1. Department of Meteorology, University of Maryland, College Park, MD 20742, USA.
2. Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
3. National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL), Code R/CMDL1, 325 Broadway, Boulder, CO 80305, USA.