Roughness Length Research Articles

Estimation of wind energy potential using WAsP model in the sub watershed of Oued-Sakni region-Algeria

This paper is a case study aiming at the determination of wind power potential in the Oued-Sakni sub-watershed. The objective is to provide wind energy for agro-pastoral regions, specially designed for irrigation systems, considering the fact that this region is far from the electricity grid. Statistical methods and the WAsP model were used to determine the climatological and energy characteristics of wind in this region. The results of this study are presented in the form of four tables containing the values of the parameters of the Weibull statistical distribution ‘ A’ and ‘ k’ for levels 10, 25and 50 m for standard conditions. Each table corresponds to a type of roughness length. A fifth table contains the calculated values of the mean speed and the mean total wind power available at the same levels and same type of roughness length.

Local climate effects of urban wind corridors in Beijing

Based on a summer observational experiment and the meteorological data from 2009 to 2018, this paper investigated the local climate effects of representative 1 Level One and 2 Level Two wind corridors (LOC and LTCs, respectively) in the central urban area (CUA) of Beijing by introducing the wind speed ratio (WsR) and urban heat island (UHI) intensity indices. In addition, the impacts on the ventilation of wind corridors from the 8 spatial landscape parameters were analyzed. The results indicated that LOC and LTCs have significant ventilation benefit and certain UHI mitigation benefit varying with season, day and night and weather，which were able to accelerate winds when wind speeds exceed 0.57 m/s and 0.75 m/s, respectively. The most important influencing parameters on the ventilation capability were frontal area index and roughness length. The multiyear average WsRs of the LOC and LTCs were 57% and 31% higher than that of the CUA, respectively. The UHI reduction did not occur along the whole corridor or throughout the whole day. The maximum seasonal mean UHI reduction of the LOC during daytime and nighttime were 0.89 °C and 0.75 °C, respectively. The LTCs mainly reduced UHI during daytime with the maximum reduction of 0.30 °C.

Evolution of and Disparity among Biomimetic Superhydrophobic Surfaces with Gecko, Petal, and Lotus Effect (Small 18/2022)

Biomimetic Superhydrophobic Surfaces In article number 2200349, Masanobu Naito and co-workers unveil the mystery of superhydrophobic surfaces with gecko, petal and lotus effects. The study discloses a facile topography evolution path to achieve these surfaces by controlling roughness factor and length scale of roughness. It presents a manifest correlation between surface topography and water droplet dynamics. The paper provides insight into superhydrophobic science and application guidelines.

A wall-modeled approach accounting for wave stress in Large Eddy Simulations of offshore wind farms

In the context of offshore wind farms, accurate predictions of surface fluxes in the marine atmospheric boundary layer are critical for Large Eddy Simulations (LES) of airflow over waves. The effect of the waves on the airflow is often modeled by prescribing the roughness length in the framework of Monin-Obukhov similarity theory. However, such approaches lack generalizability over different wave conditions due to reliance on model coefficients tuned to specific datasets. Wave phase-resolving simulations on the other hand have higher accuracy but also a higher computational cost. In this work, a sea surface-based hydrodynamic drag model is applied to model the pressure-based surface drag felt by the wind due to the waves. An offshore wind farm configuration is simulated using a wall-modeled LES, with the effect of the waves represented using the wave-drag model and the wind turbines represented by an actuator disk model. The approach is validated with data from high fidelity wave-resolving Large Eddy Simulations. The effect of the waves and the farm configuration on the mean velocity profiles power production, and kinetic energy budget are quantified.

The WRF model and the diurnal cycle of orographically-influenced flow: A case study

The Weather Research and Forecasting (WRF) model is widely used in modelling for wind energy purposes. Previous research indicated that under some situations, the WRF model fails to properly represent the diurnal cycle in wind speed. Here we present a case study with a North Sea jet where the WRF-simulated wind diurnal cycle is compared with observations at the Østerild tall mast in Northern Jutland, Denmark. The diurnal cycle in wind speed has different shapes at different heights and the WRF model can replicate the diurnal cycle at higher levels (∼ 250 m a.g.l.) but fails to replicate it near surface. We investigate the effect of using different Planetary Boundary Layer (PBL) and Surface Layer (SL) schemes and compare stability data between model and observations. The results suggest a weak link between the model’s ability to replicate stability, probably linked to incorrect surface roughness length specification, and the model’s ability to replicate diurnal cycle at lower levels.

Assessment of Air Quality and Meteorological Changes Induced by Future Vegetation in Madrid

Nature-based solutions and green urban infrastructures are becoming common measures in local air quality and climate strategies. However, there is a lack of analytical frameworks to anticipate the effect of such interventions on urban meteorology and air quality at a city scale. We present a modelling methodology that relies on the weather research and forecasting model (WRF) with the building effect parameterization (BEP) and the community multiscale air quality (CMAQ) model and apply it to assess envisaged plans involving vegetation in the Madrid (Spain) region. The study, developed within the VEGGAP Life project, includes the development of two detailed vegetation scenarios making use of Madrid’s municipality tree inventory (current situation) and future vegetation-related interventions. An annual simulation was performed for both scenarios (considering constant anthropogenic emissions) to identify (i) variations in surface temperature and the reasons for such changes, and (ii) implications on air-quality standards according to EU legislation for the main pollutants (PM10, PM2.5, NO2 and O3). Our results suggest that vegetation may have significant effects on urban meteorology due to changes induced in relevant surface properties such as albedo, roughness length or emissivity. We found a net-heating effect of around +0.18 °C when trees are introduced in dry, scarcely vegetated surfaces in the city outskirts. In turn, this enhances the planetary boundary layer height (PBLH), which brings about reductions in ambient concentrations of relevant pollutants such as NO2 (in the range of 0.5–0.8 µg m−3 for the annual mean, and 2–4 µg m−3 for the 19th highest 1 h value). Conversely, planting new trees in consolidated urban areas causes a cooling effect (up to −0.15 °C as an annual mean) that may slightly increase concentration levels due to less-effective vertical mixing and wind-speed reduction caused by increased roughness. This highlights the need to combine nature-based solutions with emission-reduction measures in Madrid.

Changes of the aerodynamic characteristics of a flux site after an extensive windthrow

Abstract. A maritime pine plantation in central Portugal that has been continuously monitored using the eddy-covariance technique for carbon fluxes since a wildfire in 2017 was significantly affected by two storms during December 2019 that resulted in a large-scale windthrow. This study analyses the impacts of this windthrow on the aerodynamic characteristics of zero-plane displacement and roughness length and, ultimately, their implications for the turbulent fluxes. The turbulent fluxes were only affected to a minor degree by the windthrow, but the footprint area of the flux tower changed markedly so that the target area of the measurements had to be redetermined.

An Isogeometric analysis based method for frictional elastic contact problems with randomly rough surfaces

The problem addressed in this paper concerns the frictional contact between an elastomer and a rigid body with randomly rough surfaces. The work is accomplished on the basis of two crucial elements: a framework for generating random geometries and a robust frictional contact algorithm, both of which are realised in the approach of Isogeometric analysis (IGA) for its high accuracy and robustness. For the former, a new Isogeometric framework for random geometry modelling is proposed, which combines the random field generation based on Karhunen–Loève expansion theory with Non-Uniform Rational B-Spline (NURBS) interpolation method. For the latter, a mortar-based frictional contact algorithm in 2D large deformation regime is adopted incorporating a modified closest point projection method for detection of contact. Numerical experiments are conducted with several settings such as ‘rough–smooth’, ‘smooth–rough’ and ‘rough–rough’ contact, depending on which side of the contact pair the randomly rough surface belongs to. The ratio of the global coefficient of friction to the prescribed local one and the ratio of true contact area to the nominal contact area are characterised under these settings, and factors like the root mean square roughness and correlation length of the random surface and the external traction are discovered to have a significant influence on the two ratios.

A Hybrid Bulk Algorithm to Predict Turbulent Fluxes over Dry and Wet Bare Soils

Abstract Measurements made in the Columbia River basin (Oregon) in an area of irregular terrain during the second Wind Forecast Improvement Project (WFIP2) field campaign are used to develop an optimized hybrid bulk algorithm to predict the surface turbulent fluxes from readily measured or modeled quantities over dry and wet bare or lightly vegetated soil surfaces. The hybrid (synthetic) algorithm combines (i) an aerodynamic method for turbulent flow, which is based on the transfer coefficients (drag coefficient and Stanton number), roughness lengths, and Monin–Obukhov similarity; and (ii) a modified Priestley–Taylor (P-T) algorithm with physically based ecophysiological constraints, which is essentially based on the surface energy budget (SEB) equation. Soil heat flux in the latter case was estimated from measurements of soil temperature and soil moisture. In the framework of the hybrid algorithm, bulk estimates of the momentum flux and the sensible heat flux are derived from a traditional aerodynamic approach, whereas the latent heat flux (or moisture flux) is evaluated from a modified P-T model. Direct measurements of the surface fluxes (turbulent and radiative) and other ancillary atmospheric/soil parameters made during WFIP2 for different soil conditions (dry and wet) are used to optimize and tune the hybrid bulk algorithm. The bulk flux estimates are validated against the measured eddy-covariance fluxes. We also discuss the SEB closure over dry and wet surfaces at various time scales based on the modeled and measured fluxes. Although this bulk flux algorithm is optimized for the data collected during the WFIP2, a hybrid approach can be used for similar flux-tower sites and field campaigns.

A measure–correlate–predict model based on neural networks and frozen flow hypothesis for wind resource assessment

In this paper, a measure–correlate–predict (MCP) model based on neural networks (NN) and frozen flow hypothesis, which is abbreviated as the MCPNN-frozen model, is proposed for wind resource assessment and tested using turbulent channel flows with three different surface roughness lengths, i.e., k0=0.001, 0.01, and 0.1 m. The predictions from the MCPNN-frozen model are compared with the real data for different separations (s) between the reference point and the target point. The results show that the correlation coefficients C.C. between the model predictions and real data are roughly higher than 0.5 for small separations s/δ≤3 (where δ is the boundary layer thickness), and the coefficients of determination (R2) are approximately higher than 0.3 when s/δ≤2. The generalization capacity of the MCPNN-frozen model is tested for different roughness lengths and different velocity components. Further analyses show that, even though C.C. and R2 decrease when increasing s, the large-scale variations of velocity fluctuations are well captured by the MCPNN-frozen model especially for the one trained using the data filtered in time. Furthermore, it is found that the model trained using the filtered data without a spanwise offset can well predict the large-scale variations at the target point when the spanwise offsets between the target point and the reference point are small (e.g., 0.1δ and 0.2δ). The proposed model leverages the power of neural networks and physical understanding. Further development of the model for complex scenarios will be carried out in the future work.

A study of function-based wind profiles based on least squares method: A case in the suburbs of Hohhot

Since wind energy is a key source of clean energy in suburban areas where multiple energy sources complement each other, it is crucial to be able to accurately predict the inlet boundary of wind turbines in order to transform the efficiency of wind turbines. Therefore, in this paper, by setting up testing stations in the suburban area of Hohhot and collecting real-time data of wind speed and wind direction in different measurement heights for three years, and using the least squares method to estimate the parameters of the exponential law, logarithmic law, and composite function model, and conducting various goodness-of-fit tests, the optimal composite function is obtained as the wind profile form in the suburban area of the city, which is named as the Suburban Exponential of Wind profile (SE). And the inter-parameter analysis of this function, using linear regression statistical method to regress between parameters and parameters and between parameters and wind speed, summarize the physical significance of each parameter of this function. Respectively, parameter a is the ultimate wind speed under the vertical height of the region, parameter b is the reference height, parameter c and the roughness length z0 meet the linear relationship, and effectively reflect the influence of building dynamics roughness on the vertical distribution of wind speed, to make up for the shortcomings of the traditional wind profile formula, and provide a basis for accurate micro-siting of small wind turbines.

The effects of ocean surface waves on global intraseasonal prediction: case studies with a coupled CFSv2.0–WW3 system

Abstract. This article describes the implementation of a coupling between a global forecast model (CFSv2.0) and a wave model (WW3) and investigates the effects of ocean surface waves on the air–sea interface in the new framework. Several major wave-related processes, including the Langmuir mixing, the Stokes–Coriolis force with entrainment, air–sea fluxes modified by the Stokes drift, and momentum roughness length, are evaluated in two groups of 56 d experiments, one for boreal winter and the other for boreal summer. Comparisons are made against in situ buoys, satellite measurements, and reanalysis data to evaluate the influence of waves on intraseasonal prediction of sea surface temperature (SST), 2 m air temperature (T02), mixed layer depth (MLD), 10 m wind speed (WSP10), and significant wave height (SWH). The wave-coupled experiments show that overestimated SSTs and T02s, as well as underestimated MLDs at mid-to-high latitudes in summer from original CFSv2.0, are significantly improved due to enhanced vertical mixing generated by the Stokes drift. For WSP10s and SWHs, the wave-related processes generally reduce biases in regions where WSP10s and SWHs are overestimated. On the one hand, the decreased SSTs stabilize the marine atmospheric boundary layer and weaken WSP10s and then SWHs. On the other hand, the increased roughness length due to waves reduces the originally overestimated WSP10s and SWHs. In addition, the effects of the Stokes drift and current on air–sea fluxes also rectify WSP10s and SWHs. These cases are helpful for the future development of the two-way CFSv2.0–wave coupled system.

Evolution of and Disparity among Biomimetic Superhydrophobic Surfaces with Gecko, Petal, and Lotus Effect.

It is desirable to turn one kind of superhydrophobic (SHPO) surfaces into another by changing surface topography alone and attaining solid surfaces with tunable properties. Herein, gecko-, petal-, and lotus-like SHPO surfaces, composed of ZnO tetrapods and polydimethylsiloxane, are realized by adjusting the roughness factor and length scale of roughness, while keeping the surface chemistry the same. Afterward, water droplet sliding and impacting are investigated. The surfaces behave similarly in spreading but deviate from each other in sliding, receding, jetting, and rebounding due to their different adhesive properties. Moreover, the disparity between surfaces with petal and lotus effects is well explained by Furmidge's and Young-Dupre equations. On the other hand, these formulas fail to elucidate the surface with gecko effect because of its inside sealed air that produces negative pressure upon droplet motion. This paper provides a facile topography evolution path and a manifest correlation between topography and performance in water droplet dynamics for SHPO surfaces with gecko, petal, and lotus effects.

Simulation of sensible and latent heat fluxes on the Tibetan Plateau from 1981 to 2018

The Noah-MP land surface model with modified thermal roughness length parameterization scheme was used for the simulation of land surface process on the Tibetan Plateau (TP) during 1981–2018, using the high-resolution China Meteorological Forcing Data. Characteristics of distributions and changes of surface sensible and latent heat (SH and LH) fluxes on the TP were analyzed using the simulation results after verification against the in situ observations. It is found that the Noah-MP model can well simulate the SH and LH fluxes. The inter-annual variations of SH fluxes on the central and western TP were larger than those on the eastern TP. All the regions showed a decreasing trend in SH flux from mid-1980s, but an increasing trend since about 2002. The LH fluxes on the eastern TP were larger than those on the central and western TP. But the inter-annual variations were smaller than those of central and western TP. The western and eastern TP got an increasing trend in LH flux during the 38 yrs. On the central TP, LH fluxes exhibited an increasing trend from mid-1980s to the beginning of the 21st century, and then decreased. For the whole TP, LH fluxes increased from mid-1980s to the beginning of the 21st century, and showed a decreasing trend from about 2003. The asynchronous changes of wind speed, Ts-Ta (land-air temperature difference) and water vapor condensation may increase the uncertainties of the trends in SH and LH fluxes.

Uncertainties Associated With Simulating Regional Sea Surface Height and Tides: A Case Study of the East China Seas

Modeling sea surface height (SSH) and tides is important but challenging in shelf seas. The Eastern China Seas (ECSs) is such a shelf sea with large inter-model deviations in prior studies. In order to assess and compare the possible uncertainty sources, numerical scenarios of varying the open boundary forcing, bottom roughness length scale, atmospheric forcing, grid resolution, and regional bathymetry were conducted in a hydrodynamic model of the ECSs. Results indicate that bathymetry data and open boundary forcing with inadequate accuracy generate uncertainties in SSH and tides locally and throughout the basin. An increase in bottom roughness enhances tidal dissipation and shifts amphidromes to the left relative to the incoming Kelvin waves, causing SSH variations in the ECSs. Refining the model resolution from 4 to 2 km mainly affects nearshore SSH and tides due to minor changes in depicted coastlines. Using different reanalysis meteorological data appears more important on the episodic than annual scale. It is highlighted that some uncertainty sources have opposing effects on SSH or tides and counteract their individual biases, making it difficult to achieve a realistic simulation. For example, increasing bottom roughness can not only compensate effects of overestimated tidal amplitude at open boundaries, but also balance out the overestimated M2 phase along the West Coast of Korea in a coarser-resolution model. Based on findings in this study, suggestions are provided for further reducing uncertainties in SSH and tide modeling in the ECSs and other shelf seas.

The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

Abstract The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Last, a brief analysis of in situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally homogeneous surface types—which have been well documented by previous studies—are inappropriate for use near strong frictional heterogeneity.

The Impact of a Reduced High‐Wind Charnock Parameter on Wave Growth With Application to the North Sea, the Norwegian Sea, and the Arctic Ocean

AbstractAs atmospheric models move to higher resolution and resolve smaller scales, the maximum modeled wind speed also tends to increase. Wave models tuned to coarser wind fields tend to overestimate the wave growth under strong winds. A recently developed semiempirical parameterization of the Charnock parameter, which controls the roughness length over surface waves, substantially reduces the aerodynamic drag of waves in high winds (above a threshold of 30 m s−1). Here, we apply the formulation in a recent version of the wave model WAM (Cycle 4.7), which uses a modified version of the physics parameterizations by Ardhuin et al. (2010, https://doi.org/10.1175/2010jpo4324.1) as well as subgrid obstructions for better performance around complex topography. The new Charnock formulation is tested with wind forcing from NORA3, a recently completed nonhydrostatic atmospheric downscaling of the global reanalysis ERA5 for the North Sea, the Norwegian Sea and the Barents Sea. Such high‐resolution atmospheric model integrations tend to have stronger (and more realistic) upper‐percentile winds than what is typically found in coarser atmospheric models. A 2‐year comparison (2011–2012) of a control run against the run with the modified Charnock parameter shows a dramatic reduction of the wave height bias in high‐wind cases. The added computational cost of the new physics and the reduction of the Charnock parameter compared to the earlier WAM physics is modest (14%). A longer (1998–2020) hindcast integration with the new Charnock parameter is found to compare well against in situ and altimeter wave measurements both for intermediate and high sea states.

Effects of seed morphology and orientation on secondary seed dispersal by wind

Abstract Understanding how diaspore (hereafter ‘seed’) morphology and orientation affect secondary seed dispersal by wind is important to link seed dispersal and post-dispersal processes, such as seed lodging, predation and germination. This study aims to describe the effects of seed morphology and orientation on secondary seed dispersal by wind via mechanistic modelling. We extend the mechanistic model of Schurr et al. (2005) in order to describe how secondary seed dispersal by wind is affected by wind conditions, ground surface, seed morphology and orientation. The model simulates the initial landing orientations, dispersal distances and stopping orientations of individual seeds. To parameterize the model, we measured orientation-specific vertical seed projection and seed lift-off velocity (the wind speed at which a seed starts moving on the ground) of the asymmetric seeds of heterocarpous Zygophyllum xanthoxylon, and determined orientation-specific model parameters that depend on properties of seeds and/or the environment. To validate the model, we conducted wind channel experiments in which we released seeds of Z. xanthoxylon onto a sand-coated tar paper, and recorded the initial landing orientations, dispersal distances and stopping orientations of the seeds. The extended model could precisely predict secondary dispersal distance, and explain up to 99% of variation in the observed proportions of seeds which stopped in various orientations. The model predicts that secondary dispersal distance increases with wind speed and decreases with aerodynamic roughness length, and that there might be a positive correlation between dispersal distance and germination success.

A Source of WRF Simulation Error for the Early‐Summer Warm‐Sector Heavy Rainfall Over South China Coast: Land‐Sea Thermal Contrast in the Boundary Layer

AbstractQuantitative prediction of the early‐summer warm‐sector heavy rainfall over South China coast (SCC) is still a challenge. This study conducts convection‐permitting Weather Research and Forecast model simulations for a representative nocturnal‐to‐morning heavy rainfall event related to low‐level jets during 29–30 May 2020, and explores sources of simulated rainfall bias using multi‐source observations, reanalysis data and sensitivity experiments. Results show that the model initiated about 12 hr before the coastal convection initiation only produces sporadic convection and misses the heavy rainfall at SCC, although it captures the major synoptic features as in previous studies. The underestimated convection development is closely related to insufficient convergence of boundary layer (BL) wind due to overestimated speed of the onshore BL wind. Increasing the land surface friction effects in the simulations through either multiplying the surface roughness length by 10 or including sub‐grid topography drag effects could help little suppress the simulated biases. Using hourly data assimilation to improve the simulation of marine/land BL temperature from the afternoon to nighttime produces a desired change in the nocturnal BL wind across the coastline and significantly improves rainfall simulation with the maximum and average rainfall over SCC increased by about 4 and 6.5 times, respectively. These findings indicate that, to improve the prediction of the nocturnal heavy rainfall in the warm‐sector over SCC during early summer, it is necessary to more accurately predict the BL thermal structure during the daytime, in addition to the previously emphasized nocturnally enhanced marine BL flow.

Intra-Seasonal and Intra-Annual Variation of the Latent Heat Flux Transfer Coefficient for a Freshwater Lake

In the case of lakes, evaporation is one of the most significant losses of water and energy. Based on high-frequency eddy-covariance (EC) measurements between May and September of 2019, the offshore heat and water vapor exchanges are evaluated for the large (~600 km2) but shallow (~3.2 m deep) Lake Balaton (Transdanubian region, Hungary). The role of local driving forces of evaporation in different time scales (from 20 min to one month) is explored, such as water surface and air temperatures, humidity, atmospheric stability, net radiation, and energy budget components. EC-derived water vapor roughness lengths and transfer coefficients (Cq) show an apparent intra-seasonal variation. Different energy balance-based evaporation estimation methods (such as the Priestley-Taylor and the Penman-Monteith) confirm this observation. Furthermore, this has suggested the existence of an intra-annual variation in these parameters. This hypothesis is verified using ten years of water balance measurements, from which, as a first step, evaporation rates and, second, transfer coefficients are derived on a monthly scale. Cq is highly reduced in winter months (~1 × 10−3) compared to summer months (~2.5 × 10−3) and strongly correlated with net radiation. The application of time-varying Cq significantly increases the accuracy of evaporation estimation when the Monin-Obukhov similarity theory-based aerodynamic method is applied. The determination coefficient increases to 0.84 compared to 0.52 when a constant Cq is employed.