Search Results (18)

The estimation of sensible heat flux (H) in drylands is important because it constitutes a significant portion of the net available surface energy. A model to estimate H half-hourly measurements for bare soils was derived by combining the surface renewal (SR) theory and the Monin–Obukhov similarity theory (MOST), involving the land surface temperature (LST), wind speed, and the air temperature in a period of half an hour, H_SR-LST. The surface roughness lengths for momentum ($z_{om}$) and for heat (z_0h) were estimated at neutral conditions. The dataset included dry climates and different measurement heights (1.5 m up to 20 m). Root mean square error ($\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}$) over the mean actual sensible heat flux estimate (H_EC), E $={\displaystyle \frac{\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}}{\overline{\mathrm{H}\mathrm{E}\mathrm{C}}}}100\%$, was considered excellent, good, and moderate for E values of up to 25%, 35%, and 40%, respectively. In stable conditions, H_SR-LST and H_MOST values were comparable and both were unacceptable (E > 40%). However, the RMSE using H_SR-LST ranged between 8 Wm² and 12 Wm² and performed slightly better than H_MOST. In unstable conditions, H_SR-LST was in excellent, good, and moderate agreement in 3, 6, and 5 cases, respectively; H_MOST was good in 3 cases; and the remaining 11 cases were intolerable because they required site-specific calibration. Full article

The Monin-Obukhov similarity theory (MOST) is widely used for surface layer parameterization. Discrepancies in MOST highlight the need to account for large eddy effects. A possible solution is to introduce the boundary layer depth $z_i$ as a new scaling parameter, which may enhance the applicability of the theory. A novel similarity scheme has recently been proposed to explicitly account for large eddy effects under unstable conditions. In this study, we estimated the impact of large eddies on the unstable surface layer using field measurements from a summer experiment on the Tibetan Plateau. Furthermore, we evaluated the proposed scheme and suggested simplifications for its improvements. In this study, the non-dimensional wind shear, ${\varphi }_m$, exhibited greater scatter and larger deviations from MOST than the non-dimensional temperature gradient, ${\varphi }_h$. Additionally, the normalized wind gradient ${\varphi }_m$ is found to depend on both $z/L$ and $z_i/L$, where $z$ is height above ground and $L$ is the Monin-Obukhov length. The additional dependence on $z_i/L$ suggests that it may serve as a crucial missing scaling parameter in the MOST under unstable conditions. Ultimately, we observed that the $z_i$-scaling parameter $C_m$ derived from the proposed scheme maintains a linear correlation with the stability parameter ($-z_i/L$), confirming the scheme’s effectiveness. Moreover, vertical wind gradients, friction velocity, and momentum flux predicted by this new scheme align more closely with observations than those estimated using the classical similarity function, thereby strengthening its feasibility and offers valuable insights for its simplification for Earth System modeling. Full article

This study proposes a microscale flow model to estimate mean wind speed, fluctuating wind speed and wind direction over complex terrain considering the effects of topography, atmospheric stability, and turbine wakes. Firstly, the effect of topography is considered using Computational Fluid Dynamics (CFD). Next, a mesoscale model is presented to account for the effect of atmospheric stability. The effect of turbine wakes on the mean and fluctuating wind speeds are then represented by an advanced wake model. The model is validated using the measurement data of a wind farm located in the North of Japan. The measured wind data by Lidar at a reference height are horizontally extrapolated to a nearby met mast hub height and validated by a cup anemometer. Moreover, a novel averaging method is proposed to calculate a directional equivalent Monin–Obukhov length scale to account for the effect of atmospheric stability. Finally, the measured wind data at the reference height are vertically extrapolated and validated at the lidar location. The predicted mean and fluctuating wind speeds show good agreement with the measurements. Full article

An accurate wind shear model is an important prerequisite in extrapolating the wind resource from lower heights to the increasing hub height of wind turbines. Based on the 1-year dataset (collected in 2014) consisting of 15-minute intervals collected at heights of 2, 10, 50, 100, and 150 m on an anemometer tower in northern China, the present study focuses on the time-varying relationship between the wind shear coefficient (WSC) and atmospheric stability and proposes a wind shear model considering atmospheric stability. Through the relationship between Monin–Obukhov (M-O) length and gradient Richardson number, the M-O length is directly calculated by wind data, and the WSC is calculated by combining the Panofsky and Dutton (PD) models, which enhances the engineering practicability of the model. Then, the performance of the model is quantified and compared with two alternative methods: the use of annual average WSC and the use of stability change WSC extrapolation. The analysis demonstrates that the proposed model outperforms the other approaches in terms of normal root mean square error (NRMSE) and normal bias (NB). More specifically, this method reduces the NRMSE and NB by 24–29% and 76–95%, respectively. Meanwhile, it reaches the highest extrapolation accuracy under unstable and stable atmospheric conditions. The results are verified using the Weibull distribution. Full article

The mean velocity distributions of unstably and stably stratified atmospheric surface layers (ASLs) are investigated here using the symmetry approach. Symmetry groups for the mean momentum and the Reynolds stress equations of ASL are searched under random dilation transformations, which, with different leading order balances in different flow regions, lead to a set of specific scalings for the characteristic length ${\ell }_{13}$ (defined by Reynolds shear stress and mean shear). In particular, symmetry analysis shows that in the shear-dominated region, ${\ell }_{13}$ scales linearly with the surface height z, which corresponds to the classical log law of mean velocity. In the buoyancy-dominated region, ${\ell }_{13}/L\sim {\left(z,/,L\right)}^{4/3}$ for unstably stratified ASL and ${\ell }_{13}/L\sim const$ for stably stratified ASL, where L is the Obukhov length. The specific formula of the celebrated Monin–Obukhov similarity function is obtained, and hence an algebraic model of mean velocity profiles in ASL is derived, showing good agreement with the datum from the QingTu Lake observation array (QLOA) in China. Full article

We revisit two recent methodologies based on Monin–Obukhov Similarity Theory (MOST), the 2D method and Hybrid-Wind (HW), which are aimed at estimation of the Obukhov length, friction velocity and kinematic heat flux within the surface layer. Both methods use wind-speed profile measurements only and their comparative performance requires assessment. Synthetic and observational data are used for their quantitative assessment. We also present a procedure to generate synthetic noise-corrupted wind profiles based on estimation of the probability density functions for MOST-related variables (e.g., friction velocity) and the statistics of the noise-corrupting perturbational amplitude found during an 82-day IJmuiden observational campaign. In the observational part of the study, 2D and HW parameter retrievals from floating Doppler wind lidar measurements are compared against those from a reference mast. Overall, the 2D algorithm outperformed the HW in the estimation of all the three parameters above. For instance, when assessing the friction-velocity retrieval performance with reference to sonic anemometers, determination coefficients of ${\rho }_{2D}^2=0.77$ and ${\rho }_{HW}^2=0.33$ were found under unstable atmospheric stability conditions, and ${\rho }_{2D}^2=0.81$ and ${\rho }_{HW}^2=0.07$ under stable conditions, which suggests the 2D algorithm as a prominent method for estimating the above-mentioned surface-layer parameters. Full article

Wind energy production mainly depends on atmospheric conditions. The atmospheric stability can be described through different parameters, such as wind shear, turbulence intensity, bulk Richardson number, and the Monin–Obukhov length. Although they are frequently used in micrometeorology and the wind industry, there is no standard comparison method. This study describes the atmospheric stability of a coastal region of Yucatan, Mexico, using these four parameters. They are calculated using six-month data from a meteorological mast and a marine buoy to determine atmospheric stability conditions and compare their results. The unstable atmospheric condition was predominant at the site, with an 80% occurrence during the measurement period, followed by 12% in neutral and 6% in stable conditions. Wind speed estimations were performed for each atmospheric stability scenario, and the variation in the energy produced was derived for each case. Unstable atmospheric conditions deliver up to 8% more power than stable conditions, while neutral conditions deliver up to 9% more energy than stable conditions. Therefore, considering a neutral state may lead to a considerably biased energy production estimation. Finally, an example calculation indicates that atmospheric stability is a crucial parameter in estimating wind energy production more accurately. Full article

Accurate turbulent inflow conditions are needed to broaden the application of the large-eddy simulation technique to predict winds around arbitrarily complex terrain. We investigate the concept of buoyancy perturbations with colored noise to trigger turbulence in upstream flows approaching complex terrain regions. Random perturbations are imposed on the source term in the pseudo-temperature transport equation. These perturbations are effective within three-dimensional boxes and scaled using a bulk Richardson number defined for each box. We apply the turbulent inflow generation technique to predict winds around the Askervein and Bolund Hills under neutrally stratified conditions. We find that a common value for the bulk Richardson number works well for a variety of flow problems. Additionally, we show that the height of the perturbation box plays an important role in the accuracy of the predictions around complex terrain. We consistently obtained good results for both simulation cases when the perturbation box height was made a fraction of the Obukhov length scale. Full article

To explore the influence of complex terrain on wind characteristics of the surface layer and to better develop and utilize wind energy resources of high-altitude regions in central and western China, two typical topographies: the Qiaodi Village in Sichuan (in western China, site 1) and the Nanhua Mountain in Shanxi (in central China, site 2), were selected for this study. The diurnal and monthly variations of the atmospheric stability were contrasted at the two sites, according to the Obukhov length calculated by the eddy covariance data. The energy exchange process between complex underlying surfaces and the atmospheric boundary layer can be reflected to a certain extent by investigating the diurnal variation differences of the turbulent fluxes at the two sites. The results show that: (1) the dominant boundary layer at site 1 during nighttime is the neutral boundary layer, while at site 2 it is the stable; (2) the horizontal wind speed at 10 m above the ground is the highest (lowest) in the neutral (unstable) boundary layer at site 1, while it is the highest (lowest) in the neutral and weak-unstable (stable) boundary layer at site 2, and (3) the momentum flux, sensible heat flux, and latent heat flux all show unimodal diurnal characteristics. There is a 1 h lag in the flux peak at site 1 compared to site 2. Full article

This work presents a parametric-solver algorithm for estimating atmospheric stability and friction velocity from floating Doppler wind lidar (FDWL) observations close to the mast of IJmuiden in the North Sea. The focus of the study was two-fold: (i) to examine the sensitivity of the computational algorithm to the retrieved variables and derived stability classes (the latter through confusion-matrix theory), and (ii) to present data screening procedures for FDWLs and fixed reference instrumentation. The performance of the stability estimation algorithm was assessed with reference to wind speed and temperature observations from the mast. A fixed-to-mast Doppler wind lidar (DWL) was also available, which provides a reference for wind-speed observations free from sea-motion perturbations. When comparing FDWL- and mast-derived mean wind speeds, the obtained determination coefficient was as high as that of the fixed-to-mast DWL against the mast (${\rho }^2=0.996$) with a root mean square error (RMSE) of 0.25 m/s. From the 82-day measurement campaign at IJmuiden (10,833 10 min records), the parametric algorithm showed that the atmosphere was neutral (31% of the cases), stable (28%), or near-neutral stable (19%) during most of the campaign. These figures satisfactorily agree with values estimated from the mast measurements ($31\%$, $27\%$, and $19\%$, respectively). Full article

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 (C_q) 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. C_q 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 C_q 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 C_q is employed. Full article

This present work investigates several local and synoptic meteorological aspects associated with two wintertime haze episodes in Greater Bangkok using observational data, covering synoptic patterns evolution, day-to-day and diurnal variation, dynamic stability, temperature inversion, and back-trajectories. The episodes include an elevated haze event of 16 days (14–29 January 2015) for the first episode and 8 days (19–26 December 2017) for the second episode, together with some days before and after the haze event. Daily PM_2.5 was found to be 50 µg m⁻³ or higher over most of the days during both haze events. These haze events commonly have cold surges as the background synoptic feature to initiate or trigger haze evolution. A cold surge reached the study area before the start of each haze event, causing temperature and relative humidity to drop abruptly initially but then gradually increased as the cold surge weakened or dissipated. Wind speed was relatively high when the cold surge was active. Global radiation was generally modulated by cloud cover, which turns relatively high during each haze event because cold surge induces less cloud. Daytime dynamic stability was generally unstable along the course of each haze event, except being stable at the ending of the second haze event due to a tropical depression. In each haze event, low-level temperature inversion existed, with multiple layers seen in the beginning, effectively suppressing atmospheric dilution. Large-scale subsidence inversion aloft was also persistently present. In both episodes, PM_2.5 showed stronger diurnality during the time of elevated haze, as compared to the pre- and post-haze periods. During the first episode, an apparent contrast of PM_2.5 diurnality was seen between the first and second parts of the haze event with relatively low afternoon PM_2.5 over its first part, but relatively high afternoon PM_2.5 over its second part, possibly due to the role of secondary aerosols. PM_2.5/PM₁₀ ratio was relatively lower in the first episode because of more impact of biomass burning, which was in general agreement with back-trajectories and active fire hotspots. The second haze event, with little biomass burning in the region, was likely to be caused mainly by local anthropogenic emissions. These findings suggest a need for haze-related policymaking with an integrated approach that accounts for all important emission sectors for both particulate and gaseous precursors of secondary aerosols. Given that cold surges induce an abrupt change in local meteorology, the time window to apply control measures for haze is limited, emphasizing the need for readiness in mitigation responses and early public warning. Full article

Since 2017, the operational high-resolution air quality forecasting system FORAIR_IT, developed and maintained by the Italian National Agency for New Technologies, Energy and Sustainable Economic Development, has been providing three-day forecasts of concentrations of atmospheric pollutants over Europe and Italy, on a daily basis, with high spatial resolution (20 km on Europe, 4 km on Italy). The system is based on the Atmospheric Modelling System of the National Integrated Assessment Model for Italy (AMS-MINNI), which is a national modelling system evaluated in several studies across Italy and Europe. AMS-MINNI, in its forecasting setup, is presently a candidate model for the Copernicus Atmosphere Monitoring Service’s regional production, dedicated to European-scale ensemble model forecasts of air quality. In order to improve the quality of the meteorological input into the chemical transport model component of FORAIR_IT, several tests were carried out on daily forecasts of NO₂ and O₃ concentrations for January and August 2019 (representative of the meteorological seasons of winter and summer, respectively). The aim was to evaluate the sensitivity to the meteorological input in NO₂ and O₃ concentration forecasting. More specifically, the Weather Research and Forecasting model (WRF) was tested to potentially improve the meteorological driver with respect to the Regional Atmospheric Modelling System (RAMS), which is currently embedded in FORAIR_IT. In this work, the WRF chain is run in several setups, changing the parameterization of several micrometeorological variables (snow, mixing height, albedo, roughness length, soil heat flux + friction velocity, Monin–Obukhov length), with the main objective being to take advantage of WRF’s consistent physics in the calculation of both mesoscale variables and micrometeorological parameters for air quality simulations. Daily forecast concentrations produced by the different meteorological model configurations are compared to the available measured concentrations, showing the general good performance of WRF-driven results, even if performance skills are different according to the single meteorological configuration and to the pollutant type. WRF-driven forecasts clearly improve the model reproduction of the temporal variability of concentrations, while the bias of O₃ is higher than in the RAMS-driven configuration. The results suggest that we should keep testing WRF configurations, with the objective of obtaining a robust improvement in forecast concentrations with respect to RAMS-driven forecasts. Full article

We report estimations of the Mexicali Valley (Mexico) mixing height for three seasons. Surface and upper air meteorological measurements were carried out nearby Cerro Prieto geothermal power plant during July 2010 (summer), January 2012 (winter), and October 2016 (autumn). Four different methods were applied to estimate the convective boundary layer (CBL) height from radiosonde (RS) profiles: the parcel method, the gradients method, the least-squares variational approach based on the slab model of the CBL structure, and a covariance method. For nocturnal conditions, we used diagnostic models based on friction velocity and Monin–Obukhov length. Under unstable conditions, we obtained (on average) mixing heights of 497 m at 06:00 LST, 1242 m at 12:00 LST, 1404 m at 15:00 LST, and 482 at 18:00 LST during summer; 754 m at 12:00 LST during winter; and 1195 m at 12:00 LST and 13:00 m between the 14:00 and 15:00 LST during the autumn. The results allowed adjusting a semiempirical model to evaluate mixing height from turbulent sensible heat flux and friction velocity data. Our results provide practical tools that could facilitate the application of regulatory dispersion models to assess air quality in the region. Full article

Atmospheric stability has been studied for decades. There are several methodologies that evolved over the years. In this study, a special experimental meteorological mast that has been erected to a complex site has been used to calculate dimensionless Obukhov length ( $\zeta =\frac{z}{L})$ , dimensionless momentum ( ${\phi }_m$ ), and heat coefficients ( ${\phi }_h$ ). The results are compared with the ones from average value approaches: Richardson number, flux-profile (F-P) relations, and wind shear exponent methods. The results show that the estimated $\zeta$ values, using the bulk Richardson number, get along well with the reference $\zeta$ within the neutral and stable regimes. F-P relations and wind shear exponent methods result in the best agreement for stable and neutral regimes. Nevertheless, average oriented methods are not reliable for the other regimes. Full article