| 1. |
- Andersson, Andreas, et al.
(författare)
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Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a High Arctic Fjord During Unstable Very-Close-to-Neutral Conditions
- 2019
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 170:3, s. 471-488
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Tidskriftsartikel (refereegranskat)abstract
- Eddy-covariance measurements made in the marine atmospheric boundary layer above a high Arctic fjord (Adventfjorden, Svalbard) are analyzed. When conditions are unstable, but close to neutral −0.1 < z/L < 0, where z is the height, and L is the Obukhov length, the exchange coefficient for sensible heat CH is significantly enhanced compared with that expected from classical surface-layer theory. Cospectra of the vertical velocity component (w) and temperature (T) reveal that a high-frequency peak develops at f ≈ 1 Hz for z/L > − 0.15. A quadrant analysis reveals that the contribution from downdrafts to the vertical heat flux increases as conditions become close to neutral. These findings are the signature of the evolving unstable very-close-to-neutral (UVCN) regime previously shown to enhance the magnitude of sensible and latent heat fluxes in the marine surface layer over the Baltic Sea. Our data reveal the significance of the UVCN regime for the vertical flux of the carbon dioxide (CO2) concentration (C). The cospectrum of w and C clearly shows how the high-frequency peak grows in magnitude for z/L > − 0.15, while the high-frequency peak dominates for z/L > − 0.02. As found for the heat flux, the quadrant analysis of the CO2 flux shows a connection between the additional small-scale turbulence and downdrafts from above. In contrast to the vertical fluxes of sensible and latent heat, which are primarily enhanced by the very different properties of the air from aloft (colder and drier) during UVCN conditions, the increase in the air–sea transfer of CO2 is possibly a result of the additional small-scale turbulence causing an increase in the water-side turbulence. The data indicate an increase in the gas-transfer velocity for CO2 for z/L > − 0.15 but with a large scatter. During the nearly 2 months of continuous measurements (March–April 2013), as much as 36% of all data are associated with the stability range −0.15 < z/L < 0, suggesting that the UVCN regime is of significance in the wintertime Arctic for the air–sea transfer of heat and possibly also CO2.
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| 2. |
- Arnqvist, Johan, et al.
(författare)
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Wind Statistics from a Forested Landscape
- 2015
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:1, s. 53-71
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Tidskriftsartikel (refereegranskat)abstract
- An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40 wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.
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| 3. |
- Balsley, Ben, et al.
(författare)
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On the scale-dependence of the gradient Richardson number in the residual layer
- 2008
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 127, s. 57-72
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Tidskriftsartikel (refereegranskat)abstract
- We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre ‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less. Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ric of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions.
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| 4. |
- Bocquet, Florence, et al.
(författare)
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Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer
- 2011
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 138:1, s. 43-60
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Tidskriftsartikel (refereegranskat)abstract
- Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (epsilon) are reasonably well correlated with concurrent measurements of vertical velocity variance (sigma(2)(w)) obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in the first 100m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent background turbulence exists even during the most stable atmospheric conditions, where e can exhibit values as low as 10(-7) m(2) s(-3). We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and e to provide a proxy measurement for sigma(2)(w) at altitudes higher than tower heights.
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| 5. |
- Bosveld, Fred C., et al.
(författare)
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The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B : Results and Process Understanding
- 2014
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 152:2, s. 157-187
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Tidskriftsartikel (refereegranskat)abstract
- We describe and analyze the results of the third global energy and water cycle experiment atmospheric boundary layer Study intercomparison and evaluation study for single-column models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL; the diagrams include the results of a number of sensitivity runs performed with one of the models. The models are characterized in terms of thermal coupling to the soil, longwave radiation and turbulent mixing. It is shown that differences in longwave radiation schemes among the models have only a small effect on the simulations; however, there are significant variations in downward radiation due to different boundary-layer profiles of temperature and humidity. The differences in modelled thermal coupling to the land surface are large and explain most of the variations in 2-m air temperature and longwave incoming radiation among models. Models with strong turbulent mixing overestimate the boundary-layer height, underestimate the wind speed at 200 m, and give a relatively large downward sensible heat flux. The result is that 2-m air temperature is relatively insensitive to turbulent mixing intensity. Evening transition times spread 1.5 h around the observed time of transition, with later transitions for models with coarse resolution. Time of onset in the morning transition spreads 2 h around the observed transition time. With this case, the morning transition appeared to be difficult to study, no relation could be found between the studied processes, and the variation in the time of the morning transition among the models.
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| 6. |
- Cummins, Donald P., et al.
(författare)
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Reducing Parametrization Errors for Polar Surface Turbulent Fluxes Using Machine Learning
- 2024
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Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 190:3
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent exchanges between sea ice and the atmosphere are known to influence the melting rate of sea ice, the development of atmospheric circulation anomalies and, potentially, teleconnections between polar and non-polar regions. Large model errors remain in the parametrization of turbulent heat fluxes over sea ice in climate models, resulting in significant uncertainties in projections of future climate. Fluxes are typically calculated using bulk formulae, based on Monin-Obukhov similarity theory, which have shown particular limitations in polar regions. Parametrizations developed specifically for polar conditions (e.g. representing form drag from ridges or melt ponds on sea ice) rely on sparse observations and thus may not be universally applicable. In this study, new data-driven parametrizations have been developed for surface turbulent fluxes of momentum, sensible heat and latent heat in the Arctic. Machine learning has already been used outside the polar regions to provide accurate and computationally inexpensive estimates of surface turbulent fluxes. To investigate the feasibility of this approach in the Arctic, we have fitted neural-network models to a reference dataset (SHEBA). Predictive performance has been tested using data from other observational campaigns. For momentum and sensible heat, performance of the neural networks is found to be comparable to, and in some cases substantially better than, that of a state-of-the-art bulk formulation. These results offer an efficient alternative to the traditional bulk approach in cases where the latter fails, and can serve to inform further physically based developments.
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| 7. |
- Esters, Leonie, et al.
(författare)
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Non-local Impacts on Eddy-Covariance Air–Lake CO2 Fluxes
- 2021
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Ingår i: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 178:2, s. 283-300
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Tidskriftsartikel (refereegranskat)abstract
- Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations.
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| 8. |
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| 9. |
- Gutierrez-Loza, Lucia, et al.
(författare)
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The Effect of Breaking Waves on Air-Sea Fluxes in the Coastal Zone
- 2018
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Ingår i: Boundary-layer Meteorology. - : SPRINGER. - 0006-8314 .- 1573-1472. ; 168:2, s. 343-360
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Tidskriftsartikel (refereegranskat)abstract
- The influence of wave-associated parameters controlling turbulent fluxes through the air-sea interface is investigated in a coastal region. A full year of high-quality data of direct estimates of air-sea fluxes based on eddy-covariance measurements is presented. The study area located in Todos Santos Bay, Baja California, Mexico, is a net sink of with a mean flux of (). The results of a quantile-regression analysis computed between the flux and, (1) wind speed, (2) significant wave height, (3) wave steepness, and (4) water temperature, suggest that the significant wave height is the most correlated parameter with the magnitude of the flux but the behaviour of the relation varies along the probability distribution function, with the slopes of the regression lines presenting both positive and negative values. These results imply that the presence of surface waves in coastal areas is the key factor that promotes the increase of the flux from and into the ocean. Further analysis suggests that the local characteristics of the aqueous and atmospheric layers might determine the direction of the flux.
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| 10. |
- Högström, Ulf, et al.
(författare)
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Air–Sea Interaction Features in the Baltic Sea and at a Pacific Trade-Wind Site : An Inter-comparison Study
- 2013
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 147:1, s. 139-163
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Tidskriftsartikel (refereegranskat)abstract
- A systematic comparison of wind profiles and momentum exchange at a trade wind site outside Oahu, Hawaii and corresponding data from the Baltic Sea is presented. The trade wind data are to a very high degree swell dominated, whereas the Baltic Sea data include a more varied assortment of wave conditions, ranging from a pure growing sea to swell. In the trade wind region swell waves travel predominantly in the wind direction, while in the Baltic, significant cross-wind swells are also present. Showing the drag coefficient as a function of the 10-m wind speed demonstrates striking differences for unstable conditions with swell for the wind-speed range 2 m s−1 < U 10 < 7 m s−1, where the trade-wind site drag values are significantly larger than the corresponding Baltic Sea values. In striking contrast to this disagreement, other features studied are surprisingly similar between the two sites. Thus, exactly as found previously in Baltic Sea studies during unstable conditions and swell, the wind profile in light winds (3 m s−1) shows a wind maximum at around 7–8 m above the water, with close to constant wind speed above. Also, for slightly higher wind speeds (4 m s−1 < U 10 < 7 m s−1), the similarity between wind profiles is striking, with a strong wind-speed increase below a height of about 7–8 m followed by a layer of virtually constant wind speed above. A consequence of these wind-profile features is that Monin–Obukhov similarity is no longer valid. At the trade-wind site this was observed to be the case even for wind speeds as high as 10 m s−1. The turbulence kinetic energy budget was evaluated for four cases of 8–16 30- min periods at the trade-wind site, giving results that agree very well with corresponding figures from the Baltic Sea.
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| 11. |
- Inagaki, A., et al.
(författare)
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Thermal Image Velocimetry
- 2013
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 149:1, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- A method for measuring the two-dimensional distribution of wind velocity vectors near a surface exposed to solar radiation, by tracking brightness temperature images instead of particle images, is proposed. It is based on time-sequential thermography with the algorithm used for particle image velocimetry. This thermal image velocimetry (TIV) was tested on a full-scale building wall covered by polystyrene boards attached side-by-side over a vertically elongated area measuring 22.2 m by 2.73 m. A thermal infrared camera was installed 8 m from the test wall to capture the wall-surface temperature at 30 Hz frequency. A sonic anemometer was also installed 35 mm from the surface used for validation of the TIV. The advection velocity estimated from thermal infrared imagery had a linear relationship with the wind velocity measured by the sonic anemometer, irrespective of the wind speed and direction. This linear slope was multiplied by the advection velocity of the thermal infrared image to rescale it to the wind velocity, and the term 'TIV velocity' was then used. A histogram and power spectra of the TIV velocity showed quantitatively good agreement with the velocity measured by the sonic anemometer, except for the high-frequency region of the spectra, where the TIV velocity was overestimated compared with that of the sonic anemometer. The method was also tested on ground covered by artificial turf to demonstrate its application to a horizontal plane with a wider area, extending for more than 80 m by 60 m.
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| 12. |
- Johansson, Cecilia, et al.
(författare)
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Double-layer structure in the boundary layer over the Baltic Sea
- 2005
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Ingår i: Boundary Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 114:2, s. 389-412
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Tidskriftsartikel (refereegranskat)abstract
- Double-layered structures found over the Baltic Sea are investigated using radiosoundings and lidar measurements. Situations with double-layer structures are also simulated with the regional model REMO in a realistic manner. The double layer consists of two adjacent well-mixed layers, with a sharp inversion in between.Results from radiosoundings show that the double-layer structure over the Baltic Sea mainly occurs during the autumn with thermally unstable stratification near the surface. The structure is present in about 50% of the radiosoundings performed during autumn. The presence of the double-layer structure cannot be related to any specific wind direction, wind speed or sea surface temperature.The lidar measurements give a more continuous picture of the time evolution of the double layer structure, and show that the top of the lower layer is not a rigid lid for vertical transport.Two possible explanations of the double-layer structure are given, (i) the structure is caused by ‘advection’ of land boundary-layer air over the convective marine boundary layer or, (ii) bydevelopment of Sc clouds in weak frontal zones connected to low pressure systems. Also the forming of Cu clouds is found to be important for the development of a double-layer structure.
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| 13. |
- Kent, Christoph W., et al.
(författare)
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Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas
- 2017
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 164:2, s. 183-213
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Tidskriftsartikel (refereegranskat)abstract
- © 2017 The Author(s)Nine methods to determine local-scale aerodynamic roughness length (Formula presented.) and zero-plane displacement (Formula presented.) are compared at three sites (within 60 m of each other) in London, UK. Methods include three anemometric (single-level high frequency observations), six morphometric (surface geometry) and one reference-based approach (look-up tables). A footprint model is used with the morphometric methods in an iterative procedure. The results are insensitive to the initial (Formula presented.) and (Formula presented.) estimates. Across the three sites, (Formula presented.) varies between 5 and 45 m depending upon the method used. Morphometric methods that incorporate roughness-element height variability agree better with anemometric methods, indicating (Formula presented.) is consistently greater than the local mean building height. Depending upon method and wind direction, (Formula presented.) varies between 0.1 and 5 m with morphometric (Formula presented.) consistently being 2–3 m larger than the anemometric (Formula presented.). No morphometric method consistently resembles the anemometric methods. Wind-speed profiles observed with Doppler lidar provide additional data with which to assess the methods. Locally determined roughness parameters are used to extrapolate wind-speed profiles to a height roughly 200 m above the canopy. Wind-speed profiles extrapolated based on morphometric methods that account for roughness-element height variability are most similar to observations. The extent of the modelled source area for measurements varies by up to a factor of three, depending upon the morphometric method used to determine (Formula presented.) and (Formula presented.).
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| 14. |
- Ketterer, C., et al.
(författare)
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Investigation of the Planetary Boundary Layer in the Swiss Alps Using Remote Sensing and In Situ Measurements
- 2014
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 151:2, s. 317-334
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Tidskriftsartikel (refereegranskat)abstract
- The development of the planetary boundary layer (PBL) has been studied in a complex terrain using various remote sensing and in situ techniques. The high-altitude research station at Jungfraujoch (3,580 m a.s.l.) in the Swiss Alps lies for most of the time in the free troposphere except when it is influenced by the PBL reaching the station, especially during the summer season. A ceilometer and a wind profiler were installed at Kleine Scheidegg, a mountain pass close to Jungfraujoch, located at an altitude of 2,061 m a.s.l. Data from the ceilometer were analyzed using two different algorithms, while the signal-to-noise ratio of the wind profiler was studied to compare the retrieved PBL heights. The retrieved values from the ceilometer and wind profiler agreed well during daytime and cloud-free conditions. The results were additionally compared with the PBL height estimated by the numerical weather prediction model COSMO-2, which showed a clear underestimation of the PBL height for most of the cases but occasionally also a slight overestimation especially around noon, when the PBL showed its maximum extent. Air parcels were transported upwards by slope winds towards Jungfraujoch when the PBL was higher than 2,800 m a.s.l. during cloud-free cases. This was confirmed by the in situ aerosol measurements at Jungfraujoch with a significant increase in particle number concentration, particle light absorption and scattering coefficients when PBL-influenced air masses reached the station in the afternoon hours. The continuous aerosol in situ measurements at Jungfraujoch were clearly influenced by the local PBL development but also by long-range transport phenomena such as Saharan dust or pollution from the south.
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| 15. |
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| 16. |
- Kovalets, Ivan, et al.
(författare)
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Verification of a One-Dimensional Model of CO2 Atmospheric Transport Inside and Above a Forest Canopy Using Observations at the Norunda Research Station
- 2018
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 168:1, s. 103-126
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Tidskriftsartikel (refereegranskat)abstract
- A model of (Formula presented.) atmospheric transport in vegetated canopies is tested against measurements of the flow, as well as (Formula presented.) concentrations at the Norunda research station located inside a mixed pine–spruce forest. We present the results of simulations of wind-speed profiles and (Formula presented.) concentrations inside and above the forest canopy with a one-dimensional model of profiles of the turbulent diffusion coefficient above the canopy accounting for the influence of the roughness sub-layer on turbulent mixing according to Harman and Finnigan (Boundary-Layer Meteorol 129:323–351, 2008; hereafter HF08). Different modelling approaches are used to define the turbulent exchange coefficients for momentum and concentration inside the canopy: (1) the modified HF08 theory—numerical solution of the momentum and concentration equations with a non-constant distribution of leaf area per unit volume; (2) empirical parametrization of the turbulent diffusion coefficient using empirical data concerning the vertical profiles of the Lagrangian time scale and root-mean-square deviation of the vertical velocity component. For neutral, daytime conditions, the second-order turbulence model is also used. The flexibility of the empirical model enables the best fit of the simulated (Formula presented.) concentrations inside the canopy to the observations, with the results of simulations for daytime conditions inside the canopy layer only successful provided the respiration fluxes are properly considered. The application of the developed model for radiocarbon atmospheric transport released in the form of (Formula presented.) is presented and discussed.
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| 17. |
- Lazeroms, Werner M. J., et al.
(författare)
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Study of Transitions in the Atmospheric Boundary Layer Using Explicit Algebraic Turbulence Models
- 2016
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Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 161:1, s. 19-47
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Tidskriftsartikel (refereegranskat)abstract
- We test a recently developed engineering turbulence model, a so-called explicit algebraic Reynolds-stress (EARS) model, in the context of the atmospheric boundary layer. First of all, we consider a stable boundary layer used as the well-known first test case from the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study (GABLS1). The model is shown to agree well with data from large-eddy simulations (LES), and this agreement is significantly better than for a standard operational scheme with a prognostic equation for turbulent kinetic energy. Furthermore, we apply the model to a case with a (idealized) diurnal cycle and make a qualitative comparison with a simpler first-order model. Some interesting features of the model are highlighted, pertaining to its stronger foundation on physical principles. In particular, the use of more prognostic equations in the model is shown to give a more realistic dynamical behaviour. This qualitative study is the first step towards a more detailed comparison, for which additional LES data are needed.
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| 18. |
- Loridan, Thomas, et al.
(författare)
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High Resolution Simulation of the Variability of Surface Energy Balance Fluxes Across Central London with Urban Zones for Energy Partitioning
- 2013
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Ingår i: Boundary-Layer Meteorology. - : Springer Netherlands. - 0006-8314 .- 1573-1472. ; 147:3, s. 493-523
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Tidskriftsartikel (refereegranskat)abstract
- The parameterization of surface heat-flux variability in urban areas relies on adequate representation of surface characteristics. Given the horizontal resolutions (e.g. ≈ 0.1–1 km) currently used in numerical weather prediction (NWP) models, properties of the urban surface (e.g. vegetated/built surfaces, street-canyon geometries) often have large spatial variability. Here, a new approach based on Urban Zones to characterize Energy partitioning (UZE) is tested within a NWP model (Weather Research and Forecasting model; WRF v3.2.1) for Greater London. The urban land-surface scheme is the Noah/Single-Layer Urban Canopy Model (SLUCM). Detailed surface information (horizontal resolution 1 km) in central London shows that the UZE offers better characterization of surface properties and their variability compared to default WRF-SLUCM input parameters. In situ observations of the surface energy fluxes and near-surface meteorological variables are used to select the radiation and turbulence parameterization schemes and to evaluate the land-surface scheme and choice of surface parameters. For radiative fluxes, improved performance (e.g. > 25 W m −2 root-mean-square error reduction for the net radiation) is attained with UZE parameters compared to the WRF v3.2.1 default for all three methods from the simplest to the most detailed. The UZE-based spatial fluxes reproduce a priori expectations of greater energy storage and less evaporation in the dense city centre compared to the residential surroundings. Problems in Noah/SLUCM partitioning of energy between the daytime turbulent fluxes are identified with the overestimation of the turbulent sensible heat and underestimation of the turbulent latent heat fluxes.
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| 19. |
- Mahrt, Larry, et al.
(författare)
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Sea-Surface Stress Driven by Small-Scale Non-stationary Winds
- 2020
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Ingår i: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 176:1, s. 13-33
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Tidskriftsartikel (refereegranskat)abstract
- A number of studies have indicated that non-stationarity of the wind field over the sea significantly disrupts the equilibrium between the wind, stress, and wave fields. However, no studies have systematically examined the impact of non-stationarity using a large dataset. Here, we examine the effect of non-stationarity of the wind field on the stress by analyzing six years of turbulent flux data from the Östergarnsholm site. On average, the impact of the non-stationarity on the stress and drag coefficient becomes important for wind speeds lessthan about 6 ms-1. Differences between the acceleration and deceleration stages are revealed by phase shifting the calculation of the non-stationarity with respect to the calculation of the stress and drag coefficient. The time structure of events is examined by selecting samples of large increases and decreases of the wind speed. Analyses needed for further progress are noted.
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| 20. |
- Mauritsen, Thorsten, et al.
(författare)
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Wave flow simulations over Arctic leads
- 2005
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 117:2, s. 259-273
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the flow over Arctic leads using a mesoscale numerical model, typical of both summer and winter, under idealised conditions. We find that Arctic leads may be the source of standing atmospheric internal gravity waves during both seasons. The summertime wave may be compared with the wave generated by a small ridge, though with the phase reversed. The mechanism for exciting the wave is found to be the internal boundary layer developing due to horizontal variations in surface temperature and roughness length. During the more exploratory wintertime simulations, with substantial temperature difference between the lead and the ice surface, we find that secondary circulations and intermittent wave-breaking may occur. The effects of the lead appear far downstream.
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| 21. |
- Monnier, Bruno, et al.
(författare)
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Turbulent Structure of a Simplified Urban Fluid Flow Studied Through Stereoscopic Particle Image Velocimetry
- 2018
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Ingår i: Boundary-layer Meteorology. - : SPRINGER. - 0006-8314 .- 1573-1472. ; 166:2, s. 239-268
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Tidskriftsartikel (refereegranskat)abstract
- Stereoscopic particle image velocimetry was used to provide a three-dimensional characterization of the flow around a simplified urban model defined by a 5 by 7 array of blocks, forming four parallel streets, perpendicular to the incoming wind direction corresponding to a zero angle of incidence. Channeling of the flow through the array under consideration was observed, and its effect increased as the incoming wind direction, or angle of incidence (AOI), was changed from to , , and . The flow between blocks can be divided into two regions: a region of low turbulence kinetic energy (TKE) levels close to the leeward side of the upstream block, and a high TKE area close to the downstream block. The centre of the arch vortex is located in the low TKE area, and two regions of large streamwise velocity fluctuation bound the vortex in the spanwise direction. Moreover, a region of large spanwise velocity fluctuation on the downstream block is found between the vortex legs. Our results indicate that the reorientation of the arch vortex at increasing AOI is produced by the displacement of the different TKE regions and their interaction with the shear layers on the sides and top of the upstream and downstream blocks, respectively. There is also a close connection between the turbulent structure between the blocks and the wind gusts. The correlations among gust components were also studied, and it was found that in the near-wall region of the street the correlations between the streamwise and spanwise gusts were dominant for all four AOI cases. At higher wall-normal positions in the array, the correlation decreased with increasing AOI, whereas the coefficient increased as AOI increased, and at all three correlations exhibited relatively high values of around 0.4.
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| 22. |
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| 23. |
- Nebenführ, Bastian, 1984, et al.
(författare)
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Large-Eddy Simulation Study of Thermally Stratified Canopy Flow
- 2015
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:2, s. 253-276
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Tidskriftsartikel (refereegranskat)abstract
- A number of large-eddy simulations (LES) are performed for the calculation of the airflow over a horizontally homogeneous forest canopy for a wide range of thermal stability classes. For the first time, results from LES of a stably stratified canopy are also presented. Simulation results compare favourably to recent field measurements over a pine forest in south-eastern Sweden. The simple heat source model was found to perform adequately and to yield within-canopy heat-flux profiles typically observed for stable conditions in the field. Evidence was found for a layer of unstably stratified air in the canopy trunk space under stable stratification. The importance of a secondary wind-speed maximum is emphasized in stable conditions. Examination of the budget equation of turbulent kinetic energy (TKE) revealed that, during stable stratification, pressure transport plays an increasingly important role in supplying the canopy region with TKE.
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| 24. |
- Norman, Maria, 1973-, et al.
(författare)
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Methods for Estimating Air-Sea Fluxes of CO2 Using High-Frequency Measurements
- 2012
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 144:3, s. 379-400
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Tidskriftsartikel (refereegranskat)abstract
- The most direct method for flux estimation uses eddy covariance, which is also the most commonly used method for land-based measurements of surface fluxes. Moving platforms are frequently used to make measurements over the sea, in which case motion can disturb the measurements. An alternative method for flux estimation should be considered if the effects of platform motion cannot be properly corrected for. Three methods for estimating CO2 fluxes are studied here: the eddy-covariance, the inertial-dissipation, and the cospectral-peak methods. High-frequency measurements made at the land-based Ostergarnsholm marine station in the Baltic Sea and measurements made from a ship during the Galathea 3 expedition are used. The Kolmogorov constant for CO2, used in the inertial-dissipation method, is estimated to be 0.68 and is determined using direct flux measurements made at the Ostergarnsholm site. The cospectral-peak method, originally developed for neutral stratification, is modified to be applicable in all stratifications. With these modifications, the CO2 fluxes estimated using the three methods agree well. Using data from the Ostergarnsholm site, the mean absolute error between the eddy-covariance and inertial-dissipation methods is 0.25 mu mol m(-2) s(-1). The corresponding mean absolute error between the eddy-covariance and cospectral-peak methods is 0.26 mu mol m(-2) s(-1), while between the inertial-dissipation and cospectral-peak methods it is 0.14 mu mol m(-2) s(-1).
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| 25. |
- Offerle, Brian, 1967, et al.
(författare)
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Surface heating in relation to air temperature, wind and turbulence in an urban street canyon
- 2007
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 122:2, s. 273-292
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Tidskriftsartikel (refereegranskat)abstract
- Wind and temperature measurements from within and above a deep urban canyon (height/width = 2.1) were used to examine the thermal structure of air within the canyon, exchange of heat with the overlying atmosphere, and the possible impacts of surface heating on within-canyon air flow. Measurements were made over a range of seasons and primarily analysed for sunny days. This allowed the study of temperature differences between opposing canyon walls and between wall and air of more than 15 degrees C in summer. The wall temperature patterns follow those of incoming solar radiation loading with a secondary daytime effect from the longwave exchange between the walls. In winter, the canyon walls receive little direct solar radiation, and temperature differences are largely due to anthropogenic heating of the building interiors. Cool air from aloft and heated air from canyon walls is shown to circulate within the canyon under cross-canyon flow. Roofs and some portions of walls heat up rapidly on clear days and have a large influence on heat fluxes and the temperature field. The magnitude and direction of the measured turbulent heat flux also depend strongly on the direction of flow relative to surface heating. However, these spatial differences are smoothed by the shear layer at the canyon top. Buoyancy effects from the heated walls were not seen to have as large an impact on the measured flow field as has been shown in numerical experiments. At night canyon walls are shown to be the source of positive sensible heat fluxes. The measurements show that materials and their location, as well as geometry, play a role in regulating the heat exchange between the urban surface and atmosphere.
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| 26. |
- Parmhed, Oskar, et al.
(författare)
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An improved Ekman layer Approximation For Smooth Eddy Diffusity Profiles
- 2005
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Ingår i: Boundary-Layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 115:3, s. 399-407
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Tidskriftsartikel (refereegranskat)abstract
- The Ekman boundary-layer model is extended analytically for a gradually varying eddy diffusivity K(z) ≥ 0, z ≥ 0. A solution for the Ekman layer is provided having similar structure to the constant-K case; that is, exponentially decaying sine functions for the two horizontal wind components. The analytical asymptotic solution compares well with its numerical counterpart for various K(z). The result can be useful in theoretical studies such as Ekman pumping, for efficient estimation of the Ekman layer profiles in various analyses with near-neutral stratifications, or for a rapid initialization of mesoscale models
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| 27. |
- Rutgersson, Anna, et al.
(författare)
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Modelling sensible and latent heat fluxes over sea during unstable, very close to neutral conditions
- 2007
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 123:3, s. 395-415
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Tidskriftsartikel (refereegranskat)abstract
- During slightly unstable but still very close to neutral conditions new results from two previous investigations have shown a significant increase of sensible and latent heat fluxes over the sea. The vertical heat transport during these conditions is dominated by detached eddies originating at the top of the boundary layer, bringing relatively cold and dry air to the surface. This effect can be described in numerical models by either enhanced heat transfer coefficients for sensible and latent heat (Stanton and Dalton numbers respectively) or with an additional roughness length, added to the original roughness lengths for heat and humidity. Such new expressions are developed using turbulence measurements from the Baltic Sea valid for wind speeds up to 14 m s−1. The effect of including the increased heat fluxes is investigated using two different numerical models: a regional three-dimensional climate model covering northern Europe, and a process-oriented ocean model for the Baltic Sea. During periods of several days, the latent heat flux can be increased by as much as 100 W m−2. The increase in sensible heat flux is significantly smaller since the process is only of importance in the very near-neutral regime where the sensible heat flux is very small. The long-term average effect over the Baltic Sea is of the order of several W m−2.
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| 28. |
- Sahlée, Erik, et al.
(författare)
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Influence from Surrounding Land on the Turbulence Measurements Above a Lake
- 2014
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 150:2, s. 235-258
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Tidskriftsartikel (refereegranskat)abstract
- Turbulence measurements taken at a Swedish lake are analyzed. Although the measurements took place over a relatively large lake with several km of undisturbed fetch, the turbulence structure was found to be highly influenced by the surrounding land during daytime. Variance spectra of both horizontal velocity and scalars during both unstable and stable stratification displayed a low frequency peak. The energy at lower frequencies showed a daily variation, increasing in the morning and decreasing in the afternoon. This behaviour is explained by spectral lag, where the low frequency energy due to large eddies that originate from the convective boundary layer above the surrounding land. When the air is advected over the lake the small eddies rapidly equilibrate with the new surface forcing. However, the large eddies remain for an appreciable distance and influence the turbulence in the developing lake boundary layer. The variances of the horizontal velocity and scalars are increased by these large eddies, while the turbulent fluxes are mainly unaffected. The drag coefficient, Stanton number and Dalton number used to parametrize the momentum flux, heat flux and latent heat flux respectively all compare well with current parametrizations developed for open sea conditions. The diurnal cycle of the partial pressure of methane, TeX , observed at this site is closely related to the diurnal cycle of the lake-air methane flux. An idealized two-dimensional model simulation of the boundary layer at a lake site indicates that the strong response of TeX to the surface methane flux is due to the shallow internal boundary layer that develops above the lake, allowing methane to accumulate in a relatively small volume.
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| 29. |
- Sahlée, Erik, et al.
(författare)
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Measurements of the damping of temperature fluctuations in a tube
- 2009
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 132:2, s. 339-348
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Tidskriftsartikel (refereegranskat)abstract
- Using field measurements we evaluate a previously deduced semi-empirical model for the dampening of temperature fluctuations in a circular tube. The measurements show that the model is a reasonably good approximation, and the previous recommendation of using a tube length of the order of 1,000 times the inner diameter of the tube to remove most of the temperature fluctuations seems to hold. Part of the difference between the empirical results and the model is possibly due to deviations from the idealized conditions assumed in the model such as no inclusion of the influence from the ambient atmospheric stability. Limitations of the measurements are also likely to affect the evaluation.
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| 30. |
- Sahlée, Erik, et al.
(författare)
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Spectra of CO2 and water vapour in the marine atmospheric surface layer
- 2008
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 126:2, s. 279-295
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Tidskriftsartikel (refereegranskat)abstract
- Spectra of CO2 and water vapour fluctuations from measurements made in the marine atmospheric surface layer have been analyzed. A normalization of spectra based on Monin-Obukhov similarity theory, originally developed for wind speed and temperature, has been successfully extended also to CO2 and humidity spectra. The normalized CO2 spectra were observed to have somewhat larger contributions from low frequencies compared to humidity spectra during unstable stratification. However, overall, the CO2 and humidity spectra showed good agreement as did the cospectra of vertical velocity with water vapour and CO2 respectively. During stable stratification the spectra and cospectra displayed a well-defined spectral gap separating the mesoscale and small-scale turbulent fluctuations. Two-dimensional turbulence was suggested as a possible source for the mesoscale fluctuations, which in combination with wave activity in the vertical wind is likely to explain the increase in the cospectral energy for the corresponding frequency range. Prior to the analysis the turbulence time series of the density measurements were converted to time series of mixing ratios relative to dry air. Some differences were observed when the spectra based on the original density measurements were compared to the spectra based on the mixing ratio time series. It is thus recommended to always convert the density time series to mixing ratio before performing spectral analysis.
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| 31. |
- Sedlar, Joseph, et al.
(författare)
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Stratiform Cloud—Inversion Characterization During the Arctic Melt Season
- 2009
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 132:3, s. 455-474
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Tidskriftsartikel (refereegranskat)abstract
- Data collected during July and August from the Arctic Ocean Experiment 2001illustrated a common occurrence of specific-humidity (q) inversions, where moistureincreases with height, coinciding with temperature inversions in the central Arctic boundarylayer and lower troposphere. Low-level stratiform clouds and their relationship to temperatureinversions are examined using radiosonde data and data from a suite of remote sensinginstrumentation. Two low-level cloud regimes are identified: the canonical case of stratiformclouds, where the cloud tops are capped by the temperature inversion base (CCI—CloudsCapped by Inversion) and clouds where the cloud tops were found well inside the inversion(CII—Clouds Inside Inversion). The latter case was found to occur more than twiceas frequently than the former. The characteristic of the temperature inversion is shown tohave an influence on the cloud regime that was supported. Statistical analyses of the cloudregimes using remote sensing instruments suggest that CCI cases tend to be dominated bysingle-phase liquid cloud droplets; radiative cooling at the cloud top limits the vertical extentof such clouds to the inversion base height. The CII cases, on the other hand, display characteristicsthat can be divided into two situations—(1) clouds that only slightly penetrate thetemperature inversion and exhibit a microphysical signal similar to CCI cases, or (2) cloudsthat extend higher into the inversion and show evidence of a mixed-phase cloud structure.An important interplay between the mixed-phase structure and an increased potential for turbulentmixing across the inversion base appears to support the lifetime of CII cases existingwithin the inversion layer.
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| 32. |
- Segalini, Antonio, et al.
(författare)
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A Linearized k-ϵ Model of Forest Canopies and Clearings
- 2016
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Ingår i: Boundary-layer Meteorology. - : Springer Netherlands. - 0006-8314 .- 1573-1472. ; 161:3, s. 439-460
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Tidskriftsartikel (refereegranskat)abstract
- A linearized analysis of the Reynolds-averaged Navier–Stokes (RANS) equations is proposed where the (Formula presented.) turbulence model is used. The flow near the forest is obtained as the superposition of the undisturbed incoming boundary layer plus a velocity perturbation due to the forest presence, similar to the approach proposed by Belcher et al. (J Fluid Mech 488:369–398, 2003). The linearized model has been compared against several non-linear RANS simulations with many leaf-area index values and large-eddy simulations using two different values of leaf-area index. All the simulations have been performed for a homogeneous forest and for four different clearing configurations. Despite the model approximations, the mean velocity and the Reynolds stress (Formula presented.) have been reasonably reproduced by the first-order model, providing insight about how the clearing perturbs the boundary layer over forested areas. However, significant departures from the linear predictions are observed in the turbulent kinetic energy and velocity variances. A second-order correction, which partly accounts for some non-linearities, is therefore proposed to improve the estimate of the turbulent kinetic energy and velocity variances. The results suggest that only a region close to the canopy top is significantly affected by the forest drag and dominated by the non-linearities, while above three canopy heights from the ground only small effects are visible and both the linearized model and the simulations have the same trends there.
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| 33. |
- Segalini, Antonio, et al.
(författare)
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An Asymptotic Theory for the Flow over Heterogeneous Roughness
- 2023
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Ingår i: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 186:3, s. 637-658
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Tidskriftsartikel (refereegranskat)abstract
- The flow over arbitrary roughness changes is investigated, revisiting the analysis of Belcher et al. (Q J R Meteorol Soc 116:611–635, 1990) regarding surface-roughness heterogeneity. The proposed theory is restricted to steady neutral boundary layers over flat regions with changes of roughness sufficiently slow and mild to inhibit the growth of nonlinear terms. The approach is based on a triple-deck decomposition of the flow above the roughness, although only the first two layers are interactive at leading order. Two experimental datasets (one with a smooth-to-rough and the other with a rough-to-smooth transition) are used to validate the theory. The latter is further compared against two large-eddy simulations featuring chessboard patterns of alternating surface roughness with relatively short and long length scales, respectively. All the comparisons show that the proposed theory is able to reasonably assess the wind-field perturbation due to the roughness heterogeneity, supporting the use of the model to quickly assess the effect of roughness changes in the flow field.
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| 34. |
- Segalini, Antonio, et al.
(författare)
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Scaling Laws in Canopy Flows : A Wind-Tunnel Analysis
- 2013
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Ingår i: Boundary-layer Meteorology. - : Springer Netherlands. - 0006-8314 .- 1573-1472. ; 148:2, s. 269-283
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and integral time scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness.
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| 35. |
- Segalini, Antonio, et al.
(författare)
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Techniques for the Eduction of Coherent Structures from Flow Measurements in the Atmospheric Boundary Layer
- 2012
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Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 143:3, s. 433-450
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Tidskriftsartikel (refereegranskat)abstract
- Two empirical methods to detect coherent motions embedded in the flow field have been compared, namely the variable interval time average (VITA) method and a wavelet-based technique, both with artificial signals as well as velocity measurements from the atmospheric boundary layer over a forest canopy. It has been found that the wavelet method is slightly better than the VITA approach in coherent structure eduction, even if the results of both techniques are comparable. Also the application of the present approach to simultaneous conditionally sampled wind data has highlighted some important features of coherent structures and gust generation in canopy flows.
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| 36. |
- Sjöblom, Anna, et al.
(författare)
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Comparison Between Eddy-Correlation and Inertial Dissipation Methods in the Marine Atmospheric Surface Layer
- 2004
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Ingår i: Boundary-layer Meteorology. - 0006-8314 .- 1573-1472. ; 110:2, s. 141-164
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Tidskriftsartikel (refereegranskat)abstract
- A comparison of momentum fluxes determined by the eddy-correlation method(ECM) and the inertial dissipation method (IDM) has been performed using longterm measurements over the sea. The measurements were made on the island ofÖstergarnsholm in the middle of the Baltic Sea. The results show that a`classical' form of the inertial dissipation method, i.e., assuming that the transportterms are negligible, and using an effective value for the Kolmogorov constant of0.55, can be used with a mean relative difference between the two methods of about15% for -1 < z/L < 0.5 (z being height and L the Obukhov length). The IDMmethod works best for high wind speeds and neutral conditions. For low windspeeds (U < 6 m s-1) the relation between the two methods is morecomplex. IDM then gives higher values than ECM on the average (about 20%),especially for swell conditions, indicating the need for an imbalance function inthe turbulent kinetic energy budget. Calculations of the effective Kolmogorovconstant, αa, suggest a dependence upon the wave age, αa increasing with increasing wave age, where the value 0.59 fits the data well for saturated waves.
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| 37. |
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| 38. |
- Sjöblom, Anna, et al.
(författare)
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Vertical Structure in the Marine Atmospheric Boundary Layer and its Implication for the Inertial Dissipation Method
- 2003
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Ingår i: Boundary-layer Meteorology. - 0006-8314 .- 1573-1472. ; 109:1, s. 1-25
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Tidskriftsartikel (refereegranskat)abstract
- The structure of the marine atmospheric boundarylayer and the validity ofMonin–Obukhov similarity theory over the seahave been investigated using longterm measurements. Three levels of turbulencemeasurements (at 10 m, 18 mand 26 m) at Östergarnsholm in themiddle of the Baltic Sea have beenanalysed. The results show that turbulentparameters have a strong dependenceon the actual height due to wave influence.The wind profile and thus thenormalised wind gradient are very sensitiveto wave state. The lower part of theboundary layer can be divided into three heightlayers, a wave influenced layerclose to the surface, a transition layer andan undisturbed ‘ordinary’ surfacelayer; the depth of the layers is determinedby the wave state. This heightstructure can, however, not be found for thenormalised dissipation, which is onlya function of the stability, except duringpronounced swell where the actualheight also has to be accounted for. Theresults have implications for the heightvariation of the turbulent kinetic energy(TKE) budget. Thus, the imbalancebetween production and dissipation willalso vary with height according to thevariation of wave state. This, in turn,will of course have strong implicationsfor the inertial dissipation method, inwhich a parameterisation of the TKEbudget is used.
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Stöckl, Stefan, et al.
(författare)
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Including the Urban Canopy Layer in a Lagrangian Particle Dispersion Model
- 2022
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Ingår i: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 185:1, s. 1-34
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Tidskriftsartikel (refereegranskat)abstract
- In this study we introduce a novel extension of an existing Lagrangian particle dispersion model for application over urban areas by explicitly taking into account the urban canopy layer. As commonly done, the original model uses the zero-plane displacement as a lower boundary condition, while the extension reaches to the ground. To achieve this, spatially-averaged parametrizations of flow and turbulence characteristics are created by fitting functions to observational and numerical data. The extended model is verified with respect to basic model assumptions (well-mixed condition) and its behaviour is investigated for unstable/neutral/stable atmospheric stabilities. A sensitivity study shows that the newly introduced model parameters characterizing the canopy turbulence impact the model output less than previously existing model parameters. Comparing concentration predictions to the Basel Urban Boundary Layer Experiment—where concentrations were measured near roof level—shows that the modified model performs slightly better than the original model. More importantly, the extended model can also be used to explicitly treat surface sources (traffic) and assess concentrations within the urban canopy and near the surface (pedestrian level). The small improvement with respect to roof level concentrations suggests that the parametrized canopy profiles for flow and turbulence characteristics realistically represent the dispersion environment on average.
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| 43. |
- Svensson, Gunilla, 1966-, et al.
(författare)
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Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer
- 2009
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 132:2, s. 261-277
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Tidskriftsartikel (refereegranskat)abstract
- The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer.
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| 44. |
- Svensson, Gunilla, et al.
(författare)
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Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models : The Second GABLS Experiment
- 2011
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 140:2, s. 177-206
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Tidskriftsartikel (refereegranskat)abstract
- We present the main results from the second model intercomparison within the GEWEX (Global Energy and Water cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today's numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled near-surface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identified.
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| 45. |
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| 46. |
- Söderberg, Stefan, et al.
(författare)
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Numerical simulations and analytical estimates of katabatic flow over a melting outflow glacier
- 2006
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 120:3, s. 509-534
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Tidskriftsartikel (refereegranskat)abstract
- A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.
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| 47. |
- Tjernström, Michael, 1955-, et al.
(författare)
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Mesoscale variability in the summer Arctic boundary layer
- 2009
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Ingår i: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 130:3, s. 1573-1472
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Tidskriftsartikel (refereegranskat)abstract
- Observations from the summer Arctic Ocean Experiment 2001 (AOE-2001) are analysed with a focus on the interactions between mesoscale and boundary-layer dynamics. Wavelet analyses of surface-pressure variations show daylong periods with different characteristics, some featuring episodes of pronounced high-frequency surface-pressure variability, here hypothesized to be caused by trapped gravity waves. These episodes are accompanied by enhanced boundary-layer turbulence and an enhanced spectral gap, but with only minor influence on the surface stress. During these episodes, mesoscale phenomena were often encountered and usually identified as front-like features in the boundary layer, with a peak in drizzle followed by changing temperature. These phenomena resemble synoptic fronts, though they are generally shallow, shorter-lasting, have no signs of frontal clouds, and do not imply a change in air mass. Based on this analysis, we hypothesize that the root cause of the episodes with high-frequency surface-pressure variance are shallow, mesoscale fronts moving across the pack ice. They may be formed due to local-to-regional horizontal contrasts, for example, between air with different lifetimes over the Arctic or with perturbations in the cloud field causing differential cooling of the boundary layer. Thermal contrasts sharpen as the air is transported with the mean flow. The propagating mesoscale fronts excite gravity waves, which affect the boundary-layer turbulence and also seem to favour entrainment of free tropospheric air into the boundary layer.
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| 48. |
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| 49. |
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| 50. |
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