University of Bremen

About: University of Bremen is a education organization based out in Bremen, Germany. It is known for research contribution in the topics: Population & Context (language use). The organization has 14563 authors who have published 37279 publications receiving 970381 citations. The organization is also known as: Universität Bremen.

Linear Convergence of Iterative Soft-Thresholding

Abstract: In this article a unified approach to iterative soft-thresholding algorithms for the solution of linear operator equations in infinite dimensional Hilbert spaces is presented. We formulate the algorithm in the framework of generalized gradient methods and present a new convergence analysis. As main result we show that the algorithm converges with linear rate as soon as the underlying operator satisfies the so-called finite basis injectivity property or the minimizer possesses a so-called strict sparsity pattern. Moreover it is shown that the constants can be calculated explicitly in special cases (i.e. for compact operators). Furthermore, the techniques also can be used to establish linear convergence for related methods such as the iterative thresholding algorithm for joint sparsity and the accelerated gradient projection method.

MAX‐DOAS O4 measurements: A new technique to derive information on atmospheric aerosols: 2. Modeling studies

Abstract: [1] A new retrieval algorithm for the determination of aerosol properties using Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements based on nonlinear optimal estimation is presented. Using simulated MAX-DOAS measurements of the optical depth of the collision complex of oxygen (O4) as well as the variation of the intensity of diffuse skylight measured at different viewing directions and wavelengths, the capability of this measurement technique to derive the aerosol extinction profile as well as information on the phase function and single scattering albedo is demonstrated. The information content, vertical resolution and retrieval errors under various atmospheric conditions are discussed. Furthermore, it is demonstrated that the assumption of a smooth variation of the aerosol properties between successive measurements can be used to improve the quality of the retrieval by applying a Kalman smoother. The results of these model studies suggest that the achievable precision of MAX-DOAS measurements of the aerosol total optical depth is better than 0.01 and thus comparable with established methods of aerosol detection by Sun photometers (e.g., within the AERONET network) over a wide range of atmospheric conditions. Moreover, MAX-DOAS measurements contain information on the vertical profile of the aerosol extinction, and can be performed with relatively simple, robust and self-calibrating instruments.

The sedimentary and tectonic evolution of the Yinggehai-Song Hong basin and the southern Hainan margin, South China Sea : Implications for Tibetan uplift and monsoon intensification

Abstract: [ 1] The Yinggehai - Song Hong basin is one of the world's largest pull-apart basins, lying along the trace of the Red River fault zone in the South China Sea. South of Hainan Island this basin crosscuts the rifted margin of the northern South China Sea. In this paper we present for the first time a regional compilation of multichannel seismic reflection data from both the strike slip and rifted margins. The basins started to open after similar to 45 Ma, especially after similar to 34 Ma. The Yinggehai basin was folded and inverted in the middle Miocene, after 21 Ma in the north and 14 Ma in the south, before rapidly subsiding again after similar to 5 Ma because of continued tectonism. This subsidence has caused shale diapirism, especially driven by associated sedimentation in the late Pliocene (2.6 - 2.0 Ma). Extension along the adjacent south Hainan margin shows preferential lower crustal extension, suggestive of lower crustal flow increasing toward the continent-ocean transition during breakup. Sediment supply is reconstructed to peak in the middle Miocene, then falls between 14 and 10.3 Ma to reach a low in the late Miocene. However, rates rose again in the Pliocene-Pleistocene. The Red River sediment budget is incompatible with climate models that propose stronger monsoon rains starting at 8 Ma or with large-scale river capture away from the Red River after similar to 10 Ma. Both lines of evidence point to major uplift in the Red River drainage being middle Miocene or older. The recent, preindustrial Red River carried much more sediment than the average Pleistocene accumulation rate, indicating modest sediment buffering onshore, at least in recent geologic time.

The EISMINT benchmarks for testing ice-sheet models

Abstract: We present a series of benchmark experiments designed for testing and comparing numerical ice sheet models. Following the outcome of two EISMINTworkshops organized to intercompare large-scale ice-sheet models currently in operation, model benchmark experiments are described for ice sheets underfixed and moving margin conditions. These address both steady-state and time-dependent behavior under schematic boundary conditions and with prescribedphysics. A comparison was made of each model's prediction of basic geophysical variables such as ice thickness, velocity and temperature. Consensusachieved in the model intercomparison provides reference solutions against which the accuracy and consistency of ice-sheet modeling codes can be assessed.