1. CNR, Istituto di Scienze dell’Atmosfera e del Clima, Rome, and Istituto Nazionale di Fisica Nucleare, Lecce, Italy
2. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts
3. Department of Physics, and Department of Mathematics and Computer Science, Eindhoven University of Technology, Eindhoven, Netherlands, and Istituto per le Applicazioni del Calcolo, Rome, and Istituto Nazionale di Fisica Nucleare, Ferrara, Italy