Objective verifications and false alarm analyses of western North Pacific tropical cyclone event forecasts by the ECMWF 32-day ensemble

Abstract

An objective tropical cyclone (TC) track analog verification technique has been developed to select all ensemble storm tracks predicted by the ECMWF 32-day ensemble that match the overall Joint Typhoon Warning Center (JTWC) post-season best-tracks. Ensemble storms within specified time and space differences of each JTWC track are first extracted as potential analogs, and four metrics of shortest distance, average distance, distance at formation time, and distance at ending time are calculated. An objective quality measure that assesses the overall track similarity between the potential analogs and each JTWC track is calculated in terms of membership functions for the four track metrics. Weighting factors multiplying these membership functions are adjusted to match with the quality measures for the ECMWF ensemble storm forecasts in a previous subjective evaluation. Objective verifications for the 2009 and 2010 seasons have been summarized in terms of Hits, Misses, False Alarms, and Correct Negatives that no TC would be present in the western North Pacific. The most important result is that the ECMWF ensemble was able to predict nearly all of the TCs in both seasons with only a small number of Misses that generally were short-lived tropical depressions. Good performance in terms of Correct Negatives was achieved during the 2010 season. False alarms are defined to be all ensemble storms that could not be matched any JTWC tracks within the specified thresholds. Evaluations of the characteristics of the false alarms indicate seasonal and geographic biases and that about 50% of the false alarm in the Week 1 forecasts originate from the initial the initial conditions in the model. A minimum of false alarms created in Week 2 forecasts is attributed to the decrease in horizontal resolution in the model that occurs at day 10. A steady and nearly uniform increase in false alarms in the Week 3 and Week 4 forecasts may be attributed to net convective heating in response to persistent environmental forcing in the tropics.