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Johannes M. L. Dahl and Jannick Fischer

Abstract

The authors investigate the origin of prefrontal, warm-season convergence lines over western Europe using the Weather Research and Forecasting Model. These lines form east of the cold front in the warm sector of an extratropical cyclone, and they are frequently the focus for convective development. It is shown that these lines are related to a low-level thermal ridge that accompanies the base of an elevated mixed layer (EML) plume generated over the Iberian Peninsula and northern Africa. Using Q-vector diagnostics, including the components that describe scalar and rotational quasigeostrophic frontogenesis, it is shown that the convergence line is associated with the rearrangement of the isentropes especially at the western periphery of the EML plume. The ascending branch of the resulting ageostrophic circulation coincides with the surface velocity convergence. The modeling results are supported by a 3-yr composite analysis of cold fronts with and without preceding convergence lines using NCEP–NCAR Reanalysis-1 data.

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D. P. Billesbach, M. L. Fischer, M. S. Torn, and J. A. Berry

Abstract

To facilitate the study of flux heterogeneity within a region, the authors have designed and field-tested a portable eddy covariance system to measure exchange of CO2, water vapor, and energy between the land surface and the atmosphere. The combination of instrumentation used in this system allows high precision flux measurements without requiring on-site infrastructure such as prepositioned towers or line power. In addition, the system contains sensors to measure a suit of soil, climatic, and energy-related parameters that are needed to quality control the fluxes and to characterize the flux footprint. The physical design and instrument packaging used in the system allows for simple transport (fits in a standard minivan) and for rapid deployment with a minimal number of field personnel (usually less than a day for one person). The power requirement for the entire system (instruments and data loggers) is less than 35 W, which is provided by a companion solar power system.

Side-by-side field comparisons between this system and two permanent AmeriFlux sites and between the roving AmeriFlux intercomparison system are described here. Results of these comparisons indicate that the portable system is capable of absolute flux resolutions of about ±1.2 μmol m−2 s−1 for CO2, ±15 W m−2 for LE, ±7 W m−2 for H, and ±0.06 m s−1 for u* between any given 30-min averaging periods. It is also found that, compared to a permanent Ameriflux site, the relative accuracy of this flux estimates is between 1% and 7%. Based on these results, it is concluded that this portable system is capable of making ecosystem flux measurements with an accuracy and precision comparable to most permanent AmeriFlux systems.

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C. D. Hewitt, E. Allis, S. J. Mason, M. Muth, R. Pulwarty, J. Shumake-Guillemot, A. Bucher, M. Brunet, A. M. Fischer, A. M. Hama, R. K. Kolli, F. Lucio, O. Ndiaye, and B. Tapia

Abstract

There is growing awareness among governments, businesses, and the general public of risks arising from changes to our climate on time scales from months through to decades. Some climatic changes could be unprecedented in their harmful socioeconomic impacts, while others with adequate forewarning and planning could offer benefits. There is therefore a pressing need for decision-makers, including policy-makers, to have access to and to use high-quality, accessible, relevant, and credible climate information about the past, present, and future to help make better-informed decisions and policies. We refer to the provision and use of such information as climate services. Established programs of research and operational activities are improving observations and climate monitoring, our understanding of climate processes, climate variability and change, and predictions and projections of the future climate. Delivering climate information (including data and knowledge) in a way that is usable and useful for decision-makers has had less attention, and society has yet to optimally benefit from the available information. While weather services routinely help weather-sensitive decision-making, similar services for decisions on longer time scales are less well established. Many organizations are now actively developing climate services, and a growing number of decision-makers are keen to benefit from such services. This article describes progress made over the past decade developing, delivering, and using climate services, in particular from the worldwide effort galvanizing around the Global Framework for Climate Services under the coordination of UN agencies. The article highlights challenges in making further progress and proposes potential new directions to address such challenges.

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C. A. M. Brenninkmeijer, P. J. Crutzen, H. Fischer, H. Güsten, W. Hans, G. Heinrich, J. Heintzenberg, M. Hermann, T. Immelmann, D. Kersting, M. Maiss, M. Nolle, A. Pitscheider, H. Pohlkamp, D. Scharffe, K. Specht, and A. Wiedensohler

Abstract

The deployment of measurement equipment in passenger aircraft for the observation of atmospheric trace constituents is described. The package of automated instruments that is installed in a one-ton-capacity aircraft freight container positioned in the forward cargo bay of a Boeing 767 ER can register a vast amount of atmospheric data during regular long-distance flights. The air inlet system that is mounted on the fuselage directly below the container comprises an aerosol inlet, a separate inlet for trace-gas sampling, and an air exhaust. All instruments, the central computer, and power supply are mounted in aviation-approved racks that slide into the reinforced container. The current instrument package comprises a fast-response chemiluminescence sensor and a conventional UV absorption detector for O3; a gas chromatograph for CO; two condensation nuclei counters for particles larger than 5 and 12 nm; and a 12-canister large-capacity whole air sampler for laboratory trace-gas analysis and isotopic analysis of CO2, CO, CH4, and N20. First measurement results of the operational Civil Aircraft for Remote Sensing and In Situ Measurements in Troposphere and Lower Stratosphere Based on the Instrumentation Container Concept (CARIBIC) container are reported.

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M. Rhein, J. Fischer, W. M. Smethie, D. Smythe-Wright, R. F. Weiss, C. Mertens, D.-H. Min, U. Fleischmann, and A. Putzka

Abstract

In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50°N, the LSW thickness decreased considerably from 1994–97, while the mean CFC concentrations did not change much. South of 50°N and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period.

The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40° to 65°N, and from 10° to 60°W. In total, 2300 ± 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45°W and 31% of the inventory was located in the eastern Atlantic.

The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4–5.6 Sv (Sv ≡ 106 m3 s−1) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988–94 is 8.1–10.8 Sv, and 1.8–2.4 Sv in 1995–97.

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K. Dethloff, M. Schwager, J. H. Christensen, S. Kiilsholm, A. Rinke, W. Dorn, F. Jung-Rothenhäusler, H. Fischer, S. Kipfstuhl, and H. Miller

Abstract

The accumulation defined as “precipitation minus evaporation” over Greenland has been simulated with the high-resolution limited-area regional climate model HIRHAM4 applied over an Arctic integration domain. This simulation is compared with a revised estimate of annual accumulation rate distribution over Greenland taking into account information from a new set of ice core analyses, based on surface sample collections from the North Greenland Traverse. The region with accumulation rates below 150 mm yr−1 in central-northwest Greenland is much larger than previously assumed and extends about 500 km farther to the south. It is demonstrated that good agreement between modeled and observed regional precipitation and accumulation patterns exists, particularly concerning the location and the values of very low accumulation in the middle of Greenland. The accumulation rates in the northern part of Greenland are reduced in comparison to previous estimates. These minima are connected with a prevailing blocking high over the Greenland ice sheet and katabatic wind systems preventing humidity transports to central Greenland. The model reasonably represents the synoptic situations that lead to precipitation. Maxima of precipitation and accumulation occur at the southwestern and southeastern coasts of Greenland and are connected with cyclonic activity and the main storm tracks around Greenland. The central region of the Greenland ice sheet acts as a blocking barrier on moving weather systems and prohibits cyclones moving from west to east across this region and, thus prevents moisture transports.

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N. Glatthor, T. von Clarmann, H. Fischer, B. Funke, U. Grabowski, M. Höpfner, S. Kellmann, M. Kiefer, A. Linden, M. Milz, T. Steck, G. P. Stiller, G. Mengistu Tsidu, and D-Y. Wang

Abstract

In late September 2002, an Antarctic major stratospheric warming occurred, which led to a strong distortion of the southern polar vortex and to a split of its mid- and upper-stratospheric parts. Such an event had never before been observed since the beginning of regular Antarctic stratospheric temperature observations in the 1950s. The split is studied by means of nonoperational level-2 CH4, N2O, CFC-11, and O3 data, retrieved at the Institute for Meteorology and Climate Research Karlsruhe (IMK) from high-resolution atmospheric limb emission spectra from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European research satellite, Environmental Satellite (ENVISAT). Retrieved horizontal and vertical distributions of CH4 and N2O show good consistency with potential vorticity fields of the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis for the entire period under investigation, even for fine structures such as vortex filaments. Tracer correlation analysis suggests that mixing into the vortex had already occurred before the major warming and that vortex fragments were transported into the surrounding air masses on potential temperature levels above 400 K during the split. Correlation analysis of ozone with the source gases indicates slight ongoing ozone destruction in the lower-stratospheric vortex (below ∼500 K) after the beginning of the warming event.

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M. Susan Lozier, Sheldon Bacon, Amy S. Bower, Stuart A. Cunningham, M. Femke de Jong, Laura de Steur, Brad deYoung, Jürgen Fischer, Stefan F. Gary, Blair J. W. Greenan, Patrick Heimbach, Naomi P. Holliday, Loïc Houpert, Mark E. Inall, William E. Johns, Helen L. Johnson, Johannes Karstensen, Feili Li, Xiaopei Lin, Neill Mackay, David P. Marshall, Herlé Mercier, Paul G. Myers, Robert S. Pickart, Helen R. Pillar, Fiammetta Straneo, Virginie Thierry, Robert A. Weller, Richard G. Williams, Chris Wilson, Jiayan Yang, Jian Zhao, and Jan D. Zika

Abstract

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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