Lemke, Peter
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Lemke, Peter
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Lemke, Peter
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Item-typ:Veröffentlichung, Interannual and decadal variability of sea ice drift, concentration and thickness in the Weddell Sea(2012-03-12); ; ; Sea ice concentrations in the Weddell Sea are subject to regional climate variability. The magnitude and origin of local trends in the sea ice coverage were studied using the bootstrap algorithm sea ice concentration data from the NSIDC for 1979-2006. The impact of atmospheric forcing such as air temperature, wind speed, sea level pressure and cloud coverage, gained from NCEP/NCAR reanalysis, was assessed by analyzing correlation coefficients between the respective atmospheric component and the sea ice concentrations. In addition, the variability of sea ice drift was analyzed using the Polar Pathfinder sea ice motion vectors, and the correlation with sea ice concentration was tested after an assessment of the product s uncertainties. The connection to the variability of sea ice thicknesses was derived by model simulations from the Finite Element Sea ice-Ocean Model (FESOM). It was found that sea ice concentrations increased in the eastern and decreased in the western Weddell Sea, predominantly in the marginal sea ice zone. There, and in coastal regions, temperatures are strongly negatively correlated to sea ice concentrations, whereas in the central Weddell Sea, mostly a positive correlation was assessed, especially during winter. From analyses of the wind field it was found that the prevailing westerlies at the Antarctic Peninsula frequently show a shift towards the south. The enhanced southward winds are expected to bring warmer air into the western and central Weddell Sea and are further expected to redistribute the sea ice from the west into the central and eastern regions. This would increase the sea ice concentrations in the central Weddell Sea due to enhanced compactness, although temperatures are increasing. The correlation between sea ice concentrations and sea ice drift is only robust for the central Weddell Sea, where both parameters are mainly anti-correlated. Hence, strong sea ice drift is connected to lower sea ice concentrations and vice versa. This finding is consistent with the connection to the wind fields, since stronger northerly winds would reduce the north-eastward drift of sea ice in this region and enhance the sea ice concentrations. From model simulations with FESOM it was found that sea ice thicknesses predominantly show the same tendencies for changes as the simulated sea ice concentrations, which are basically decreasing in the central Weddell Sea and increasing in the eastern Weddell Sea. The overall changes in sea ice thickness and concentration result in an increase of the total sea ice volume by 1 % per decade in the simulations. A sensitivity study with a free drift model, forced by 10-m wind speeds and ocean currents from FESOM showed that the trends in the modelled sea ice drift are driven by the atmospheric fields, since ocean currents show barely any trends. Further it was revealed that sea ice drift velocities in the model are overestimated, especially in the zonal direction. Nevertheless, despite the overestimation, the mean sea ice export rate of 22 x 10^3 m^3/s is only about half of the export rates found in previous studies, which is certainly an effect of underestimated sea ice thicknesses in the western Weddell Sea.Dissertation357 156 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Data Assimilation in a Regional Finite Element Sea-Ice Model for the Arctic - Application of the Singular Evolutive Interpolated Kalman Filter(2008-05-29); ; ; The Arctic region is sensitive to climate change. Since the Arctic sea-ice cover influences the surface heat budget of the Earth the observed sea-ice decline is seen as an indication of global warming. Furthermore, the dynamics of sea ice plays an important role for the sea-ice mass distribution in the Arctic, for the production of dense, cold, and salty water in the Arctic Ocean, which contributes to the thermohaline circulation, and also for the freshwater budget of the Nordic Seas. Thus, a realistic description of sea-ice motion is important to draw conclusions for the mass transport and sea-ice mass distribution.The Finite-Element Sea-Ice Model simulates the large-scale physical sea-ice processes like the sea-ice growth and circulation realistically. The model domain covers the entire Arctic Ocean and its marginal seas. Together with the Singular Evolutive Interpolated Kalman (SEIK) Filter and remotely sensed sea-ice drift observations this sea-ice model is applied for data assimilation to investigate details of the sea-ice dynamics. So far, drift assimilation has been carried out to analyze and modify only the drift field with subsequent computation of the advection or redistribution of ice mass which corresponds more to the physical model behavior than a statistical analysis that the SEIK Filter provides. The sea-ice drift data assimilation with the SEIK Filter achieves drift modification and furthermore changes in the two other sea-ice variables concentration and thickness. The modifications of these "unobserved variables" (within the meaning of data assimilation) are validated and it is found that they are in good agreement for at least 2 months for the sea-ice thickness and even 4 months for the sea-ice concentration which is the longest period examined. The drift improvement is achieved due to the sea-ice concentration and thickness changes which leads to a sustainable effect for further sea-ice drift simulation. Furthermore, the assimilation results suggest a higher thickness variability that the model alone is not able to produce. A localized version of the SEIK Filter leads to a more pronounced drift correction which is not sustainable in course of further model integration because in this case the sea-ice concentration and thickness are not much affected by the assimilation method. This thesis describes the initial work of the sea ice drift assimilation with the SEIK Filter and further examines the ability of the SEIK Filter to modify the model state using the observation data. The applicability, capability, limitation of the assimilation method and suggestions are discussed.Dissertation467 142 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Antarctic Specific Features of the Greenhouse Effect : a Radiative Analysis Using Measurements and Models(2015-03-02); ; ; CO2 is the strongest anthropogenic forcing agent for climate change since pre-industrial times. Like other greenhouse gases, CO2 absorbs terrestrial surface radiation and causes emission from the atmosphere to space. As the surface is generally warmer than the atmosphere, the total long-wave emission to space is commonly less than the surface emission. However, this does not hold true for the high elevated areas of central Antarctica. For this region, it is shown that the greenhouse effect of CO2 is around zero or even negative. Moreover, for central Antarctica an increase in CO2 concentration leads to an increased long-wave energy loss to space, which cools the earth-atmosphere system. These unique findings for central Antarctica are in contrast to the well known general warming effect of increasing CO2 . The work contributes to explain the non-warming of central Antarctica since 1957.Dissertation679 196 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Arctic Sea Ice Dynamics: Drift and Ridging in Numerical Models and Observations(2007-05-11); ; ; The Arctic sea ice cover is constantly in motion driven by the wind and ocean currents. The transport of freshwater and latent heat is associated with the ice drift. Furthermore, the drift causes deformation of the sea ice cover under compressive and shear forces and pressure ridges form. Ridges in turn affect the momentum and---to a minor degree---the heat exchange between sea ice and atmosphere and ocean because they strongly increase the local surface roughness and thickness of the ice. Therefore, the sea ice drift and deformation interact with the climate system and its changes, and it is a key issue to both the remote-sensing and modelling community to provide products of good quality. The present thesis splits into three parts: a study of modelled and observed drift estimates, an analysis of sea ice ridge quantities derived from laser altimeter and airborne electromagnetic measurements and an investigation of different numerical algorithms for the representation of ridges in a large-scale sea ice model.The study of sea ice drift focuses on the comparison of different sea ice-ocean coupled models and the validation with buoy and remote-sensing data of the period 1979--2001 on the basis of monthly averages. According to drift speed distributions the group of models, which matches best the observations, has a mode at drift speeds around 0.03 m/s and a short tail towards higher speeds. However, there are also models with much larger drift speeds. In general, all models are capable of producing realistic drift pattern variability although differences are found between models and observations. Reasons for these differences are manifold and lie in discrepancies of wind stress forcing as well as sea ice model characteristics and sea ice-ocean coupling.The investigation of sea ice ridges is based on Arctic-wide in situ measurements of the period 1995--2005 which include different sea ice roughness regimes. While sail density is found to emphasise local deformation events sail height features a large-scale, positive gradient from the Siberian shelf seas towards the Lincoln Sea, where sails of up to 10 m height were found. However, regionally averaged sail heights are found to vary little between 1.1 m and 1.6 m. Rather large ratios of 10 sails per keel and 1:6.3 m for sail height to keel depth are derived. Linear relationships are determined for sail to keel density and sail height to keel depth. Furthermore, functional relationships of sail height and level ice thickness are found.Three different approaches to the simulation of pressure ridge formation are introduced and tested in idealised experiments and for realistic Arctic conditions. Simulations are evaluated with airborne laser profiles of the sea ice surface roughness. The main characteristics of the respective ridging algorithms are: (1) a prognostic derivation of deformation energy from which ridge parameters are deduced, (2) a redistribution function, transforming level ice to a second, ridged ice category, combined with a stochastic simulation of ridge quantities, and (3) prognostic equations for ridge density and height resulting in the formation of ridged ice volume. The model results show that the ridge density is mainly related to the sea ice drift whereas the mean sail height relates to the parent ice thickness. Most deformation occurs at coastlines. In general, all of the three algorithms produce realistic distributions of ridges. Finally, the second ridging scheme is regarded to be most appropriate for climate modelling while the third scheme is found to be advantageous for short-term sea ice forecasting.Dissertation774 337 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Titel: Sea Level Variations derived from Mass Conserving Finite Element Sea-Ice Ocean Model; Untertitel: Study of Major Contributions to Sea Level Change in the Recent Past(2011-02-17); ; ; During the last century sea level rise strongly increased compared to sea level change in the last 2000 years. The present study investigates global and regional sea level change, simulated with the finite element sea-ice ocean model (FESOM). The major goal is to separate sea level change into steric and eustatic contributions and to estimate the influence of Greenland and Antarctic ice sheet melt on global and regional sea level. Modeled steric height variations show realistic regional geophysical patterns compared with steric height variations derived from altimetry measurements and GRACE. Compared to the time before the 1990 s, an increased global trend in steric sea level rise is found in estimates derived from the model and from satellite measurements. Modeled ocean mass exhibits reasonable spatial structures. However, the trend in the global model mean cannot be trusted in FESOM as it strongly depends on the mass budget of the model, which is determined by uncertain mass fluxes. To account for this, global mean ocean mass variations need to be optimized to realistic values. To this end results from GRACE in combination with GPS data is used. Greenland and Antarctic ice sheet melting influence the global sea level mainly through the additional mass. The eustatic sea level rises by about 0.3 mm/yr for 100 Gt/yr of melt water. Additionally, the fresh water causes local steric variations in sea level that are transported farther by ocean currents. The ice sheet mass loss yields a decrease in gravitational attraction causing a sea level fall near the source of mass loss but also to a slight increase at long distance. This effect is computed for the Greenland ice sheet mass loss using Green s functions. It leads to a decreased sea level near the Greenland coast and to a slightly increased sea level in the Southern Ocean. The effect of different melting scenarios is investigated.Dissertation481 185 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Lagrangian circulation and transports of the Antarctic Intermediate Water in the south and tropical Atlantic(2005-09-22); ; ; This study uses a data set of 611 float years in the south and tropical Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). The data set includes pop-up and acoustically tracked floats that drifted within more than one decade in the area 60 degrees W to 30 degrees E and 70 degrees S to 10 degrees N. Float data is constrained in the vertical according to two isoneutral surfaces (gn = 27.25 and gn = 27.55), according to the characteristics of AAIW. Velocity space-time averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW layer). Judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a cell size of 3 degrees (latitude) by 4 degrees (longitude) with cells deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Since these space-time averages and the corresponding objective maps were unable to reveal the Intermediate Western Boundary Current (iWBC), an alternative approach based on an objective mapping with primal data in overlapping subsets was also applied. With this goal, an isotropic longitudinal covariance function was estimated considering cylinDissertation302 108 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Convective processes in the polar atmospheric boundary layer: a study based on measurements and modeling(2015-12-10); ; ; Climate change is especially pronounced over the Arctic Ocean, where the atmosphere warmed twice as fast as in lower latitudes in the last few decades. This warming is associated with a rapid decline of the Arctic sea ice cover. For future predictions of changes in the Arctic climate system, profound knowledge of all processes influencing the surface energy budget in polar regions is essential. The focus of this thesis lies on improving our current understanding of convective processes and the related turbulent fluxes in the polar atmospheric boundary layer (ABL) over both the sea ice covered regions and over the open ocean at the sea ice edge. A major part of the analysis is based on aircraft measurements from the campaign STABLE, which was carried out over the pack ice in the northern Fram Strait in March 2013. These results are supplemented by modeling studies using a simple boxmodel and a one-dimensional mesoscale model. For the first time, comprehensive aircraft measurements over leads were conducted during the campaign STABLE. They are used to study the formation of convective plumes over leads and their impact on the polar ABL. It is found that the conditions over four wide leads are highly variable with respect to turbulent fluxes, as well as to the mean variables temperature, humidity, and wind. In one of the cases large entrainment fluxes exceeding 30 % of the surface fluxes are observed. The convective plumes over leads have a large influence on the vertical profiles of sensible heat and momentum fluxes, which are non-linear downstream of the leads with a distinct flux maximum in the core of the convective plumes. For the first time, it it shown based on measurements that the plume also affects the wind field by diminishing low level jets in the region influenced by the plume. In addition to the small scale impact of individual leads the regional impact of lead ensembles is studied using long transect flights. The analysis shows that near-surface atmospheric temperatures are clearly related to the ice concentration in the considered region. The impact of a heterogeneous sea ice cover and of the related surface temperature changes on atmospheric temperatures is also analysed using a Lagrangian box model. The model uses reanalysis winds as well as sea ice concentration and surface temperature from satellites as input data. The box model is used to calculate the evolution of the near-surface air temperature along backward-trajectories, which are then compared to measured temperatures at three different Arctic sites. The results suggest that a large amount of the observed air temperature variability can be attributed to heterogeneous surface temperatures and that the characteristic length of the upstream region influencing air temperatures at a specific location is 200 km. Convection during cold air outbreaks at the sea ice edge has a much stronger impact on the polar ABL than convective plumes over leads. Dropsonde measurement of four cold air outbreaks during STABLE are used to analyse the downstream development of meteorological variables and the ABL growth. Two of the considered cases are influenced by the size of the Whaler's Bay polynya north of Svalbard, which was unusually large in the three winters from 2012 to 2014 compared to the previous 20 years. The analysis of the dropsonde measurements shows that the unusual ice conditions lead to strong atmospheric convection in a region north of Svalbard that was typically ice-covered in the last decades. This leads to extreme convective ABL heights and modifies local temperature conditions considerably. Convective processes in the ABL have to be parametrised in climate models. Therefore, in addition to the measurements, the performance of three different sensible heat flux parametrisations is tested in a 1D mesoscale model and results are compared to those of a large eddy simulation model (LES). Both the considered counter-gradient and eddy-diffusivity mass-flux (EDMF) approach reproduce the shape of the temperature profile of the LES better than a classical mixing length approach. A sensitivity analysis shows that the EMDF approach is the least sensitive to changes of the vertical grid spacing, which can be attributed to the derivation of the ABL height using a diagnostic equation of the updraft velocity. The sensitivity of the counter-gradient closure to the grid spacing can be significantly reduced when the updraft velocity equation of the EDMF approach is included and used to derive the ABL height.Dissertation971 197 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Coastal polynyas in the southwestern Weddell Sea: Surface fluxes, sea ice production and water mass modification(2013-09-18); ; ; Coastal polynyas in the southwestern Weddell Sea were simulated using the Finite Element Sea Ice-Ocean Model forced with NCEP/NCAR Reanalysis data. The period 1990-2009 was used in analysis. Also, shorter model runs forced with GME and COSMO model data were branched off and compared. Depending on the region, the 20-year mean winter heat flux to the atmosphere is 310-510 W/m^2, whereof 50-60 W/m^2 are supplied by cooling the ocean and the remainder induces a sea ice production of 7-13 cm/(d m^2). The coastal polynyas (0.6% of the area) contribute 11% to the southwestern Weddell Sea sea ice production. In the mean 2 Sv High Salinity Shelf Water are exported from the continental shelves. Different forcing data sets can cause substantial disparities in the regional results.Dissertation289 162 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Bestimmung verschiedener Eisklassen durch statistische Analyse der Rauigkeit von Meereis(2007-02-23); ; ; Among the properties of sea ice, roughness is an important parameter. It affects the interactions between ice, atmosphere and ocean. The morphological properties of the top and underside surface influence the transfer of energy and momentum. In satellite remote sensing, the knowledge of surface roughness characteristics is important, because they influence the measured signal in a complex way.Based on in-depth statistical analyses of sea-ice roughness, two classification methods are investigated in this work regarding their potential to separate different ice types. These methods are discriminant and cluster analysis.In order to take different aspects of roughness into account, datasets from four different geographical locations are used. These comprise data from the Lincoln Sea north of Greenland, the Arctic Ocean near Svalbard, and the Baltic Sea. One dataset from the Arctic Ocean was obtained during summer. The available data thus enable investigations of regional as well as seasonal changes of sea-ice roughness.The statistical analyses reveal regional differences in sea-ice roughness. Surface roughness profiles are found to be nonstationary and to display fractal properties on length scales below 20~m. The distributions of height and spacing of pressure ridges are approximately exponential or lognormal, respectively. Pressure ridges are not distributed randomly over the ice surface but appear in clusters. Significant correlations exist between profiles of the sea-ice surface and draft. Spatial scales that contribute most to the surface roughness are found to be smaller than 50~m. The surface roughness is thus largely influenced by length scales comparable to observed pressure ridge widths. The statistical analyses lead to a set of parameters, consisting of mean height, RMS height, skewness, kurtosis, fractal dimension and RMS slope, which characterize the roughness and form the basis for the classification analysis. The discriminant analysis shows that the thickest ice classes can be distinguished from one another and from thinner ice using the surface roughness parameters to separate the classes. The cluster analysis reveals that different types of surface roughness cannot be distinguished clearly from one another. Synthetic roughness profiles are important for studies of the interactions between the sea-ice surface and the atmosphere. In this work a numerical model for simulations of sea-ice draft is assessed regarding its potential to generate realistic sea-ice surface profiles. It is shown that the model is capable of reproducing many of the properties of real surface roughness profiles.Dissertation585 114