We investigate the accretion disk geometry in Galactic black hole sources by measuring the time delay between soft and hard X-ray emissions. Similar to the recent discoveries of anticorrelated hard X-ray time lags in Cygnus X-3 and GRS 1915+105, we find that the hard X-rays are anticorrelated with soft X-rays with a significant lag in another source, XTE J1550À564. We also find the existence of pivoting in the model-independent X-ray spectrum during these observations. We investigate time-resolved X-ray spectral parameters and find that the variation in these parameters is consistent with the idea of a truncated accretion disk. The QPO frequency, which is a measure of the size of truncated accretion disks, also changes, indicating that the geometric size of the hard X-ray emitting region changes along with the spectral pivoting and soft X-ray flux. A similar kind of delay is also noticed in 4U 1630À47.

This article describes the main features of the Brazilian Global Atmospheric Model (BAM), analyses of its performance for tropical rainfall forecasting, and its sensitivity to convective scheme and horizontal resolution. BAM is the new global atmospheric model of the Center for Weather Forecasting and Climate Research [Centro de Previsão de Tempo e Estudos Climáticos (CPTEC)], which includes a new dynamical core and state-of-the-art parameterization schemes. BAM’s dynamical core incorporates a monotonic two-time-level semi-Lagrangian scheme, which is carried out completely on the model grid for the tridimensional transport of moisture, microphysical prognostic variables, and tracers. The performance of the quantitative precipitation forecasts (QPFs) from two convective schemes, the Grell–Dévényi (GD) scheme and its modified version (GDM), and two different horizontal resolutions are evaluated against the daily TRMM Multisatellite Precipitation Analysis over different tropical regions. Three main results are 1) the QPF skill was improved substantially with GDM in comparison to GD; 2) the increase in the horizontal resolution without any ad hoc tuning improves the variance of precipitation over continents with complex orography, such as Africa and South America, whereas over oceans there are no significant differences; and 3) the systematic errors (dry or wet biases) remain virtually unchanged for 5-day forecasts. Despite improvements in the tropical precipitation forecasts, especially over southeastern Brazil, dry biases over the Amazon and La Plata remain in BAM. Improving the precipitation forecasts over these regions remains a challenge for the future development of the model to be used not only for numerical weather prediction over South America but also for global climate simulations.

In this work we address the issue of shock formation in black hole accretion disks. We provide a generalized two-parameter solution scheme for multitransonic accretion and wind around Schwarzschild black holes, mainly by concentrating on accretion solutions that may contain steady, standing isothermal shocks. Such shocks conserve flow temperature by dissipating energy at the shock location. We use a vertically integrated 1.5-dimensional model to describe the disk structure, where the equations of motion apply to the equatorial plane of the central accretor, assuming the flow to be in hydrostatic equilibrium in the transverse direction. Unlike previous works in this field, our calculation is not restricted to any particular kind of postNewtonian gravitational potentials; rather, we use all available pseudo-Schwarzschild potentials to formulate and solve the equations governing the accretion and wind only in terms of the flow temperature T and specific angular momentum of the flow. The accretion flow is assumed to be nondissipative everywhere, except possibly at the shock location, if any. We observe that a significant region of parameter space spanned by {, T} allows shock formation. Our generalized formalism ensures that the shock formation is not just an artifact of a particular type of gravitational potential; rather, the inclusion of all available black hole potentials demonstrates a substantially extended zone of parameter space allowing for the possibility of shock formation. We thus arrive at the conclusion that the standing shocks are essential components of rotating, advective accretion flows of isothermal fluid around a nonspinning astrophysical black hole. We identify all possible shock solutions that may be present in isothermal disk accretion and thoroughly study the dependence of various shock parameters on fundamental dynamical variables governing the accretion flow for all possible initial boundary conditions. Types of shocks discussed in this paper may appear to be '' bright '' because of the huge amount of energy dissipation at the shock, and the quick removal of such energy to maintain isothermality may power the strong X-ray flairs recently observed to be emerging from our Galactic center. The results are discussed in connection with other astrophysical phenomena of related interest, such as the quasi-periodic oscillation behavior of galactic black hole candidates.

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