12,580 research outputs found
Nonlinear stability and ergodicity of ensemble based Kalman filters
The ensemble Kalman filter (EnKF) and ensemble square root filter (ESRF) are
data assimilation methods used to combine high dimensional, nonlinear dynamical
models with observed data. Despite their widespread usage in climate science
and oil reservoir simulation, very little is known about the long-time behavior
of these methods and why they are effective when applied with modest ensemble
sizes in large dimensional turbulent dynamical systems. By following the basic
principles of energy dissipation and controllability of filters, this paper
establishes a simple, systematic and rigorous framework for the nonlinear
analysis of EnKF and ESRF with arbitrary ensemble size, focusing on the
dynamical properties of boundedness and geometric ergodicity. The time uniform
boundedness guarantees that the filter estimate will not diverge to machine
infinity in finite time, which is a potential threat for EnKF and ESQF known as
the catastrophic filter divergence. Geometric ergodicity ensures in addition
that the filter has a unique invariant measure and that initialization errors
will dissipate exponentially in time. We establish these results by introducing
a natural notion of observable energy dissipation. The time uniform bound is
achieved through a simple Lyapunov function argument, this result applies to
systems with complete observations and strong kinetic energy dissipation, but
also to concrete examples with incomplete observations. With the Lyapunov
function argument established, the geometric ergodicity is obtained by
verifying the controllability of the filter processes; in particular, such
analysis for ESQF relies on a careful multivariate perturbation analysis of the
covariance eigen-structure.Comment: 38 page
Geometric phase in open systems: beyond the Markov approximation and weak coupling limit
Beyond the quantum Markov approximation and the weak coupling limit, we
present a general theory to calculate the geometric phase for open systems with
and without conserved energy. As an example, the geometric phase for a
two-level system coupling both dephasingly and dissipatively to its environment
is calculated. Comparison with the results from quantum trajectory analysis is
presented and discussed
SGXIO: Generic Trusted I/O Path for Intel SGX
Application security traditionally strongly relies upon security of the
underlying operating system. However, operating systems often fall victim to
software attacks, compromising security of applications as well. To overcome
this dependency, Intel introduced SGX, which allows to protect application code
against a subverted or malicious OS by running it in a hardware-protected
enclave. However, SGX lacks support for generic trusted I/O paths to protect
user input and output between enclaves and I/O devices.
This work presents SGXIO, a generic trusted path architecture for SGX,
allowing user applications to run securely on top of an untrusted OS, while at
the same time supporting trusted paths to generic I/O devices. To achieve this,
SGXIO combines the benefits of SGX's easy programming model with traditional
hypervisor-based trusted path architectures. Moreover, SGXIO can tweak insecure
debug enclaves to behave like secure production enclaves. SGXIO surpasses
traditional use cases in cloud computing and makes SGX technology usable for
protecting user-centric, local applications against kernel-level keyloggers and
likewise. It is compatible to unmodified operating systems and works on a
modern commodity notebook out of the box. Hence, SGXIO is particularly
promising for the broad x86 community to which SGX is readily available.Comment: To appear in CODASPY'1
Biophysical controls on light response of net CO<inf>2</inf>exchange in a winter wheat field in the North China Plain
To investigate the impacts of biophysical factors on light response of net ecosystem exchange (NEE), CO2 flux was measured using the eddy covariance technique in a winter wheat field in the North China Plain from 2003 to 2006. A rectangular hyperbolic function was used to describe NEE light response. Maximum photosynthetic capacity (Pmax) was 46.6±4.0 μmol CO2 m-2 s-1 and initial light use efficiency (α) 0.059±0.006 μmol μmol-1 in April-May, two or three times as high as those in March. Stepwise multiple linear regressions showed that Pmax increased with the increase in leaf area index (LAI), canopy conductance (gc) and air temperature (Ta) but declined with increasing vapor pressure deficit (VPD) (P25°C or VPD>1.1-1.3 kPa, NEE residual increased with the increase in Ta and VPD (P<0.001), indicating that temperature and water stress occurred. When gc was more than 14 mm s-1 in March and May and 26 mm s-1 in April, the NEE residuals decline disappeared, or even turned into an increase in gc(P<0.01), implying shifts from stomatal limitation to non-stomatal limitation on NEE. Although the differences between sunny and cloudy sky conditions were unremarkable for light response parameters, simulated net CO2 uptake under the same radiation intensity averaged 18% higher in cloudy days than in sunny days during the year 2003-2006. It is necessary to include these effects in relevant carbon cycle models to improve our estimation of carbon balance at regional and global scales. © 2014 Tong et al
High Performance Perovskite Solar Cells
Perovskite solar cells fabricated from organometal halide light harvesters have captured significant attention due to their tremendously low device costs as well as unprecedented rapid progress on power conversion efficiency (PCE). A certified PCE of 20.1% was achieved in late 2014 following the first study of long-term stable all-solid-state perovskite solar cell with a PCE of 9.7% in 2012, showing their promising potential towards future cost-effective and high performance solar cells. Here, notable achievements of primary device configuration involving perovskite layer, hole-transporting materials (HTMs) and electron-transporting materials (ETMs) are reviewed. Numerous strategies for enhancing photovoltaic parameters of perovskite solar cells, including morphology and crystallization control of perovskite layer, HTMs design and ETMs modifications are discussed in detail. In addition, perovskite solar cells outside of HTMs and ETMs are mentioned as well, providing guidelines for further simplification of device processing and hence cost reduction
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