Surface-layer Wind Research Articles

Nonstationary near-ground wind characteristics and wind-induced pressures on the roof of a low-rise building during a typhoon

Wind fields in the surface layer during typhoons usually have strong nonstationary features, which cause damage to low-rise buildings, especially their roofs. It is thus required to establish accurate models of typhoon-generated near-surface wind fields and assess typhoon effects on low-rise buildings. In this paper, a framework of extracting time-varying mean component and determining time-varying fluctuating component of nonstationary signals is established, which is applied to investigate the on-site measured near-ground nonstationary wind velocities and wind-induced pressures on the roof of a typical low-rise building during a typhoon. It is observed that near-ground wind characteristic parameters (including turbulence intensities, gust factors, peak factors and turbulence integral length scales, wind spectra) and wind pressure coefficients on the roof obtained by the nonstationary analytical approach are more reasonable than those determined by the conventional stationary analytical approach. Besides, the evolutionary power spectral density is used to examine time-frequency spectra of wind velocities and wind pressures. This comparative study based on the nonstationary and stationary analysis approaches aims to further the understanding of the nonstationary surface layer wind characteristics and wind pressures on typical low-rise buildings for reduction of windstorm damages to residential buildings in typhoon-prone regions.

Surface-Layer Wind Shear and Momentum Transport From Clear-Sky to Cloudy Weather Regimes Over Land.

This study investigates how wind shear and momentum fluxes in the surface‐ and boundary layer vary across wind and cloud regimes. We use a 9‐year‐long data set from the Cabauw observatory complemented by (8.2 × 8.2 km2) daily Large Eddy Simulation (LES) hindcasts. An automated algorithm classifies observed and simulated days into different cloud regimes: (a) clear‐sky days, (b) days with shallow convective clouds rooted in the surface layer, with two ranges of cloud cover, and (c) non‐convective cloud days. Categorized days in observations and LES do not always match, particularly the number of non‐convective cloud days are underestimated in the LES, which likes to develop convection. However, the climatology and diurnal cycle of winds for each regime are very similar in LES and observations, strengthening our confidence in LES’ skill to reproduce certain clouds for certain atmospheric states. Along‐wind momentum flux profiles are similar across all regimes, but large cloud cover (convective and non‐convective) days have larger total momentum flux distributed over a deeper layer, with up to 30% of the surface flux still present near cloud base. The clear‐sky and especially shallow cumulus regime with low cloud cover have notably larger crosswind momentum fluxes in the boundary layer. Surface‐layer wind shear at daytime is smallest in the shallow cumulus regimes, having deeper boundary layers and a steady increase in surface layer wind speed during daytime. Compared to clear‐sky days at a similar stability, convective cloud regimes have smaller surface‐layer wind shear and larger surface friction than estimated by Monin‐Obukhov Similarity Theory.

The influence of mesoscale land–sea breeze circulation on local wind climatology in the Svalbard fjords of Kongsfjorden and Hornsund

AbstractWe analyse data series (1992–2013) of wind measurements from meteorological stations in Ny‐Ålesund and Hornsund on Svalbard and then have them compared to surface layer winds from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. We find significant discrepancies between the local wind direction and directions of wind compatible with the analysis of large‐scale pressure fields. We argue that one of the most important factors controlling wind directions in the Svalbard fjords is the difference in temperature between the neighbouring glaciers and surface sea temperatures of open waters warmed by the west Spitsbergen current. This creates atmospheric circulation patterns similar to the land breeze in temperate climates. We show that the frequency of simultaneous breeze‐type circulation events on the northern and southern fjords of Spitsbergen is highly correlated with sea–land temperature difference on monthly timescales. A Monte Carlo analysis of breeze probabilities rejects the null hypothesis of independent breeze events in both fjords. This result shows that breeze events are not independent where two fjords placed at opposite ends of Spitsbergen's west coast are concerned, and we also posit that the breeze occurrence is largely controlled by the synoptic situation. Such large‐scale wind phenomena should have an impact on air–sea heat fluxes to the east of Spitsbergen.

Non-Gaussian Lagrangian Stochastic Model for Wind Field Simulation in the Surface Layer

Wind field simulation in the surface layer is often used to manage natural resources in terms of air quality, gene flow (through pollen drift), and plant disease transmission (spore dispersion). Although Lagrangian stochastic (LS) models describe stochastic wind behaviors, such models assume that wind velocities follow Gaussian distributions. However, measured surface-layer wind velocities show a strong skewness and kurtosis. This paper presents an improved model, a non-Gaussian LS model, which incorporates controllable non-Gaussian random variables to simulate the targeted non-Gaussian velocity distribution with more accurate skewness and kurtosis. Wind velocity statistics generated by the non-Gaussian model are evaluated by using the field data from the Cooperative Atmospheric Surface Exchange Study, October 1999 experimental dataset and comparing the data with statistics from the original Gaussian model. Results show that the non-Gaussian model improves the wind trajectory simulation by stably producing precise skewness and kurtosis in simulated wind velocities without sacrificing other features of the traditional Gaussian LS model, such as the accuracy in the mean and variance of simulated velocities. This improvement also leads to better accuracy in friction velocity (i.e., a coupling of three-dimensional velocities). The model can also accommodate various non-Gaussian wind fields and a wide range of skewness–kurtosis combinations. Moreover, improved skewness and kurtosis in the simulated velocity will result in a significantly different dispersion for wind/particle simulations. Thus, the non-Gaussian model is worth applying to wind field simulation in the surface layer.

On the Application of Linear Regression to Surface-Layer Wind Profiles for Deducing Roughness Length and Friction Velocity

In a steady, spatially homogeneous neutral stability flow dominated by mechanical turbulence, the friction velocity ($$ u_{*} $$) and the roughness length (z0) can be estimated by applying linear regression to measurements of wind speed (u) at several heights (z). Surface-layer wind profile plots in the scientific literature normally have ln(z) on the vertical axis and u on the abscissa, which might suggest a linear regression, such as the ordinary least-squares method, which minimizes the vertical residuals, i.e., the ln(z) deviations, from the fitted line. Here, we show that ordinary least-squares fitting of the profile data is sensitive to the choice of the variable whose residuals are being minimized, and that the linear regression should be computed as u versus ln(z), i.e., minimizing the u deviations. This is equivalent to ln(z) being the independent variable and u the dependent variable. The differences in the estimated values of $$ u_{*} $$ and z0 compared to those resulting from the ln(z) versus u linear regression can be expressed as a function of the coefficient of determination (r2) of the wind-profile data. Applying the ordinary least-squares method while minimizing the deviations of ln(z) leads to systematic overestimation of the $$ u_{*} $$ and z0 values. Using these values as input into the atmospheric dispersion model AERMOD leads to increased shear-induced turbulence and consequently enhanced dilution of the plume.

Seasonal variations of sea breeze and its effect on the spectral behaviour of surface layer winds in the coastal zone near Visakhapatnam, India

Seasonal variation of sea breeze (SB) characteristics and its effect on turbulence spectra at Visakhapatnam (17.7°N, 83.3°E) located at east coast of Peninsular India are investigated by considering 244 sea breeze events during December 2012 to March 2014. The delayed onset of backdoor SB during winter occurs due to the southward component of shore-parallel background winds while the prevailing northward component of background wind enable the early onset of corkscrew SB during pre-monsoon and summer monsoon. The turbulence spectral peak of horizontal winds shifts to higher frequency side after SB onset.

WRF model sensitivity to choice of PBL and microphysics parameterization for an advection fog event at Barkachha, rural site in the Indo-Gangetic basin, India

The present study evaluates the performance of four planetary boundary layer (PBL) parameterization schemes combined with five cloud microphysics schemes in Weather Research Forecasting (WRF) model, specifically for an advection fog event occurred during 4–6 December 2014 at Barkachha, rural site in the Indo-Gangetic plain (IGP). For this purpose, the model was configured over the IGP with 2-km horizontal resolution, and results are compared with detailed micrometeorological data (surface meteorological parameters and fluxes, radiative fluxes, and surface layer wind profiles) gathered during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Integrated Ground Observational Campaign (IGOC) site located in the IGP. The meteorological conditions conducive for the fog formation have been evaluated. All of the tested PBL-microphysics combination showed substantial bias for surface temperature, radiation fluxes, and wind speed. None of the combination found to be superior in predicting the fog event; however, the local MYNN2.5 combination with the WSM3, WSM6, and Lin microphysics obtained slightly better result at the study location. In general, judging from all simulations of liquid water content (as an indicator for the fog), the above combinations were able to simulate the current fog event but the fog onset, duration, and dissipation were particularly offset.

A new gust parameterization for weather prediction models

We analyze Uruguyan measurements, focusing on gusts at turbine hub height. Large gusts, exceeding 15 m/s, are observed to occur when the surface layer (as assessed by the bulk Richardson number Ri) is neutral, and are uncommon when the layer is stable or even unstable. Gust factors (the ratio of the gusts to the mean winds) are inversely related to the mean winds but increase as the atmosphere becomes more unstable. Numerical simulations using different planetary boundary layer (PBL) schemes and mesoscale grid resolutions are employed in the development of a gust parameterization (GP) utilizing forecasts of surface layer Ri and winds above hub height. The GP, which provides gust factors to be applied to predicted turbine-level winds, provides higher skill at relatively coarser resolution than a simpler algorithm based solely on surface layer information, although its success is strongly dependent on the PBL and surface layer schemes selected.

Field measurements of wind effects on a low-rise building with roof overhang during typhoons

Typhoon-induced disasters caused significant economic losses and heavy causalities, most of which resulted from the damage or collapse of low-rise buildings. It is thus necessary to understand the characteristics of typhoon-generated near-surface wind and assess its effects on typical low-rise buildings. In this study, a full-scale low-rise building built in a typhoon-prone region has been instrumented for investigation of the typhoon-induced wind effects on it. This paper presents analysed results measured from the instrumented low-rise building during Typhoons Mujigae and Sarika. The near-surface turbulence intensity, gust factor, turbulence integral length scale and power spectral density were analyzed and discussed. Moreover, the wind-induced pressures on the roof ridge and overhang of the building where were subjected to the most severe suctions under quartering winds are presented and discussed. This field measurement study aims to enhance the understanding of surface-layer wind structures and the wind effects on typical low-rise buildings during typhoons in an effort to reduce windstorm damages to residential buildings.

In-flight wind identification and soft landing control for autonomous unmanned powered parafoils

ABSTRACTFor autonomous unmanned powered parafoil, the ability to perform a final flare manoeuvre against the wind direction can allow a considerable reduction of horizontal and vertical velocities at impact, enabling a soft landing for a safe delivery of sensible loads; the lack of knowledge about the surface-layer winds will result in messing up terminal flare manoeuvre. Moreover, unknown or erroneous winds can also prevent the parafoil system from reaching the target area. To realize accurate trajectory tracking and terminal soft landing in the unknown wind environment, an efficient in-flight wind identification method merely using Global Positioning System (GPS) data and recursive least square method is proposed to online identify the variable wind information. Furthermore, a novel linear extended state observation filter is proposed to filter the groundspeed of the powered parafoil system calculated by the GPS information to provide a best estimation of the present wind during flight. Simulation experiments and real airdrop tests demonstrate the great ability of this method to in-flight identify the variable wind field, and it can benefit the powered parafoil system to fulfil accurate tracking control and a soft landing in the unknown wind field with high landing accuracy and strong wind-resistance ability.

Extending the diabatic surface layer wind shear profile for offshore wind energy

In this research the diabatic surface layer wind shear model is extended for offshore wind energy purposes to higher altitudes based on Gryning's wind profile and the resistance functions proposed by Byun. The wind profile is in theory applicable up to the boundary layer height, which is parametrized with the Rossby-Montgommery equation. The coefficient c of the Rossby-Montgommery equation is found to be stability dependent with decreasing values up to 0.04 for stable conditions and increasing values up to 0.17 for unstable conditions. The proposed shear profile has been validated with 1 year of offshore observation data, and a significant improvement in accuracy is found compared to traditional surface layer shear profiles or power laws. The influence of adopting this extended shear profile for wind energy is analysed in terms of the kinetic energy flux and blade root fatigue loads experienced by a wind turbine. It is found that, especially for stable conditions, results deviate significantly compared to using the traditional surface layer shear profile. The kinetic energy flux decreases by up to 15%.

Observed variability in the upper layers at the Equator, 90°E in the Indian Ocean during 2001–2008, 1: zonal currents

The observed variability of zonal currents (ZC) at the Equator, 90°E shows a strong seasonal cycle in the near-surface 40–350 m water column with periodic east–west reversals most pronounced at semiannual frequency. Superposed on this, a strong intraseasonal variability of 30–90 day periodicity is also prominently seen in the near-surface layer (40–80 m) almost throughout the year with the only exception of February–March. An eastward flowing equatorial undercurrent (EUC) is present in the depth range of 80–160 m during March–April and October–November. The observed intraseasonal variability in the near-surface layer is primarily determined by the equatorial zonal westerly wind bursts (WWBs) through local frictional coupling between the zonal flow in the surface layer and surface zonal winds and shows large interannual variability. The eastward flowing EUC maintained by the ZPG set up by the east–west slope of the thermocline remotely controlled by the zonal wind (ZW) and zonally propagating wave fields also shows significant interannual variability. This observed variability on interannual time scales appears to be controlled by the corresponding variability in the alongshore winds off the Somalia coast during the preceding boreal winter, the ZW field along the equator, and the associated zonally propagating Kelvin and Rossby waves. The salinity induced vertical stratification observed in the near-surface layer through barrier layer thickness (BLT) effects also shows a significant influence on the ZC field on intraseasonal time scale. Interestingly, among all the 8 years (2001–2008), relatively weaker annual cycle is seen in both ZC in the 40–350 m water column and boreal spring sea surface temperature (SST) only during 2001 and 2008 along the equator caused through propagating wave dynamics.

A surface-layer wind speed correction: A case-study of Darling station

In previous study, the vertical wind speed extrapolation from measurement station to modern turbine hubs over an open homogenous terrain was considered. It was presented that an assumption of wind shear exponent under different stability conditions was an inaccurate representation of the actual wind climates as the precise knowledge of the site's wind characteristics at different levels and seasons are essential for planning and implementation of a proposed energy project. In this study, the surface-layer wind speed correction at Darling using the WRF modeling with mesoscale terrain corrections is presented. An hourly mesoscale modeled winds at 3 km grid spacing obtained for one month are postprocessed for estimation of local wind speed profiles at 10 and 50 m height AGL. The sensitivity of the modeled winds to surface terrain corrections is investigated using mesoscale topography parameterizations. Furthermore, 6-hourly mesoscale modeled and satellite observed winds as well as measurements from Darling station are utilized for validation of the statistical downscaling method utilized for the postprocessing of the boundary layer winds over land. It is presented that the precision of the mesoscale modeled winds for local wind speed estimates at potential site without historical measurements can be significantly improved. The confidence in the validity of this methodology for local wind speed correction is estimated at 96–98%.

Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer*

Abstract Density currents (i.e., cold pools or outflows) beneath marine stratocumulus clouds are characterized using 30 days of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An air density increase criterion applied to the Improved Meteorological (IMET) sensor data identified 71 density current front, core (peak density), and tail (dissipating) zones. The similarity in speeds of the mean density current propagation speed (1.8 m s−1) and the mean cloud-level advection relative to the surface layer wind (1.9 m s−1) allowed drizzle cells to deposit elongated density currents in their wakes. Scanning Doppler lidar captured prefrontal updrafts with a mean intensity of 0.91 m s−1 and an average vertical extent of 800 m. Updrafts were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. The observed density currents were 5–10 times thinner and weaker than typical continental thunderstorm cold pools. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm day−1 over a 30-km diameter. Rather than peaking when rain rates were highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurred with shallow subcloud dry and stable layers. The dry layers may have contributed to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occurred in a large region of predominantly open cells but also occurred in regions of closed cells.

Addressing Spatial Variability of Surface-Layer Wind with Long-Range WindScanners

AbstractThis paper presents an analysis of mean wind measurements from a coordinated system of long-range WindScanners. From individual scan patterns the mean wind field was reconstructed over a large area, and hence it highlights the spatial variability. From comparison with sonic anemometers, the quality of the WindScanner data is high, although the fidelity of the estimated vertical velocity component is significantly limited by the elevation angles of the scanner heads. The system of long-range WindScanners presented in this paper is close to being fully operational, with the pilot study herein serving not only as a proof of concept but also verifying expectations of reliable wind measurements over arbitrary three-dimensional volumes, in future sustained meteorological campaigns.

Impacts of subgrid‐scale orography parameterization on simulated surface layer wind and monsoonal precipitation in the high‐resolution WRF model

AbstractThis paper reports on the first attempt to investigate whether excessive precipitation over mountainous areas, which is a common problem in model simulations, could be remedied by the implementation of a more realistic surface wind field in the high‐resolution Weather Research and Forecasting (WRF) model. A series of 48 h short‐range forecasts was conducted for the month of July 2006 within the triple‐nested WRF configuration, for which the highest resolution of 3 km was focused on areas with complex orography over East Asian monsoonal regions. For accurate surface wind simulations, the subgrid‐scale (SGS) orography parameterization scheme was employed. It was found that the simulated surface wind showed negative (positive) bias over mountainous (flat) regions when the SGS orography parameterization was excluded. After inclusion of the SGS orography parameterization, wind speed over mountainous (flat) regions increased (decreased), implying that the bias was mitigated. Moisture divergence (convergence) over the mountains (on the leeward side of the mountains) was induced, and surface latent heat flux increased along the mountain ranges following the improvement in the representation of the surface wind by the inclusion of the SGS orography parameterization. Eventually, excessive precipitation simulated over mountainous areas of East Asia, which is a feature commonly observed in numerical model studies, was alleviated because of the moisture divergence and increased surface latent heat flux.

Improve the Forecast of Surface-Layer Wind in Wind Power Farm with WRF-3DVAR

Numerical Weather Predication (NWP) is the main way to forecast the surface-layer wind of wind power farm. In order to improve the accuracy of the forecast with the existing observations, the WRF-3DVAR is used in two-way nested simulations with the data from National Centers for Environmental Prediction (NCEP) Final (FNL) Operational Global Analyses as initial and boundary conditions. Conventional data (sounding and surface observation) are assimilated through 3h assimilation cycles. The impacts of assimilation are examined through direct verification of the forecasts against the observations from conventional stations and the wind tower. It is found that the data assimilation can improve the accuracy of the forecast of surface-Layer wind up to 36h, and the improvement at conventional stations is more prominent than that at the wind tower.

Connections between the Spring Breakup of the Southern Hemisphere Polar Vortex, Stationary Waves, and Air–Sea Roughness

Abstract A robust connection between the drag on surface-layer winds and the stratospheric circulation is demonstrated in NASA's Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM). Specifically, an updated parameterization of roughness at the air–sea interface, in which surface roughness is increased for moderate wind speeds (4–20 m s−1), leads to a decrease in model biases in Southern Hemispheric ozone, polar cap temperature, stationary wave heat flux, and springtime vortex breakup. A dynamical mechanism is proposed whereby increased surface roughness leads to improved stationary waves. Increased surface roughness leads to anomalous eddy momentum flux convergence primarily in the Indian Ocean sector (where eddies are strongest climatologically) in September and October. The localization of the eddy momentum flux convergence anomaly in the Indian Ocean sector leads to a zonally asymmetric reduction in zonal wind and, by geostrophy, to a wavenumber-1 stationary wave pattern. This tropospheric stationary wave pattern leads to enhanced upward wave activity entering the stratosphere. The net effect is an improved Southern Hemisphere vortex: the vortex breaks up earlier in spring (i.e., the spring late-breakup bias is partially ameliorated) yet is no weaker in midwinter. More than half of the stratospheric biases appear to be related to the surface wind speed biases. As many other chemistry–climate models use a similar scheme for their surface-layer momentum exchange and have similar biases in the stratosphere, the authors expect that results from GEOSCCM may be relevant for other climate models.

Response of spectral characteristics of wind and temperature of atmospheric surface layer to the noontime annular solar eclipse

The influence of annular solar eclipse of 15 January 2010 on the spectral behaviour of surface layer winds and temperature has been investigated. At the peak of the eclipse, the magnitudes of spectral power of wind and the rate of dissipation of turbulent kinetic energy decreased by an order whereas, the temperature decreased by more than one order. A distinct spectral characteristic of wind components is noticed during the peak phase and for the first time, influence of surface wind speed and wind direction associated with the eclipse on the spectral characteristics of wind components has been investigated.

Improving the Representation of the Nocturnal Near-Neutral Surface Layer in the Urban Environment with a Mesoscale Atmospheric Model

A new approach to improve the representation of surface processes in the Global Environmental Multiscale (GEM) atmospheric model associated with the exchanges between the urban canopy and the atmosphere is presented. Effects of the urban canopy on the evolution of surface-layer wind, temperature, moisture, and turbulence are directly parametrized in order to allow realistic interactions between the canopy elements (i.e., roofs, roads, and walls) and the atmosphere at GEM’s multiple vertical levels that are positioned inside the canopy. Surface energy budgets as implemented in the Town Energy Balance (TEB) scheme have been used to determine temperatures of the urban canopy elements for the proposed multilayer scheme. Performance of the multilayer scheme is compared against standard implementations of the TEB scheme for one nighttime intensive observation period of the Joint Urban 2003 experiment held in Oklahoma City, USA. Although the new approach is found to have a negligible impact on urban surface-layer wind profiles, it improves the prediction of near-neutral nocturnal profiles of potential temperature close to the surface. The urban heat island effect is simulated with a better accuracy by the multilayer approach. The horizontal temperature gradient across the central business district of the city along the direction of flow is also reasonably well captured by the proposed scheme.