Prediction of tropical cyclone formation in the western North Pacific using the Navy global model
Bower, Caroline A.
Harr, Patrick A.
Elsberry, Russell L.
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The Tropical Cyclone Vorticity Tracking Program is used to identify vortices in the western North Pacific from the Navy Operational Global Atmospheric Prediction System (NOGAPS) analyses and forecasts during May- October 2002 and 2003. Based on the NOGAPS analyses, several parameters are different between the 23vortices that developed into storms during 2002 according to the Joint Typhoon Warning Center (JTWC) and the231 vortices that did not develop. After eliminating 127 vortices that did not persist at least 24 h, this left 104 nondevelopingcases. For the developing circulations, the average 850-mb relative vorticity value at the first JTWCwarningtime was 5.0 x 10-5 s-1, with an easterly deep layer wind shear of -1.8 m s-1. The average 850-mb relativevorticity maximum for the non-developing cases was 3.3 x 10-5 s-1, with a westerly vertical shear of 4.1 m s-1. TheNOGAPS model tends to over-forecast relative vorticity prior to formation time for both developers and nondevelopers.Especially for the 72-h and 96-h forecasts, the over-forecasting tendency leads to non-developingvortices meeting the threshold vorticity value of the developing vortices. The tendency for NOGAPS to forecastthe non-developing deep layer wind shear to become increasingly easterly with time is considered to be a majorfactor in these over-forecasts of formation. Some adjustments in the cumulus parameterization heating andmoistening plus convective momentum transport may improve these forecasts of tropical cyclone formation.
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