scholarly journals Wind Regimes Associated with a Mountain Gap at the Northeastern Adriatic Coast

2013 ◽  
Vol 52 (9) ◽  
pp. 2089-2105 ◽  
Author(s):  
Danijel Belušić ◽  
Mario Hrastinski ◽  
Željko Večenaj ◽  
Branko Grisogono
Keyword(s):  
Model Simulation ◽  
Single Case ◽  
Sea Breeze ◽  
Geostrophic Wind ◽  
Wind Speeds ◽  
Lee Wave ◽  
Adriatic Coast ◽  
Northeasterly Wind ◽  
Bora Wind ◽  
Wind Regimes

AbstractWinds through the Vratnik Pass, a mountain gap in the Dinaric Alps, Croatia, are polarized along the gap axis that extends in the northeast–southwest direction. Although stronger northeasterly wind at the Vratnik Pass is frequently related to the Adriatic bora wind, especially at the downstream town of Senj, there are many cases in which the wind at Senj is directionally decoupled from the wind at the Vratnik Pass. A cluster analysis reveals that this decoupling is sometimes related to lower wind speeds or a shallow southeasterly sirocco wind along the Adriatic, but in many cases the bora blows over a wider region, while only Senj has a different wind direction. Several mechanisms can be responsible for the latter phenomenon, including the formation of a lee wave rotor. A numerical model simulation corroborates the decoupling caused by a rotor for a single case. The prevalence of northeasterly winds at the Vratnik Pass during southeasterly sirocco episodes is another result that challenges the current understanding. It is shown that, at least in one of these episodes, this phenomenon is related to a secondary mesoscale low pressure center in the eastern lee of the Apennines that forms as a subsystem of a broader Genoa cyclone. Less frequent southwesterly winds through the gap are predominantly related to the thermal sea breeze and anabatic circulations that are sometimes superimposed on the geostrophic wind.

Northeast Atlantic storminess in centennial reanalyses

2021 ◽  
Author(s):  
Oliver Krueger ◽  
Frauke Feser ◽  
Christopher Kadow ◽  
Ralf Weisse
Keyword(s):  
Climate Models ◽  
Numerical Models ◽  
Geostrophic Wind ◽  
Model Systems ◽  
Storm Activity ◽  
Northeast Atlantic ◽  
Wind Speeds ◽  
Long Term Trends ◽  
Storm Index

<p>Global atmospheric reanalyses are commonly applied for the validation of climate models, diagnostic studies, and driving higher resolution numerical models with the emphasis on assessing climate variability and long-term trends. Over recent years, longer reanalyses spanning a period of more than hundred years have become available. In this study, the variability and long-term trends of storm activity is assessed over the northeast Atlantic in modern centennial reanalysis datasets, namely ERA-20cm, ERA-20c, CERA-20c, and the 20CR-reanalysis suite with 20CRv3 being the most recent one. All reanalyses, except from ERA-20cm, assimilate surface pressure observations, whereby ERA-20C and CERA-20c additionally assimilate surface winds. For the assessment, the well-established storm index of higher annual percentiles of geostrophic wind speeds derived from pressure observations at sea level over a relatively densely monitored marine area is used.</p><p>The results indicate that the examined centennial reanalyses are not able to represent long-term trends of storm activity over the northeast Atlantic, particularly in the earlier years of the period examined when compared with the geostrophic wind index based on pressure observations. Moreover, the reanalyses show inconsistent long-term behaviour when compared with each other. Only in the latter half of the 20th century, the variability of reanalysed and observed storminess time series starts to agree with each other. Additionally, 20CRv3, the most recent centennial reanalysis examined, shows markedly improved results with increased uncertainty, albeit multidecadal storminess variability does not match observed values in earlier times before about 1920.</p><p>The behaviour shown by the centennial reanalyses are likely caused by the increasing number of assimilated observations, changes in the observational databases used, and the different underlying numerical model systems. Furthermore, the results derived from the ERA-20cm reanalysis that does not assimilate any pressure or wind observations suggests that the variability and uncertainty of storminess over the northeast Atlantic is high making it difficult to determine storm activity when numerical models are not bound by observations. The results of this study imply and reconfirm previous findings that the assessment of long-term storminess trends and variability in centennial reanalyses remains a rather delicate matter, at least for the northeast Atlantic region.</p>

scholarly journals Local Characteristics of the Nocturnal Boundary Layer in Response to External Pressure Forcing

2017 ◽  
Vol 56 (11) ◽  
pp. 3035-3047 ◽  
Author(s):  
Steven J. A. van der Linden ◽  
Peter Baas ◽  
J. Antoon van Hooft ◽  
Ivo G. S. van Hooijdonk ◽  
Fred C. Bosveld ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Stable Boundary Layer ◽  
Dynamical Behavior ◽  
Geostrophic Wind ◽  
External Pressure ◽  
Near Surface ◽  
Stable Boundary ◽  
Wind Speeds ◽  
Turbulent Characteristics

AbstractGeostrophic wind speed data, derived from pressure observations, are used in combination with tower measurements to investigate the nocturnal stable boundary layer at Cabauw, the Netherlands. Since the geostrophic wind speed is not directly influenced by local nocturnal stability, it may be regarded as an external forcing parameter of the nocturnal stable boundary layer. This is in contrast to local parameters such as in situ wind speed, the Monin–Obukhov stability parameter (z/L), or the local Richardson number. To characterize the stable boundary layer, ensemble averages of clear-sky nights with similar geostrophic wind speeds are formed. In this manner, the mean dynamical behavior of near-surface turbulent characteristics and composite profiles of wind and temperature are systematically investigated. The classification is found to result in a gradual ordering of the diagnosed variables in terms of the geostrophic wind speed. In an ensemble sense the transition from the weakly stable to very stable boundary layer is more gradual than expected. Interestingly, for very weak geostrophic winds, turbulent activity is found to be negligibly small while the resulting boundary cooling stays finite. Realistic numerical simulations for those cases should therefore have a comprehensive description of other thermodynamic processes such as soil heat conduction and radiative transfer.

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 Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

2020 ◽  
Vol 8 (11) ◽  
pp. 931
Author(s):  
Syeda Rafiq ◽  
Charitha Pattiaratchi ◽  
Ivica Janeković
Keyword(s):  
Surface Current ◽  
Field Measurements ◽  
Sea Breeze ◽  
Wind Forcing ◽  
Current Response ◽  
Surface Currents ◽  
The South ◽  
Wind Speeds ◽  
North East ◽  
South West

The land–sea breeze (LSB) system, driven by the thermal contrast between the land and the adjacent ocean is a widely known atmospheric phenomenon, which occurs in coastal regions globally. South-west Australia experiences a persistent and one of the strongest LSB systems globally with maximum wind speeds associated with the LSB system often exceeding 15 ms−1. In this paper, using field measurements and numerical simulations, we examine: (1) the local winds associated with the land–sea breeze with an emphasis on the ocean; and, (2) the response of the surface currents to the diurnal wind forcing. The measurements indicated that the wind speeds decreased between midnight and 0400 and increased rapidly after 1100, reaching maxima >10 ms−1 around 1800) associated with the sea breeze and decreased to midnight. Wind directions were such that they were blowing from south-east (120°) in the morning and changed to almost southerly (~200°) in the afternoon. Decomposition of the wind record to the diurnal and synoptic components indicated that the diurnal component of winds (i.e., LSB) was oriented along the south-west to north-east axis. However, the stronger synoptic winds were from the south-east to south quadrant and in combination with the LSB, the winds consisted of a strong southerly component. We examined the evolution, horizontal extent, and propagation properties of sea breeze fronts for characteristic LSB cycles and the sea breeze cell propagating offshore and inland. The results indicated that the sea breeze cell was initiated in the morning in a small area, close to 33° S, 115.5° E, with a width of ~25 km and expanded onshore, offshore and alongshore. The sea breeze cell expanded faster (30 kmh−1) and farther (120 km) in the offshore direction than in the onshore direction (10 kmh−1 and 30–40 km). Winds during the LSB cycle followed a counterclockwise rotation that was also reflected in the surface currents. The winds and surface currents rotated anticlockwise with the surface currents responding almost instantaneously to changes in wind forcing but were modified by topography. The diurnal surface currents were enhanced due to the resonance between the LSB forcing and the inertial response.

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 Recovery processes in coastal wind farms under sea-breeze conditions

2021 ◽  
Vol 56 ◽  
pp. 129-139
Author(s):  
Tanvi Gupta ◽  
Somnath Baidya Roy
Keyword(s):  
Wind Farm ◽  
Wrf Model ◽  
Wind Farms ◽  
Sea Breeze ◽  
Offshore Wind ◽  
Dominant Factor ◽  
Offshore Wind Farms ◽  
Sea Breezes ◽  
Wind Speeds ◽  
Recovery Processes

Abstract. With the rapid growth in offshore wind energy, it is important to understand the dynamics of offshore wind farms. Most of the offshore wind farms are currently installed in coastal regions where they are often affected by sea-breezes. In this work, we quantitatively study the recovery processes for coastal wind farms under sea-breeze conditions. We use a modified Borne's method to identify sea breeze days off the west coast of India in the Arabian Sea. For the identified sea breeze days, we simulate a hypothetical wind farm covering 50×50 km2 area using the Weather Research and Forecasting (WRF) model driven by realistic initial and boundary conditions. We use three wind farm layouts with the turbines spaced 0.5, 1, and 2 km apart. The results show an interesting power generation pattern with a peak at the upwind edge and another peak at the downwind edge due to sea breeze. Wind farms affect the circulation patterns, but the effects of these modifications are very weak compared to the sea breezes. Vertical recovery is the dominant factor with more than half of the momentum extracted by wind turbines being replenished by vertical turbulent mixing. However, horizontal recovery can also play a strong role for sparsely packed wind farms. Horizontal recovery is stronger at the edges where the wind speeds are higher whereas vertical recovery is stronger in the interior of the wind farms. This is one of the first studies to examine replenishment processes in offshore wind farms under sea breeze conditions. It can play an important role in advancing our understanding wind farm-atmospheric boundary layer interactions.

scholarly journals Infragravity Wave Energy Partitioning in the Surf Zone in Response to Wind-Sea and Swell Forcing

2019 ◽  
Vol 7 (11) ◽  
pp. 383
Author(s):  
Stephanie Contardo ◽  
Graham Symonds ◽  
Laura Segura ◽  
Ryan Lowe ◽  
Jeff Hansen
Keyword(s):  
Surf Zone ◽  
Pressure Sensors ◽  
Low Frequency ◽  
Sea Breeze ◽  
Peak Frequency ◽  
Short Wave ◽  
Edge Waves ◽  
Leaky Waves ◽  
Wind Speeds ◽  
Wind Sea

An alongshore array of pressure sensors and a cross-shore array of current velocity and pressure sensors were deployed on a barred beach in southwestern Australia to estimate the relative response of edge waves and leaky waves to variable incident wind wave conditions. The strong sea breeze cycle at the study site (wind speeds frequently > 10 m s−1) produced diurnal variations in the peak frequency of the incident waves, with wind sea conditions (periods 2 to 8 s) dominating during the peak of the sea breeze and swell (periods 8 to 20 s) dominating during times of low wind. We observed that edge wave modes and their frequency distribution varied with the frequency of the short-wave forcing (swell or wind-sea) and edge waves were more energetic than leaky waves for the duration of the 10-day experiment. While the total infragravity energy in the surf zone was higher during swell forcing, edge waves were more energetic during wind-sea periods. However, low-frequency (0.005–0.023 Hz) edge waves were found to be dominant in absence of wind-sea conditions, while higher-frequency (0.023–0.050 Hz) edge waves dominated when wind-sea conditions were present.

Interaction of Sea Breeze and Deep Convection over the Northeastern Adriatic Coast: An Analysis of Sensitivity Experiments Using a High-Resolution Mesoscale Model

2017 ◽  
Vol 174 (11) ◽  
pp. 4197-4224 ◽  
Author(s):  
Gabrijela Kehler-Poljak ◽  
Maja Telišman Prtenjak ◽  
Marko Kvakić ◽  
Kristina Šariri ◽  
Željko Večenaj
Keyword(s):  
High Resolution ◽  
Mesoscale Model ◽  
Deep Convection ◽  
Sea Breeze ◽  
Sensitivity Experiments ◽  
Adriatic Coast

scholarly journals Revisiting an Old Concept: The Gradient Wind*

2014 ◽  
Vol 142 (4) ◽  
pp. 1460-1471 ◽  
Author(s):  
Keith F. Brill
Keyword(s):  
Wind Speed ◽  
Upper Bound ◽  
Full Range ◽  
Geostrophic Wind ◽  
Horizontal Wind ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Natural Gradient ◽  
Wind Speeds ◽  
Gradient Wind

Abstract The gradient wind is defined as a horizontal wind having the same direction as the geostrophic wind but with a magnitude consistent with a balance of three forces: the pressure gradient force, the Coriolis force, and the centrifugal force arising from the curvature of a parcel trajectory. This definition is not sufficient to establish a single way of computing the gradient wind. Different results arise depending upon what is taken to be the parcel trajectory and its curvature. To clarify these distinctions, contour and natural gradient winds are defined and subdivided into steady and nonsteady cases. Contour gradient winds are based only on the geostrophic streamfunction. Natural gradient winds are obtained using the actual wind. Even in cases for which the wind field is available along with the geostrophic streamfunction, it may be useful to obtain the gradient wind for comparison to the existing analyzed or forecast wind or as a force-balanced reference state. It is shown that the nonanomalous (normal) solution in the case of nonsteady natural gradient wind serves as an upper bound for the actual wind speed. Otherwise, supergradient wind speeds are possible, meaning that a contour gradient wind or the steady natural gradient wind used as an approximation for an actual wind may not be capable of representing the full range of actual wind magnitude.

scholarly journals Calculated wind climatology of the South-Saxonian/North-Czech mountain topography including improved resolution of mountains

1996 ◽  
Vol 14 (7) ◽  
pp. 767-772 ◽  
Author(s):  
D. Hinneburg ◽  
G. Tetzlaff
Keyword(s):  
Mesoscale Model ◽  
Surface Wind ◽  
Geostrophic Wind ◽  
Horizontal Resolution ◽  
Special Treatment ◽  
Frequency Distributions ◽  
Wind Speeds ◽  
Radiosonde Station ◽  
Wind Climatology ◽  
Speed Up

Abstract. A mesoscale model has been applied to calculate climatological means of the surface wind. A reliable average requires more than 40 model runs, which are differentiated by the direction and speed of the geostrophic wind under the assumption of neutral stratification. The frequency distributions of the geostrophic wind have been taken from observations of the 850-hPa winds at the radiosonde station in Prague for a 10-year period. The simulation results have been averaged over all sectors and speed classes of the geostrophic wind according to their frequencies. A comparison of the calculated mean wind speeds with observed ones shows deviations of about 0.4 ms–1 outside the mountains. The representation of steep topography and isolated mountains on the basis of a 3-km horizontal resolution of the simulations needs special treatment in order to reduce the gap of up to 4 ms–1 between observed and simulated mean wind speeds over mountains. Therefore, an empiric speed-up formula has been applied to the isolated mountains that otherwise would fall through the 3-km meshes. The corresponding deviations have been reduced to 1.5 ms–1.

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