compatible with the CFD-based retrieval method.

Simulating aerosols over Arabian Peninsula with CHIMERE: Sensitivity to soil, surface parameters and anthropogenic emission inventories

A three dimensional chemistry transport model, CHIMERE, was used to simulate the aerosol optical depths (AOD) over the Arabian Peninsula desert with an offline coupling of Weather Research and Forecasting (WRF) model. The simulations were undertaken with: (i) different horizontal and vertical configurations, (ii) new datasets derived for soil/surface properties, and (iii) EDGAR-HTAP anthropogenic emissions inventories. The model performance evaluations were assessed: (i) qualitatively using MODIS (Moderate-Resolution Imaging Spectroradiometer) deep blue (DB) AOD data for the two local dust events of August 6th and 23rd (2013), and (ii) quantitatively using AERONET (Aerosol Robotic Network) AOD observations, CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) aerosol extinction profiles, and AOD simulations from various forecast models. The model results were observed to be highly sensitive to erodibility and aerodynamic surface roughness length. The use of new datasets on soil erodibility, derived from the MODIS reflectance, and aerodynamic surface roughness length (z0), derived from the ERA-Interim datasets, significantly improved the simulation results. Simulations with the global EDGAR-HTAP anthropogenic emission inventories brought the simulated AOD values closer to the observations. Performance testing of the adapted model for the Arabian Peninsula domain with improved datasets showed good agreement between AERONET AOD measurements and CHIMERE simulations, where the correlation coefficient (R) is 0.6. Higher values of the correlation coefficients and slopes were observed for the dusty periods compared to the non-dusty periods.

Modeling vegetation and wind erosion from a millet field and from a rangeland: Two Sahelian case studies

Quantifying wind erosion and dust emissions in the semi-arid Sahel remains challenging because of the large seasonal and interannual dynamics of surface properties. The increasing conversion of rangelands into croplands raises issues for quantifying wind erosion over these two contrasted surfaces. Whereas wind erosion models have been so far applied to these two surface types separately, this study proposes a common modeling approach to represent the horizontal flux from Sahelian rangelands and croplands. Pair simulations of both typical Sahelian land surface types investigate the horizontal flux due to wind erosion over a 3-year period for two instrumented sites in Mali and Niger. Two different vegetation models simulate the specific phenology and growth of a rangeland grass and a millet crop. These models also account for the local cropping and pastoral practices. Compared to field measurements, the vegetation cover is satisfyingly simulated by the models, especially the strong seasonal dynamics. Specific parameterizations of the aerodynamic surface roughness length (Z(0)) as a function of vegetation variables are established using measurements from the two sites. The simulated horizontal flux turns out to be higher for a cropland than for a rangeland by approximately a factor 1.5, implying that increasing Sahelian cropped areas would increase dust emissions from the Sahel. This difference is mainly due to the time shift between grass and millet growth: the latter starts growing about 2-3 weeks later than annual grass. The amount of dry vegetation remaining during late dry season is also important for Sahelian wind erosion.

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 micrometeorological 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 consider 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 windflow conditions, thermodynamic characteristics of the surface 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 affect 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 aerodynamic 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−1 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 parameterization scheme for numerical atmospheric models.

A summertime near-ground velocity profile of the Bora wind

While effects of the atmospheric boundary layer flow on engineering infrastructure are more or less known, some local transient winds create difficulties for structures, traffic and human activities. Hence, further research is required to fully elucidate flow characteristics of some of those very unique local winds. In this study, important characteristics of observed vertical velocity profiles along the main wind direction for the Bora wind blowing along the eastern Adriatic coast are presented. Commonly used empirical power-law and the logarithmic-law profiles are compared against unique 3-level high- frequency Bora measurements. The experimental data agree well with the power-law and logarithmic-law approximations. An interesting feature observed is a decrease in the power-law exponent and aerodynamic surface roughness length, and an increase in friction velocity with increasing Bora wind velocity. This indicates an urban-like velocity profile for smaller wind velocities and rural-like velocity profile for larger wind velocities, which is due to a stronger increase in absolute velocity at each of the heights observed as compared to the respective velocity gradient (difference in average velocity among two different heights). The trends observed are similar both during the day and night. The thermal stratification is near neutral due to a strong mechanical mixing. The differences in aerodynamic surface roughness length are negligible for different time averaging periods when using the median.For friction velocity the arithmetic mean proved to be independent of the time record length, while for the power-law exponent both the arithmetic mean and the median are not influenced by the time averaging period. Another issue is a large difference in aerodynamic surface roughness length when calculating using the arithmetic mean and the median. This indicates that the more robust median is a more suitable parameter to determine the aerodynamic surface roughness length than the arithmetic mean value. Variations in velocity profiles at the same site during different wind periods are interesting because, in the engineering community, it has been commonly accepted that the aerodynamic characteristics at a particular site remain the same during various wind regimes.

Temporal changes in small scale snowpack surface roughness length for sublimation estimates in hydrological modelling

Snowpack aerodynamic surface roughness length (zo) is a critical variable in estimating heat transfers to and from a snow surface and thus sublimation rates. This variable has been shown to be site specific. To illustrate a temporal variation in zo, laboratory experiments were performed using a small evaporation pan sitting on a load cell with a constant wind flow over the snow surface. Comparing multi-layer meteorological data above the pan to sublimation measured from mass change showed a decrease in the snowpack surface roughness length as the snow metamorphosed. The sensitivity of snowpack zo changes over time in modeling of sublimation was examined using hourly meteorological data for the winter of 2000-2001 at Syracuse, New York and Leadville Colorado for several scenarios, including increasing or decreasing zo after a snowfall event, considering directionality of zo as a function of the wind direction, and a ratio of latent heat to momentum roughness lengths. The base case used a constant zo of 0.01 metres. The modeled differences were a function of the values of zo, which varied with the frequency of occurrence of fresh snow and the distribution of wind from various directions. The temporal and spatial variability in surface roughness is crucial in computing the energy and mass balance of a snowpack.

Interaction of aerodynamic roughness length and windflow conditions and its parameterization over vegetation surface

Surface aerodynamic roughness length is usually taken as a constant. In fact, it displays a remarkable dynamic change over underlying vegetation surfaces, because of the coupling of land surface roughness elements and windflow conditions. Current international research on this dynamic change and associated mechanisms is very limited. Using observations from different underlying surfaces (including forest, farmland and grassland) provided by a northern China coordinated observation test, the variation of aerodynamic roughness length, along with wind speed and friction velocity, is analyzed. We introduce two relationship fits, between aerodynamic roughness length and wind speed u, and dynamic variable u 2/u*. Results show that aerodynamic roughness length has a clear dynamic change, and has complicated interactions with near-surface windflow. Further, the relationship fits between aerodynamic roughness length, u and u 2/u*, are not only related to the roughness properties of the underlying vegetation surface (e.g. plant height), but also to plant dynamic response characteristics (e.g. flexibility). Aerodynamic roughness length decreases with increasing wind speed, because near-surface windflow conditions can change both plant roughness properties and airflow. However, the change of aerodynamic roughness length with friction velocity is complicated, and its sensitivities and transition points significantly depend on vegetation type. For underlying surfaces of forest and corn, with relatively substantial vegetative cover, roughness length correlates well with wind speed. For a surface with short vegetative cover, like natural lawn, the correlation is low. However, for all of the three vegetative surfaces, there is a close relation between roughness length and u 2/u*, and their coefficients of fit from testing essentially represent the plant height and flexibility of different vegetation types. The test results also indicate that the parameterized relationships of roughness length over the underlying vegetation surface hold prospects for application.

A comparison between latent heat fluxes over grass using a weighing lysimeter and surface renewal analysis

An experiment to study the performance of the hourly sensible ( H ) and latent heat ( LE ) estimates over grass using surface renewal, SR , analysis was carried out at the University of California West Side Field Station near Five Points (San Joaquin Valley, CA). Hourly eddy covariance, EC , and SR sensible heat fluxes were averaged from their respective half-hourly fluxes. They were used to estimate hourly latent heat flux, LE , by calculating the residual of the energy balance equation, i.e., LE EC = R n − G − H EC and LE SR = R n − G − H SR , where R n is the net radiation; G is the soil heat flux; H is the buoyant sensible heat flux and the H subscripts denote measurements values from the EC and SR methods. Regardless of the weather conditions and time of day, H SR was comparable to H EC ; especially from sunrise until the afternoon formation of a capping inversion due to regional advection of sensible heat flux. The main source of energy contributing to LE was ( R n − G ). When ( R n − G ) was positive, on average, H contributed to 9% of ( R n − G ) when data were collected under unstable conditions, but it accounted for 21% under stable conditions . In general, correlation between measured LE using a weighing lysimeter, LE Lys , and LE SR was R 2 = 0.97 which was substantially higher than between LE Lys and ( R n − G ), R 2 = 0.93. In principle, the observed performance can be extrapolated to all irrigated, short, and dense-canopy vegetation that is mostly decoupled from the environment. Because for these surfaces the zero-plane displacement and the aerodynamic surface roughness length can be estimated as a portion (about 0.7 and 0.12, respectively) of the canopy height, using a two dimensional sonic anemometer capable to record high-frequency sonic temperature provides all the input to determine H SR which in conjunction with ( R n − G ) is required to estimate LE .

An improvement on the dust emission scheme in the global aerosol-climate model ECHAM5-HAM

Abstract. Formulation of the dust emission scheme in the global aerosol-climate modeling system ECHAM5-HAM has been improved. Modifications on the surface aerodynamic roughness length, soil moisture and East-Asian soil properties are included in the parameterization, which result in a large impact on the threshold wind friction velocity for aeolian erosion and thus influence the simulated dust emission amount. The annual global mean of dust emission in the year 2000 is reduced by 76.5% and 2.2%, respectively, due to changes in the aerodynamic roughness length and the soil moisture. An inclusion of detailed East-Asian soil properties leads to an increase of 16.6% in the annual global mean of dust emission, which exhibits mainly in the arid and semi-arid areas of northern China and southern Mongolia. Measurements of the surface dust concentrations are collected in remote marine sites globally and in dust source regions of East Asia. The averaged relative differences between model results and measurements are reduced from 17% to 12% in global remote marine sites and from 69% to 30% in East Asia, by including the improvements. Comparisons between model results and available measurements verify a more realistic dust distribution with the improved emission scheme.

On the aeolian saltation bed shear stress and saltation roughness length

The aerodynamic surface roughness length, z 0 , is an input parameter in models to calculate the bed shear stress from the geostrophic wind. The bed shear stress is a fundamental variable to link flow conditions to sediment transport and dust emission. This study examines by numerical integration of the equations of motion for grains how the cloud of grains in transport modifies the wind profile. Transport parameters as the ejection angle, the lift-off velocity of the grains and the transport rate are systematically varied and the resulting air borne bed shear stresses profile τ a (z) and effective roughness length z 0s during saltation are evaluated. The results for z 0s and τ a (z) are compared with the analytical model of Raupach. Our grain-scale analysis also provides valuable information about the splash function which is an important input for numerical models. Using a constant sand transport rate estimated from wind tunnel data yields different τ a (z) and z 0s dependent on the mean saltation height and mean ejection angle. The higher the mean saltation height of the grains, the higher is To and the lower is z 0s . Assuming proportionality of the vertical grain velocity to the shear stress above the saltation layer, as often done in numerical modelling, is shown to be inconsistent.

Comparison among sea surface roughness schemes

Based on the measurements from the US National Data Buoy Center 3-m discus buoy site No. 44004 (38.5°N, 70.47°W) from January 1 to March 31 of 2003, with the COARE algorithm (Version 3.0), the results from four parameterization schemes developed recently for sea surface aerodynamic roughness length were compared with each other. Calculations of frictional speedu*, drag coefficientC d and wind stress τ indicate that the calculated frictional velocities from the four schemes (8.50%–16.20%, the normalized standard error estimate, or NSEE), the computed drag coefficients and wind stress (respectively 15.08%–28.67% and 17.26%–50.59% NSEE) are reasonable. Schemes YT96 and GW03 are consistent. The O02 scheme gives over-estimated values foru* andC d. Schemes TY01 and GW03 display discontinuous characteristics in handling young wave data.

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, λ) of roughness elements. But at a higher free stream velocity, the height is more important than the shape in affecting roughness length.

Measurements Of Turbulence Transfer In The Near-Surface Layer Over The Southeastern Tibetan Plateau

Turbulent flux measurements at Qamdo site over the Tibetan Plateau during TIPEX from May 18 to June 30, 1998 are presented. Sensible heat dominated,accounting for about 66% of the available energy (the sum of net radiation and soil heat flux) prior to the monsoon(dry period), reducing to about 31%, with latent heat increased to about 56% of available energy,in the monsoon season (wet period). Surface energy budget closure on average was about 0.80 (0.85)prior to the monsoon and 0.89 (0.76) during the monsoon using eddy correlation (profile) methods. The sum of latent and sensible heat fluxes calculated from the flux-profilemethod was smaller by about 15% than that from eddy correlation. Martano's method is used toestimate the surface aerodynamic roughness length z0 and zero plane displacement d from singlelevel sonic anemometer data, giving d = 0.12 m and z0 = 0.08 m. The overall neutral dragcoefficient (CDN) and scalar coefficient (CHN) were found to be CDN = 0.0055and CHN = 0.0059 in the southeastern area of Tibet. Their variations with the mean wind speed at 10 m are discussed.

NOTES AND CORRESPONDECEScaling Reasoning and Field Data on the Sea Surface Roughness Lengths for Scalars

The heat and mass transfer over the sea is considered in terms of the sea surface roughness lengths for scalars, z 0T for potential temperature u, and z 0q for specific humidity q, or alternatively, in terms of the roughness-layer scalar increments, du and dq. A new scaling reasoning is proposed in support of the familiar square root dependence of the above increments on the roughness Reynolds number, Re0u 5 z 0uu * /n, where z 0u is the sea surface aerodynamic roughness length, u * is the friction velocity, and n is the molecular viscosity of the air. Scaling predictions are validated using data from measurements made by the National Oceanic and Atmospheric Administration’s Environmental Technology Laboratory aboard the R/V Moana Wave in the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment in 1992‐93 and the R/PFLIP in the San Clemente Ocean Probing Experiment in September 1993. Data presented as the dimensionless scalar increments (or the ratios z 0u/z 0T and z 0u/z 0q) versus Re 0u show a good agreement with theoretical predictions, especially at Re0u . 2 (over stormy sea). The resulting roughness-length formulations are recommended for practical use in climate and mesoscale air‐sea interaction models.

Chaos and “Bursting” in the Planetary Boundary Layer

Abstract A theoretical study of the intermittent breakdowns of the planetary boundary layer (PBL) has been carried out using a one-dimensional, “flat” terrain, PBL model of the type originally developed by A.K. Blackadar. It was found that the turbulent burst effects predominate in the lowest tens of meters of the PBL where an alternation between successive regimes of laminar and turbulent flow drive either rapid cooling or rapid heating, respectively, of the near-surface air temperature under certain conditions. This rather discontinuous change of the air temperature occurs with variable amplitudes at irregular intervals depending critically on the specified surface aerodynamic roughness length, the prevailing geostrophic wind, the magnitude of the radiation cooling with respect to the soil and its thermal properties, etc. For an aerodynamic roughness length of 1 m, collapse was found to occur within only a very narrow range of geostrophic winds (between 1.5 and 3 m s−1, with a maximum response in the ai...