Lars Wiegand, Arwen Twitchett, Conny Schwierz & Peter Knippertz

11th EMS Annual Meeting, Berlin, 12–16 Sept. 2011 Heavy Precipitation at the Alpine South Side and Saharan Dust over Central Europe: A Predictability Study using TIGGE Lars Wiegand, Arwen Twitchett, Conny Schwierz & Peter Knippertz

Outline • Motivation • Upper-Level Development • Forecast performance • Analysis uncertainty • High-Impact Weather • Saharan dust storm • Heavy precipitation at the Alpine south side • Conclusions

Motivation Motivation 2008/05/27 10 UTC 2008/05/27 13 UTC

Motivation Motivation Gorner glacier August 2008 with dust from the May event

Motivation Motivation mm • flooding on the Alpine south side of Switzerland & Italy • Lago Maggiore rose by 10 cm in 24 hours 4-day precipitation sum 26–29 May 2008 source: ENSEMBLE gridded rain gauge data

Motivation DATA • Thorpex Interactive Grand Global Ensemble (TIGGE) • 10 forecasting centres: Australia, Brazil, China, Canada, ECMWF, France, Japan, Korea, UK, USA • ~500 multi-model ensemble members per day • Exciting new dataset for operational forecasting and research

Upper-Level Development Upper-level potential vorticity

Upper-Level Development Upper-level potential vorticity region of interest (29°-55°N, 15°W-2°E)

Upper-Level Development Root mean square errors (RMSEs) ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk calculated with ECMWF analysis

Upper-Level Development Root mean square errors (RMSEs) ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk calculated with every center’s own analysis

Upper-Level Development RMSE/spread comparison non-dispersivemodels

Upper-Level Development RMSE/spread comparison under dispersivemodels

High Impact Weather Saharan dust outbreak • RGB composite: purple color = dust • from 2008/05/26: 09UTC – 23UTC

High Impact Weather 925-hPa wind fields

High Impact Weather Diagnostic approach Define “Potential dust uplift” (PDU) Set dust emission threshold: Ut = 10 m s–1 at 925hPa for U > Ut for U < Ut

High Impact Weather Potential dust uplift forecast 103m3s-3 PDU analyses

High Impact Weather Precipitation observation mm 4-day precipitation sum 06 UTC 26 – 06 UTC 29 May 2008 source: ENSEMBLE (gridded rain gauges data)

High Impact Weather Precipitation forecast observed precipitation (ENSEMBLE data set) mm in 4 days

Conclusions Conclusions • TIGGE offers interesting new insight into multi-model ensemble prediction. • Case study of unusual weather event in May 2008. • Forecast performance metrics sensitive to analysis used. • Some TIGGE models with under-dispersive behaviour. • Winds for dust mobilization over Sahara general underpredicted but spread rather large. • Heavy precipitation at Alpine south side fairly well forecast on regional scale. • Multi model approach increases skill! • More in a recently published paper in Weather & Forecasting

Lars Wiegand, Arwen Twitchett, Conny Schwierz & Peter Knippertz