| 1. |
- Hemer, Mark A., et al.
(författare)
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Projected changes in wave climate from a multi-model ensemble
- 2013
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Ingår i: Nature Climate Change. - 1758-678X .- 1758-6798. ; 3:5, s. 471-476
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Tidskriftsartikel (refereegranskat)abstract
- Future changes in wind-wave climate have broad implications for the operation and design of coastal, near-and off-shore industries and ecosystems, and may further exacerbate the anticipated vulnerabilities of coastal regions to projected sea-level rise(1,2). However, wind waves have received little attention in global assessments of projected future climate change. We present results from the first community-derived multi-model ensemble of wave-climate projections. We find an agreed projected decrease in annual mean significant wave height (H-S) over 25.8% of the global ocean area. The area of projected decrease is greater during boreal winter (January-March, mean; 38.5% of the global ocean area) than austral winter (July-September, mean; 8.4%). A projected increase in annual mean H-S is found over 7.1% of the global ocean, predominantly in the Southern Ocean, which is greater during austral winter (July-September; 8.8%). Increased Southern Ocean wave activity influences a larger proportion of the global ocean as swell propagates northwards into the other ocean basins, observed as an increase in annual mean wave period (T-M) over 30.2% of the global ocean and associated rotation of the annual mean wave direction (theta(M)). The multi-model ensemble is too limited to systematically sample total uncertainty associated with wave-climate projections. However, variance of wave-climate projections associated with study methodology dominates other sources of uncertainty (for example, climate scenario and model uncertainties).
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| 4. |
- Ranjha, Raza, 1984-, et al.
(författare)
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Global distribution and seasonal variability of coastal low-level jets derived from ERA-Interim reanalysis
- 2013
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 65, s. 20412-
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Tidskriftsartikel (refereegranskat)abstract
- A low-level wind maximum is often found over the oceans near many coasts around the world. These Coastal Low-Level Jets (CLLJs) play an important role in the coastal weather and have significant impacts on regional climate and ecology as well as on a number of human activities. The presence of CLLJs is related to various local circumstances such as land-sea temperature contrasts, upwelling, coastal terrain, orientation of the coast, etc., but also to the large-scale atmospheric dynamics. This makes studies of CLLJs not only interesting but also challenging.In the present study, based on ERA-Interim reanalysis data, the global distribution, spatio-temporal structure, and the seasonal variability of CLLJs are documented. Seasonal data from 1980 to 2011 are used to identify areas where CLLJs are frequently found in the lowest 2 km, following criteria based on the vertical profiles of wind speed and temperature. The results are analyzed to highlight the fundamental aspects and distinctive features of the CLLJs across the globe, including their occurrence rate, jet height, wind-speed maximum and horizontal extent.Global maps of CLLJs are constructed for the summer and winter seasons. The west coasts of North America, the Iberian Peninsula, north-western Africa and the south-eastern coast of the Arabian Peninsula make up the northern-hemispheric CLLJ regions, while the west coasts of South America, Australia and southern Africa comprise the south-hemispheric equivalents. The existence and characteristics of CLLJs along the southern coast of Oman and the western coast of the Iberian Peninsula regions are also discussed, not fully envisaged before in the context of CLLJs. The highest occurrence of CLLJs is found during the summer in both hemispheres, and the coast of Oman has the globally highest CLLJ frequency, with also the highest maximum wind speeds. The most commonly found CLLJ has a maximum wind speed between 9 and 15 m s-1, and occurs at heights between 500 and 700 m a.s.l.
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| 5. |
- Ranjha, Raza, 1984-, et al.
(författare)
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Modelling coastal low-level wind-jets : does horizontal resolution matter?
- 2016
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Ingår i: Meteorology and atmospheric physics (Print). - : Springer Science and Business Media LLC. - 0177-7971 .- 1436-5065. ; 128:2, s. 263-278
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Tidskriftsartikel (refereegranskat)abstract
- Atmospheric flows in coastal regions are impacted by land-sea temperature contrasts, complex terrain, shape of the coastline, among many things. Along the west coast of central North America, winds in the boundary layer are mainly from north or northwest, roughly parallel to the coastline. Frequently, the coastal low-level wind field is characterized by a sharp wind maximum along the coast in the lowest kilometer. This feature, commonly referred to as a Coastal Low-Level Jet (CLLJ), has significant impact on the climatology of the coastal region, influencing aviation, shipping, fishing and a number of other human activities along the littoral zone, and better understanding and forecasting of this is vital.The purpose of this study is to elucidate the issue of proper resolution in a mesoscale numerical model to describe the physics of a CLLJ, and its impact on the upper ocean. The COAMPS® model is used for a summer event to determine the realism of the model results compared to observations, from an area of supercritical flow adjustment between Pt. Sur and Pt. Conception, California. Simulations at different model horizontal resolutions, from 54 km to 2 km are performed. While the model produces realistic results with increasing details at higher resolution, the results do not converge even at a resolution of only few kilometers and an objective analysis of model errors do not show an increased skill with increasing resolution. New methods may thus have to be developed to evaluate models at very high resolution. Based on all available information, a compromise resolution appears to be at least 6 km.
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| 6. |
- Ranjha, Raza, 1984-, et al.
(författare)
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Structure and Variability of the Oman Coastal Low-Level Jet
- 2015
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 67
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Tidskriftsartikel (refereegranskat)abstract
- During the boreal summer strong southwesterly, coast parallel low-level winds prevail off the coast of Oman and over the Arabian Sea. The offshore large-scale structure and variability of these coastal winds has been a topic of numerous studies because of their strong connection to the south-Asian summer monsoon. However, low-level coastal jets along the coast of Oman have not been studied in detail, especially on a mesoscale level, despite their globally high frequency of occurrence. In the current study, regional atmospheric modeling has been utilized in an attempt to resolve the mesoscale structure, spatial variability and temporal characteristics of the Oman coastal low-level jet (CLLJ). The limited area model COAMPS® has been used at 6-km resolution for a five month period from May through September, during 2009. The model output compares favorably with the seasonal climatology for the coast of Oman.In this study, analysis of high resolution model fields reveals the mesoscale structure of the Oman CLLJ, clearly distinguishing it from the large-scale southwesterly monsoon circulation farther offshore over the Arabian Sea. The Oman CLLJ is closer to the coast and spread northeastwards along the coast of Oman. Although the synoptic forcing that preconditions the Oman CLLJ is different from other areas of CLLJ occurrences, nevertheless, the jet is still driven primarily by the coastal baroclinicity due to sharp land-sea thermal contrasts. Within the study period, July exhibits the highest CLLJ frequency with ~80% occurrences and also manifests highest winds around 27 m s-1.
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| 7. |
- Rutgersson, Anna, et al.
(författare)
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Impact of surface waves in a regional climate model
- 2010
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Ingår i: Meteorologische Zeitschrift. - : Schweizerbart. - 0941-2948 .- 1610-1227. ; 19:3, s. 247-257
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Tidskriftsartikel (refereegranskat)abstract
- A coupled regional atmosphere-wave model system is developed with the purpose of investigating the impactof climate changes on the wave field, as well as feed-back effects of the wave field on the atmosphericparameters. This study focuses on the effects of introducing a two-way atmosphere-wave coupling on theatmosphere as well as on wave parameters. The model components are the regional climate model RCA, andthe third generation wave modelWAM. Two different methods are used for the coupling, using the roughnesslength and only including the effect of growing sea, and using the wave age and introducing the reductionof roughness due to decaying sea (swell). Introducing a two-way coupling results in an altered frequencydistribution of wind speed and wave heights. When only including growing sea the impact of waves on thelong term mean atmospheric parameters is limited, inducing a reduction of wind speed and significant waveheight. When also the impact of swell is introduced, there is a shift towards higher wind speeds as well ashigher significant wave heights in the four investigated areas. There is a reduction of surface heat fluxesand a decrease in near surface temperature as well as a significant increase in near surface humidity. Themajor conclusion is that when introducing a more realistic surface description over sea, the air-sea interactionrepresented by waves has a significant impact also on long term averages of parameters in the atmosphere.Waves should thus be introduced in climate models for a realistic description of processes over sea.
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| 8. |
- Semedo, Alvaro, 1966-, et al.
(författare)
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A Global View on the Wind Sea and Swell Climate and Variability from ERA-40
- 2011
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 24:5, s. 1461-1479
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Tidskriftsartikel (refereegranskat)abstract
- In this paper a detailed global climatology of wind sea and swell parameters, based on the ERA-40 wave reanalysis, is presented. The spatial pattern of the swell dominance of the Earth’s Oceans, in terms of the wave field energy balance and wave field characteristics, is also investigated. Statistical analysis shows that the Global Ocean is strongly dominated by swell waves. The inter-annual variability of the wind sea and swell significant wave heights, and how they are related to the resultant significant wave height, is analyzed over the Pacific, Atlantic, and Indian Oceans. The leading modes of variability of wind sea and swell demonstrate noticeable differences, particularly in the Pacific and Atlantic Oceans. During the Northern Hemisphere winter a strong north-south swell propagation pattern is observed in the Atlantic Ocean. Statistically significant secular increases in the wind sea and swell significant wave heights are found in the North Pacific and North Atlantic Oceans.
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| 9. |
- Semedo, Alvaro
(författare)
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Atmosphere-ocean Interactions in Swell Dominated Wave Fields
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ocean wind waves represent the atmosphere-ocean boundary, playing a central role in the air-sea exchanging processes. Heat, mass and momentum are transferred across this boundary, with waves mediating the exchange of principally the momentum between the winds and the ocean surface. During the generation process waves are called wind sea. When they leave their generation area or outrun their generating wind they are called swell. The wave field can be said to be dominated either by wind sea or swell. Depending on the wave regime the momentum and energy exchanging processes and the degree of coupling between the waves and the wind is different. During the growing process, waves act as a drag on the surface wind and the momentum flux is directed downward. When swell dominates the wave field a reverse momentum flux mechanism occurs triggered by swell waves traveling considerably faster than the surface winds. The momentum transfer is now directed from the waves to the atmosphere, and takes place because swell waves perform work on the atmosphere as part of their attenuation process. This upward momentum transfer has an impact on the lower atmosphere dynamics, and on the overall turbulence structure of the boundary layer. A detailed qualitative climatology of the global wind sea and swell fields from wave reanalysis data, is presented, revealing a very strong swell dominance of the World Ocean. The areas of larger potential impact of swell on the atmosphere, from a climatological point of view, are also studied. A model that reproduces the swell impact on the lower atmosphere dynamics, conceptually based on the energy transfer from the waves to the atmosphere, is presented – a new parameterization for the wave-induced stress is also proposed. The model results are compared with field observations. A modeling simulation, using a coupled wave-atmosphere model system, is used to study the impact of swell in a regional climate model, by using different formulations on how to introduce the wave state effect in the modeling system.
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| 10. |
- Semedo, Alvaro, 1966-, et al.
(författare)
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Global Distribution of the Wave Age Parameter
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper a detailed global climatology of the wave age parameter, based on the ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-40 reanalysis is presented. The global annual and seasonal mean climatological patterns of the wave age confirm, in line with previous studies, the global dominance of the World Ocean by swell waves. It is also shown, from a climatological perspective, that the state of equilibrium of the World Ocean is a relatively rare event. The leading modes of variability of the wave age demonstrate that the areas of higher explained variability (the centers of action) occur mostly along the equator, coinciding with the equatorial swell pools. Statistically significant secular changes of the annual wave age parameter are analyzed, revealing dominant upward trend in the equatorial and southeast Pacific Ocean, and partially in the extratropical storm tracks in the Southern Ocean, although in some areas, mainly in the Indian Ocean, some negative trends of the wave age are also observed.
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