178 research outputs found
Nonlinear data-assimilation using implicit models
International audienceWe show how the traditional 4D-Var method can be adapted for implicit time-integration and extended for multi-parameter estimation. We present the algorithm for this new method, which we call I4D-Var, and demonstrate its performance using a fully-implicit barotropic quasi-geostrophic model of the wind-driven double-gyre ocean circulation. For the latter model, the different regimes of flow behavior and the regime boundaries (i.e. bifurcation points) are well known and hence the parameter estimation problem can be systematically studied. It turns out that I4D-Var is able to correctly estimate parameter values, even when background flow and "observations" are in different dynamical regimes
Working with farmers; towards a service approach in irrigation; training; the socio-economic base line survey
Water management in rural areas is becoming more complex, increasing the need to communicate more intensely with water users. The trend to decentralize decision making in water management and to make those who render water management services answerable to the clients of their services, only enforces this. The intention of this report is to acquaint the community with a technical training in the methods and techniques that social scientists apply when working with farmers. They can be either managers responsible for all operations of a project or water management organization, staff charged with the task to supervise social scientists, or staff working together with social scientists and applying their results. The report may also be appreciated by social scientists who wish to refresh their knowledg
A tailored solver for bifurcation analysis of ocean-climate models
In this paper, we present a new linear system solver for use in a fully-implicit ocean model. The new solver allows to perform bifurcation analysis of relatively high-resolution primitive-equation ocean-climate models. It is based on a block-ILU approach and takes special advantage of the mathematical structure of the governing equations. In implicit models Jacobian matrices have to be constructed. Analytical construction is hard for complicated but more realistic representations of mixing. This is overcome by evaluating the Jacobian in part numerically. The performance of the new implicit ocean model is demonstrated using (i) a high-resolution model of the wind-forced double-gyre flow problem in a (relatively small) midlatitude spherical basin, and (ii) a medium-resolution model of thermohaline and wind-driven flows in an Atlantic size single-hemispheric basin.
Substrate-Assisted Catalysis Unifies Two Families of Chitinolytic Enzymes
Hen egg-white lysozyme has long been the paradigm for enzymatic glycosyl hydrolysis with retention of configuration, with a protonated carboxylic acid and a deprotonated carboxylate participating in general acid-base catalysis. In marked contrast, the retaining chitin degrading enzymes from glycosyl hydrolase families 18 and 20 all have a single glutamic acid as the catalytic acid but lack a nucleophile on the enzyme. Both families have a catalytic (βα)8-barrel domain in common. X-ray structures of three different chitinolytic enzymes complexed with substrates or inhibitors identify a retaining mechanism involving a protein acid and the carbonyl oxygen atom of the substrate’s C2 N-acetyl group as the nucleophile. These studies unambiguously demonstrate the distortion of the sugar ring toward a sofa conformation, long postulated as being close to that of the transition state in glycosyl hydrolysis.
The formation of CO through consumption of gas-phase CO on vacuum-UV irradiated water ice
[Abridged] Observations of protoplanetary disks suggest that they are
depleted in gas-phase CO. It has been posed that gas-phase CO is chemically
consumed and converted into less volatile species through gas-grain processes.
Observations of interstellar ices reveal a CO component within HO ice
suggesting co-formation. The aim of this work is to experimentally verify the
interaction of gas-phase CO with solid-state OH radicals above the sublimation
temperature of CO. Amorphous solid water (ASW) is deposited at 15 K and
followed by vacuum-UV (VUV) irradiation to dissociate HO and create OH
radicals. Gas-phase CO is simultaneously admitted and only adsorbs with a short
residence time on the ASW. Products in the solid state are studied with
infrared spectroscopy and once released into the gas phase with mass
spectrometry. Results show that gas-phase CO is converted into CO, with an
efficiency of 7-27%, when interacting with VUV irradiated ASW. Between 40 and
90 K, CO production is constant, above 90 K, O production takes over.
In the temperature range of 40-60 K, the CO remains in the solid state,
while at temperatures 70 K the formed CO is released into the gas
phase. We conclude that gas-phase CO reacts with solid-state OH radicals above
its sublimation temperature. This gas-phase CO and solid-state OH radical
interaction could explain the observed CO embedded in water-rich ices. It
may also contribute to the observed lack of gas-phase CO in planet-forming
disks, as previously suggested. Our experiments indicate a lower water ice
dissociation efficiency than originally adopted in model descriptions of
planet-forming disks and molecular clouds. Incorporation of the reduced water
ice dissociation and increased binding energy of CO on a water ice surfaces in
these models would allow investigation of this gas-grain interaction to its
full extend.Comment: Accepted for publication in Astronomy & Astrophysic
Chemo-dynamical modeling of Cha-mms1 to predict new solid-phase species for detection with JWST
Laboratory astrophysics and astrochemistr
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