Overforecasts of tropical cyclogenesis in the FIM global model

During the summer 2009 Hurricane Forecast Improvement Project (HFIP) real-time demonstration, a serious, but not uncommon, problem of excessive tropical cyclone (TC) genesis was found in high-resolution runs (30, 15, 10 km) of the FIM (fim.noaa.gov) global atmospheric model. While there were no obvious impacts on traditional measures of TC forecast skill (track and intensity), the TC overforecasts did lead to a better understanding of the role of resolution/dynamics in the model physical processes, in particular deep convection. Extensive model experiments, for a case in the eastern North Pacific, indicated that the primary factor may be the partitioning between large-scale and convective precipitation; and how this partitioning may depend on both noise from model dynamics and horizontal spatial resolution (as it affects the underlying assumptions of the convective parametrization).

We first objectively measure TC genesis in the FIM, and other operational global models including those from ECMWF, UKMO, NCEP and the US Navy, and then apply newly developed diagnostic tools to calculate the partitioning relative to the TC and to compare model precipitation to satellite estimates. The analysis will consider all northern Hemisphere TC basins, to understand how the excessive formation depends on the tropical general circulation, e.g., monsoon trough (western North Pacific) v easterly waves (Atlantic)