Preconditioning of Improved and ``Perfect'' Fermion Actions

We construct a locally-lexicographic SSOR preconditioner to accelerate the parallel iterative solution of linear systems of equations for two improved discretizations of lattice fermions: the Sheikholeslami-Wohlert scheme where a non-constant block-diagonal term is added to the Wilson fermion matrix and renormalization group improved actions which incorporate couplings beyond nearest neighbors of the lattice fermion fields. In case (i) we find the block llssor-scheme to be more effective by a factor about 2 than odd-even preconditioned solvers in terms of convergence rates, at beta=6.0. For type (ii) actions, we show that our preconditioner accelerates the iterative solution of a linear system of hypercube fermions by a factor of 3 to 4.Comment: 27 pages, Latex, 17 Figures include

Ground-based measurements of tropospheric and stratospheric bromine monoxide above Nairobi (1° S, 36° E)

International audienceGround based observations of stratospheric and tropospheric bromine monoxide, BrO, from a multi axial differential optical absorption spectrometer, MAXDOAS, located at the UNEP/UNON site in Nairobi (1° S, 36° E) are presented for the year 2003. Differences in BrO slant column densities at 90° and 80° solar zenith angle retrieved from the zenith-sky measurements are used to study stratospheric BrO. They show only small variations with season, as expected for the small seasonality in stratospheric Bry and NO2 in this region. A pronounced diurnal variation can be observed, the average value for the morning being 1.3×1014 molecules/cm2 and for the evening 1.5×1014 molecules/cm2. The measurements are compared with simulations from a one-dimensional photochemical stacked box model which is coupled with a radiative transfer model to allow direct comparisons between the observations and the model calculations. In general the model reproduces the measurements very well. The differences in the absolute values are 15% for the evening and 20% for the morning which is within the limits of the combined uncertainties. Both seasonality and diurnal variation are well reproduced by the model. A sensitivity study shows that inclusion of the reaction BrONO2 + O(3P) significantly improves the agreement between model calculations and measurements, indicating an important role of this reaction in the stratosphere near to the equator. Tropospheric BrO columns and profile information is derived from the combined results obtained in the different viewing directions for the average over several clear days. The resulting tropospheric BrO columns are in the range of 4?7.5×1012 molecules/cm2 which is significant but lower than in previous studies at mid and high latitudes. The vertical distribution of the tropospheric BrO peaks at about 3 km indicating the absence of local sources at this high altitude site

Preconditioning of Improved and "Perfect" Fermion Actions

We construct a locally-lexicographic SSOR preconditioner to accelerate the parallel iterative solution of linear systems of equations for two improved discretizations of lattice fermions: (i) the Sheikholeslami-Wohlert scheme where a non-constant block-diagonal term is added to the Wilson fermion matrix and (ii) renormalization group improved actions which incorporate couplings beyond nearest neighbors of the lattice fermion fields. In case (i) we find the block ll -SSOR-scheme to be more effective by a factor ß 2 than odd-even preconditioned solvers in terms of convergence rates, at fi = 6:0. For type (ii) actions, we show that our preconditioner accelerates the iterative solution of a linear system of hypercube fermions by a factor of 3 to 4. Keywords: lattice QCD, improved actions, perfect actions, hypercube fermions, SSOR preconditioning 1 Introduction Traditionally, simulations of lattice quantum chromodynamics (QCD) were based on nearest-neighbor finite difference approximations..

A Preconditioner for Improved Fermion Actions

Introduction Recently, the symmetric successive overrelaxed preconditioner (SSOR) turned out to be parallelizable by means of the l ocally- l exicographic ordering technique [1]. In this way, SSOR preconditioning has been made applicable to the acceleration of standard Wilson fermion inversions on high performance massively parallel systems and it outperforms o/e preconditioning. It appears intriguing to extend the range of ll - SSOR-preconditioners such as to accelerate the inversion of improved fermionic actions, which became very popular in the recent years. In Symanzik's on-shell improvement program [2], counter terms are added to both, lattice action and composite operators in order to reduce O(a) artifacts which spoil results in the instance of the Wilson fermion formulation. In the approach of Sheikholeslami and Wohlert (SWA) [3], the Wilson action is modified by adding a