| 1. |
- Andersson, Andreas, et al.
(author)
-
Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a High Arctic Fjord During Unstable Very-Close-to-Neutral Conditions
- 2019
-
In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 170:3, s. 471-488
-
Journal article (peer-reviewed)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.
|
|
| 2. |
- Arnqvist, Johan, et al.
(author)
-
Wind Statistics from a Forested Landscape
- 2015
-
In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 156:1, s. 53-71
-
Journal article (peer-reviewed)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.
|
|
| 3. |
- Balsley, Ben, et al.
(author)
-
On the scale-dependence of the gradient Richardson number in the residual layer
- 2008
-
In: Boundary-Layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 127, s. 57-72
-
Journal article (peer-reviewed)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.
|
|
| 4. |
- Bocquet, Florence, et al.
(author)
-
Comparing Estimates of Turbulence Based on Near-Surface Measurements in the Nocturnal Stable Boundary Layer
- 2011
-
In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 138:1, s. 43-60
-
Journal article (peer-reviewed)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.
|
|
| 5. |
- Bosveld, Fred C., et al.
(author)
-
The Third GABLS Intercomparison Case for Evaluation Studies of Boundary-Layer Models. Part B : Results and Process Understanding
- 2014
-
In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 152:2, s. 157-187
-
Journal article (peer-reviewed)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.
|
|
| 6. |
- Cummins, Donald P., et al.
(author)
-
Reducing Parametrization Errors for Polar Surface Turbulent Fluxes Using Machine Learning
- 2024
-
In: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 190:3
-
Journal article (peer-reviewed)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.
|
|
| 7. |
- Esters, Leonie, et al.
(author)
-
Non-local Impacts on Eddy-Covariance Air–Lake CO2 Fluxes
- 2021
-
In: Boundary-layer Meteorology. - : Springer Nature. - 0006-8314 .- 1573-1472. ; 178:2, s. 283-300
-
Journal article (peer-reviewed)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.
|
|
| 8. |
|
|
| 9. |
- Gutierrez-Loza, Lucia, et al.
(author)
-
The Effect of Breaking Waves on Air-Sea Fluxes in the Coastal Zone
- 2018
-
In: Boundary-layer Meteorology. - : SPRINGER. - 0006-8314 .- 1573-1472. ; 168:2, s. 343-360
-
Journal article (peer-reviewed)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.
|
|
| 10. |
- Högström, Ulf, et al.
(author)
-
Air–Sea Interaction Features in the Baltic Sea and at a Pacific Trade-Wind Site : An Inter-comparison Study
- 2013
-
In: Boundary-layer Meteorology. - : Springer Science and Business Media LLC. - 0006-8314 .- 1573-1472. ; 147:1, s. 139-163
-
Journal article (peer-reviewed)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.
|
|
