5,657 research outputs found

    STRATAQ: A three-dimensional Chemical Transport Model of the stratosphere

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    International audienceA three-dimensional (3-D) Chemical Transport Model (CTM) of the stratosphere has been developed and used for a test study of the evolution of chemical species in the arctic lower stratosphere during winter 1996/97. This particular winter has been chosen for testing the model's capabilities for its remarkable dynamical situation (very cold and strong polar vortex) along with the availability of sparse chlorine, HNO3 and O3 data, showing also very low O3 values in late March/April. Due to those unusual features, the winter 1996/97 can be considered an excellent example of the impact of both dynamics and heterogeneous reactions on the chemistry of the stratosphere. Model integration has been performed from January to March 1997 and the resulting long-lived and short-lived tracer fields compared with available measurements. The model includes a detailed gas phase chemical scheme and a parameterization of the heterogeneous reactions occurring on liquid aerosol and polar stratospheric cloud (PSC) surfaces. The transport is calculated using a semi-lagrangian flux scheme, forced by meteorological analyses. In such form, the STRATAQ CTM model is suitable for short-term integrations to study transport and chemical evolution related to "real" meteorological situations. Model simulation during the chosen winter shows intense PSC formation, with noticeable local HNO3 capture by PSCs, and the activation of vortex air leading to chlorine production and subsequent O3 destruction. The resulting model fields show generally good agreement with satellite data (MLS and TOMS), although the available observations, due to their limited number and time/space sparse nature, are not enough to effectively constraint the model. In particular, the model seems to perform well in reproducing the rapid processing of air inside the polar vortex on PSC converting reservoir species in active chlorine. In addition, it satisfactorily reproduces the morphology of the continuous O3 decline as shown by the satellite during the investigated period, with a tendency, however, to underestimate the total column values inside the polar vortex during late winter. As possible causes of this model/observation difference we suggest an incorrect estimation of the vertical transport and of the tropospheric contribution

    Assimilation of stratospheric ozone in the chemical transport model STRATAQ

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    International audienceWe describe a sequential assimilation approach useful for assimilating tracer measurements into a three-dimensional chemical transport model (CTM) of the stratosphere. The numerical code, developed largely according to Kha00, uses parameterizations and simplifications allowing assimilation of sparse observations and the simultaneous evaluation of analysis errors, with reasonable computational requirements. Assimilation parameters are set by using ?2 and OmF (Observation minus Forecast) statistics. The CTM used here is a high resolution three-dimensional model. It includes a detailed chemical package and is driven by UKMO (United Kingdom Meteorological Office) analyses. We illustrate the method using assimilation of Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) ozone observations for three weeks during the 1996 antarctic spring. The comparison of results from the simulations with TOMS (Total Ozone Mapping Spectrometer) measurements shows improved total ozone fields due to assimilation of MLS observations. Moreover, the assimilation gives indications on a possible model weakness in reproducing polar ozone values during springtime

    Vibrational Stability of NLC Linac accelerating structure

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    The vibration of components of the NLC linac, such as accelerating structures and girders, is being studied both experimentally and analytically. Various effects are being considered including structural resonances and vibration caused by cooling water in the accelerating structure. This paper reports the status of ongoing work.Comment: 3 pages 8 figures Presented at EPAC 2002 Paris Franc

    Baseline LHC machine parameters and configuration of the 2015 proton run

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    This paper shows the baseline LHC machine parameters for the 2015 start-up. Many systems have been upgraded during LS1 and in 2015 the LHC will operate at a higher energy than before and with a tighter filling scheme. Therefore, the 2015 commissioning phase risks to be less smooth than in 2012. The proposed starting configuration puts the focus on feasibility rather than peak performance and includes margins for operational uncertainties. Instead, once beam experience and a better machine knowledge has been obtained, a push in ÎČ∗\beta^* and performance can be envisaged. In this paper, the focus is on collimation settings and reach in ÎČ∗\beta^*---other parameters are covered in greater depth by other papers in these proceedings.Comment: submitted for publication in a CERN yellow report (Proceedings of the LHC Performance Workshop - Chamonix 2014

    Measurements of the effect of collisions on transverse beam halo diffusion in the Tevatron and in the LHC

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    Beam-beam forces and collision optics can strongly affect beam lifetime, dynamic aperture, and halo formation in particle colliders. Extensive analytical and numerical simulations are carried out in the design and operational stage of a machine to quantify these effects, but experimental data is scarce. The technique of small-step collimator scans was applied to the Fermilab Tevatron collider and to the CERN Large Hadron Collider to study the effect of collisions on transverse beam halo dynamics. We describe the technique and present a summary of the first results on the dependence of the halo diffusion coefficient on betatron amplitude in the Tevatron and in the LHC.Comment: 4 pages, 2 figures. Submitted to the Proceedings of the ICFA Mini-Workshop on Beam-beam Effects in Hadron Colliders (BB2013), Geneva, Switzerland, 18-22 March 201

    Detecting barriers to transport: A review of different techniques

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    We review and discuss some different techniques for describing local dispersion properties in fluids. A recent Lagrangian diagnostics, based on the Finite Scale Lyapunov Exponent (FSLE), is presented and compared to the Finite Time Lyapunov Exponent (FTLE), and to the Okubo-Weiss (OW) and Hua-Klein (HK) criteria. We show that the OW and HK are a limiting case of the FTLE, and that the FSLE is the most efficient method for detecting the presence of cross-stream barriers. We illustrate our findings by considering two examples of geophysical interest: a kinematic meandering jet model, and Lagrangian tracers advected by stratospheric circulation.Comment: 15 pages, 9 figures, submitted to Physica

    Analysis of water vapor LIDAR measurements during the MAP campaign: evidence of sub-structures of stratospheric intrusions

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    This paper presents two case studies of transport of dry air in the free troposphere measured by a ground based Raman LIDAR in the Northern-Italy, during the Mesoscale Alpine Programme (MAP). Two observations characterized by the presence of anomalously dry layers, below 6 km height, were analyzed using Lagrangian techniques. These events are related to upper-tropospheric, high Potential Vorticity (PV) streamers crossing the Alpine region. These are interpreted as small-scale features of stratospheric intrusions associated with the PV ridge during its phase of dissipation. One of the measurements also shows the presence of two distinct dehydrated structures associated with the same event. The water vapor concentration also suggests dilution processes of dry stratospheric air in the troposphere. Lagrangian simulations allowed to successfully reproduce the observed water vapor distribution and the air parcel histories confirmed the stratospheric origin of the dry layers