Ensemble Prediction of Oceanic Convective Hazards (epoch) Assessment: Part Ii

Abstract

This document is Part II of a report of the Quality Assessment Product Development Team’s (QA PDT) assessment of the Ensemble Prediction of Oceanic Convective Hazards (EPOCH). EPOCH was developed by the Convective Weather Product Development Team (CW PDT) at the National Center for Atmospheric Research (NCAR). EPOCH aims to forecast convection-related hazards to aviation that may affect transoceanic flights, which often need lead times of 24 hours or more for planning. The QA PDT was tasked with an independent quality assessment of the EPOCH product to establish a baseline of performance and inform stakeholders of product skill and characteristics. EPOCH consists of synoptic, 6-hourly global forecasts of the likelihood of thunderstorm occurrence and the likelihood of convective clouds exceeding 30, 35, and 40 kft, on a 1.0° grid. The EPOCH likelihoods are derived using the Global Ensemble Forecast System (GEFS) and the Canadian Meteorological Centre's Ensemble (CMCE) forecasts of accumulated precipitation, Convective Available Potential Energy (CAPE), and outgoing longwave radiation (OLR) (Stone et al. 2016). The performance and characteristics of EPOCH likelihoods were evaluated relative to the operational thunderstorm (Cb) products of the World Area Forecast System (WAFS), namely WAFS Cb horizontal extent and Cb Tops. The current WAFS Cb products are derived from the deterministic National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) model and the UK Met Office Unified Model (UM). The WAFS forecasts are produced on a 1.25? grid. The verification of EPOCH employed a diverse set of observations (Section 2.2), including: two regional radar data sets, which directly measure convective cloud hydrometeors; lightning, which defines thunderstorms; and global satellite cloud and precipitation products, which are linked, though less directly, to the identification of tall thunderstorms. The analysis comprised an evaluation of: (i) characteristics of the products’ fields, (ii) statistical performance against truth data sets, (iii) consistency of forecasts between successive forecast cycles, and (iv) performance in specific case studies. The document is organized as follows. Section 2 provides details of the datasets, including forecast products and observations. The methods and evaluations used in this assessment are described in Sections 3 and 4, respectively. Assessment results are presented in Section 5, while case studies appear in Section 6. Finally, findings are summarized in Section 7. The evaluation will serve to provide a baseline of performance for future versions of EPOCH.