scholarly journals Factors Controlling Rain on Small Tropical Islands: Diurnal Cycle, Large-Scale Wind Speed, and Topography

2017 ◽  
Vol 74 (11) ◽  
pp. 3515-3532 ◽  
Author(s):  
Shuguang Wang ◽  
Adam H. Sobel
Keyword(s):  
Wind Speed ◽  
Gravity Waves ◽  
Large Scale ◽  
Sea Breeze ◽  
Flow Regimes ◽  
Prevailing Wind ◽  
Tropical Islands ◽  
Wind Speeds ◽  
Upward Deflection ◽  
Quantitative Impact

Abstract A set of idealized cloud-permitting simulations is performed to explore the influence of small islands on precipitating convection as a function of large-scale wind speed. The islands are situated in a long narrow ocean domain that is in radiative–convective equilibrium (RCE) as a whole, constraining the domain-average precipitation. The island occupies a small part of the domain, so that significant precipitation variations over the island can occur, compensated by smaller variations over the larger surrounding oceanic area. While the prevailing wind speeds vary over flat islands, three distinct flow regimes occur. Rainfall is greatly enhanced, and a local symmetric circulation is formed in the time mean around the island, when the prevailing large-scale wind speed is small. The rainfall enhancement over the island is much reduced when the wind speed is increased to a moderate value. This difference is characterized by a change in the mechanisms by which convection is forced. A thermally forced sea breeze due to surface heating dominates when the large-scale wind is weak. Mechanically forced convection, on the other hand, is favored when the large-scale wind is moderately strong, and horizontal advection of temperature reduces the land–sea thermal contrast that drives the sea breeze. Further increases of the prevailing wind speed lead to strong asymmetry between the windward and leeward sides of the island, owing to gravity waves that result from the land–sea contrast in surface roughness as well as upward deflection of the horizontal flow by elevated diurnal heating. Small-amplitude topography (up to 800-m elevation is considered) has a quantitative impact but does not qualitatively alter the flow regimes or their dependence on wind speed.

scholarly journals Atmospheric Circulation Effects on Wind Speed Variability at Turbine Height

2007 ◽  
Vol 46 (4) ◽  
pp. 445-456 ◽  
Author(s):  
Katherine Klink
Keyword(s):  
Wind Speed ◽  
Pressure Gradient ◽  
Large Scale ◽  
Ground Level ◽  
The Arctic ◽  
Circulation Index ◽  
Regression Equations ◽  
Change Models ◽  
Wind Speeds ◽  
The North

Abstract Mean monthly wind speed at 70 m above ground level is investigated for 11 sites in Minnesota for the period 1995–2003. Wind speeds at these sites show significant spatial and temporal coherence, with prolonged periods of above- and below-normal values that can persist for as long as 12 months. Monthly variation in wind speed primarily is determined by the north–south pressure gradient, which captures between 22% and 47% of the variability (depending on the site). Regression on wind speed residuals (pressure gradient effects removed) shows that an additional 6%–15% of the variation can be related to the Arctic Oscillation (AO) and Niño-3.4 sea surface temperature (SST) anomalies. Wind speeds showed little correspondence with variation in the Pacific–North American (PNA) circulation index. The effect of the strong El Niño of 1997/98 on the wind speed time series was investigated by recomputing the regression equations with this period excluded. The north–south pressure gradient remains the primary determinant of mean monthly 70-m wind speeds, but with 1997/98 removed the influence of the AO increases at nearly all stations while the importance of the Niño-3.4 SSTs generally decreases. Relationships with the PNA remain small. These results suggest that long-term patterns of low-frequency wind speed (and thus wind power) variability can be estimated using large-scale circulation features as represented by large-scale climatic datasets and by climate-change models.

scholarly journals Statistical Downscaling of Daily Wind Speed Variations

2014 ◽  
Vol 53 (3) ◽  
pp. 660-675 ◽  
Author(s):  
Megan C. Kirchmeier ◽  
David J. Lorenz ◽  
Daniel J. Vimont
Keyword(s):  
Wind Speed ◽  
Gamma Distribution ◽  
Large Scale ◽  
Probabilistic Method ◽  
Probabilistic Approach ◽  
Local Scale ◽  
Scale Parameters ◽  
Wind Speeds ◽  
Distribution Shape ◽  
Realistic Information

AbstractThis study presents the development of a method to statistically downscale daily wind speed variations in an extended Great Lakes region. A probabilistic approach is used, predicting a daily-varying probability density function (PDF) of local-scale daily wind speed conditioned on large-scale daily wind speed predictors. Advantages of a probabilistic method are that it provides realistic information on the variance and extremes in addition to information on the mean, it allows the autocorrelation of downscaled realizations to be tuned to match the autocorrelation of local-scale observations, and it allows flexibility in the use of the final downscaled product. Much attention is given to fitting the proper functional form of the PDF by investigating the observed local-scale wind speed distribution (predictand) as a function of the decile of the large-scale wind (predictor). It is found that the local-scale standard deviation and the local-scale shape parameter (from a gamma distribution) are nonconstant functions of the large-scale predictor. As such, a vector generalized linear model is developed to relate the large-scale and local-scale wind speeds. Maximum likelihood and cross validation are used to fit local-scale gamma distribution shape and scale parameters to the large-scale wind speed. The result is a daily-varying probability distribution of local-scale wind speed, conditioned on the large-scale wind speed.

scholarly journals Study on the Influence of Baseline Control System on the Fragility of Large-Scale Wind Turbine considering Seismic-Aerodynamic Combination

2020 ◽  
Vol 2020 ◽  
pp. 1-15
Author(s):  
Chenyang Yuan ◽  
Jing Li ◽  
Jianyun Chen ◽  
Qiang Xu ◽  
Yunfei Xie
Keyword(s):  
Control System ◽  
Wind Speed ◽  
Wind Turbine ◽  
Large Scale ◽  
Fragility Curves ◽  
Operating Conditions ◽  
Ground Acceleration ◽  
Wind Speeds ◽  
Speed Range ◽  
Wind Actions

The purpose of this paper is to explore the effect of the baseline control system (BCS) on the fragility of large-scale wind turbine when seismic and wind actions are considered simultaneously. The BCS is used to control the power output by regulating rotor speed and blade-pitch angle in real time. In this study, the fragility analysis was performed and compared between two models using different peak ground acceleration, wind speeds, and specified critical levels. The fragility curves with different wind conditions are obtained using the multiple stripe analysis (MSA) method. The calculation results show that the probability of exceedance specified critical level increases as the wind speed increases in model 1 without considering BCS, while does not have an obvious change in the below-rated wind speed range and has a significant decrease in the above-rated wind speed range in model 2 with considering BCS. The comparison depicts that if the BCS is neglected, the fragility of large-scale wind turbine will be underestimated in around the cut-in wind speed range and overestimated in the over-rated wind speed range. It is concluded that the BCS has a great effect on the fragility especially within the operating conditions when the rated wind speed is exceeded, and it should be considered when estimating the fragility of wind turbine subjected to the interaction of seismic and aerodynamic loads.

scholarly journals Simulating orographic rainfall with a limited-area, non-hydrostatic atmospheric model under idealized forcing

2005 ◽  
Vol 5 (1) ◽  
pp. 215-226 ◽  
Author(s):  
A. Pathirana ◽  
S. Herath ◽  
T. Yamada
Keyword(s):  
Large Scale ◽  
Sea Breeze ◽  
Atmospheric Model ◽  
Limited Area ◽  
Moisture Profile ◽  
Wind Speeds ◽  
Orographic Rainfall ◽  
Flow Features ◽  
Accumulated Rainfall ◽  
Low Wind Speeds

Abstract. A modified version of an operational 3-dimensional, non-hydrostatic, limited-area atmospheric model (MM5) was used to perform high-resolution, idealized simulations of the interaction of an infinitely long single ridge with a steady, lateral large-scale wind field. The effect of different mountain ridge dimensions, wind speeds and patterns and moisture profiles on the quantity and distribution of orographic rainfall was investigated. The simulations demonstrated a number of commonly observed mountain flow features like formation of cap clouds, foehn wall, convective break-out associated with mountain topography, interaction of downslope winds with sea breeze, and different stages of cumulus development. It was found that the rainfall maxima associated with the mountain always occur upstream of the ridge peak. Changing mountain dimensions, wind speeds and patterns and moisture profile had clear effects on amount and pattern of accumulated rainfall. Low wind speeds resulted the maximum accumulated rainfall to occur considerable distance upstream of ridge peak. Reversal of wind directions in upper atmosphere caused rainfall to be largely restricted to the wind-side of the peak. The observed rainfall patterns are explained by the different flow patterns observed in the model output.

scholarly journals Coastal Wind and Turbulence Observations during the Morning and Evening Transitions over Tropical Terrain

2017 ◽  
Vol 56 (12) ◽  
pp. 3167-3185 ◽  
Author(s):  
Derek D. Jensen ◽  
Timothy A. Price ◽  
Daniel F. Nadeau ◽  
Jacob Kingston ◽  
Eric R. Pardyjak
Keyword(s):  
Heat Flux ◽  
Large Scale ◽  
Sensible Heat Flux ◽  
Sea Breeze ◽  
Flow Regimes ◽  
Resource Assessment ◽  
Sensible Heat ◽  
Coastal Site ◽  
Synoptic Forcing ◽  
Mean Wind Speed

AbstractData collected during a multiyear, wind-resource assessment over a multi-land-use coastal environment in Belize are used to study the development and decay of wind and turbulence through the morning and evening transitions. Observations were made on three tall masts, forming an inland transect of approximately 5 km. The wind distribution is found to be bimodal and governed by synoptic scales, with onshore and offshore flow regimes. The behavior between the coastal and inland sites is found to be very similar when the flow is directed offshore; for onshore flow, stark differences occur. The mean wind speed at the coastal site is approximately 20% greater than the most inland site and is nearly constant throughout the diurnal cycle. For both flow regimes, the influence of the land–sea breeze circulation is inconsequential relative to the large-scale synoptic forcing. Composite time series are used to study the evolution of sensible heat flux and turbulence kinetic energy (TKE) throughout the morning and evening transitions. The TKE budget reveals that at the coastal site mechanical production of TKE is much more important than buoyant production. This allows for the unexpected case in which TKE increases through the ET despite the decrease of buoyant TKE production. Multiresolution flux decomposition is used to further study this phenomenon as well as the evolution of the sensible heat flux at differing time scales. An idealized schematic is presented to illustrate the timing and structure of the morning and evening transitions for an inland site and a coastal site that are subjected to similar synoptic forcing.

scholarly journals Influencing Mechanisms of a Crosswind on the Thermo-Hydraulic Characteristics of a Large-Scale Air-Cooled Heat Exchanger

Energies ◽  
2019 ◽  
Vol 12 (6) ◽  
pp. 1128
Author(s):  
Yanqiang Kong ◽  
Weijia Wang ◽  
Zhitao Zuo ◽  
Lijun Yang ◽  
Xiaoze Du ◽  
...  
Keyword(s):  
Heat Exchanger ◽  
Wind Speed ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Large Scale ◽  
Air Mass ◽  
Heat Rejection ◽  
Wind Speeds ◽  
Air Cooled Heat Exchanger

For the large scale air-cooled heat exchanger of a natural draft dry cooling system (NDDCS) in power plants, its thermo-flow characteristics are basically dominated by crosswinds. Unfortunately however, the detailed mechanisms of the crosswind effects have yet to be fully uncovered. Therefore, in this research, the local flow and heat transfer performances of the cooling deltas, which are also termed as the fundamental cells of the large-scale air-cooled heat exchanger, are specifically investigated with full consideration for the cell structure and the water-side temperature distribution at various wind speeds. A 3D CFD method with a realizable k-ε turbulence model, heat exchanger model, and porous media model is developed, and the accuracy and credibility of the numerical model are experimentally validated. With the numerical simulation, the overall 3D outlet air temperature of the large-scale air-cooled heat exchanger, and the corresponding local air velocity and temperature fields of the cooling deltas are qualitatively analyzed. Furthermore, the air-mass flow rate and heat rejection are also quantitatively studied at both the global and local views. The results depict that with an increase in the wind speed, the air mass flow rate and heat rejection will increase greatly for the frontal deltas; however, they will drop dramatically for the middle-front deltas. As for the middle- as well as the middle-rear deltas, the thermo-flow performances vary markedly at various wind speeds, which behave in the most deteriorated manner at a wind speed of 12 m/s. The rear deltas show the best thermo-flow performances at a wind speed of 12 m/s, but the worst at 16 m/s. A detailed analysis of the variable fields for each cooling delta may contribute to the performance improvement of the large-scale air-cooled heat exchanger of NDDCS.

Temporal variations in elliptical forest fire shapes

1989 ◽  
Vol 19 (11) ◽  
pp. 1496-1500
Author(s):  
R. S. McAlpine
Keyword(s):  
Wind Speed ◽  
Point Source ◽  
Forest Fire ◽  
Ponderosa Pine ◽  
Growth Models ◽  
Temporal Variations ◽  
Width Ratio ◽  
Prevailing Wind ◽  
Fire Growth ◽  
Wind Speeds

Elliptical fire growth models are dependant on a relationship between the length to width ratio of the ellipse and the prevailing wind speed. A laboratory study of point source fires growing in two fuel types (Ponderosa Pine (Pinusponderosa Laws.) needle litter and excelsior) showed that the length to width ratio changes from the time of inception until a stabilized "equilibrium" eccentricity is established. The size of fuel bed required to allow stabilization of the length to width ratio is dependant on wind speed. Results indicate that a fuel bed 0.93 m wide is insufficient to allow length to width ratio stabilization for wind speeds above 1.6 km/h.

scholarly journals Influence of a Hailstreak on Boundary Layer Evolution

2005 ◽  
Vol 133 (4) ◽  
pp. 942-960 ◽  
Author(s):  
Zewdu T. Segele ◽  
David J. Stensrud ◽  
Ian C. Ratcliffe ◽  
Geoffrey M. Henebry
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
South Dakota ◽  
Land Surface ◽  
Surface Wind ◽  
Sea Breeze ◽  
Surface Wind Speed ◽  
Vegetation Coverage ◽  
Wind Speeds ◽  
Severe Thunderstorms

Severe thunderstorms developed on 20 June 1997 and produced heavy precipitation, damaging winds, and large hail over two swaths in southeastern South Dakota. Calculations of fractional vegetation coverage (scaled from 0 to 1) based upon composite satellite data indicate that, within the hailstreak region, vegetation coverage decreased from 0.50 to near 0.25 owing to the damaging effects of hail on the growing vegetation. The northern edge of the larger hailstreak was located a few kilometers south of Chamberlain, South Dakota, a National Weather Service surface observation site. Hourly observations from Chamberlain and several nearby surface sites in South Dakota are averaged over 7 days both before and after this hail event. These observations illustrate that the late-afternoon (nighttime) temperatures are 2°C higher (2°C lower) near the hailstreak after the event than before the event. Similarly, daily average dewpoint temperatures after the event are 2.6°C lower near the hailstreak. These changes are consistent with the influences of a recently devegetated zone on changes to the surface energy budget. To explore how these hailstreaks further affected the evolution of the planetary boundary layer in this region, two model simulations are performed using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). In the control run, climatology is used for the land surface characteristics, and hence the simulation is independent of the hailstreaks. In the hailstreak simulation (HSS), the fractional vegetation coverage and soil moisture in the hailstreak regions are modified to reflect the likely conditions within the hailstreaks. Two different days are simulated: one with low surface wind speeds and one with stronger surface wind speeds. For the low surface wind speed case, the HSS simulation produces a sea-breeze-like circulation in the boundary layer by midmorning. For the stronger surface wind speed case, this sea-breeze-like circulation does not develop in the HSS, but the simulated low-level temperatures are modified over a larger area. These results suggest that to capture and reasonably simulate the evolution of boundary layer structures, there is a need for routine daily updates of land surface information. Hailstreaks also are important to consider in the future as the focus for observational studies on nonclassical mesoscale circulations.

scholarly journals Hydrological response unit-based blowing snow modelling over an alpine ridge

2010 ◽  
Vol 7 (1) ◽  
pp. 1167-1208 ◽  
Author(s):  
M. K. MacDonald ◽  
J. W. Pomeroy ◽  
A. Pietroniro
Keyword(s):  
Wind Speed ◽  
Land Surface ◽  
Large Scale ◽  
Snow Accumulation ◽  
Blowing Snow ◽  
Hydrological Response ◽  
Wind Speeds ◽  
Response Unit ◽  
Winter Snow ◽  
Hydrological Response Unit

Abstract. Snow redistribution by wind and the resulting accumulation regimes were simulated for two winters over an alpine ridge transect located in the Canada Rocky Mountains. Simulations were performed using physically based blowing snow and snowmelt models. A hydrological response unit (HRU)-based spatial discretization was used rather than a more computationally expensive fully-distributed one. The HRUs were set up to follow an aerodynamic sequence, whereby eroded snow was transported from windswept, upwind HRUs to drift accumulating, downwind HRUs. HRUs were selected by examining snow accumulation patterns from manual snow depth measurements. Simulations were performed using two sets of wind speed forcing: (1) station observed wind speed, and (2) modelled wind speed from a widely applied empirical, terrain-based windflow model. Best results were obtained when using the site meteorological station wind speed data. The windflow model performed poorly when comparing the magnitude of modelled and observed wind speeds, though over-winter snow accumulation results obtained when using the modelled wind speeds were reasonable. However, there was a notable discrepancy (17%) between blowing snow sublimation quantities estimated when using the modelled and observed wind speeds. As a result, the end-of-winter snow accumulation was considerably underestimated (32%) when using the modelled wind speeds. That snow redistribution by wind can be adequately simulated in computationally efficient HRUs over this alpine ridge has important implications for representing snow transport in large-scale hydrology models and land surface schemes. Snow redistribution by wind was shown to significantly impact snow accumulation regimes in mountainous environments as snow accumulation was reduced to less than one-third of snowfall on windswept landscapes and nearly doubled in certain lee slope and treeline areas. Blowing snow sublimation losses were shown to be significant (approximately one-quarter of snowfall or greater).

scholarly journals Centennial-scale variability of terrestrial near-surface wind speed over China from reanalysis

2021 ◽  
pp. 1-52
Author(s):  
Cheng Shen ◽  
Jinlin Zha ◽  
Jian Wu ◽  
Deming Zhao
Keyword(s):  
Wind Speed ◽  
Large Scale ◽  
Stepwise Regression ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Near Surface ◽  
Wind Speeds ◽  
Ocean Atmosphere ◽  
Forward Stepwise ◽  
Strong Winds

AbstractInvestigations of variations and causes of near-surface wind speed (NWS) further understanding of the atmospheric changes and improve the ability of climate analysis and projections. NWS varies on multiple temporal scales; however, the centennial-scale variability in NWS and associated causes over China remains unknown. In this study, we employ the European Centre for Medium-Range Weather Forecasts (ECMWF) twentieth century reanalysis (ERA-20C) to study the centennial-scale changes in NWS from 1900–2010. Meanwhile, a forward stepwise regression algorithm is used to reveal the relationships between NWS and large-scale ocean-atmosphere circulations. The results show three unique periods in annual mean NWS over China from 1900–2010. The annual mean NWS displayed a decreasing trend of -0.87% decade-1 and -11.75% decade-1 from 1900–1925 and 1957–2010, respectively, which were caused by the decreases in the days with strong winds, with trends of -6.64 and -4.66 days decade-1, respectively. The annual mean NWS showed an upward trend of 55.47% decade-1 from 1926–1956, which was caused by increases in the days with moderate (0.43 days decade-1) and strong winds (23.55 days decade-1). The reconstructed wind speeds based on forward stepwise regression algorithm matched well with the original wind speeds; therefore, the decadal changes in NWS over China at centennial-scale were mainly induced by large-scale ocean-atmosphere circulations, with the total explanation power of 66%. The strongest explanation power was found in winter (74%), and the weakest explanation power was found in summer (46%).

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