Dynamical tropical cyclone track forecast errors. Part I: Tropical region error sources

All highly erroneous (>300 n mi or 555 km at 72 h) Navy Operational Global Atmospheric Prediction System (NOGAPS) and U. S. Navy version of the Geophy sical Fluid Dynamics Laboratory model (GFDN) tropical cyclone track forecas ts in the western North Pacific during 1997 are examined. Responsible error mechanisms are described by conceptual models that are all related to know n tropical cyclone motion processes that are being misrepresented in the dy namical models. Error mechanisms that predominantly occur while the tropica l cyclone is still in the Tropics are described in this paper, and those er rors that are more related to midlatitude circulations are addressed in a c ompanion paper. Of the 69 NOGAPS large- error cases, 39 were attributed to excessive direct cyclone interaction (E- DCI), 12 cases of excessive ridge modification by the tropical cyclone (E- RMT), and 10 cases of excessive re verse trough formation (E- RTF). Of the 50 GFDN large- error cases, 31 were E- DCI, and only two E- RMT and two E- RTF cases were found, but 9 cases i nvolving a single cyclone were attributed to excessive tropical cyclone siz e (E- TCS). Characteristics and symptoms in the forecast tracks and model f ields that accompany these frequently occurring error mechanisms are docume nted and illustrative case studies are presented. When a sudden deviation f rom previous track guidance or a track outlier from the other dynamical mod el guidance appears, the forecaster should diagnose whether this is an erro r, or is indicative of a real track change. If the conceptual models of lar ge- error mechanisms proposed from this retrospective study can be applied in real time, track forecasting will be improved.