Sounding-derived indices for neural network based short-term thunderstorm and rainfall forecasts

A neural network-based scheme to do a multivariate analysis for forecasting the occurrence and intensity of a meteo event is presented. Many sounding-derived indices are combined together to build a short-term forecast of thunderstorm and rainfall events, in the plain of the Friuli Venezia Giulia region (hereafter FVG, NE Italy).For thunderstorm forecasting, sounding, lightning strikes and mesonet station data (rain and wind) from April to November of the years 1995–2002 have been used to train and validate the artificial neural network (hereafter ANN), while the 2003 and 2004 data have been used as an independent test sample. Two kind of ANNs have been developed: the first is a “classification model” ANN and is built for forecasting the thunderstorm occurrence. If this first ANN predicts convective activity, then a second ANN, built as a “regression model”, is used for forecasting the thunderstorm intensity, as defined in a previous article.The classification performances are evaluated with the ROC diagram and some indices derived from the Table of Contingency (like KSS, FAR, Odds Ratio). The regression performances are evaluated using the Mean Square Error and the linear cross correlation coefficient R.A similar approach is applied to the problem of 6 h rainfall forecast in the Friuli Venezia Giulia plain, but in this second case the data cover the period from 1992 to 2004. Also the forecasts of binary events (defined as the occurrence of 5, 20 or 40 mm of maximum rain), made by classification and regression ANN, were compared. Particular emphasis is given to the sounding-derived indices which are chosen in the first places by the predictor forward selection algorithm.     

Gravitational wave signals from the collapse of the first stars

We study the gravitational wave emission from the first stars, which are assumed to be very massive objects (VMOs). We take into account various feedback (both radiative and stellar) effects regulating the collapse of objects in the early Universe and thus derive the VMO initial mass function and formation rate. If the final fate of VMOs is to collapse, leaving very massive black hole remnants, then the gravitational waves emitted during each collapse would be seen as a stochastic background. The predicted spectral strain amplitude in a critical density cold dark matter (CDM) universe peaks in the frequency range ν ≈5×10⁻⁴–5×10⁻³ Hz, where it has a value in the range ≈10⁻²⁰–10⁻¹⁹ Hz^−1/2, and might be detected by the Laser Interferometer Space Antenna ( LISA ). The expected emission rate is roughly 4000 event yr⁻¹, resulting in a stationary discrete sequence of bursts, i.e. a shot-noise signal.     

Scintillation as a probe of the intergalactic medium

Most of the baryons in the low-redshift Universe reside in a warm/hot component which is difficult to detect with standard absorption/emission-line techniques. We propose to use quasar refractive scintillation as a useful, complementary probe for such ionized, intergalactic gas. In particular, an application to the case of the intracluster medium is presented. We show that clusters located at z ≈0.02 should produce a source rms intensity fluctuation at 50–100 GHz of several tens of per cent and on time-scales ranging from days to months, depending on the projected location of the source on the foreground cluster. However, in order to produce such a signal, the source needs to be very compact. This effect, if observed, can be used as an independent test of the baryonic mass fraction in clusters.     

Cosmological origin of the lowest metallicity halo stars

We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars that are likely to be found within our Galaxy today. By PopIII we refer to stars formed at large redshift ( z >4), with low metallicities ([ Z /Z_⊙]<−2.5) and in small systems (total mass ≲ 2×10⁸ M_⊙) that are extremely sensitive to stellar feedback, and which through a prescribed merging history end up becoming part of the Milky Way today. An analytic, extended Press–Schechter formalism is used to obtain the mass functions of haloes which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modelled as a mini-galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disc instabilities, all of which determine the fraction of the baryons that are subject to star formation. The use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, and give accurate estimates of their expected numbers today and their location in L /L_⊙ versus T /K Hertzsprung–Russell (HR) diagrams. A first comparison with observational data suggests that the initial mass function (IMF) of the first stars was increasingly high-mass weighted towards high redshifts, levelling off at z ≳9 at a characteristic stellar mass scale m _s=10–15 M_⊙.     

Pregalactic metal enrichment

I review recent results concerning the metal enrichment of the intergalactic medium and give arguments which allow to conclude that the heavy elements detected in the Lyα forest absorbers must have been produced and injected by a population of protogalaxies with typical mass ≈ 10⁸ M_⊙ at redshift z ≈ 10. In addition, I will discuss the existence of a metallicity-induced shift in the Initial Mass Function of first stars from an extrely top-heavy distribution to a `normal' one and bring observational support to this idea. This revised version was published online in August 2006 with corrections to the Cover Date.