Evaluation of the AFWA WRF 4-km moving nest model predictions for Western North Pacific tropical cyclones

Abstract

The Air Force Weather Agency (AFWA) version of the Advanced Research Weather Research and Forecasting (ARW) model with a moving 4-km nested grid is examined for 10 track and intensity predictions of six western North Pacific tropical cyclones during 2005. In three of the 10 integrations, the ARW vortex tracker algorithm based on the 500-mb height minimum failed to appropriately move the nest and thus lost track of the storm vortex. For the other seven cases, the ARW track forecasts are more skillful than the AFWA MM5 forecasts and (except at 12 h) the CLIPER-type forecasts. The ARW intensity forecasts were less skillful than the MM5 and CLIPER-type forecasts at all forecast intervals, and were severely degraded by a large negative bias at the initial time. The deficiency in these intensity forecasts is shown to be related to model spin-up (lasting 12-54 h) problems caused by the lack of a bogus vortex and a cold start initialization from the interpolation of the NCEP Global Forecast System (GFS) analysis to the 12- km and 4-km grids. Thus, a more appropriate initial vortex representation will be required to improve intensity forecasts.