8.1    Application of a Tropical Cyclone Index to Climate Model      Downscaling

Holland, Greg, and Cindy Bruyere, National Center for Atmospheric Research

We examine the application of a Cyclone Genesis Index to both reanalyses and relatively course climate simulations for assessing potential changes in tropical cyclones. Both all cyclones and the more intense hurricanes will be examined. We first examine existing indices and show that several parameters used add no skill for our purposes. A new CGI is then applied to reanalyses and shown to accurately reproduce both the mean monthly interannual seasonal variation and the interannual variability, though care needs to be taken in the selected base region for application. We find that for assessing basin wide cyclone frequency, averaging indices over the whole basin is less skillful than is application to specific regions with high cyclone sensitivity, such as the Atlantic Main Development region. The applicability of this approach to all annual tropical cyclones and to just the more intense hurricanes will be discussed. Future trends from the application to NRCM simulations will also be described.