1,721,213 research outputs found
Simulations of the boundary layer flow over idealised patterns of leads in sea ice with (non-)lead-resolving applications
No full textThe data set consists of model data from simulations of the atmospheric flow over different configurations of leads in sea ice. Leads are elongated channels in sea ice over which strong convection can develop, especially between late autumn and spring due to large spatial temperature differences in that season. Lead-generated convection can considerably influence the structure of the atmospheric boundary layer (ABL), not only on a local but also on a more regional scale. All simulations (18 in total, see Table 1) were carried out with the non-eddy-resolving MEsoscale TRansport And Stream model (METRAS, Schlünzen et al., 2018a, b). Every simulation was forced with the same idealised initial inflow conditions of a springtime ABL typically observed in the polar ocean regions. Hence, the data is not georeferenced and the outputs are given with respect to a Cartesian coordinate system. The simulations refer to six different idealised configurations of leads in sea ice (three runs for each case). All cases consist of different domains downwind of an inflow region over 100% thick sea ice cover. In the vertical direction, the grid spacing is 20m below 350m (ABL height is 300m). The initial inflow conditions are the same in all simulations. These correspond to one of the idealised cases of Michaelis et al. (2020). The latter carried out similar simulations for the flow over individual leads, with METRAS and also with an LES model (LES data: Zhou & Gryschka, 2019). In five cases, the simulations are carried out on a microscale, lead- and convection-resolving grid with 200m horizontal grid spacing. The domains all consist of idealised series of leads, which differ by the width of the leads L (with L = {1, 2, 5, 10}km) and by the distance between the leads. These cases are abbreviated by ENS-1km, ENS-2km, ENS-5km-d20km, ENS-5km-d40km, and ENS-10km (see Table 1 in the data description file and Figure 2 in Michaelis and Lüpkes (2022)). Between the leads, which all have a surface temperature of 270K to represent leads covered by thin, new ice, 100% thick sea ice cover is assumed (250K surface temperature). All configurations in the five cases have been chosen in a way that the domain-averaged sea ice concentration is always the same (approximately 91%). The sixth case should represent a few grid cells of a regional climate model, wherein neither leads nor lead-generated convection is resolved so that the surface topography differs strongly from the previous cases. It is run on a grid with 35km horizontal grid spacing. However, the sea ice concentration prescribed in each surface grid cell is the same as the domain-averaged sea ice concentration used for the lead-resolving simulations. This case is abbreviated by ENS-C, where 'C' hints to climate models. Thus, a comparison of domain-averaged quantities is possible between the high- and coarse-resolution simulation results. For each case, different parametrizations of the subgrid-scale turbulent fluxes have been applied using local and non-local closures. These are described in detail by Michaelis and Lüpkes (2022) in their Table 1. The model output files (see Table 2) consist of results after quasi-stationary conditions had been reached. Data is shown with respect to the distance y from the inflow boundary at y = 0km (equal to the scalar grid point position of the model domains) and for 2D-variables also with respect to the height from the surface at z = 0m. In the vertical direction, two different grid variables are used. Most 2D-output variables (temperature, pressure, wind components) are available on a non-equidistant grid in vertical direction up to the model's top at approximately 9600m. The corresponding vertical coordinate is z. The remaining 2D- output variables (turbulent heat flux, turbulent momentum flux) are available on an equidistant grid in vertical direction (20m spacing) up to a height of approximately 2000m (coordinate z_uni_2000m). A scientific evaluation of the model simulations is given in the corresponding publication Michaelis and Lüpkes (2022). More details on the METRAS model are shown in the model's documentation (Schlünzen et al., 2018a, b)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A combined measurement-model approach for estimating methane emissions on the Mackenzie Delta
No full text
Modelling and parametrization of turbulent convective processes over leads in sea ice
Full text linkIn the polar ocean regions, the Earth’s climate system is characterised by many different interaction processes between atmosphere, ocean, and sea ice. Especially between late autumn and spring, the sea ice cover plays a very important role in this system due to its mainly insulating effect, which minimises the exchange of energy between the ocean and the atmosphere. Nonetheless, also in the cold season with large sea ice cover, a strong turbulent transport of heat and moisture is possible between the relatively warm ocean and the cold atmosphere, for example, through elongated open-water channels in sea ice, which are called leads. The convective atmospheric transport over leads is driven mainly by large spatial temperature differences causing plumes with enhanced turbulent transport, which strongly affect the characteristics of the atmospheric boundary layer (ABL) depending on both lead geometry and meteorological forcing. Understanding and quantifying these rather small-scale physical processes is crucial for improving model results also on larger scales and for obtaining accurate projections of the future climate.
The focus of this thesis lies on a detailed investigation of the atmospheric processes related to the flow over leads, predominantly by means of small-scale numerical modelling. The applied model uses grid sizes so that the convective plume but not the single turbulent eddies are resolved, which requires turbulence parametrization and validation of the corresponding results. The central part of this thesis is the derivation of an improved parametrization to describe the turbulent fluxes over leads of different width. The new parametrization follows a non-local approach and it is derived based on an already existing closure, but, as a new feature, the lead width is included as a parameter.
Small-scale model results obtained with the new parametrization as well as with already existing approaches are evaluated in this thesis for different idealised and observed situations. As a first step, for the idealised cases, the corresponding model results are compared with new time-averaged large eddy simulation results. It is shown that an improved representation of several ABL patterns is obtained when using the new approach for situations of a lead-perpendicular flow in a neutrally stratified ABL below a strong capping inversion.
As a second step, small-scale model results are validated using airborne observations. As compared with the idealised cases, also stable inflow conditions and shallower boundary layers are considered. A basic representation of the observed patterns is obtained by the model also for these situations, but some effects remain underestimated. Therefore, further modifications of the new parametrization are introduced, which cause an improved agreement between model results and observations.
Besides the new parametrization, also model results obtained with a local turbulence closure are evaluated for the idealised and observed cases. Several drawbacks are shown in the corresponding results, especially for the idealised cases, whereas some of the observed ABL characteristics can be basically reproduced also with this closure type. However, the advantage of applying a non-local approach is clearly shown by the physically most reasonable representation of atmospheric processes, especially in regions of both upward heat transport in neutral or even slightly stable conditions (counter-gradient transport) and vertical entrainment.
Finally, a preliminary study is carried out to point at potential implications of small-scale lead-generated atmospheric effects on larger scales. The small-scale model is applied to simulate the flow over different spatial distributions of sea ice and leads and over a region of continuous fractional sea ice cover representing a few grid cells of a regional climate model with the same sea ice concentration in each cell. It is shown by comparison of domain-averaged profiles that the distribution of leads and their geometry has a profound impact on ABL characteristics also on a larger scale. Moreover, differences are obtained depending on the applied turbulence parametrization. Although at the moment this result cannot be validated by observations, it points clearly to the necessity of an improved treatment of leads in large-scale models
Convection over sea ice leads: Airborne measurements of the campaign STABLE from March 2013
No full textThis data set consists of the airborne measurements obtained in the framework of the AWI-campaign "SpringTime Atmospheric Boundary Layer Experiment" (STABLE) that are used as supplement for the study by Michaelis et al. (2020). The measurements were performed in the atmospheric boundary layer near three different leads in the Arctic Marginal Sea Ice Zone north and northwest of Svalbard on 10, 25 and 26 March 2013. All cases represent conditions of a nearly lead-perpendicular convective flow over the leads.
For each of the three cases, different flight legs are provided (see filenames):
(a) 'upwind': a vertical profile performed upwind of the lead
(b) 'cross-lead': a low-level horizontal flight leg performed across the lead along the mean wind direction
(c) 'lead-parallel': lead-parallel flight legs performed at different distances to the upwind lead edge at different altitudes, either above the lead or further downwind
(d) 'saw-tooth': a saw-tooth flight pattern performed across the lead along the mean wind direction (only for 25 March)
(e) 'lead-parallel_avg-val_fluxes': flight-leg-averaged values of mean atmospheric quantities and turbulent fluxes for the lead-parallel legs
The measurements of wind, temperature, pressure and humidity were performed with instruments mounted on a 3m long nose-boom of the research aircraft Polar 5 (see also Tetzlaff et al., 2015). For the three wind components, temperature, and pressure, the sampling rate is 100Hz so that with the observed ground speed of the aircraft of 40 to 75 ms-1 a spatial resolution of approximately 0.4 to 0.75m was obtained (Tetzlaff et al., 2015, Michaelis et al., 2020). The three wind components and air pressure were measured using a five hole probe. Air temperature was measured with a PT-100 resistance thermometer. Relative air humidity was measured with a dew point mirror. Global Position System (GPS) and Inertial Navigation System (INS) were used to derive height and position of the aircraft. In addition, a KT-19 radiation thermometer and an infrared (IR) scanner were used to measure surface temperatures. All data is adjusted to the time measured at the nose-boom.
The turbulent fluxes were calculated using the eddy covariance method for all lead-parallel flight legs (see Tetzlaff et al., 2015, Michaelis et al., 2020). Mean values averaged over an entire flight leg and the corresponding statistical error are provided for air temperature, air density, horizontal wind speed, the sensible heat flux, the absolute value of the total vertical flux of horizontal momentum, and for the x- and y-components of the latter quantity.
A more detailed description on the measurement techniques and flight patterns is provided by Tetzlaff et al. (2015). For more details on the post-processing of the raw data and on measurement accuracy, see Hartmann et al. (2018)
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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