Diurnal Variations In Wind Speed Research Articles

Investigating the Diurnal Variation in Coastal Boundary Layer Winds on Hainan Island Using Three Tower Observations

This study analyzes wind structures up to 509 m in the atmospheric boundary layer in the coastal area of Hainan Island, using a dataset obtained from ultrasonic anemometers housed in three towers. The wind profile, consisting of the measurements from the three towers, followed logarithmic law. In a diurnal variation, the maximum wind speed occurred at night, with a greater component of northerly wind, while the minimum wind speed was observed at noon, with a greater component of easterly wind. The variation in wind speed suggests that the measurements were representative of the wind field in the upper part of the atmospheric boundary layer, and the variation in wind direction might be affected by sea and land breezes, which can be induced by the different thermal conditions of underlying surfaces. The diurnal variation in average wind speed ranged from 0.5 to 1.5 m s−1, and the diurnal variation in wind direction was 10–20 degrees. In our measurements, the diurnal trajectory of the wind vector was observed to be counterclockwise, which differs from previous studies conducted over uniform and flat underlying surfaces. This is partially due to the different thermodynamic conditions of the underlying land and sea surfaces. The impact of topographic relief on wind measurement is also discussed. The measurements suggest that wind speeds at altitudes above 50 m are less influenced by terrain. The height of the reversal layer, which is generated by the different diurnal variations in wind speed in the upper and lower parts of the boundary layer, was estimated to be around 300 m.

Spatiotemporal variations of 100 m wind in Mongolia and implications for wind energy resources

AbstractMongolia is a high‐altitude inland country, which experiences strong near‐surface winds that could be valuable resources for wind power generation. In this study, we examine the spatiotemporal variations of 100 m wind and assess wind energy potential in Mongolia using the European Centre for Medium‐Range Weather Forecasts Reanalysis version 5 (ERA5) data during 1979–2020. Mongolia exhibits two peaks in the monthly variation of 100 m wind, the higher peak in April and the lower peak in November. The eastern and southern (ES) region of the country is characterized by relatively high wind speed, night‐time diurnal maximum of wind speed and relatively small amplitudes of monthly and diurnal variations of wind speed. This region is classified to have good potential for wind energy development with a long mean episode length. On the other hand, the western and northern (WN) region is characterized by relatively low wind speed, afternoon diurnal maximum of wind speed and relatively large amplitudes of monthly and diurnal variations of wind speed. This region is classified to have less wind energy potential. Four hotspots which have the greatest wind energy potential are found near the eastern border of Mongolia, south of the Gobi‐Altai Mountains, near the southern border of Mongolia and in the southern region. Two principal modes of spatiotemporal variations of 100 m wind speed are obtained using the cyclostationary empirical orthogonal function (CSEOF) analysis method. The first CSEOF mode shows a spatial contrast between western and eastern Mongolia and is positively amplified in winter and negatively amplified in summer, which is associated with the seasonal change of the East Asian monsoon. The second mode is amplified in spring and autumn and shows mountain–basin contrasts that are reversed diurnally, which may be associated with the mountain–plains solenoid circulations and resultant vertical mixing of momentum.

Controls and rates of evaporation from a water supply reservoir in the Colorado Front Range

Evaporation can be a major component of a reservoir’s water balance in arid, populated regions such as the western United States, yet accurate measurements are scarce. In an effort to fill this observation gap, direct measurements of evaporation from Standley Lake, a water supply reservoir in the Colorado Front Range, were collected via the eddy covariance method from March 1, 2020 through February 28, 2021. Evaporation followed a seasonal pattern, with the spring, summer, and early fall experiencing daily cycles that peaked just after midday, with measurable evaporation occurring each day for approximately 4–6 h. In the late fall and winter, the daily cycles decayed and episodic enhancements in evaporation were common, most often lasting between 18 and 36 h, some longer. Evaporation peaked in June, aligning with peak net radiation, and summertime evaporation was predominantly driven by diurnal variations in wind speed and the vapor pressure gradient. Interestingly, fall and winter saw multiple periods of negative (downward) sensible heat transfer which helped drive wintertime evaporation. The study period evaporation totaled 1377 mm, with 35 % of that occurring in summer, 27 % in fall, 21 % in spring, and 17 % in winter. This study shows that evaporation is a significant part of the energy balance of Standley Lake, and with continued observations, enables a better understanding of the impacts a changing climate and increased water demands have on water resources.

Classifying diurnal changes of cyanobacterial blooms in Lake Taihu to identify hot patterns, seasons and hotspots based on hourly GOCI observations

Occurrence of cyanobacterial blooms in most lakes has dramatic changes in time and space. However, most current studies only focused on daily or seasonal scales to obtain a relatively coarse resolution result. To explore the possibility of fine changes occurring within a day in Lake Taihu (China), the area coverage of surface cyanobacterial blooms was quantified from the hourly Geostationary Ocean Color Imager (GOCI) data using a GOCI-derived cyanobacterial index. Based on that, diurnal change characteristics were explored at two scales, and the environmental impacts were investigated. For that, an classification method was first designed to identify the types of diurnal change patterns of cyanobacterial blooms automatically. This method classified the patterns into four types, including the decreasing (Type1), decreasing first and then increasing (Type2), increasing (Type3), increasing first and then decreasing (Type4). Based on that, the types of diurnal change patterns of blooms in Lake Taihu (from April 1, 2011 to October 31, 2020) were identified at pixel (500 m) and synoptic scales. Results indicated that Type1 and Type3 were two hot diurnal change patterns of blooms, and lakeshore was the hotspot occurring severe diurnal changes, and autumn was the hot season occurring frequent diurnal changes. Specifically, hotspot of Type1 was lakeshore, while hotspot of Type3 was Central Regions. Environmental impacts were analyzed at two scales. At pixel scale (500 m), diurnal variation of temperature affected the regional occurence of each type ofdiurnal changes patterns of blooms, and the afternoon temperature played the most critical role (p < 0.001, N = 8316). The occurrence frequency of Type1 was positively (R = 0.41) related with the afternoon temperature, and the occurrence frequency of Type3 was negatively (R = -0.37) related with it. Diurnal variation of wind speed was another key factor impacting the occurrence of obvious diurnal blooms changes, and the wind impacts should be distinguished when the wind speed was over or below 3.5 m/s. At synoptic scale, the interaction of multi environmental factors influenced the diurnal change degree of blooms area, and the environmental contributions were 71%.Comparing with the existing manual classifying workat synoptic scale, the designed classification method can identify the types of diurnal change patterns of blooms at a higher spatial resolution (500 m). These explorations on diurnal dynamics of cyanobacterial blooms in Lake Taihu provide a new insight for advanced cyanobacteria dynamics studies and regional water management.

Particle dynamics in stormwater treatment areas

In south Florida, USA, Stormwater Treatment Areas (STAs) are used to reduce Phosphorus (P) loading into the northern portion of the Everglades. Phosphorus is well retained in the STAs (e.g. P biological uptake and P-sorption on adequate substrata) but a small, yet significant portion of it, leaves the STAs as particulate P (PP) in the discharge water. Thus, this study was conducted to improve the understanding of mechanisms and factors that affect PP removal efficiency of the STAs. Specifically, this study employed a suite of field and analytical methods to determine the effects of hydraulic conditions on the erosion, entrainment, transport, and settling of particles in the STAs. Prediction of the flux of sediment by numerical models requires estimates of the density, settling velocity, size, and fractal dimension of the particles and estimates of the critical bed shear velocity. We found that the STA particles are porous aggregates as in other similar shallow water bodies, but have a relatively high settling velocity, likely due to the high amount of CaCO3 and relatively low amount of organic matter in the sediment. Correlations between diurnal variations in windspeed and suspended sediment concentrations suggest that wind generated turbulence was responsible for resuspended matter in the water column. However, the wind-generated shear stresses applied to the bed were much smaller that the critical shear stress measured at the bed. These results suggest that much of the suspended material came from entrainment of materials from the surfaces of the surrounding submerged aquatic vegetation rather than erosion and entrainment of sedimented materials from the bed. Using calibrated acoustic backscatter from an acoustic doppler velocimeter, an analogy of Type 2 erosion was applied to estimate the critical shear stress of particles from the stems and leaves of submerged aquatic vegetation. The estimated critical stress was close to zero, supporting the idea that this unconsolidated and easily resuspended material comprises the bulk of the suspended sediment during normal operating conditions.

Wind Characteristics in the Taiwan Strait: A Case Study of the First Offshore Wind Farm in Taiwan

This study analyzed the wind speed data of the met mast in the first commercial-scale offshore wind farm of Taiwan from May 2017 to April 2018. The mean wind speed and standard deviation, wind rose, histogram, wind speed profile, and diurnal variation of wind speed with associated changes in wind direction revealed some noteworthy findings. First, the standard deviation of the corresponding mean wind speed is somewhat high. Second, the Hellmann exponent is as low as 0.05. Third, afternoons in winter and nights and early mornings in summer have the highest and lowest wind speed in a year, respectively. Regarding the histogram, the distribution probability of wind is bimodal, which can be depicted as a mixture of two gamma distributions. In addition, the corresponding change between the hourly mean wind speed and wind direction revealed that the land–sea breeze plays a significant role in wind speed distribution, wind profile, and wind energy production. The low Hellmann exponent is discussed in detail. To further clarify the effect of the land–sea breeze for facilitating future wind energy development in Taiwan, we propose some recommendations.

Investigation of Wind Power Potential in Al-Aqiq, Saudi Arabia

Saudi Arabia authorities recognize the importance of renewable energy, especially wind, and they intend to invest in this promising power sector. This paper presents a wind data analysis in terms of annual, seasonal, and diurnal variations at Al-Aqiq, which is located in the highlands of the southwestern region of the Kingdom of Saudi Arabia. In the analysis, the diurnal variation of wind speed at 3 m above the roof surface shows that wind speed remains above 2.5 m/s during the period 10:00 AM to 10:00 PM. The overall wind speed is found to be 2.68 m/s, 4.2 m/s, and 5.48 m/s at heights of 3 m, 10 m, and 50 m, respectively, with the main wind direction ranging approximately from south-southeast to south-southwest. The highest average monthly wind speed is recorded in March, with values of 3.09 m/s, 5.11 m/s, and 6.43 m/s at heights of 3 m, 10 m, and 50 m, respectively, and with southwest being the main wind direction. The wind data analysis is then used to estimate wind energy production. Wind energy generation is considered for 21 wind machines of different sizes. At a height of 10 m above ground level, the highest energy estimate is generated using an Enercon E141/4200 wind machine, with a rate of 4898.96 MWh/Year. The lowest energy estimate is generated using a Soyut Wind 100 wind machine, with a rate of 165.54 MWh/Year. The highest capacity factor (26.0%) is recorded for an Aeronautica Windpower 33-225 wind machine and the lowest capacity factor (5.9%) is recorded for an Enercon E126/7580. The analysis shows that the power factor is low for wind turbines located at 3 m above roof surface level, but higher heights at Albaha University, Al-Aqiq, may be suitable for wind farm development.

Seasonal and diurnal variation of marine wind characteristics based on lidar measurements

AbstractThe accurate assessment of marine wind characteristics is receiving increasing attention because of its practical importance in diverse areas. However, the reliable measurement of marine winds is often insufficient mainly because of the limitation of the instrumentation used. In the present study, four year lidar measurement data, recorded continuously from March 2012 to November 2015, are collected at an offshore platform near Lamma Island in Hong Kong, China, to obtain useful marine meteorological information, with a particular interest in the seasonal and diurnal variations of wind speed, wind shear co‐efficient (WSC) and veering wind angle. It is shown that the wind speed and Weibull parameters generally exhibit evident monthly variation. The maximum monthly wind speed occurs in February, with values between 7.41 and 8.13 m·s−1 depending on the height, and the monthly minimum occurs in August, with values between 4.66 and 5.27 m·s−1. The Weibull scale parameter shows a similar monthly pattern with wind speed, while the Weibull shape parameter peaks in June. Furthermore, the diurnal pattern of both wind speed and Weibull parameters is closely tied to seasonality. The WSC covering the bottom and top layers of the wind turbine is likewise examined. Larger WSCs are obtained during hotter months, while smaller WSCs are associated with cooler months. The diurnal pattern of the WSC shows reasonable agreement among different seasons, although the magnitude is somewhat different. The veering angle generally increases with height. Its monthly pattern reveals a maximum in May, reaching up to 17.8° within the height of concern. Pronounced monthly and diurnal patterns of veering angles are observed at higher altitudes.

Multiscale modeling of the atmospheric environment over a forest canopy

Vegetation constitutes one of the fundamental types of land use on Earth. The presence of trees in urban areas can decrease local winds and exchange sensible and latent heat with the surrounding environments, thus exerting notable influences on the urban microenvironment. A better understanding of the turbulent transfer of momentum and scalars around vegetation canopy could significantly contribute to improvement of the urban environment. This work develops a large-eddy simulation (LES) method that is applicable to model the flow and scalar transport over the forest canopy. We study the atmospheric flow over complex forested areas under typical weather conditions by coupling LES to the mesoscale model. Models of radiation and energy balance have been developed with explicit treatment of the vegetation canopy. By examining the flow over a forest canopy under a range of stability conditions, we found that buoyancy enhances or suppresses turbulent mixing in unstable or stable atmosphere respectively, with decreasing or increasing wind shear, respectively. From the multiscale modeling of the Beijing Olympic Forest Park, the present coupling scheme proves to better resolve the diurnal variations in wind speed, temperature, and relative humidity over complex urban terrains. The coupling scheme is superior to the traditional mesoscale model in terms of wind field simulation. This is mainly because the coupling scheme not only takes the influences of external mesoscale flow into consideration, but also resolves the heterogeneous urban surface at a fine scale by downscaling, thus better reproducing the complex flow and turbulent transport in the urban roughness sublayer.

Diurnal Variations in Surface Wind over the Tibetan Plateau

This study uses hourly surface wind direction and wind speed observations from 53 meteorological stations on the Tibetan Plateau (TP) (70–105° E, 25–45° N) between 1995 and 2017 to investigate diurnal variations in the surface wind. The results show large diurnal variations in surface wind on the TP. The minimum wind speed occurs in the morning and the maximum in the afternoon. In all four seasons, the prevailing meridional wind is a southerly, and this is typically evident for more than two-thirds of each day. However, in the mornings during December–February and September–November, this southerly wind is replaced by a northerly, but remains southerly in the afternoon. The TP shows remarkable regional characteristics with respect to diurnal variations in wind speed. In the eastern region, the minimum and maximum daily wind speeds occur about 1 h later than in the west. Among the 53 meteorological stations, 79% observed that it took less time for the minimum speed to rise to the maximum speed than for the maximum to drop to the minimum. The blocking effect of the high surrounding terrain causes the diurnal variations seen in the surface winds at the three stations in the Qaidam Basin to differ significantly from those observed at the other stations elsewhere on the plateau. These Qaidam Basin stations recorded their maximum wind speeds around noon, with the minimum at dusk, which is around 1900 LST. The EOF1 (EOF = empirical orthogonal function) of the hourly wind speed on the TP indicates the key daily circulation feature of the region; i.e., the wind speed is high in the afternoon and low in the morning. The EOF2 reflects the regional differences in the diurnal variations of wind speed on the TP; i.e., the eastern region reaches the daily maximum and minimum wind speeds slightly later than the western region.

Contrasting variations in the surface layer structure between the convective and non-convective periods in the summer monsoon season for Bangalore location during PRWONAM

An attempt has been made here to examine the contrasting variations in mean surface layer parameters including surface fluxes, and in surface layer stability between the convective and non-convective periods in the southwest monsoon season for the Bangalore experiment location (12.54° N, 77.22° E). The micrometeorological measurements analysed during 2009 and 2010 are from the instrumentation network established during the programme, “Prediction of Regional Weather using Observational meso-Network and Atmospheric Modelling (PRWONAM)”.The Short Wave (SW) radiative flux at the surface is observed to be respectively at 799 ± 188 Wm−2 (772 ± 195 Wm−2) and 436 ± 113 Wm−2 (257 ± 101 Wm−2) at 12:00 LT (Local Time, UTC+05:30) during the non-convective and convective periods in 2009 (2010). The significant difference in SW radiative flux is due to the difference of cloud cover between the non-convective and convective periods. This significant reduction of 515 W m−2 at 12:00 LT in SW radiative flux caused maximum cooling in skin temperature (air temperature) by 6.2 °C (3.8 °C) at 12:00 LT (18:30 LT) from 30.8 ± 3.9 °C (27.1 ± 1.4 °C) in the non-convective period. The impact of convection on soil temperature is observed up to 0.2 m deep. The diurnal amplitudes in composites of air temperature are 8.4 °C (8.4 °C) and 5.7 °C (4.7 °C) during the non-convective and convective periods respectively in 2009 (2010); and the amplitudes in relative humidity are 41.5% (39.7%) and 29% (22.8%).Low wind speeds prevailed 63.4% of the time, all through the day and night, in the monsoon season. The diurnal variations in wind speed during the convective period showed higher variability than in non-convective period. The momentum flux varied in accordance with the strength of the wind speed during the monsoon seasons of both the years 2009 and 2010. The peak sensible heat flux in the convective period is noted to be smaller than that in the non-convective period by 128 W m−2 in 2009 and 124 W m−2 in 2010. The moisture flux showed no significant variations between the two periods. Sensible heat and moisture flux peaked up at 334 W m−2 and 258 W m−2 respectively at low wind strong unstable situation.

Effectiveness of Using Multisatellite Wind Speed Estimates to Construct Hourly Wind Speed Datasets with Diurnal Variations

AbstractThis study used an optimum interpolation method (OIM) to construct a sea surface wind speed (SSW) dataset with hourly resolution based on merged wind speed analysis products from four satellites [AMSR-2, ASCAT, OceanSat Scatterometer (OSCAT), and WindSat]. To validate this hourly SSW dataset, the OIM dataset was compared with observations obtained from moored buoys. These buoy observations were also compared with the products of each of the four satellites individually. The root-mean-square differences and the correlation coefficients between the buoy observations and the OIM dataset indicated that the accuracy of the dataset was slightly lower than that of the single-satellite products. However, a spectrum analysis at the buoy locations indicated that the OIM dataset was capable of resolving diurnal variations in wind speed, which was a result not reproduced by the single-satellite products. In addition, the study also found that the hourly dataset with diurnal variation was effective in obtaining accurate daily mean values by reducing the sampling error. A comparison of daily mean wind speeds derived from satellite observations with those obtained from buoy observations demonstrated that greater accuracy in daily mean SSW data could be achieved using multisatellite observations in comparison with single-satellite observations. Therefore, the application of multisatellite observations that have different observation times could be a useful and effective approach with which to construct datasets with high temporal resolution and to improve the accuracy of daily mean values.

Winds measured by the Rover Environmental Monitoring Station (REMS) during the Mars Science Laboratory (MSL) rover's Bagnold Dunes Campaign and comparison with numerical modeling using MarsWRF

A high density of REMS wind measurements were collected in three science investigations during MSL's Bagnold Dunes Campaign, which took place over ∼80 sols around southern winter solstice (Ls∼90°) and constituted the first in situ analysis of the environmental conditions, morphology, structure, and composition of an active dune field on Mars. The Wind Characterization Investigation was designed to fully characterize the near-surface wind field just outside the dunes and confirmed the primarily upslope/downslope flow expected from theory and modeling of the circulation on the slopes of Aeolis Mons in this season. The basic pattern of winds is ‘upslope’ (from the northwest, heading up Aeolis Mons) during the daytime (∼09:00–17:00 or 18:00) and ‘downslope’ (from the southeast, heading down Aeolis Mons) at night (∼20:00 to some time before 08:00). Between these times the wind rotates largely clockwise, giving generally westerly winds mid-morning and easterly winds in the early evening. The timings of these direction changes are relatively consistent from sol to sol; however, the wind direction and speed at any given time shows considerable intersol variability. This pattern and timing is similar to predictions from the MarsWRF numerical model, run at a resolution of ∼490m in this region, although the model predicts the upslope winds to have a stronger component from the E than the W, misses a wind speed peak at ∼09:00, and under-predicts the strength of daytime wind speeds by ∼2–4m/s. The Namib Dune Lee Investigation reveals ‘blocking’ of northerly winds by the dune, leaving primarily a westerly component to the daytime winds, and also shows a broadening of the 1Hz wind speed distribution likely associated with lee turbulence. The Namib Dune Side Investigation measured primarily daytime winds at the side of the same dune, in support of aeolian change detection experiments designed to put limits on the saltation threshold, and also appears to show the influence of the dune body on the local flow, though less clearly than in the lee. Using a vertical grid with lower resolution near the surface reduces the relative strength of nighttime winds predicted by MarsWRF and produces a peak in wind speed at ∼09:00, improving the match to the observed diurnal variation of wind speed, albeit with an offset in magnitude. The annual wind field predicted using this grid also provides a far better match to observations of aeolian dune morphology and motion in the Bagnold Dunes. However, the lower overall wind speeds than observed and disagreement with the observed wind direction at ∼09:00 suggest that the problem has not been solved and that alternative boundary layer mixing schemes should be explored which may result in more mixing of momentum down to the near-surface from higher layers. These results demonstrate a strong need for in situ wind data to constrain the setup and assumptions used in numerical models, so that they may be used with more confidence to predict the circulation at other times and locations on Mars.

Two distinct patterns of seasonal variation of airborne black carbon over Tibetan Plateau

Airborne black carbon (BC) mass concentrations were measured from November 2012 to June 2013 at Ranwu and Beiluhe, located in the southeastern and central Tibetan Plateau, respectively. Monthly mean BC concentrations show a winter (November–February) high (413.2ngm−3) and spring (March–June) low (139.1ngm−3) at Ranwu, but in contrast a winter low and spring high at Beiluhe (204.8 and 621.6ngm−3, respectively). By examining the meteorological conditions at various scales, we found that the monthly variation of airborne BC over the southeastern Tibetan Plateau (TP) was highly influenced by regional precipitation and over the hinterland by winds. Local precipitation at both sites showed little impact on the seasonal variation of airborne BC concentrations. Potential BC source regions are identified using air mass backward trajectory analysis. At Ranwu, BC was dominated by the air masses from the northeastern India and Bangladesh in both winter and spring, whereas at Beiluhe it was largely contributed by air masses from the south slope of Himalayas in winter, and from the arid region in the north of the TP in spring. The winter and spring seasonal peak of BC in the southern TP is largely contributed by emissions from South Asia, and this seasonal variation is heavily influenced by the regional monsoon. In the northern TP, BC had high concentrations during spring and summer seasons, which is very likely associated with more efficient transport of BC over the arid regions on the north of Tibetan Plateau and in Central Asia. Airborne BC concentrations at the Ranwu sampling site showed a significant diurnal cycle with a peak shortly after sunrise followed by a decrease before noon in both winter and spring, likely shaped by local human activities and the diurnal variation of wind speed. At the Beiluhe sampling site, the diurnal variation of BC is different and less distinct.

풍력-기상자원지도에 기반한 제주 행원 풍력발전단지 효율성 평가

This study evaluates wind farm efficiency at Haengwon in Jeju Island. The actual energy production at Haengwon wind farm is compared with the estimated energy production based on Korean wind map which is developed at the National Institute of Meteorological Research/KMA. The validation of wind map at Gujwa located near the Haengwon wind farm shows that the wind speed is overestimated. The diurnal variation of wind speed shows a maximum value in the afternoon due to the effect of sea-land breeze. The ratio of the actual energy production at Haengwon wind farm and the estimated energy production based on the Korean wind map is 24.8%, while the distribution of energy frequency is similar each other. The difference of energy production is caused by mechanical error of the turbine and the overestimation of the simulated wind map. This study will contribute to the repowering of turbines for improving the efficiency of wind farm in the future.

The effect of diurnal profile and seasonal wind regime on sizing grid-connected and off-grid wind power plants

In a high wind penetration scenario in electricity production, the availability of models for synthetically generating hourly wind speed data becomes increasingly important for both sizing and modeling integrated renewable systems. A methodology for generating hourly wind speed time series is presented in this paper, adopting a physical-statistical approach based on some known aggregate input data. The proposed approach, developed in a previous study by the same authors, has been here improved using a new diurnal wind speed profile function. The improved methodology is first described and validated through experimental wind speed data, comparing the results with those obtained using the basic model. Then, hourly wind speeds generated with the improved methodology are used as input data for the optimal design of grid-connected and off-grid wind power plants. For each configuration, the influence of the diurnal wind speed profile and wind regime on system sizing and its economic parameters has been evaluated. Results have shown that the diurnal variation of wind speed does not affect the size of wind turbine, but strongly influences the storage capacity of off-grid wind power plants.

The Persian Gulf summertime low‐level jet over sloping terrain

AbstractThere are few observational and numerical studies of the summertime low‐level jet (LLJ) over the Persian Gulf, known as the shamal. In this study, the Weather Research and Forecasting (WRF) model accurately simulates the LLJ's vertical structure, nocturnal features and strong diurnal variation of the wind. The case‐study period is divided into a period during which the shamal is spatially extended (strong winds over the Gulf and Iraq) and a period during which it is less spatially extended (strong flow only over the Gulf). Orography, mountain slope and land/sea breeze are found to be key factors for the shamal diurnal variation of wind speed and wind direction; the Blackadar mechanism appears to be secondary to the heating in forcing the shamal. The Zagros Mountains not only channel the northwesterly winds but also provide a barrier for the easterly monsoon airflow, which maximizes the wind speed. Only during the less extensive period does the Zagros Mountains slope significantly affect the shamal wind intensity and direction. Both shallow and steep mountain slope sensitivity studies feature a jet‐like flow over the Persian Gulf. The steep slopes cause larger wind speeds; however, the shallow slopes reveal a stronger diurnally varying wind direction due to larger heating and cooling of the sloping terrain. It is also shown that, during the less extensive period, the land breeze and the lower friction over the sea increase the intensity of the nocturnal jet over the Gulf. Copyright © 2011 Royal Meteorological Society

Winds and tides of Ligeia Mare, with application to the drift of the proposed time TiME (Titan Mare Explorer) capsule

We use two independent General Circulation Models (GCMs) to estimate surface winds at Titan’s Ligeia Mare (78°N, 250°W), motivated by a proposed mission to land a floating capsule in this ∼500km hydrocarbon sea. The models agree on the overall magnitude (∼0.5–1m/s) and seasonal variation (strongest in summer) of windspeeds, but details of seasonal and diurnal variation of windspeed and direction differ somewhat, with the role of surface exchanges being more significant than that of gravitational tides in the atmosphere. We also investigate the tidal dynamics in the sea using a numerical ocean dynamics model: assuming a rigid lithosphere, the tidal amplitude is up to ∼0.8m. Tidal currents are overall proportional to the reciprocal of depth—with an assumed central depth of 300m, the characteristic tidal currents are ∼1cm/s, with notable motions being a slosh between Ligeia’s eastern and western lobes, and a clockwise flow pattern.We find that a capsule will drift at approximately one tenth of the windspeed, unless measures are adopted to augment the drag areas above or below the waterline. Thus motion of a floating capsule is dominated by the wind, and is likely to be several km per Earth day, a rate that will be readily measured from Earth by radio navigation methods. In some instances, the wind vector rotates diurnally such that the drift trajectory is epicyclic.

An Observational and Modeling Study of Characteristics of Urban Heat Island and Boundary Layer Structures in Beijing

Abstract In this paper, the characteristics of urban heat island (UHI) and boundary layer structures in the Beijing area, China, are analyzed using conventional and Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The Weather Research and Forecasting (WRF) model coupled with a single-layer urban canopy model (UCM) is used to simulate these urban weather features for comparison with observations. WRF is also used to test the sensitivity of model simulations to different urban land use scenarios and urban building structures to investigate the impacts of urbanization on surface weather and boundary layer structures. Results show that the coupled WRF/Noah/UCM modeling system seems to be able to reproduce the following observed features reasonably well: 1) the diurnal variation of UHI intensity; 2) the spatial distribution of UHI in Beijing; 3) the diurnal variation of wind speed and direction, and interactions between mountain–valley circulations and UHI; 4) small-scale boundary layer convective rolls and cells; and 5) the nocturnal boundary layer lower-level jet. The statistical analyses reveal that urban canopy variables (e.g., temperature, wind speed) from WRF/Noah/UCM compare better with surface observations than the conventional variables (e.g., 2-m temperature, 10-m wind speed). Both observations and the model show that the airflow over Beijing is dominated by mountain–valley flows that are modified by urban–rural circulations. Sensitivity tests imply that the presence or absence of urban surfaces significantly impacts the formation of horizontal convective rolls (HCRs), and the details in urban structures seem to have less pronounced but not negligible effects on HCRs.

Evaluation of the Wind Shear at a Site in the North-West of the Yucatan Peninsula, Mexico

Extrapolations from 10m above the ground up to the wind turbine hub height are frequently made to the data available for wind power assessments. Because of its simplicity, the power law profile has been one of the most popular mathematical formulations to predict the vertical wind shear. This paper presents an analysis of wind speed and wind shear in terms of the directional, diurnal and seasonal patterns for a site at the Autonomous University of Yucatan which experiences the tropical conditions of the Yucatan Peninsula in Mexico. This analysis takes a detailed look at frequency distributions to facilitate a comprehensive understanding of the local climatic conditions. Diurnal wind speed variations are shown to be affected in particular by the differing wind conditions associated with fetches over two distinct offshore regions. Seasonal behaviour suggests some departure from the oscillations expected from temperature variation. In addition, the use of rate of change of temperature at one height is proposed as an alternative to vertical temperature gradient inferred from two heights as an indicator of atmospheric stability which will affect the wind shear. The work presented is part of a regionally funded research program to evaluate the onshore and offshore wind potential in the north of the Yucatan Peninsula.