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Exeter, United Kingdom

Jung T.,Alfred Wegener Institute for Polar and Marine Research | Doblas-Reyes F.,Barcelona Supercomputing Center | Goessling H.,Alfred Wegener Institute for Polar and Marine Research | Guemas V.,Institute Catala Of Ciencies Del Clima Ic3 | And 11 more authors.
Bulletin of the American Meteorological Society | Year: 2015

The International Workshop on Polar Lower-Latitude Linkages and Their Role in Weather and Climate Predic?tion was held in Barcelona, Spain from December 10-12, 2014. The workshop, which was attended by 80 participants from 20 countries and included early career scientists, was motivated by the fact that the polar regions were anticipated to undergo rapid changes in a warming world. A unique aspect of the Barcelona workshop was that polar lower-latitude linkages were also discussed from a prediction and services perspective.

Philipp A.,University of Augsburg | Bartholy J.,Eotvos Lorand University | Beck C.,University of Augsburg | Erpicum M.,University of Liege | And 16 more authors.
Physics and Chemistry of the Earth | Year: 2010

A new database of weather and circulation type catalogs is presented comprising 17 automated classification methods and five subjective classifications. It was compiled within COST Action 733 " Harmonisation and Applications of Weather Type Classifications for European regions" in order to evaluate different methods for weather and circulation type classification. This paper gives a technical description of the included methods using a new conceptual categorization for classification methods reflecting the strategy for the definition of types. Methods using predefined types include manual and threshold based classifications while methods producing types derived from the input data include those based on eigenvector techniques, leader algorithms and optimization algorithms. In order to allow direct comparisons between the methods, the circulation input data and the methods' configuration were harmonized for producing a subset of standard catalogs of the automated methods. The harmonization includes the data source, the climatic parameters used, the classification period as well as the spatial domain and the number of types. Frequency based characteristics of the resulting catalogs are presented, including variation of class sizes, persistence, seasonal and inter-annual variability as well as trends of the annual frequency time series. The methodological concept of the classifications is partly reflected by these properties of the resulting catalogs. It is shown that the types of subjective classifications compared to automated methods show higher persistence, inter-annual variation and long-term trends. Among the automated classifications optimization methods show a tendency for longer persistence and higher seasonal variation. However, it is also concluded that the distance metric used and the data preprocessing play at least an equally important role for the properties of the resulting classification compared to the algorithm used for type definition and assignment. © 2010.

Kjellsson J.,British Antarctic Survey | Holland P.R.,British Antarctic Survey | Marshall G.J.,British Antarctic Survey | Mathiot P.,British Antarctic Survey | And 5 more authors.
Ocean Modelling | Year: 2015

We examine the sensitivity of the Weddell and Ross seas to vertical mixing and surface freshwater forcing using an ocean-sea ice model. The high latitude Southern Ocean is very weakly stratified, with a winter salinity difference across the pycnocline of only ~0.2 PSU. We find that insufficient vertical mixing, freshwater supply from the Antarctic Ice Sheet, or initial sea ice causes a high salinity bias in the mixed layer which erodes the stratification and causes excessive deep convection. This leads to vertical homogenisation of the Weddell and Ross seas, opening of polynyas in the sea ice and unrealistic spin-up of the subpolar gyres and Antarctic Circumpolar Current. The model freshwater budget shows that a ~30% error in any component can destratify the ocean in about a decade. We find that freshwater forcing in the model should be sufficient along the Antarctic coastline to balance a salinity bias caused by dense coastal water that is unable to sink to the deep ocean. We also show that a low initial sea ice area introduces a salinity bias in the marginal ice zone. We demonstrate that vertical mixing, freshwater forcing and initial sea ice conditions need to be constrained simultaneously to reproduce the Southern Ocean hydrography, circulation and sea ice in a model. As an example, insufficient vertical mixing will cause excessive convection in the Weddell and Ross seas even in the presence of large surface freshwater forcing and initial sea ice cover. © 2015 The Authors.

Bosveld F.C.,Royal Netherlands Meteorological Institute | Baas P.,Royal Netherlands Meteorological Institute | Steeneveld G.-J.,Wageningen University | Holtslag A.A.M.,Wageningen University | And 10 more authors.
Boundary-Layer Meteorology | Year: 2014

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. © 2014 The Author(s).

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