MATLAB and Practical Applications on Climate Variability Studies

In every three-hour session of the tutorial, students will be introduced to practical applications for the study of the climate system. Those applications will be based on Matlab. For those students that are not familiar on using the Matlab, in every three-hour sessions there will be an introduction to the working environment, dealing with matrices, useful functions, logical conditions, saving and loading, data management, functions & scripts, loops and vectorizaton, etc.Main objective of the course (5 sessions) is the transfer of know-how in practical applications and management of statistical tools commonly used to explore meteorological time series, using MATLAB, focusing on applications to study issues related with the climate variability and climate change.Download esercizi.zip from the following alternative location, in order to create your "tutorial work environment".CLARIS EU Project (A Europe-South America Network for Climate Change Assessment and Impact Studies; GOCE-CT-2003-01454).Unpublishedope

The Matter Bispectrum in N-body Simulations with non-Gaussian Initial Conditions

We present measurements of the dark matter bispectrum in N-body simulations with non-Gaussian initial conditions of the local kind for a large variety of triangular configurations and compare them with predictions from Eulerian Perturbation Theory up to one-loop corrections. We find that the effects of primordial non-Gaussianity at large scales, when compared to Perturbation Theory, are well described by the initial component of the matter bispectrum, linearly extrapolated at the redshift of interest. In addition, we find that, for f_NL=100, the nonlinear corrections due to non-Gaussian initial conditions are of the order of ~3, 4% for generic triangles up to ~20% for squeezed configurations, at any redshift. We show that the predictions of Perturbation Theory at tree-level fail to describe the simulation results at redshift z=0 already at scales corresponding to k ~ 0.02 - 0.08 h/Mpc, depending on the triangle, while one-loop corrections can significantly extend their validity to smaller scales. At higher redshift, one-loop Perturbation Theory provides indeed quite accurate predictions, particularly with respect to the relative correction due to primordial non-Gaussianity.Comment: 17 pages, 7 figures. Revised to match journal version with updated references. Accepted for publication in MNRAS

Constraints on Galaxy Bias, Matter Density, and Primordial Non--Gausianity from the PSCz Galaxy Redshift Survey

We compute the bispectrum for the \IRAS PSCz catalog and find that the galaxy distribution displays the characteristic signature of gravity. Assuming Gaussian initial conditions, we obtain galaxy biasing parameters $1/b_1=1.20^{+0.18}_{-0.19}$ and $b_2/b_1^2=-0.42\pm0.19$, with no sign of scale-dependent bias for $k\leq 0.3$ h/Mpc. These results impose stringent constraints on non-Gaussian initial conditions. For dimensional scaling models with $\chi^2_N$ statistics, we find N>49, which implies a constraint on primordial skewness $B_3<0.35$.Comment: 4 pages, 3 embedded figures, uses revtex style file, minor changes to reflect published versio

Generation of Vorticity and Velocity Dispersion by Orbit Crossing

We study the generation of vorticity and velocity dispersion by orbit crossing using cosmological numerical simulations, and calculate the backreaction of these effects on the evolution of large-scale density and velocity divergence power spectra. We use Delaunay tessellations to define the velocity field, showing that the power spectra of velocity divergence and vorticity measured in this way are unbiased and have better noise properties than for standard interpolation methods that deal with mass weighted velocities. We show that high resolution simulations are required to recover the correct large-scale vorticity power spectrum, while poor resolution can spuriously amplify its amplitude by more than one order of magnitude. We measure the scalar and vector modes of the stress tensor induced by orbit crossing using an adaptive technique, showing that its vector modes lead, when input into the vorticity evolution equation, to the same vorticity power spectrum obtained from the Delaunay method. We incorporate orbit crossing corrections to the evolution of large scale density and velocity fields in perturbation theory by using the measured stress tensor modes. We find that at large scales (k~0.1 h/Mpc) vector modes have very little effect in the density power spectrum, while scalar modes (velocity dispersion) can induce percent level corrections at z=0, particularly in the velocity divergence power spectrum. In addition, we show that the velocity power spectrum is smaller than predicted by linear theory until well into the nonlinear regime, with little contribution from virial velocities.Comment: 27 pages, 14 figures. v2: reorganization of the material, new appendix. Accepted by PR

Modelling large-scale halo bias using the bispectrum

We study the relation between the halo and matter density fields -- commonly termed bias -- in the LCDM framework. In particular, we examine the local model of biasing at quadratic order in the matter density. This model is characterized by parameters b_1 and b_2. Using an ensemble of N-body simulations, we apply several statistical methods to estimate the parameters. We measure halo and matter fluctuations smoothed on various scales and find that the parameters vary with smoothing scale. We argue that, for real-space measurements, owing to the mixing of wavemodes, no scale can be found for which the parameters are independent of smoothing. However, this is not the case in Fourier space. We measure halo power spectra and construct estimates for an effective large-scale bias. We measure the configuration dependence of the halo bispectra B_hhh and reduced bispectra Q_hhh for very large-scale k-space triangles. From this we constrain b_1 and b_2. Using the lowest-order perturbation theory, we find that for B_hhh the best-fit parameters are in reasonable agreement with one another as the triangle scale is varied, but that the fits become poor as smaller scales are included. The same is true for Q_hhh. The best-fit parameters depend on the discreteness correction. This led us to consider halo-mass cross-bispectra. The results from these statistics support our earlier findings. We develop a test to explore the importance of missing higher-order terms in the models. We prove that low-order expansions are not able to correctly model the data, even on scales k_1~0.04 h/Mpc. If robust inferences are to be drawn from galaxy surveys, then accurate models for the full nonlinear matter bispectrum and trispectrum will be essential.Comment: 23 pages, 7 figures; accepted for publication in MNRA

Observed shift towards earlier spring discharge in the main Alpine rivers

In this study, we analyse the observed long-term discharge time-series of the Rhine, the Danube, the Rhone and the Po rivers. These rivers are characterised by different seasonal cycles reflecting the diverse climates and morphologies of the Alpine basins. However, despite the intensive and varied water management adopted in the four basins, we found common features in the trend and low-frequency variability of the spring discharge timings. All the discharge time-series display a tendency towards earlier spring peaks of more than two weeks per century. These results can be explained in terms of snowmelt, total precipitation (i.e. the sum of snowfall and rainfall) and rainfall variability. The relative importance of these factors might be different in each basin. However, we show that the change of seasonality of total precipitation plays a major role in the earlier spring runoff over most of the Alps

Changes in Tropical Cyclone Activity due to Global Warming in a General Circulation Model

This study investigates the possible changes that the greenhouse global warming might generate in the characteristics of the tropical cyclones (TCs). The analysis has been performed using scenario climate simulations carried out with a fully coupled high-resolution global general circulation model. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the 20th Century with observations. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link the TC occurrence with the large scale circulation. The results from the climate scenarios reveal a substantial general reduction of the TC frequency when the atmospheric CO2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical North West Pacific (NWP) and North Atlantic (ATL). In the NWP the weaker TC activity seems to be associated with a reduced amount of convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Despite the overall warming of the tropical upper ocean and the expansion of warm SSTs to the subtropics and mid-latitudes, the action of the TCs remains well confined to the tropical region and the peak of TC number remains equatorward of 20° latitude in both Hemispheres. An extended version of this work is in available on Journal of Climate (Gualdi et al.,2008 - DOI:10.1175/2008JCLI1921.1

The Pacific Decadal Oscillation modulates tropical cyclone days on the interannual timescale in the North Pacific Ocean

The North Pacific Ocean is the most active region on our planet in terms of tropical cyclone (TC) activity. These storms are responsible for numerous fatalities and economic damages, affecting the livelihood of those living in the impacted areas. Historically the examination of TCs in the North Pacific Ocean has been performed separately for its two main sub-basins: the West North Pacific and the East North Pacific. Here, we consider the TC activity in the North Pacific as a single basin and examine the climate processes responsible for its number of TC days. We show that the Pacific Decadal Oscillation modulates the number of TC days in the North Pacific Ocean through its connection to the sea surface temperature. The insights from this work will advance the understanding of the climate processes responsible for these storms, and will provide valuable information toward our preparation and adaptation efforts on long timescales