Descripción: The effect of rotation is considered to become important when the Rossby number is sufficiently small, as is the case in many geophysical and astrophysical flows. Here we present direct numerical simulations to study the effect of rotation in flows with moderate Rossby numbers (down to Ro ≈ 0.03) but at Reynolds numbers large enough to observe the beginning of a turbulent scaling at scales smaller than the energy injection scale. We use coherent forcing at intermediate scales, leaving enough room in the spectral space for an inverse cascade of energy to also develop. We analyze the spectral behavior of the simulations, the shell-to-shell energy transfer, scaling laws and intermittency, as well as the geometry and the anisotropy of the structures in the flow. At late times, the direct transfer of energy at small scales is mediated by interactions with the largest scale in the system, the energy containing eddies with k⊥ ≈ 1, where ⊥ refers to wavevectors perpendicular the axis of rotation. The transfer between modes with wavevector parallel to the rotation is strongly quenched. The inverse cascade of energy at scales larger than the energy injection scale is nonlocal, and energy is transferred directly from small scales to the largest available scale. We observe both a direct and inverse cascade of energy at high rotation rate, indicative that these cascades can take place simultaneously. Also, as time evolves and the energy piles up at the large scales, the intermittency of the direct cascade of energy is preserved while corrections due to intermittency are found to be the same (within error bars) as in homogeneous nonrotating turbulence. © 2009 American Institute of Physics.

Descripción: We investigate the performance of one stretched-grid atmospheric global model, five different regional climate models and a statistical downscaling technique in simulating 3 months (January 1971, November 1986, July 1996) characterized by anomalous climate conditions in the southern La Plata Basin. Models were driven by reanalysis (ERA-40). The analysis has emphasized on the simulation of the precipitation over land and has provided a quantification of the biases of and scatter between the different regional simulations. Most but not all dynamical models underpredict precipitation amounts in south eastern South America during the three periods. Results suggest that models have regime dependence, performing better for some conditions than others. The models' ensemble and the statistical technique succeed in reproducing the overall observed frequency of daily precipitation for all periods. But most models tend to underestimate the frequency of dry days and overestimate the amount of light rainfall days. The number of events with strong or heavy precipitation tends to be under simulated by the models. © The Author(s) 2009.

Descripción: Above the southern Andes range and its prolongation in the Antarctic Peninsula, large-amplitude mountain and shear gravity waves observed with Weather Research and Forecasting (WRF) mesoscale model simulations during winter 2009 are analyzed. Two specific reasons motivated this study: (1) a decade of satellite observations of temperature fluctuations in the stratosphere, allowing us to infer that this region may be launching the largest-amplitude gravity waves into the upper atmosphere, and (2) the recent design of a research program to investigate these features in detail, the Southern Andes Antarctic Gravity wave Initiative (SAANGRIA). The simulations are forced with ERA-Interim data from the European Centre for Medium-Range Weather Forecasts. The approach selected for the regional downscaling is based on consecutive integrations with weekly reinitialization with 24 h of spin-up, and the outputs during this period are excluded from the analysis. From 1 June to 31 August 2009, five case studies were selected on the basis of their outstanding characteristics and large wave amplitudes. In general, one or two prevailing modes of oscillation are identified after applying continuous wavelet transforms at constant pressure levels and perpendicularly to the nominal orientation of the dominant wave crests. In all cases, the dominant modes are characterized by horizontal wavelengths around 50 km. Their vertical wavelengths, depending on a usually strong background wind shear, are estimated to be between 2 and 11 km. The corresponding intrinsic periods range between 10 and 140 min. In general, the estimated vertical wavelength (intrinsic period) maximizes (minimizes) around 250-300 hPa. The synoptic circulation for each case is described. Zonal and meridional components of the vertical flux of horizontal momentum are shown in detail for each case, including possible horizontal wavelengths between 12 and 400 km. Large values of this flux are observed at higher pressure levels, decreasing with increasing height after a progressive deposition of momentum by different mechanisms. As expected, in the wintertime upper troposphere and lower stratosphere in this region, a prevailing zonal component is negative almost everywhere, with the exception of one case above the northern tip of the Antarctic Peninsula. A comparison with previous experimental results reported in the region from in situ and remote sensing measurements suggests a good agreement with the momentum flux profiles computed from the simulations. Partial wave reflection near the tropopause was found, as considerable departures from equipartition between potential and kinetic wave energy are obtained in all cases and at all pressure levels. This ratio was always less than 1 below the lower stratosphere. Copyright 2012 by the American Geophysical Union.