Extended Abstract

PSS = H – FETS = ( a – a r ) / ( a+b+c - a r )Peirce Skill ScoreEquitable Threat Scorewhere a r is the number of hits for random forecasts.Figure 1 shows seven-year (2003-09) accumulated and averaged results of ECMWFQPFs for the forecast accumulation period of +18-42 hours, averaged over 100 raingauge stations in Finland. Due to the large dataset there are quite many extreme heavyprecipitation cases (in the Finnish climate), i.e. over 300 cases with 24-houraccumulated precipitation exceeding 30 mm. One can immediately note how thetraditional verification scores degenerate towards zero beyond 5-10 mm of dailyprecipitation. On the other hand, the novelty scores defined in the previous section,behave remarkably different. At the lowest precipitation rates they are somewhatdifferent to each other and qualitatively comparable to the PSS. When the precipitationevents become more extreme they tend to converge to each other but not triviallytowards zero. The EDS is somewhat lower compared to the others when dailyprecipitation amounts exceed 10 mm, and EDI and SEDI are almost identical at highestprecipitation rates as can be interpreted from their definitions.Figure 1. Verification scores as a function of precipitation threshold for 42 hour ECMWF 24-houraccumulated (+18-42 hrs) QPFs, averaged over seven years (2003-2009) and c. 100 rain gauge stationsin Finland. The lowermost numbers indicate the number of cases in each category. See text for theexplanation of acronyms.Figure 2 presents a comparison between the EDS, SEDS and EDI scores at twodifferent ECMWF QPF ranges, c. two days (+18-42 hour accumulation) and c. five days(+114-138 hour accumulation). (SEDI was not available at the longer forecast range).The two pairs of three curves behave qualitatively in a similar manner and as one wouldhope to expect the longer range forecasts produce the poorer verification results. Goingto the most extreme (and rare) events of precipitation exceeding 50 mm per day all ofthe scores fall down to zero level at day D+5 forecast range.-224-