Verification and Improvement of Impact-based Forecast Using Multi Model Ensemble

  The Korea Meteorological Administration(KMA) is producing an impact-based forecast data based on both the deterministic forecast and ensemble forecast for heat waves (HW) and cold waves (CW). Ensemble prediction system for impact-based forecast is Multi-Model ensemble system which integrates UM(global, global ensemble, local, and local ensemble models), ECMWF(global and global ensemble models) and KIM(Korean Integrated Model) global model(Hereafter, impact-based forecast  based on the deterministic forecast and Multi-Model ensemble system are called `IMPC` and ‘MEPS’, respectively.). MEPS determine the risk level by using the probability of occurrence of abnormal temperatures on the Korean Peninsula. Once maximum perceived temperature(HW) or lowest temperature(CW) from all 93 Multi-Model Ensemble members were extracted, their probability distribution was determined by using a Generalized Extreme Value (GEV) distribution.
  The performance of MEPS for HW was compared with IMPC for July and August 2022. Verification was conducted by evaluating how well impact-based forecast level(safe, concern, caution, warning, alarm) was matched to the observed risk level in 175 regions. 
  As a result of verification it was found that IMPC forecasts ‘concern’ more frequently and MEPS forecasts ‘caution’ more frequently. In July, IMPC's prediction performance is excellent for ‘safe’ and ‘concern’ level and MEPS is excellent at a ‘caution’ and ‘warning’. IMPC underestimates ‘caution’ and ‘warning’ while MEPS tends to overestimate ‘safe’ and ‘concern’. In August, ETS score of MEPS is excellent for ‘safe’ levels as well as ‘caution’ and ‘warning’ levels. In case studies, there is many cases in which MEPS detected HW better than IMPC when a high-level heat wave was observed on the Korean Peninsula. Overall, MEPS is expected to be a good reference data in the impact-based forecast where predictive ability for high risk levels is important. 
  The MEPS guidance uses only daily temperatures, but according to KMA’s forecast guideline, HW is defined a phenomenon in which a high temperature lasts for more than 2 days. By reflecting these conditions, future guidance needs to be improved. In the HW guidance, the risk level re-established by considering whether the risk level lasts longer tha two days improved the predictive performance of ‘safe’ and ‘concern’, and the CSI (Critical Success Index) and ETS increased at all risk level.