Subtropical cyclones (ST) have a checkered history in the North Atlantic. They are intermittently recorded in the National Hurricane Center (NHC) Hurricane Database (HURDAT), but these records are incomplete (Guishard et al. 2007). Recent ST landfalls, such as Hurricane Karen (2001) in Bermuda, highlight the severe weather impacts associated with these events. Indeed, the media outcry after Karen’s landfall revealed that this storm was interpreted by the general public to be an unnamed tropical storm (Guishard et al. 2007).
In this study, we utilize a series of North Atlantic ST case studies to develop a uniform set of ST criteria that are well resolved using operational analyses. The structural characteristics and life cycle evolution of these storms are examined using Global Forecast System (GFS) operational analyses (Kanamitsu 1989) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis fields (for anomaly calculations; Kalnay et al. 1996), as well as a variety of satellite and other data sources. A conceptual model for ST formation based on a hybrid thermal structure in the cyclone phase space (CPS; Hart 2003) is introduced here. This conceptual model is applied to differentiate these case studies from other candidate storms. The ST criteria presented here were applied to the 40-yr European Centre forMedium- RangeWeather Forecasts Re-Analysis (ERA-40;Uppala et al. 2004) in an accompanying study (Guishard et al. 2009, hereafter Part II) to derive a 45-yr climatology of North Atlantic ST for the period September 1957– August 2002.Evans, Jenni L., Mark P. Guishard, 2009: Atlantic Subtropical Storms. Part I: Diagnostic Criteria and Composite Analysis. Mon. Wea. Rev., 137, 2065–2080.