Title: Seasonal Predictions of Tropical cyclones Using a 25-km resolution General Circulation Model

Authors: Jan-Huey Chen and Shian-Jiann Lin

Submitted to Journal of Climate

Key science issue:

 Retrospective seasonal predictions of tropical cyclones (TCs) in the three major ocean basins of the Northern Hemisphere are performed from 1990 to 2010 using the Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM) at 25-km resolution.

        Atmospheric states are initialized for each forecast, with the sea surface temperature anomaly (SSTA) persisted from that at the starting time during the 5-month forecast period (July-November).

Key findings:

• Using a 5-member ensemble, it is shown that the storm counts of both tropical storm (TS) and hurricane categories are highly predictable in the North Atlantic basin during the 21-year period.
• The correlations between the 21-year observed and model predicted storm counts are 0.88 and 0.89 for hurricanes and TSs, respectively. 
• The prediction in the eastern North Pacific is skillful, but it is not as outstanding as that in the North Atlantic.
• The persisted SSTA assumption appears to be less robust for the western North Pacific, contributing to less skillful predictions in that region. 
• The persisted SSTA assumption used in the predictions is shown to be valid up to the end of the hurricane season in the Atlantic Main Development Region; but it is much poor for the main TC genesis areas in both the eastern and western North Pacific basins.
• The intensity distribution of TCs can be captured well by our model if the central sea-level pressure were used as the threshold variable instead of the commonly used 10-meter wind speed.
• The feasibility of using the 25-km resolution HiRAM, a general circulation model designed initially for long-term climate simulations, to study the impacts of climate-change on the intensity distribution of TC is demonstrated.