Vertical Wind Component Research Articles

Impact of Atmospheric Turbulence on Dynamic Wind Loads on Heliostats

This study analyses the turbulent wind properties in the lower 10 m of the atmospheric surface layer close to the ground where heliostats are located. Power spectral densities of streamwise and vertical components of wind are analysed, and the frequencies associated with the peak of the spectra are determined using field data collected at two open-country terrains in addition to spectral models of ESDU85020. Variations of the peak frequencies with height from the ground, terrain type and average wind speed are discussed and the impact on heliostat wind loads are described. It is found that while the peak frequency of the streamwise turbulence is between 0.01–0.1 Hz, the peak of the vertical turbulence occurs at frequencies in the range of 0.1–1 Hz. It is shown that smaller heliostats that are located closer to the ground are exposed to turbulent wind fluctuations at a higher frequency and are thus expected to experience wind loads with a higher dominant frequency compared to larger heliostats located higher above the ground. These frequencies need to be considered in design of heliostat drives and structural components

Rotary-wing drone-induced flow – comparison of simulations with lidar measurements

Abstract. Ultrasonic anemometers mounted on rotary-wing drones have the potential to provide a cost-efficient alternative to the classical meteorological mast-mounted counterpart for atmospheric boundary layer research. However, the propeller-induced flow may degrade the accuracy of free-stream wind velocity measurements by wind sensors mounted on drones – a fact that needs to be investigated for optimal sensor placement. Computational fluid dynamics (CFD) simulations are an alternative to experiments for studying characteristics of the propeller-induced flow but require validation. Therefore, we performed an experiment using three short-range continuous-wave Doppler lidars (light detection and ranging; DTU WindScanners) to measure the complex and turbulent three-dimensional wind field around a hovering drone at low ambient wind speeds. Good agreement is found between experimental results and those obtained using CFD simulations under similar conditions. Both methods conclude that the disturbance zone (defined as a relative deviation from the mean free-stream velocity by more than 1 %) on a horizontal plane located at 1 D (rotor diameter D of 0.71 m) below the drone extends about 2.8 D upstream from the drone center for the horizontal wind velocity and more than 7 D for the vertical wind velocity. By comparing wind velocities along horizontal lines in the upstream direction, we find that the velocity difference between the two methods is ≤ 0.1 m s−1 (less than a 4 % difference relative to the free-stream velocity) in most cases. Both the plane and line scan results validate the reliability of the simulations. Furthermore, simulations of flow patterns in a vertical plane at the ambient speed of 1.3 m s−1 indicate that it is difficult to accurately measure the vertical wind component with less than a 1 % distortion using drone-mounted sonic anemometers.

CORNICE MODULAR WIND COLLECTOR © FOR COLLECTION AND AMPLIFICATION OF THE VERTICAL WIND COMPONENT IN BUILDINGS FOR GENERATION OF SMALL WIND ELECTRIC ENERGY

The generation of small wind electric energy in urban spaces in general and buildings in particular has an indisputable potential for practical applications. Notwithstanding, up until now, advances in this field have been less than expected. In order to take advantage of small wind energy, research in buildings requires the construction of a scale model for wind tunnel simulation. The difficulties regarding the construction of scale models and the availability of wind tunnels have slowed down a great deal this kind of research. New CDF (COMPUTATIONAL FLUID DYNAMICS) software techniques have made it possible to do research in buildings for small wind energy exploitation. A virtual scale model of the building under study is developed utilizing CAD. Subsequently, the model created with CAD is inserted into the CFD software and a simulation is conducted as if in a virtual wind tunnel. Considering the results obtained from research conducted with CFD software as well as from experimental validations in a wind tunnel, the research groups in La Rioja University known as “Modeling simulation and optimization of electrical industrial and automated fabrication systems” and “Integral Design Group“ have designed an optimization system for collection of small wind energy in buildings, for electric energy production through renewable sources.

Snowfall deposition in mountainous terrain: a statistical downscaling scheme from high-resolution model data on simulated topographies

One of the primary causes of non-uniform snowfall deposition on the ground in mountainous regions is the preferential deposition of snow, which results from the interaction of near-surface winds with topography and snow particles. However, producing high-resolution snowfall deposition patterns can be computationally expensive due to the need to run full atmospheric models. To address this, we developed two statistical downscaling schemes that can efficiently downscale near-surface, low-resolution snowfall data to fine-scale snow deposition accounting for the effect of preferential deposition in mountainous regions. Our approach relies on a comprehensive, model database generated using 3D wind fields from an atmospheric model and a preferential deposition model on several thousand simulated topographies covering a broad range in terrain characteristics. Both snowfall downscaling schemes rely on fine-scale topographic scaling parameters and low-resolution wind speed as input. While one scheme, referred to as the “wind scheme”, further necessitates fine-scale vertical wind components, a second scheme, referred to as the “aspect scheme”, does not require fine-scale temporal input. We achieve this by additionally downscaling near-surface vertical wind speed solely using topographic scaling parameters and low-resolution wind direction. We assess the performance of our downscaling schemes using an independent subset of the model database on simulated topographies, model data on actual terrain, and spatially measured new snow depth obtained through a photogrammetric drone survey following a snowfall event on previously snow-free ground. While the assessments show that our downscaling schemes perform well (relative errors ≤ ±3% with modeled and ≤ ±6% with measured snowfall deposition), they also demonstrate comparable results to benchmark downscaling models. However, our schemes notably outperform the benchmark models in representing fine-scale patterns. Our downscaling schemes possess several key features, including high computational efficiency, versatility enabled by the comprehensive model database, and independence from fine-scale temporal input data (aspect scheme), indicating their potential for widespread applicability. Therefore, our downscaling schemes for near-surface snowfall and vertical wind speed can be beneficial for various applications at fine grid resolutions such as in atmospheric and climate sciences, snow hydrology, glaciology, remote sensing, and avalanche sciences.

Diurnal Cycle of Tropospheric Winds over West Sumatra and Its Variability Associated with Various Climate and Weather Modes

The typical diurnal variability of tropospheric winds over West Sumatra and their changes associated with El Niño Southern Oscillation, Quasi-Biennial Oscillation, Madden–Julian Oscillations and convectively coupled Kelvin waves during the extended boreal winter season are investigated based on nineteen years of observations from Equatorial Atmosphere Radar in Kototabang, Indonesia. Sub-diurnal wind variability is assessed based on the amplitude and phase of the diurnal (24 h) and semidiurnal (12 h) modes.The results show that composite diurnal variability is dominated by cloud-induced circulation and thermal tides. Although these sub-diurnal modes do not change the daily mean wind direction, they modulate velocities throughout the day. Typical diurnal evolution of the vertical wind component is consistent with changes in the latent heating profiles associated with the evolution of a cloud field from cumulus before noon to deep convection in the afternoon and stratiform clouds in the evening. El Niño Southern Oscillation and Quasi-Biennial Oscillation affect the mean tropospheric winds, throughout the troposphere and above 250 hPa, respectively, but do not affect sub-diurnal amplitudes. Eastward propagating Madden–Julian Oscillations and convectively coupled Kelvin waves impact both the mean and sub-diurnal tropospheric wind variability. Both horizontal and vertical winds show the largest variability in the lower and mid troposphere (below 400 hPa). The observed variability in the vertical wind component highlights that large-scale phenomena interact with both the local evolution and progression of a cloud field through dynamical feedback.

The Performance of a Time-Varying Filter Time Under Stable Conditions over Mountainous Terrain

Eddy-covariance data from five stations in the Inn Valley, Austria, are analyzed for stable conditions to determine the gap scale that separates turbulent from large-scale, non-turbulent motions. The gap scale is identified from (co)spectra calculated from different variables using both Fourier analysis and multi-resolution flux decomposition. A correlation is found between the gap scale and the mean wind speed and stability parameter z/L that is used to determine a time-varying filter time, whose performance in separating turbulent and non-turbulent motions is compared to the performance of constant filter times between 0.5 and 30 min. The impact of applying different filter times on the turbulence statistics depends on the parameter and location, with a comparatively smaller impact on the variance of the vertical wind component than on the horizontal components and the turbulent fluxes. Results indicate that a time-varying filter time based on a multi-variable fit taking both mean wind speed and stability into account and a constant filter time of 2–3 min perform best in that they remove most of the non-turbulent motions while at the same time capturing most of the turbulence. For the studied sites and conditions, a time-varying filter time does not outperform a well chosen constant filter time because of relatively small variations in the filter time predicted by the correlation with mean flow parameters.

Modeling Tool for Estimating Carbon Dioxide Fluxes over a Non-Uniform Boreal Peatland

We present a modeling tool capable of computing carbon dioxide (CO2) fluxes over a non-uniform boreal peatland. The three-dimensional (3D) hydrodynamic model is based on the “one-and-a-half” closure scheme of the system of the Reynolds-Averaged Navier–Stokes and continuity equations. Despite simplifications used in the turbulence description, the model allowed obtaining the spatial steady-state distribution of the averaged wind velocities and coefficients of turbulent exchange within the atmospheric surface layer, taking into account the surface heterogeneity. The spatial pattern of CO2 fluxes within and above a plant canopy is derived using the “diffusion–reaction–advection” equation. The model was applied to estimate the spatial heterogeneity of CO2 fluxes over a non-uniform boreal ombrotrophic peatland, Staroselsky Moch, in the Tver region of European Russia. The modeling results showed a significant effect of vegetation heterogeneity on the spatial pattern of vertical and horizontal wind components and on vertical and horizontal CO2 flux distributions. Maximal airflow disturbances were detected in the near-surface layer at the windward and leeward forest edges. The forest edges were also characterized by maximum rates of horizontal CO2 fluxes. Modeled turbulent CO2 fluxes were compared with the mid-day eddy covariance flux measurements in the southern part of the peatland. A very good agreement of modeled and measured fluxes (R2 = 0.86, p < 0.05) was found. Comparisons of the vertical profiles of CO2 fluxes over the entire peatland area and at the flux tower location showed significant differences between these fluxes, depending on the prevailing wind direction and the height above the ground.

New Parameterizations of Turbulence Statistics for the Atmospheric Surface Layer

Abstract Recent work has shown that bulk-Richardson (Rib) parameterizations for friction velocity, sensible heat flux, and latent heat flux have similar, and in some instances better, performance than long-standing parameterizations from Monin–Obukhov similarity theory (MOST). In this work, we expanded upon new Rib parameterizations and developed parameterizations of turbulence statistics, i.e., standard deviations in the 30-min u (horizontal), υ (meridional), and w (vertical) wind components (i.e., σu, συ, and σw, respectively), which allowed us to derive Rib-based parameterizations of turbulent kinetic energy (e), and standard deviations in the 30-min temperature and moisture measurements (σθ and σq, respectively). We used datasets from three 10-m micrometeorological towers installed during the Land Atmosphere Feedback Experiment (LAFE) conducted in Oklahoma from 1 to 31 August 2017 and evaluated the new parameterizations by comparing them against parameterizations from MOST. We used the LAFE datasets and fully independent datasets obtained from two micrometeorological towers installed in Alabama between February 2016 and April 2017 to evaluate the performance of the parameterizations. Based on the slope of the relationship between the observed and parameterized turbulence statistics (mb) and the coefficient of correlation (r), we found that the Rib relationships generally performed better than MOST at parameterizing συ, σw, σθ, and σq, and the Rib relationships performed better at low wind speeds than at high wind speeds. These results, coupled with recent developments of Rib parameterizations for surface-layer momentum, heat, and moisture fluxes, provide further evidence to consider using Rib-based parameterizations in weather forecasting models. Significance Statement Deficiencies in Monin–Obukhov similarity theory (MOST) are well known, yet MOST forms the basis in weather forecasting models for describing heat, moisture, and momentum transfer between the land surface and atmosphere. We expanded upon previous work suggesting a MOST alternative called the bulk-Richardson approach. We used data collected from meteorological towers installed in Oklahoma and compared the bulk-Richardson approach with MOST. We evaluated these two approaches using data from meteorological towers installed in Oklahoma and Alabama and found that, overall, the bulk-Richardson approach performed better than MOST in determining the 30-min variability in temperature, moisture, and wind. This result provides additional motivation to use a bulk-Richardson approach in weather forecasting models because doing so will likely yield improved forecasts.

Particulate Matter Concentrations over South Korea: Impact of Meteorology and Other Pollutants

Air pollution is a serious challenge in South Korea and worldwide, and negatively impacts human health and mortality rates. To assess air quality and the spatiotemporal characteristics of atmospheric particulate matter (PM), PM concentrations were compared with meteorological conditions and the concentrations of other airborne pollutants over South Korea from 2015 to 2020, using different linear and non-linear models such as linear regression, generalized additive, and multivariable linear regression models. The results showed that meteorological conditions played a significant role in the formation, transportation, and deposition of air pollutants. PM2.5 levels peaked in January, while PM10 levels peaked in April. Both were at their lowest levels in July. Further, PM2.5 was the highest during winter, followed by spring, autumn, and summer, whereas PM10 was the highest in spring followed by winter, autumn, and summer. PM concentrations were negatively correlated with temperature, relative humidity, and precipitation. Wind speed had an inverse relationship with air quality; zonal and vertical wind components were positively and negatively correlated with PM, respectively. Furthermore, CO, black carbon, SO2, and SO4 had a positive relationship with PM. The impact of transboundary air pollution on PM concentration in South Korea was also elucidated using air mass trajectories.

Simulation of a Hailstorm Event over Bangladesh using WRF Model

An attempt is made to simulate a pre-monsoon hailstorm event over Bangladesh using the high-resolution Weather Research and Forecasting (WRF) model. A severe hailstorm event embedded with multi-cell thunderstorm occurred over the North-central region of Bangladesh and adjacent West Bengal state of India on 30 March 2018, which moved towards the South-East direction up to its highest intensity stage during 0900-1030 UTC is selected for the study. The WRF model is used to investigate the dynamical and thermodynamical characteristics of the selected hail producing thunderstorm event. The model is run on a double nested domain at 9 and 3 km horizontal resolutions using the Kain-Fritsch (new Eta) cumulus and Mellor-Yamada Nakanishi and Niino Level 2.5 (MYNN2) planetary boundary layer parameterization schemes. The analysis of the accumulated hail, mean sea level pressure, vertical wind shear, vertical (w) component of wind, relative humidity along with low level wind, convective available potential energy (CAPE) and convective inhibition energy (CINE), rainfall (mm) parameters are investigated. Rainfall is compared with that of observed value. The high amount of relative humidity (>90%), CAPE (>3000 JKg-1) and vertical wind shear (>40 ms-1) help to form severe convective storm which vertically developed upto 150 hPa level. In addition, the high value of w component of winds (~0.4 ms-1) helps updraft process which is favorable for hail formation. The present combination of parameterization schemes of model is found suitable to capture the selected hailstorm event reasonably well, but more similar hailstorm events need to be studied to make any final conclusions. The Dhaka University Journal of Earth and Environmental Sciences, Centennial Special Volume June 2022: 123-130

How does the rotational direction of an upwind turbine affect its downwind neighbour?

Wind-turbine blades rotate in clockwise direction looking downstream on the rotor. During daytime conditions of the atmospheric boundary layer, the rotational direction has no influence on the turbine wakes. In stably stratified conditions occurring during night, the atmospheric inflow is often characterized by a veering inflow describing a clockwise wind direction change with height in the Northern Hemisphere. A changing wind direction with height interacting with the rotor impacts its wake characteristics (wake elongation, width and deflection). We investigate the impact on the turbine performance (streamwise velocity for power, turbulence kinetic energy for loading) of a downwind turbine considering the four possible combinations of rotational directions of two 5 MW NREL rotors by means of large-eddy simulations. A counterclockwise rotating upwind turbine results in a 4.1% increase of the rotor averaged inflow velocity at the downwind rotor in comparison to a common clockwise rotating upwind turbine rotor. In case of two counterclockwise rotating rotors, the increase is 4.5%. This increase in streamwise velocity is accompanied by a 3.7% increase in rotor averaged turbulence kinetic energy. The performance difference of the downwind rotor (+4.8% increase of cumulative power of both wind turbines, if the upwind rotor rotates counterclockwise) results from the rotational direction dependent amplification or weakening of the spanwise and the vertical wind components, which is the result of the superposition of veering inflow and upwind rotor rotation.

The effect of static pressure-wind covariance on vertical carbon dioxide exchange at a windy subalpine forest site

Accounting for turbulent temperature and water vapor fluctuations on the vertical flux of CO2 measured by an open-path infrared gas analyzer are commonly known as the Webb–Pearman–Leuning (WPL) corrections. Static pressure fluctuations also affect air sample density, but the magnitude and effect of these changes on CO2 flux is not usually considered. In our study, the turbulent static pressure p and the vertical wind component w covariance w′p′¯ from just-above a Rocky Mountain subalpine forest are examined. The magnitude of w′p′¯ was highly correlated to the mean horizontal wind speed U with similar characteristics during the daytime and nighttime. The pressure term was calculated from w′p′¯ and compared to other terms in the equation for the vertical net ecosystem exchange of CO2 (NEE). We found the following: (1) for U≲6 m s−1, the pressure term was small, (2) as U increased beyond 6 m s−1, the pressure term became more and more important reaching a magnitude of ≈ 2 μmol m−2 s−1 at U of 12 m s−1 (for leaf area index LAI ≈ 2.6 m2 m−2), (3) for a more dense forest (LAI ≈ 4.8 m2 m−2), the magnitude of the pressure term at U of 12 m s−1 was ≈ 1 μmol m−2 s−1, or about half the open forest value, and (4) based on 14 years of measurements, the interannual mean and standard deviation of the yearly cumulative pressure and NEE terms were −32.8± 24.5 g C m−2 year−1 and −147.4± 231.7 g C m−2 year−1, respectively. Therefore, on average, including the pressure term in the NEE calculation reduces the annual carbon uptake by the GLEES forest from 147.4 g C m−2 year−1 to 114.6 g C m−2 year−1. This implies that carbon uptake by forests in windy locations that ignore the pressure term can be overestimated by as much as 20%. However, the year-to-year variability of both NEE and the pressure term was large due to changes in the forest structure and LAI from beetle attack and tree die-off. Finally, we present results from sensor-manipulation experiments examining the effect of tilting the quad-disk pressure ports on the turbulent static pressure.

Characterization of vertical wind velocity variability based on fractal dimension analysis

Abstract Modelling and forecasting the temporal behaviour of wind is of substantial importance in a variety of applications, which depend largely on the understanding and quantification of its variability. Existing literature focusing on wind speed variability is predominantly associated with horizontal wind component, whereas the knowledge of vertical wind component is much less documented. This paper investigates the variability of vertical wind velocity in the context of fractal dimension analysis. It is shown that the time series of vertical wind velocity are monofractals and weak anti-persistent. The median value of fractal dimension decreases from 1.582 ​at 10 ​m height to 1.547 ​at 320 ​m height. The vertical profile of fractal dimension is strongly tied with atmospheric stability condition, in terms of both magnitude and profile shape, and the deviation is most pronounced at higher levels. Moreover, the fractal dimension is found to increase with the increase in the mean horizontal wind speed, which can be partly attributed to the effect of atmospheric stability. Such relationship is also height-dependent. Similar to the horizontal wind component, the fractal dimension of vertical wind component also exhibits a positive correlation with window width.

Wave-slope soaring of the brown pelican

BackgroundFrom the laboratory at Scripps Institution of Oceanography, it is common to see the brown pelican (Pelecanus occidentalis) traveling along the crests of ocean waves just offshore of the surf-zone. When flying in this manner, the birds can travel long distances without flapping, centimeters above the ocean’s surface. Here we derive a theoretical framework for assessing the energetic savings related to this behavior, ‘wave-slope soaring,’ in which an organism in flight takes advantage of localized updrafts caused by traveling ocean surface gravity waves.MethodsThe energy cost of steady, constant altitude flight in and out of ground effect are analyzed as controls. Potential flow theory is used to quantify the ocean wave-induced wind associated with near-shoaling, weakly nonlinear, shallow water ocean surface gravity waves moving through an atmosphere initially at rest. Using perturbation theory and the Green’s function for Laplace’s equation in 2D with Dirichlet boundary conditions, we obtain integrals for the horizontal and vertical components of the wave-induced wind in a frame of reference moving with the wave. Wave-slope soaring flight is then analyzed using an energetics-based approach for waves under a range of ocean conditions and the body plan of P. occidentalis.ResultsFor ground effect flight, we calculate a ∼15 - 25% reduction in cost of transport as compared with steady, level flight out of ground effect. When wave-slope soaring is employed at flight heights ∼2m in typical ocean conditions (2m wave height, 15s period), we calculate 60-70% reduction in cost of transport as compared with flight in ground effect. A relatively small increase in swell amplitude or decrease in flight height allows up to 100% of the cost of transport to be offset by wave-slope soaring behavior.ConclusionsThe theoretical development presented here suggests there are energy savings associated with wave-slope soaring. Individual brown pelicans may significantly decrease their cost of transport utilizing this mode of flight under typical ocean conditions. Thus wave-slope soaring may provide fitness benefit to these highly mobile organisms that depend on patchy prey distribution over large home ranges.

The dual-field-of-view polarization lidar technique: a new concept in monitoring aerosol effects in liquid-water clouds – case studies

Abstract. In a companion article (Jimenez et al., 2020), we introduced a new lidar method to derive microphysical properties of liquid-water clouds (cloud extinction coefficient, droplet effective radius, liquid-water content, cloud droplet number concentration Nd) at a height of 50–100 m above the cloud base together with aerosol information (aerosol extinction coefficients, cloud condensation nuclei concentration NCCN) below the cloud layer so that detailed studies of the influence of given aerosol conditions on the evolution of liquid-water cloud layers with high temporal resolution solely based on lidar observations have become possible now. The novel cloud retrieval technique makes use of lidar observations of the volume linear depolarization ratio at two different receiver field of views (FOVs). In this article, Part 2, the new dual-FOV polarization lidar technique is applied to cloud measurements in pristine marine conditions at Punta Arenas in southern Chile. A multiwavelength polarization Raman lidar, upgraded by integrating a second polarization-sensitive channel to permit depolarization ratio observations at two FOVs, was used for these measurements at the southernmost tip of South America. Two case studies are presented to demonstrate the potential of the new lidar technique. Successful aerosol–cloud-interaction (ACI) studies based on measurements with the upgraded aerosol–cloud lidar in combination with a Doppler lidar of the vertical wind component could be carried out with 1 min temporal resolution at these pristine conditions. In a stratocumulus layer at the top of the convective boundary layer, we found values of Nd and NCCN (for 0.2 % water supersaturation) ranging from 15–100 and 75–200 cm−3, respectively, during updraft periods. The studies of the aerosol impact on cloud properties yielded ACI values close to 1. The impact of aerosol water uptake on the ACI studies was analyzed with the result that the highest ACI values were obtained when considering aerosol proxies (light-extinction coefficient αpar or NCCN) measured at heights about 500 m below the cloud base (and thus for dry aerosol conditions).

Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer?

Abstract. Stably stratified atmospheric boundary layers are often characterized by a veering wind profile, in which the wind direction changes clockwise with height in the Northern Hemisphere. Wind-turbine wakes respond to this veer in the incoming wind by stretching from a circular shape into an ellipsoid. We investigate the relationship between this stretching and the direction of the turbine rotation by means of large-eddy simulations. Clockwise rotating, counterclockwise rotating, and non-rotating actuator disc turbines are embedded in wind fields of a precursor simulation with no wind veer and in wind fields with a Northern Hemispheric Ekman spiral, resulting in six combinations of rotor rotation and inflow wind condition. The wake strength, extension, width, and deflection depend on the interaction of the meridional component of Ekman spiral with the rotational direction of the actuator disc, whereas the direction of the disc rotation only marginally modifies the wake if no veer is present. The differences result from the amplification or weakening/reversion of the spanwise and the vertical wind components due to the effect of the superposed disc rotation. They are also present in the streamwise wind component of the wake and in the total turbulence intensity. In the case of an counterclockwise rotating actuator disc, the spanwise and vertical wind components increase directly behind the rotor, resulting in the same rotational direction in the whole wake while its strength decreases downwind. In the case of a clockwise rotating actuator disc, however, the spanwise and vertical wind components of the near wake are weakened or even reversed in comparison to the inflow. This weakening/reversion results in a downwind increase in the strength of the flow rotation in the wake or even a different rotational direction in the near wake in comparison to the far wake. The physical mechanism responsible for this difference can be explained by a simple linear superposition of a veering inflow with a Rankine vortex.

Validation of the quasi-steady performance model for pumping airborne wind energy systems

The quasi-steady performance model (QSM) has been developed specifically for pumping airborne wind energy systems using flexible membrane wings. In this study, we validate this model using a comprehensive set of flight data that includes 87 consecutive pumping cycles and is acquired with the development platform of Kitepower B.V. The aerodynamic properties of the kite are determined using onboard measurements of the relative flow velocity. We found that neglecting the vertical wind component and straightening and slacking motion of the tether lead to substantial errors in the kite velocity calculated using the system model. A reasonable agreement between the QSM simulations and flight data can be obtained by multiplying the kite’s drag coefficient by a fudge factor and thereby turning the QSM into a grey-box model. The model accuracy is statistically evaluated as opposed to only evaluating a single pumping cycle per system configuration as done in earlier research.

Impacts of MJO on the Intraseasonal Temperature Variation in East Asia

AbstractThe Madden–Julian oscillation (MJO) is closely related to the intraseasonal variability of surface temperature in East Asia. It has been shown that significant cold surface temperature anomalies are observed in East Asia during MJO phase 3. However, the cooling tendency develops prior to phase 3, suggesting that the cold surface anomalies in East Asia are a delayed and accumulated response to the MJO forcings prior to phase 3. Here, using a thermodynamic equation, it is shown that both meridional advection and adiabatic cooling terms associated with the MJO flow are the dominant contributors to the cooling tendency. The meridional cold advection initially manifests in East Asia in the form of northerly wind anomalies in the eastern part of anticyclonic circulation anomalies that are centered over eastern Europe and develop before the establishment of the cold anomalies. It is suggested that the enhanced convection in the western North Pacific Ocean is responsible for the anomalous anticyclonic flow over eastern Europe with about a 10-day lag via a meridionally propagating Rossby wave train. Further, cooling by the vertical wind component in East Asia is a result of adiabatic cooling interpreted as a reversed local overturning circulation, with downward motion in the tropics and upward motion in the subtropics. This anomalous meridional overturning circulation process initiated from suppressed convection spanning the tropical Indian Ocean to East Asia also takes about 10 days. Therefore, both the Rossby wave propagation and a local overturning circulation induced by the tropical convections play an important role in driving the lagged response of cold surface anomalies in East Asia. Interestingly, these tropical convection forcings are similar to the typical dipole pattern in convection during MJO phase 7, with suppressed Indian Ocean convection and enhanced western North Pacific convection. This implies that the dipole convective forcing during MJO phase 7 possibly leads to the cold anomalies in East Asia following phase 3.

An analytical model for rapid estimation of hurricane supergradient winds

The supergradient winds that may have severe implications on the wind design of high-rise buildings have been commonly observed in the hurricane boundary layer. However, the widely-used log-law or power-law wind profile excludes the supergradient-wind region in which the tangential winds are larger than the gradient winds. Although high-fidelity, nonlinear hurricane wind models may well capture the supergradient winds, high computational demand is needed for each simulation. Recently developed linear, height-resolving hurricane wind models, while can efficiently consider the existence of supergradient winds, significantly underestimate them due essentially to the ignorance of vertical advection term in the governing equations. A number of studies have actually demonstrated that the vertical advection is a major contributor to the transfer of horizontal momentum to the supergradient region. To this end, a refined analytical model that simultaneously integrates the horizontal advection, vertical advection and vertical diffusion terms into the governing equations is developed for accurately and efficiently estimating the hurricane supergradient winds. The important role of the vertical wind speed in determining the horizontal wind speeds (including supergradient winds) in the hurricane boundary layer is highlighted. Since the horizontal and vertical wind components are mutually dependent, the iteration technique is utilized to solve the proposed analytical model. The consideration of the vertical advection results in intensified supergradient winds that are consistent with the observations. Furthermore, a strong outflow region in the vicinity of the radius of maximum winds due to the supergradient winds can be obtained. Due to its simplicity and computational efficiency, the developed analytical model can be easily implemented in the Monte Carlo simulations for the rapid assessment of hurricane wind risk to coastal structures, especially to high-rise buildings.

Retrieval of Horizontal Visibility Using MODIS Data: A Deep Learning Approach

Horizontal visibility (HVIS) is a primary index used for assessing air quality. Although satellite images provide information regarding atmospheric aerosols, atmospheric visibility is not directly measured. In this paper, a deep learning approach is proposed to retrieve HVIS using moderate-resolution imaging spectroradiometer (MODIS) aerosol optical depth (AOD) data, the European Centre for Medium-Range Weather Forecasts reanalysis dataset, and ground-based visibility observations. The deep neural network model comprises a multi-layer unsupervised restricted Boltzmann machine (RBM) and a layer for supervised learning. The dropout mechanism was used in the training process to overcome the errors caused by over-fitting. The results demonstrate that the correlation coefficient values between HVIS observations and retrievals during training, pre-validating, and evaluation were 0.74, 0.723, and 0.697, respectively. The retrieved HVIS in Eastern China exhibited a north-to-south increasing trend, increasing and decreasing in summer and winter, respectively. In conclusion, the proposed model presents an effective and more reliable method for HVIS retrieval. However, the small samples, low AOD, low albedo, high total column water, high longitude, and the low vertical wind component at 10 m likely cause HVIS bias.