A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface

Svensson, Nina; Bergström, Hans; Nilsson, Erik; Badger, Merete; Rutgersson, Anna

doi:10.1155/2017/9015891

Cited by 14 publications

(11 citation statements)

References 40 publications

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“…Additionally, as shown by Högström et al, the stratification during conditions with large land‐water temperature difference might be extremely strong, such that a special turbulence regime ensues described as quasi–two dimensional, decorrelated from layers in the vertical. In Svensson et al, it is shown that advected convective structures persist up to 100 km over the stable sea surface. The offshore environment can therefore be complex with effects with different origins taking place in different layers of the atmosphere.…”

Section: Mesoscale Wind Field Modificationsmentioning

confidence: 98%

Modification of the Baltic Sea wind field by land‐sea interaction

2019

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The wind and turbulence fields over a small, high-latitude sea are investigated. These fields are highly influenced by the proximity to the coast, which is never more than 200 km away. Simulations with the WRF model over the Baltic Sea are compared with a simplified, stationary wind model driven by the synoptic forcing. The difference between the models is therefore representative of the mesoscale influence. The results show that the largest wind-field modifications compared with a neutral atmosphere occur during spring and summer, with a mean monthly increase of up to approximately 1 ms −1 at typical hub heights and upper rotor area (120-170 m height) in the WRF model. The main reason for this is large-scale low-level jets caused by the land-sea temperature differences, likely increasing in strength due to inertial oscillations. These kind of events can be persistent for approximately 12 hours and cover almost the entire basin, causing wind speed and wind shear to increase considerably. The strongest effect is around 2000 to 2300 local time. Sea breezes and coastal low-level jets are of less importance, but while sea breezes are mostly detected near the coastline, other types of coastal jets can extend large distances off the coast. During autumn and winter, there are fewer low-level jet occurrences, but the wind profile cannot be explained by the classical theory of the one-dimensional model. This indicates that the coastal environment is complex and may be affected by advection from land surfaces to a large degree even when unstable conditions dominate. KEYWORDSBaltic Sea, coastal meteorology, low-level jet, sea breeze INTRODUCTIONOver the open ocean, surface conditions are relatively horizontally homogeneous and mostly influenced by the synoptic weather and ocean circulation. In coastal areas, on the other hand, land-sea interaction plays a significant role in modifying the wind, temperature, and turbulence fields, as has been shown in many earlier studies. 1-3 This is especially true in inland seas, like the Baltic Sea in Northern Europe. The Baltic Sea is a small, semienclosed sea at high latitudes, which makes it to a large degree affected by the surrounding land areas. The effect this has on the atmospheric conditions have been investigated in several studies. [3][4][5][6][7][8][9] The land-sea temperature differences cause warm-air advection over the sea surface in spring and summer, creating stable atmospheric conditions and an increased low-level jet (LLJ) occurrence. 9 The positive land-sea-temperature differences also make it possible for sea breezes to form.A future expansion of offshore wind energy in the Baltic Sea requires accurate estimates of the wind climate for siting and production estimates of wind farms. Given the specific meteorological conditions described above, it is worthwhile to assess the coastal influence on the wind field in this region, for several reasons: LLJs increase the wind speed compared with the assumption of a neutral atmosphere. Since LLJs are frequently observed below ...

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Section: Mesoscale Wind Field Modificationsmentioning

confidence: 98%

Modification of the Baltic Sea wind field by land‐sea interaction

2019

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“…Examples are processes generated by air-sea temperature contrasts, such as sea breeze circulations, low-level jets (LLJs, e.g. K€ allstrand et al, 2000) or horizontal rolls (Svensson et al, 2017). Coastal areas are influenced by riverine outflow resulting in respiration of terrestrial organic carbon inputs, benthic-pelagic coupling, variability in surfactant abundance and near-surface vertical stratification (Rutgersson et al, 2011).…”

Section: Category Characterizationmentioning

confidence: 99%

Using land-based stations for air–sea interaction studies

Rutgersson¹,

Pettersson²,

Bergström³

et al. 2020

Tellus A: Dynamic Meteorology and Oceanography

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In situ measurements representing the marine atmosphere and air-sea interaction are taken at ships, buoys, stationary moorings and land-based towers, where each observation platform has structural restrictions. Air-sea fluxes are often small, and due to the limitations of the sensors, several corrections are applied. Land-based towers are convenient for long-term observations, but one critical aspect is the representativeness of marine conditions. Hence, a careful analysis of the sites and the data is necessary. Based on the concept of flux footprint, we suggest defining flux data from land-based marine micrometeorological sites in categories depending on the type of land influence: 1. CAT1: Marine data representing open sea, 2. CAT2: Disturbed wave field resulting in physical properties different from open sea conditions and heterogeneity of water properties in the footprint region, and 3. CAT3: Mixed land-sea footprint, very heterogeneous conditions and possible active carbon production/consumption. Characterization of data would be beneficial for combined analyses using several sites in coastal and marginal seas and evaluation/comparison of properties and dynamics. Aerosol fluxes are a useful contribution to characterizing a marine micrometeorological field station; for most conditions, they change sign between land and sea sectors. Measured fluxes from the land-based marine station € Ostergarnsholm are used as an example of a land-based marine site to evaluate the categories and to present an example of differences between open sea and coastal conditions. At the € Ostergarnsholm site the surface drag is larger for CAT2 and CAT3 than for CAT1 when wind speed is below 10 m/s. The heat and humidity fluxes show a distinctive distinguished seasonal cycle; latent heat flux is larger for CAT2 and CAT3 compared to CAT1. The flux of carbon dioxide is large from the coastal and land-sea sectors, showing a large seasonal cycle and significant variability (compared to the open sea sector). Aerosol fluxes are partly dominated by sea spray emissions comparable to those observed at other open sea conditions.

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“…Their analyses of airborne data suggest that roll vortices may be a significant factor modulating the air-sea momentum exchange. Recently, Svensson et al (2017) presented results concerning roll persistence during situations with advection to a strongly stratified boundary layer via SAR and aircraft data. Gao et al (2017) studied the impact of rolls on the intensification process of the TC's and identified a pathway by which rolls can affect the structure and the intensification of the simulated TC's.…”

Section: Introductionmentioning

confidence: 99%

Tropical Cyclone Boundary Layer Rolls in Synthetic Aperture Radar Imagery

Huang

Liu

et al. 2018

JGR Oceans

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Marine atmospheric boundary layer (MABL) roll plays an important role in the turbulent exchange of momentum, sensible heat, and moisture throughout MABL of tropical cyclone (TC). Hence, rolls are believed to be closely related to TC's development, intensification, and decay processes. Spaceborne synthetic aperture radar (SAR) provides a unique capability to image the sea surface imprints of quasi‐linear streaks induced by the MABL rolls within a TC. In this study, sixteen SAR images, including three images acquired during three major hurricanes: Irma, Jose, and Maria in the 2017 Atlantic hurricane season, were utilized to systematically map the distribution and wavelength of MABL rolls under the wide range of TC intensities. The images were acquired by SAR onboard RADARSAT‐1/2, ENVISAT, and SENTINEL‐1 satellites. Our findings are in agreement with the previous one case study of Hurricane Katrina (2005), showing the roll wavelengths are between 600 and 1,600 m. We also find that there exist roll imprints in eyewall and rainbands, although the boundary layer heights are shallower there. Besides, the spatial distribution of roll wavelengths is asymmetrical. The roll wavelengths are found to be the shortest around the storm center, increase and then decrease with distance from storm center, reaching the peak values in the range of d∗−2d∗, where d∗ is defined as the physical location to TC centers normalized by the radius of maximum wind. These MABL roll characteristics cannot be derived using conventional aircraft and land‐based Doppler radar observations.

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A Case Study of Offshore Advection of Boundary Layer Rolls over a Stably Stratified Sea Surface

Cited by 14 publications

References 40 publications

Modification of the Baltic Sea wind field by land‐sea interaction

Modification of the Baltic Sea wind field by land‐sea interaction

Using land-based stations for air–sea interaction studies

Tropical Cyclone Boundary Layer Rolls in Synthetic Aperture Radar Imagery

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