Validation and assimilation of Seasat altimeter wave heights using the WAM wave model

The mutual consistency of the Seasat global data sets of scatterometer winds and altimeter wave heights is investigated for the complete Seasat period using the third-generation wave model WAM. The wave model was driven by surface (1000 hPa) wind and surface stress fields constructed by the Goddard Laboratory for Atmospheres (GLA) by assimilation of the scatterometer winds in an atmospheric model. For the 10-day period September 7?17 the intercomparison was extended to two further scatterometer wind fields: a 1000-hPa assimilated wind field from the European Centre for Medium-Range Weather Forecasts and a subjectively analyzed 19.5-m-height surface wind field from the Jet Propulsion Laboratory. On the global average, the modeled and observed wave heights agree reasonably well. Regional differences, however, can be large and sometimes exceed 40%. The errors are attributed mainly to deficiencies in the forcing wind fields. Low wind speeds are found to be overestimated and high wind speeds underestimated by the Seasat scatterometer algorithm. The friction velocities of the GLA model are found to be significantly underestimated in the high-wind belt of the southern hemisphere. The analysis demonstrates the diagnostic advantages of applying a wave model for the quality assessment of satellite wind and wave data. A preliminary wave data assimilation scheme is presented in which the wave field is updated without changing the forcing wind field. A considerable improvement of the computed wave field is achieved, particularly in regions in which the wave energy is dominated by swell. However, a more general assimilation scheme including modifications of the wind field is needed to upgrade wind sea forecasts

Correlated versus Uncorrelated Stripe Pinning: the Roles of Nd and Zn Co-Doping

We investigate the stripe pinning produced by Nd and Zn co-dopants in cuprates via a renormalization group approach. The two dopants play fundamentally different roles in the pinning process. While Nd induces a correlated pinning potential that traps the stripes in a flat phase and suppresses fluctuations, Zn pins the stripes in a disordered manner and promotes line meandering. We obtain the zero temperature phase diagram and compare our results with neutron scattering data. A good agreement is found between theory and experiment.Comment: To appear at the proceedings of the LLD2K Conference Tsukuba, July 2000, Japan. 4 pages, 2 figure

Mechanisms of wave transformation in finite-depth water

Mechanisms of wave transformation in finite-depth water are investigated. The linear mechanisms ex- amined are percolation, bottom motion, shoaling, and refraction. The nonlinear mechanisms examined are wave-wave interaction and bottom friction. New exact computations of the nonlinear transfer for fi- nite-depth waves are presented for some directional wave spectra. These mechanisms are found to ex- plain satisfactorily wave decay observations obtained at several sites with different bottom sediment properties. The decay rates at these sites are found to be dominated by different mechanisms which are determined by the bottom conditions. As an example, detailed calculations are presented for data ob- tained at the Jonswap site

Investigation of processes governing the large-scale variability of the atmosphere using low-order barotropic spectral models as a statistical tool

Idealized low-order spectral models based on quasi-geostrophy have been proposed by Charney, Egger, and others for a qualitative explanation of quasi-stationary flow patterns in the troposphere, such as blocking highs. To test these concepts, we consider spectral, quasi-geostrophic, barotropic models of a slightly higher, but still relatively low resolution (28 and 68 degrees of freedom). The models are treated as regression models, the predictors being the individual components of the prognostic vorticity equation, namely the beta, advection, orography, friction and forcing terms. A 10-year record of observed north-hemispheric geopotential data is used as data basis for the statistical regression analysis. The regression model may be interpreted as a truncated model in which the individual terms have been modified by (a) projecting the truncation and other systematic model errors onto the terms retained in the simplified system, and (b) keeping in the retained terms only the contributions which are correlated with the observed change in the atmospheric state. The model is applied in the inverse mode as a diagnostic tool to determine which processes are most important for the evolution of the system, and how much of the observed large-scale variance of the atmosphere can be explained by such a low-order system. Despite the strong spectral truncation, the model is found to explain a reasonable percentage of the observed variance (of the order of 30%, or 0.55 correlation, for 68 components). However, it was not possible to explain a significant fraction of the variance by still more strongly truncated models of the idealized form proposed by Charney and Egger. Our analysis indicates that all degrees of freedom of the truncated system (28 or 68) contribute significantly to the dynamics of the large-scale components. The most important processes are the wave/mean-flow interaction and the beta effect, followed by the nonlinear interactions among waves and the annually varying thermal forcing. Interactions with orography and frictional effects are generally negligible. The residual variance not represented by the model cannot be parameterized in a simple manner in terms of the components of the truncated model itself and must be treated as external stochastic forcing. Thus for a realistic description of large-scale atmospheric variability, low-order spectral models must be augmented by a significant stochastic forcing component in addition to the internal interactions

Stripe dynamics in presence of disorder and lattice potentials

We study the influence of disorder and lattice pinning on the dynamics of a charged stripe. Starting from a phenomenological model of a discrete quantum string, we determine the phase diagram for this system. Three regimes are identified, the free phase, the flat phase pinned by the lattice, and the disorder pinned phase. In the absence of impurities, the system can be mapped onto a 1D array of Josephson junctions. The results are compared with measurements on nickelates and cuprates and a good qualitative agreement is found between our results and the experimental data.Comment: 4 pages, 2 figure

Statistical prediction of seasonal air temperature over Eurasia

Statistical models for the prediction of seasonal surface air temperature anomalies over Eurasia were constructed. The models were designed to test the relative predictive skill of Atlantic sea surface temperatures (SST), sea level pressure (SLP) and persistence in a cyclostationary setting. Significant forecast skill was found for the spring season in central and eastern Europe. The main predictors were persistence and SLPs (the north Atlantic oscillation). SSTs had little predictive value. All results were confirmed with independent forecast experiments. The statistical results were attributed to (a) a positive feedback between given winter atmospheric circulation regimes, the snow cover they produce and the snow-induced enhancement/retardation of normal season warming and (b) the persistence of large-scale circulation patterns over the Atlantic Ocean