Forecasting exceedance probabilities of extreme sea level events based on ensemble forecasts

Transcription

1 Forecasting exceedance probabilities of extreme sea level events based on ensemble forecasts Jani Särkkä, Laura Tuomi and Antti Kangas Finnish Meteorological Institute September 12, 217

2 Introduction Sea level forecasts are made for the Baltic Sea Sea level forecasts needed from 2 to 1-day length Planning of ship cargo loading and ship route Preparing for sea level extremes at the coast Deterministic forecast too uncertain if forecast length is longer than 4-5 days Probability forecasts more useful for longer time periods J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 2/18

3 Baltic Sea Kemi 65 Bothnian Sea Föglö Bothnian Bay Oulu Raahe Pietarsaari Vaasa Kaskinen Mäntyluoto Rauma Turku Hanko Helsinki Gulf of Finland Hamina Danish Straits Baltic Sea J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 3/18

4 Combined model of Baltic Sea level Sea level components are modeled separately Internal fluctuations of the Baltic Sea level are calculated using 2-D Hansen sea level model and assuming Baltic Sea a closed basin Water balance of the Baltic Sea is estimated from the daily means of zonal surface winds v B on Bornholm (55 N, 15 E) z(t) = a(t )v B (t T ) T =1 a(t ) = 2.9mm/ms 1, 1 T 4 a(t ) = 4.mm/ms 1 exp(.72t ), 4 < T. J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 4/18

5 Sea level at Kemi model observations Sea level at Kemi (cm) Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 5/18

6 Sea level probability forecast 1 control and 5 perturbed forecasts from the ECMWF Ensemble Prediction System 15-day forecasts made daily at and 12 UTC 1-day forecasts containing surface wind and air pressure data in the sea level model grid delivered to FMI Grid size 15 nmi, atmospheric input data time step 6 h 51 1-day sea level simulations take 3 minutes with a laptop Results saved for tide gauge locations (in the future the entire grid) Different sea level probability fractiles calculated from simulation ensemble J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 6/18

7 Sea level at Kemi December 216-January Sea level at Kemi (cm) Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 7/18

8 Sea level probability forecast at Kemi Sea level at Kemi (cm) 8 4 min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 8/18

9 Sea level at Kemi (cm) min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 9/18

10 Sea level at Kemi (cm) min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 1/18

11 Sea level at Kemi (cm) min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 11/18

12 Sea level at Kemi (cm) 8 4 min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 12/18

13 Sea level at Kemi (cm) 8 4 min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 13/18

14 Sea level at Kemi (cm) min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 14/18

15 Sea level at Kemi (cm) min max observations Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 15/18

16 Exceedance probabilities of threshold levels Exceedance probability at Kemi cm 1 cm 12 cm 14 cm Days after J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 16/18

17 Summary Spread of forecast is small for length under 2 days Forecast is most useful for 2-5 days Spread increases after 5 days Sharp sea level peaks are underestimated due to 6-hour time step in input data Water balance variations are well described by statistical model For details of the sea level model, see Särkkä et al. (217), Boreal Environment Research 22: Sea level probability forecast is already operational at FMI J. Särkkä, L. Tuomi and A. Kangas: Sea level probability forecasts 17/18

18 Ilmatieteen laitos Meteorologiska institutet Finnish Meteorological Institute Erik Palménin aukio 1 Erik Palméns plats 1 Erik Palménin aukio 1 5 Helsinki 5 Helsingfors 5 Helsinki PL 53, 11 Helsinki PB 53, 11 Helsingfors P.O.Box 53, 11 Helsinki puh tel tel » Facebook: FMIBeta