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Third THORPEX International Science Symposium, Monterey: 14-18 Sept. 2009. An unusual Saharan dust outbreak into central Europe and heavy precipitation at the southern side of the Alps in May 2008: A TIGGE case study. Lars Wiegand 1,2 , Arwen Twitchett 2 , Conny Schwierz 3 & Peter Knippertz 2.
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Third THORPEX International Science Symposium, Monterey: 14-18 Sept. 2009 An unusual Saharan dust outbreak into central Europe and heavy precipitation at the southern side of the Alps in May 2008:A TIGGE case study Lars Wiegand1,2, Arwen Twitchett2, Conny Schwierz3 & Peter Knippertz2 1 2 3
Outline • Motivation • Upper-Level Development • RMSE/SPREAD • analysis errors • position of forecasted streamers • High Impact Weather • Saharan dust storm • heavy precipitation in Southern Switzerland • Conclusions
Motivation Motivation article in newspaper „Süddeutsche“ 2008/05/29: „Die Sahara über Deutschland“ Gornergletscher 08/2008, dust from 05/2008 albedo increase ablation increase mass balance decrease 2008/05/27 10UTC 2008/05/27 13UTC
Motivation Motivation mm • widespread heavy precipitation caused: • flooding on the Alpine south side of Switzerland and Italy • Lake Maggiore rised by 10 cm in just 24 hours 4-day sum of precipitation (26th – 29th of May 2008) data: ENSEMBLE (gridded data set from rain gauges)
Upper-Level Development Analysis End of May 2008 (21st-26th) region of interest (29-55N, 15W-2E) upper level PV operational analysis ECMWF
Upper-Level Development RMSE of all models (Box: 29-55N, 15W-2E) ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk RMSE calculated with every Centers own Analysis RMSE calculated with Analysis from ECMWF
Upper-Level Development 0,23 (UK, 4Dvar) 0,44 (Korea, 3Dvar) 0,22 (USA, Interpolation) 0,41 (Australia, Interpolation) 0,23 (Brazil, USA Interpolation) 0,26 (Japan, 4Dvar) RMSE in box 0,59 (China, Interpolation) 0,25 (Canada, 4Dvar) (4Dvar) -1.5 -1.0 -0.5 0.5 1.0 1.5
Upper-Level Development RMSE & SPREAD (slightly) overdispersive “non-dispersive” underdispersive CMA_china CMC_canada ECMWF_europe JMA_japan BoM_australia CPTEC_brazil NCEP_usa UKMO_uk KMA_korea RMSE SPREAD
Upper-Level Development Position of Streamer (center of mass) position of ECMWF analysis envelope of member positions ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk 48h forecast 96h forecast 120h forecast 168h forecast
High Impact Weather Saharan dust outbreak • RGB composite: purple color = dust • from 2008/05/26: 09UTC – 23UTC • needed for dust mobilization: u925hPa > 10 m/s
High Impact Weather Region of interest (5W-10E & 23N-35N) number of grid points in box: 208 number of gp, wind > 10m/s: 113
High Impact Weather Grid points where wind > 10m/s median of centres forecasts ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk ECMWF analysis
High Impact Weather Precipitation forecast (box average) median of centres forecasts observed precipitation (ENSEMBLE data set) ECMWF_europe BoM_australia CMA_china CMC_canada CPTEC_brazil JMA_japan KMA_korea NCEP_usa UKMO_uk
Conclusion Conclusions • Open question: which analysis is the “best” one to use? • RMSE smaller with own analysis • Upper-level development • increase of RMSE with lead time • SPREAD of similar magnitude as RMSE in 4 models, 4 models are underdispersive and 1 model is slightly overdispersive • forecasted positions of streamers are mostly too far north • Wind for dust mobilization over Sahara • winds systematically too weak • Precipitation at southern side of Alps • systematically too high in medium-range • mostly too low in short-range
