SENSITIVITY OF PERTURBATION GROWTH TO FLOW CHARACTERISTICS AND SAMPLING STRATEGY

The chaotic nature of the weather/climate attractor intrinsically limits the deterministic skill of weather forecasts by promoting rapid growth of errors. In this study, such error growth is simulated by artificially perturbing the atmosphere at initial time, and its sensitivity to the chosen perturbation methodology and to the flow characteristics is investigated. The different simulations are integrated with the limited-area model LM run on a convection-resolving grid. Results demonstrate that the locations of growing disturbances are insensitive to the definition of the initial temperature perturbation. This can be explained through an analysis of the perturbation growth and propagation mechanisms. In particular, rapid radiation of the imposed initial disturbance through a sound wave and presence of specific flow characteristics (e.g. convective instability) appear to force localized error growth far remote from the initial perturbation

The role of soil states in medium-range weather predictability

International audienceCurrent day operational ensemble weather prediction systems generally rely upon perturbed atmospheric initial states, thereby neglecting the eventual effect on the atmospheric evolution that uncertainties in initial soil temperature and moisture fields could bring about during the summer months. The purpose of this study is to examine the role of the soil states in medium-range weather predictability. A limited area weather prediction model is used with the atmosphere/ land-surface system in coupled or uncoupled mode. It covers Europe and part of the north Atlantic, and is driven by prescribed sea-surface temperatures over the sea, and by atmospheric reanalyses at its lateral boundaries. A series of 3 member ensembles of summer simulations are used to assess the predictability of a reference simulation assumed to be perfect. In a first step, two ensembles are simulated: the first with the atmosphere coupled to the land-surface model, the second in the uncoupled mode with perfect soil conditions prescribed every 6 hours. Subsequent experiments are combinations thereof, in which the uncoupled and coupled modes alternate in the course of a simulation. The results show that there are "stable" and "unstable" periods in the weather evolution under consideration. During the stable periods, the predictability (measured in terms of ensemble spread at 500 hPa) of the coupled and uncoupled dynamical systems is almost identical; prescribing the perfect soil conditions has a negligible impact upon the atmospheric predictability. In contrast, the predictability during an unstable phase is found to be remarkably improved in the uncoupled ensembles. This effect results from guiding the atmospheric phase-space trajectory along its perfect evolution. It persists even when switching back from the uncoupled to the coupled mode prior to the onset of the unstable phase, a result that underlines the importance of soil moisture and temperature in data assimilation systems

DNA Repair in Mammalian Cells: Mismatched repair: variations on a theme

Abstract.: Complementary base pairing underlies the genetic template function of the DNA double helix. Therefore, to assure faithful DNA transactions, cells must adhere to a strict application of the Watson-Crick base pairing principle.Yet, mispairing does arise in DNA, most frequently as a result of DNA polymerase errors or base damage. These mismatches need be rectified to avoid mutation. Sometimes, however, mispairing is actively induced to trigger mutagenesis. This happens in activated B-lymphocytes, where the targeted generation and processing of G·U mismatches contributes to somatic hypermutation and antibody diversification. Non-mutagenic mismatches arise in heteroduplex intermediates of homologous recombination, and their processing helps restrict homeologous recombination. Depending on the type of mismatch and the biological context of its occurrence, cells must apply appropriate strategies of repair to properly control mutagenesis. This review will illustrate conceptual and functional challenges of cellular mismatch correction on typical examples of mutagenic base-base mismatches. (Part of a Multi-author Review

European summer climate variability in a heterogeneous multi-model ensemble

Recent results from an enhanced greenhouse-gas scenario over Europe suggest that climate change might not only imply a general mean warming at the surface, but also a pronounced increase in interannual surface temperature variability during the summer season (Schär etal., Nature 427:332-336, 2004). It has been proposed that the underlying physical mechanism is related to land surface-atmosphere interactions. In this study we expand the previous analysis by including results from a heterogeneous ensemble of 11 high-resolution climate models from the PRUDENCE project. All simulations considered comprise 30-year control and enhanced greenhouse-gas scenario periods. While there is considerable spread in the models' ability to represent the observed summer variability, all models show some increase in variability for the scenario period, confirming the main result of the previous study. Averaged over a large-scale Central European domain, the models simulate an increase in the standard deviation of summer mean temperatures between 20 and 80%. The amplification occurs predominantly over land points and is particularly pronounced for surface temperature, but also evident for precipitation. It is also found that the simulated changes in Central European summer conditions are characterized by an emergence of dry and warm years, with early and intensified depletion of root-zone soil moisture. There is thus some evidence that the change in variability may be linked to the dynamics of soil-moisture storage and the associated feedbacks on the surface energy balance and precipitatio

Noncontact atomic force microscopy simulator with phase-locked-loop controlled frequency detection and excitation

A simulation of an atomic force microscope operating in the constant amplitude dynamic mode is described. The implementation mimics the electronics of a real setup including a digital phase-locked loop (PLL). The PLL is not only used as a very sensitive frequency detector, but also to generate the time-dependent phase shifted signal driving the cantilever. The optimum adjustments of individual functional blocks and their joint performance in typical experiments are determined in detail. Prior to testing the complete setup, the performances of the numerical PLL and of the amplitude controller were ascertained to be satisfactory compared to those of the real components. Attention is also focused on the issue of apparent dissipation, that is, of spurious variations in the driving amplitude caused by the nonlinear interaction occurring between the tip and the surface and by the finite response times of the various controllers. To do so, an estimate of the minimum dissipated energy that is detectable by the instrument upon operating conditions is given. This allows us to discuss the relevance of apparent dissipation that can be conditionally generated with the simulator in comparison to values reported experimentally. The analysis emphasizes that apparent dissipation can contribute to the measured dissipation up to 15% of the intrinsic dissipated energy of the cantilever interacting with the surface, but can be made negligible when properly adjusting the controllers, the PLL gains and the scan speed. It is inferred that the experimental values of dissipation usually reported in the literature cannot only originate in apparent dissipation, which favors the hypothesis of "physical" channels of dissipation