Western North Pacific Tropical Cyclones in the ECMWF 32-day Ensemble Prediction System

In retrospective studies of the 2008 and 2009 western North Pacific typhoon seasons, Elsberry et al. (2010, 2011) found that the European Center for Medium-range Weather Forecasts (ECMWF) 32-day, 51-member ensemble forecasts made once a week provide outlooks of formations and subsequent track type for some typhoons for as long as three or four weeks. Ensemble storm tracks that were formed by combining nearby ensemble member vortices with a weighted-mean vector motion technique were validated relative to the Joint Typhoon Warning Center tracks. Based on these studies, a near-real time version of the technique was prepared to support mission planning activities for the Impact of Typhoons on the Ocean in the Pacific (ITOP) field experiment during August to October 2010. The ITOP support requirement was to provide outlooks that a typhoon of at least 100 kt intensity would exist within 1000 km of the operations base in Guam. Enough ocean instrumentation was available for two such deployments during the 60-day experiment period. An upgraded version of the ECMWF ensemble was available in 2010 with horizontal resolution of T639 (32 km) Day 1-10 and T319 (63 km) Days 10-32 with a coupled ocean model after Day 10. The 2010 typhoon season was marked by the fewest number of tropical cyclones in recent times. The performance of the weekly ECMWF 32-day ensemble forecast will be summarized with special focus on Typhoons Fanapi and Malakas and Supertyphoon Megi. An unusual period of no tropical cyclone activity occurred within 1000 km of Guam between 23 September and 12 October, as Malakas was ending and before Megi formed. The ability to forecast this period of no activity was vital because the second deployment and recovery of the ocean drifters had to be completed prior to 20 October. Thus the capability of the ECMWF ensemble to predict weekly periods in which there would be no Tropical Storm or higher intensity will also be evaluated. Post-ITOP experiment improvements of the technique including fitted vortex tracks and cluster analysis for pre- and post-processing of the tracks will be illustrated by comparison with the “operational” version available during ITOP.