| 1. |
- Aires, F., et al.
(författare)
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Microwave hyperspectral measurements for temperature and humidity atmospheric profiling from satellite: The clear-sky case
- 2015
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202 .- 2169-897X .- 2169-8996. ; 120:21, s. 11334-11351
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the benefits of a satellite HYperspectral Microwave Sensor (HYMS) for the retrieval of atmospheric temperature and humidity profiles, in the context of numerical weather prediction (NWP). In the infrared, hyperspectral instruments have already improved the accuracy of NWP forecasts. Microwave instruments so far only provide observations for a limited number of carefully selected channels. An information content analysis is conducted here to assess the impact of hyperspectral microwave measurements on the retrieval of temperature and water vapor profiles under clear-sky conditions. It uses radiative transfer simulations over a large variety of atmospheric situations. It accounts for realistic observation (instrument and radiative transfer) noise and for a priori information assumptions compatible with NWP practices. The estimated retrieval performance of the HYMS instrument is compared to those of the microwave instruments to be deployed on board the future generation of European operational meteorological satellites (MetOp-SG). The results confirm the positive impact of a HYMS instrument on the atmospheric profiling capabilities compared to MetOp-SG. Temperature retrieval uncertainty, compared to a priori information, is reduced by 2 to 10%, depending on the atmospheric height, and improvement rates are much higher than what will be obtained with MetOp-SG. For humidity sounding these improvements can reach 30%, a significant benefit as compared to MetOp-SG results especially below 250 hPa. The results are not very sensitive to the instrument noise, under our assumptions. The main impact provided by the hyperspectral information originates from the higher resolution in the O2 band around 60 GHz. The results are presented over ocean at nadir, but similar conclusions are obtained for other incidence angles and over land. Key Points A hyperspectral MW instrument could improve temperature & humidity retrieval compared to MetOp-SG The main impact from HYMS comes from higher resolution in the O2 band around 60 GHz Hyperspectral information is not really sensitive to instrument noise.
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| 2. |
- Alm, Love, et al.
(författare)
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Electron density and parallel electric field distribution of the auroral density cavity
- 2015
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 120:11, s. 9428-9441
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Tidskriftsartikel (refereegranskat)abstract
- We present an event study in which Cluster satellites C1 and C3 encounters the flux tube of a stable auroral arc in the pre-midnight sector. C1 observes the mid cavity, while C3 enters the flux tube of the auroral arc at an altitude which is below the acceleration region, before crossing into the top half of the acceleration region. This allows us to study the boundary between the ionosphere and the density cavity, as well as large portion of the upper density cavity. The position of the two satellites, in relation to the acceleration region, is described using a pseudo altitude derived from the distribution of the parallel potential drop above and below the satellites.The electron density exhibits an anti-correlation with the pseudo altitude, indicating that the lowest electron densities are found near the top of the density cavity. Over the entire pseudo altitude range, the electron density distribution is similar to a planar sheath, formed out of a plasma sheet dominated electron distribution, in response to the parallel electric field of the acceleration region. This indicates that the parallel electric fields on the ionosphere-cavity boundary, as well as the mid cavity parallel electric fields, are part of one unified structure rather than two discrete entities.The results highlight the strong connection between the auroral density cavity and auroral acceleration as well as the necessity of studying them in a unified fashion.
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| 3. |
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| 4. |
- Benediktsson, Ivar Örn, et al.
(författare)
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Architecture and structural evolution of an early Little Ice Age terminal moraine at the surge-type glacier Mulajokull, Iceland
- 2015
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Ingår i: Journal of Geophysical Research - Earth Surface. - 2169-9011 .- 0148-0227 .- 2156-2202. ; 120:9, s. 1895-1910
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Tidskriftsartikel (refereegranskat)abstract
- The internal architecture and structural evolution of the Arnarfellsmular terminal moraine at Mulajokull, a surge-type glacier in central Iceland, is described in order to demonstrate submarginal and proglacial glaciotectonic processes during glacier surging, as well as constraining the age of the maximum extent of the glacier. The moraine is 4-7m high, 50-100m wide, and composed of a highly deformed sequence of loess, peat, and tephra that is draped by till up to the crest. The internal architecture is dominated by steep, high-amplitude overturned folds and thrusts in the crest zone but open, low-amplitude folds on the distal slope. Section balancing suggests a basal detachment (decollement) depth of 1.4m and a total horizontal shortening of around 59%. This implies that the glacier coupled to the foreland about 70m up glacier from its terminal position to initiate the formation of the moraine. The structural evolution is polyphase in that the formation commenced with low-amplitude open folding of the foreland, followed by overfolding and piggyback thrusting. Radiocarbon dating and analysis of tephra layers, along with historical references, indicate that the most likely time of moraine formation was between A.D. 1717 and 1760, which suggests that Mulajokull had its Little Ice Age maximum and most extensive surge earlier than many other surge-type glaciers in Iceland.
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| 5. |
- Bobrowski, N., et al.
(författare)
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Gas emission strength and evolution of the molar ratio of BrO/SO2 in the plume of Nyiragongo in comparison to Etna
- 2015
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-897X. ; 120:1, s. 277-291
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Tidskriftsartikel (refereegranskat)abstract
- Airborne and ground-based differential optical absorption spectroscopy observations have been carried out at the volcano Nyiragongo (Democratic Republic of Congo) to measure SO2 and bromine monoxide (BrO) in the plume in March 2004 and June 2007, respectively. Additionally filter pack and multicomponent gas analyzer system (Multi-GAS) measurements were carried out in June 2007. Our measurements provide valuable information on the chemical composition of the volcanic plume emitted from the lava lake of Nyiragongo. The main interest of this study has been to investigate for the first time the bromine emission flux of Nyiragongo (a rift volcano) and the BrO formation in its volcanic plume. Measurement data and results from a numerical model of the evolution of BrO in Nyiragongo volcanic plume are compared with earlier studies of the volcanic plume of Etna (Italy). Even though the bromine flux from Nyiragongo (2.6t/d) is slightly greater than that from Etna (1.9t/d), the BrO/SO2 ratio (maximum 7x10(-5)) is smaller than in the plume of Etna (maximum 2.1x10(-4)). A one-dimensional photochemical model to investigate halogen chemistry in the volcanic plumes of Etna and Nyiragongo was initialized using data from Multi-GAS and filter pack measurements. Model runs showed that the differences in the composition of volcanic volatiles led to a smaller fraction of total bromine being present as BrO in the Nyiragongo plume and to a smaller BrO/SO2 ratio.
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| 6. |
- Chiacchio, Marc, et al.
(författare)
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On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
- 2017
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
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Tidskriftsartikel (refereegranskat)abstract
- A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.
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| 7. |
- De Moor, J. M., et al.
(författare)
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Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high-frequency gas monitoring
- 2016
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-9313. ; 121:8, s. 5761-5775
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Tidskriftsartikel (refereegranskat)abstract
- Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San Jose. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high-frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2-rich gas (CO2/S-total>4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2weeks before eruptions, which are accompanied by shallowly derived sulfur-rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is similar to 8-10km deep, whereas the shallow magmatic gas source is at similar to 3-5km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000T/d SO2 and H2S/SO2>1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2
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| 8. |
- Fredriksson, Sam, 1966, et al.
(författare)
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An evaluation of gas transfer velocity parameterizations during natural convection using DNS
- 2016
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 121:2, s. 1400-1423
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations (DNS) of free surface flows driven by natural convection are used to evaluate different methods of estimating air-water gas exchange at no-wind conditions. These methods estimate the transfer velocity as a function of either the horizontal flow divergence at the surface, the turbulent kinetic energy dissipation beneath the surface, the heat flux through the surface, or the wind speed above the surface. The gas transfer is modeled via a passive scalar. The Schmidt number dependence is studied for Schmidt numbers of 7, 150 and 600. The methods using divergence, dissipation and heat flux estimate the transfer velocity well for a range of varying surface heat flux values, and domain depths. The two evaluated empirical methods using wind (in the limit of no wind) give reasonable estimates of the transfer velocity, depending however on the surface heat flux and surfactant saturation. The transfer velocity is shown to be well represented by the expression, k(s) = A (Bv)(1/4) Sc2(n), where A is a constant, B is the buoyancy flux, m is the kinematic viscosity, Sc is the Schmidt number, and the exponent n depends on the water surface characteristics. The results suggest that A = 0.39 and n approximate to 1/2 and n approximate to 2/3 for slip and no-slip boundary conditions at the surface, respectively. It is further shown that slip and no-slip boundary conditions predict the heat transfer velocity corresponding to the limits of clean and highly surfactant contaminated surfaces, respectively.
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| 9. |
- Fredriksson, Sam, 1966, et al.
(författare)
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Surface shear stress dependence of gas transfer velocity parameterizations using DNS
- 2016
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Ingår i: Journal of Geophysical Research: Oceans. - 2169-9275 .- 2169-9291 .- 0148-0227 .- 2156-2202. ; 121:10, s. 7369-7389
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Tidskriftsartikel (refereegranskat)abstract
- Air-water gas-exchange is studied in direct numerical simulations (DNS) of free-surface flows driven by natural convection and weak winds. The wind is modeled as a constant surface-shear-stress and the gas-transfer is modeled via a passive scalar. The simulations are characterized via a Richardson number Ri=Bν/u*4 where B, ν, and u* are the buoyancy flux, kinematic viscosity, and friction velocity respectively. The simulations comprise 0Ric or kg=Ashearu*Sc-n, Ri
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| 10. |
- Gustafsson, Erik, 1977, et al.
(författare)
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The air-water CO2 exchange of a coastal sea— A sensitivity study on factors that influence the absorption and outgassing of CO2 in the Baltic Sea
- 2015
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202 .- 2169-9275 .- 2169-9291. ; 120:8, s. 5342-5357
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the BALTSEM model is used to estimate how air-water CO2 fluxes in the Baltic Sea respond to parameterizations of organic alkalinity (Aorg), gas transfer, and phytoplankton growth, and further to changes in river loads. The forcing data include the most complete compilation of Baltic river loads for dissolved inorganic and organic carbon (DIC and DOC) and total alkalinity (TA). In addition, we apply the most recent estimates of internal TA generation in the system. Our results clearly demonstrate how airwater CO2 fluxes of a coastal sea depend on river loads of carbon, TA, and nutrients as well the freshwater import itself. Long-term changes in DIC loads are shown to be compensated by corresponding changes in air-water CO2 exchange. By adding Aorg, a discrepancy in the carbonate system calculations was removed, and the simulated net CO2 absorption of the system decreased by 11%. A new parameterization for cyanobacteria growth significantly improved the seasonal development of pCO2 in the central Baltic Sea, although the net effect on CO2 fluxes was below 5%. By applying either a linear, quadratic, or cubic wind speed dependence for gas transfer, the long-term net CO2 exchange was adjusted by less than 5%. There is no clear indication that any one of these parameterizations provides a more accurate estimate of CO2 fluxes than the other two. Our findings are applicable in other coastal areas that are heavily influenced by river loads of TA, DIC, and DOC.
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