| 1. |
|
|
| 2. |
- Blomquist, B. W., et al.
(författare)
-
Wind Speed and Sea State Dependencies of Air-Sea Gas Transfer : Results From the High Wind Speed Gas Exchange Study (HiWinGS)
- 2017
-
Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 122:10, s. 8034-8062
-
Tidskriftsartikel (refereegranskat)abstract
- A variety of physical mechanisms are jointly responsible for facilitating air-sea gas transfer through turbulent processes at the atmosphere-ocean interface. The nature and relative importance of these mechanisms evolves with increasing wind speed. Theoretical and modeling approaches are advancing, but the limited quantity of observational data at high wind speeds hinders the assessment of these efforts. The HiWinGS project successfully measured gas transfer coefficients (k(660)) with coincident wave statistics under conditions with hourly mean wind speeds up to 24 m s(-1) and significant wave heights to 8 m. Measurements of k(660) for carbon dioxide (CO2) and dimethylsulfide (DMS) show an increasing trend with respect to 10 m neutral wind speed (U-10N), following a power law relationship of the form: k660CO2 approximate to U10N1.68 and k660dms approximate to U10N1.33. Among seven high wind speed events, CO2 transfer responded to the intensity of wave breaking, which depended on both wind speed and sea state in a complex manner, with k660CO2 increasing as the wind sea approaches full development. A similar response is not observed for DMS. These results confirm the importance of breaking waves and bubble injection mechanisms in facilitating CO2 transfer. A modified version of the Coupled Ocean-Atmosphere Response Experiment Gas transfer algorithm (COAREG ver. 3.5), incorporating a sea state-dependent calculation of bubble-mediated transfer, successfully reproduces the mean trend in observed k(660) with wind speed for both gases. Significant suppression of gas transfer by large waves was not observed during HiWinGS, in contrast to results from two prior field programs.
|
|
| 3. |
- Cummins, Donald P., et al.
(författare)
-
Reducing Parametrization Errors for Polar Surface Turbulent Fluxes Using Machine Learning
- 2024
-
Ingår i: Boundary-layer Meteorology. - : Springer. - 0006-8314 .- 1573-1472. ; 190:3
-
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.
|
|
| 4. |
- Elvidge, A. D., et al.
(författare)
-
Surface Heat and Moisture Exchange in the Marginal Ice Zone : Observations and a New Parameterization Scheme for Weather and Climate Models
- 2021
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 126:17
-
Tidskriftsartikel (refereegranskat)abstract
- Aircraft observations from two Arctic field campaigns are used to characterize and model surface heat and moisture exchange over the marginal ice zone (MIZ). We show that the surface roughness lengths for heat and moisture over uninterrupted sea ice vary with roughness Reynolds number (R*; itself a function of the roughness length for momentum, z0, and surface wind stress), with a peak at the transition between aerodynamically smooth (R*<0.135) and aerodynamically rough (R*>2.5) regimes. A pre-existing theoretical model based on surface-renewal theory accurately reproduces this peak, in contrast to the simple parameterizations currently employed in two state-of-the-art numerical weather prediction models, which are insensitive to R*. We propose a new, simple parameterization for surface exchange over the MIZ that blends this theoretical model for sea ice with surface exchange over water as a function of sea ice concentration. In offline tests, this new scheme performs much better than the existing schemes for the rough conditions observed during the 'Iceland Greenland Seas Project' field campaign. The bias in total turbulent heat flux across the MIZ is reduced to only 13 W m(-2) for the new scheme, from 48 and 80 W m(-2) for the Met Office Unified Model and ECMWF Integrated Forecast System schemes, respectively. It also performs marginally better for the comparatively smooth conditions observed during the 'Aerosol-Cloud Coupling and Climate Interactions in the Arctic' field campaign. The new surface exchange scheme has the benefit of being physically-motivated, comparatively accurate and straightforward to implement, although to reap the full benefits an improvement to the representation of sea ice topography via z0 is required.
|
|
| 5. |
- Gutiérrez-Loza, Lucía
(författare)
-
Mechanisms controlling air-sea gas exchange in the Baltic Sea
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon plays a major role in physical and biogeochemical processes in the atmosphere, the biosphere, and the ocean. CO2 and CH4 are two of the most common carbon-containing compounds in the atmosphere, also recognized as major greenhouse gases. The exchange of CO2 and CH4 between the ocean and the atmosphere is an essential part of the global carbon cycle. The exchange is controlled by the air–sea concentration gradient and by the efficiency of the transfer processes. The lack of knowledge about the forcing mechanisms affecting the exchange of these climate-relevant gases is a major source of uncertainty in the estimation of the global oceanic contributions. Quantifying and understanding the air–sea exchange processes is essential to constrain the estimates and to improve our knowledge about the current and future climate. In this thesis, the mechanisms controlling the air–sea gas exchange in the Baltic Sea are investigated.The viability of micrometeorological techniques for CH4 monitoring in a coastal environment is evaluated. One year of semi-continuous measurements of air–sea CH4 fluxes using eddy covariance measurements suggests that the method is useful for CH4 flux estimations in marine environments. The measurements allow long-term monitoring at high frequency rates, thus, capturing the temporal variability of the flux. The region off Gotland is a net source of CH4, with both the air–sea concentration gradient and the wind as controlling mechanisms.A sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms controlling the air–sea CO2 exchange in the Baltic Sea. This analysis shows that the spatio-temporal variability of CO2 fluxes is strongly modulated by water-side convection, precipitation, and surfactants. The effect of these factors is relevant both at regional and global scales, as they are not included in the current budget estimates.
|
|
| 6. |
- McCusker, Gillian Young, et al.
(författare)
-
Evaluating Arctic clouds modelled with the Unified Model and Integrated Forecasting System
- 2023
-
Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 23:8, s. 4819-4847
-
Tidskriftsartikel (refereegranskat)abstract
- By synthesising remote-sensing measurements made in the central Arctic into a model-gridded Cloudnet cloud product, we evaluate how well the Met Office Unified Model (UM) and the European Centre for Medium-Range Weather Forecasting (ECMWF) Integrated Forecasting System (IFS) capture Arctic clouds and their associated interactions with the surface energy balance and the thermodynamic structure of the lower troposphere. This evaluation was conducted using a 4-week observation period from the Arctic Ocean 2018 expedition, where the transition from sea ice melting to freezing conditions was measured. Three different cloud schemes were tested within a nested limited-area model (LAM) configuration of the UM – two regionally operational single-moment schemes (UM_RA2M and UM_RA2T) and one novel double-moment scheme (UM_CASIM-100) – while one global simulation was conducted with the IFS, utilising its default cloud scheme (ECMWF_IFS).Consistent weaknesses were identified across both models, with both the UM and IFS overestimating cloud occurrence below 3 km. This overestimation was also consistent across the three cloud configurations used within the UM framework, with >90 % mean cloud occurrence simulated between 0.15 and 1 km in all the model simulations. However, the cloud microphysical structure, on average, was modelled reasonably well in each simulation, with the cloud liquid water content (LWC) and ice water content (IWC) comparing well with observations over much of the vertical profile. The key microphysical discrepancy between the models and observations was in the LWC between 1 and 3 km, where most simulations (all except UM_RA2T) overestimated the observed LWC.Despite this reasonable performance in cloud physical structure, both models failed to adequately capture cloud-free episodes: this consistency in cloud cover likely contributes to the ever-present near-surface temperature bias in every simulation. Both models also consistently exhibited temperature and moisture biases below 3 km, with particularly strong cold biases coinciding with the overabundant modelled cloud layers. These biases are likely due to too much cloud-top radiative cooling from these persistent modelled cloud layers and were consistent across the three UM configurations tested, despite differences in their parameterisations of cloud on a sub-grid scale. Alarmingly, our findings suggest that these biases in the regional model were inherited from the global model, driving a cause–effect relationship between the excessive low-altitude cloudiness and the coincident cold bias. Using representative cloud condensation nuclei concentrations in our double-moment UM configuration while improving cloud microphysical structure does little to alleviate these biases; therefore, no matter how comprehensive we make the cloud physics in the nested LAM configuration used here, its cloud and thermodynamic structure will continue to be overwhelmingly biased by the meteorological conditions of its driving model.
|
|
| 7. |
- Prytherch, John, et al.
(författare)
-
Air-sea CO2 and CH4 gas transfer velocity in Arctic sea-ice regions from eddy covariance flux measurements onboard Icebreaker Oden
- 2017
-
Ingår i: Geophysical Research Abstracts. - 1029-7006 .- 1607-7962. ; 19
-
Tidskriftsartikel (refereegranskat)abstract
- The Arctic Ocean is an important sink for atmospheric CO2, and there is ongoing debate on whether seafloor seeps in the Arctic are a large source of CH4 to the atmosphere. The impact of warming waters, decreasing sea-ice extent and expanding marginal ice zones on Arctic air-sea gas exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts as a near-impermeable lid to air-sea gas exchange, but is also hypothesised to enhance gas transfer rates through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. The dependence of the gas transfer rate on sea-ice concentration remains uncertain due to a lack of in situ measurements. Here we present the first direct estimates of gas transfer rate in a wide range of Arctic sea-ice conditions. The estimates were derived from eddy covariance CO2 and CH4 fluxes, measured from the Swedish Icebreaker Oden during two expeditions: the 3-month duration Arctic Clouds in Summer Experiment (ACSE) in 2014, a component of the Swedish-Russian-US Arctic Ocean Investigation on Climate-Cryosphere-Carbon Interactions (SWERUS-C3) in the eastern Arctic Ocean shelf region; and the Arctic Ocean 2016 expedition to the high latitude Arctic Ocean. Initial CO2 results from ACSE showed that the gas transfer rate has a near-linear dependence on sea-ice concentration, and that some previous indirect measurements and modelling estimates overestimate gas transfer rates in sea-ice regions. This supports a linear sea-ice scaling approach for assessments of polar ocean carbon fluxes. Air-sea gas transfer model assumptions (e.g. Schmidt number dependence) will be examined using simultaneous CO2 and CH4 measurements, and observations in different ice conditions (e.g. summer melt, autumn freeze up, central Arctic and marginal ice zones) will be compared.
|
|
| 8. |
- Prytherch, John, 1980-, et al.
(författare)
-
Central Arctic Ocean surface-atmosphere exchange of CO2 and CH4 constrained by direct measurements
- 2024
-
Ingår i: Biogeosciences. - : Copernicus Publications. - 1726-4170 .- 1726-4189. ; 21:2, s. 671-688
-
Tidskriftsartikel (refereegranskat)abstract
- The central Arctic Ocean (CAO) plays an important role in the global carbon cycle, but the current and future exchange of the climate-forcing trace gases methane (CH4) and carbon dioxide (CO2) between the CAO and the atmosphere is highly uncertain. In particular, there are very few observations of near-surface gas concentrations or direct air-sea CO2 flux estimates and no previously reported direct air-sea CH4 flux estimates from the CAO. Furthermore, the effect of sea ice on the exchange is not well understood. We present direct measurements of the air-sea flux of CH4 and CO2, as well as air-snow fluxes of CO2 in the summertime CAO north of 82.5 N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021. Measurements of air-sea CH4 and CO2 flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden, and air-snow fluxes were determined from chambers deployed on sea ice. Gas transfer velocities determined from fluxes and surface-water-dissolved gas concentrations exhibited a weaker wind speed dependence than existing parameterisations, with a median sea-ice lead gas transfer rate of 2.5cmh-1 applicable over the observed 10m wind speed range (1-11ms-1). The average observed air-sea CO2 flux was -7.6mmolm-2d-1, and the average air-snow CO2 flux was -1.1mmolm-2d-1. Extrapolating these fluxes and the corresponding sea-ice concentrations gives an August and September flux for the CAO of -1.75mmolm-2d-1, within the range of previous indirect estimates. The average observed air-sea CH4 flux of 3.5μmolm-2d-1, accounting for sea-ice concentration, equates to an August and September CAO flux of 0.35μmolm-2d-1, lower than previous estimates and implying that the CAO is a very small (‰ 1%) contributor to the Arctic flux of CH4 to the atmosphere.
|
|
| 9. |
- Prytherch, John, et al.
(författare)
-
Direct determination of the air-sea CO2 gas transfer velocity in Arctic sea ice regions
- 2017
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:8, s. 3770-3778
-
Tidskriftsartikel (refereegranskat)abstract
- The Arctic Ocean is an important sink for atmospheric CO2. The impact of decreasing sea ice extent and expanding marginal ice zones on Arctic air-sea CO2 exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts to limit air-sea gas exchange by reducing contact between air and water but is also hypothesized to enhance gas transfer rates across surrounding open-water surfaces through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. Here we present the first direct determination of the CO2 air-sea gas transfer velocity in a wide range of Arctic sea ice conditions. We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration. We also show that previous modeling approaches overestimate gas transfer rates in sea ice regions.
|
|
| 10. |
- Prytherch, John, et al.
(författare)
-
Wind, Convection and Fetch Dependence of Gas Transfer Velocity in an Arctic Sea‐Ice Lead Determined From Eddy Covariance CO2 Flux Measurements
- 2021
-
Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 35:2
-
Tidskriftsartikel (refereegranskat)abstract
- The air‐water exchange of trace gases such as CO2 is usually parameterized in terms of a gas transfer velocity, which can be derived from direct measurements of the air‐sea gas flux. The transfer velocity of poorly soluble gases is driven by near‐surface ocean turbulence, which may be enhanced or suppressed by the presence of sea ice. A lack of measurements means that air‐sea fluxes in polar regions, where the oceanic sink of CO2 is poorly known, are generally estimated using open‐ocean transfer velocities scaled by ice fraction. Here, we describe direct determinations of CO2 gas transfer velocity from eddy covariance flux measurements from a mast fixed to ice adjacent to a sea‐ice lead during the summer‐autumn transition in the central Arctic Ocean. Lead water CO2 uptake is determined using flux footprint analysis of water‐atmosphere and ice‐atmosphere flux measurements made under conditions (low humidity and high CO2 signal) that minimize errors due to humidity cross‐talk. The mean gas transfer velocity is found to have a quadratic dependence on wind speed: k660 = 0.179 U102, which is 30% lower than commonly used open‐ocean parameterizations. As such, current estimates of polar ocean carbon uptake likely overestimate gas exchange rates in typical summertime conditions of weak convective turbulence. Depending on the footprint model chosen, the gas transfer velocities also exhibit a dependence on the dimension of the lead, via its impact on fetch length and hence sea state. Scaling transfer velocity parameterizations for regional gas exchange estimates may therefore require incorporating lead width data.
|
|
