Optical Remote Sensing, Radio and Space Science, Chalmers University of Technology, 41296 Gothenburg, Sweden

now at: Monitoring & Laboratory Division, California Air Resources Board, 9528 Telstar Avenue, El Monte, CA 91731, USA

Optical Remote Sensing, Radio and Space Science, Chalmers University of Technology, 41296 Gothenburg, Sweden

Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

now at: Atmospheric and Oceanic Program, Princeton University, Princeton, NJ 08544, USA

Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

now at: Ecological Process Modelling Unit, Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, 8903, Switzerland

Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Jet Propulsion Laboratory, Pasadena, CA 91109, USA