Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictability and predictive skill of weather systems and atmospheric flow patterns -25 years of progress- -The legacy of FGGE- Professor Lennart Bengtsson ESSC, University of Reading, UK MPI for Meteorology, Hamburg Many thanks to colleagues at ESSC and ECMWF and especially to Kevin Hodges, ESSC

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictability and predictive skill of weather systems and atmospheric flow patterns The improvements in NWP over the last 25 years The impact of observations on forecast skill The importance of data-assimilation How much better can the forecasts be? The need for a new initiative in weather forecasting

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Improvements in NWP from Miyakoda (1972) to Courtesy ECMWF How long to get to D+10 in winter?

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 What are the causes of the improvements? More accurate models Advanced data-assimilation Better observational coverage

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 The principle of error reduction in data assimilation

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Same model and data- assimilation but different observations. Results from ERA40 Courtesy ECMWF

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Note that the error growth is virtually unchanged between the time of a good forecast (r=0.90) and the time of a useful forecast (r=0.60). This fall in skill takes ca. 3.5 days

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Possible causes? When errors have reached a certain size the growth rate is determined by internal dynamics. If so the skill can only be increased by a more accurate initial state. Or, there are errors due to down-scale cascade, say from tropical forcing, which not yet is properly handled by models Or, perhaps errors, due to aliasing or to incorrect handling of the up-scale cascade of physical processes, such as convection, projecting the errors on synoptic scale modes leading to an overly rapid growth?

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 There are large variations in forecast skill ECMWF 1000 hPa height anomaly correlation Cumulative frequency distribution for Europe. Winter 1988 and 1998

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 There are large variations in forecast skill ECMWF 500 hPa height anomaly correlation, Europe. Winter 1998 and Courtesy H Böttger, ECMWF 95% 75% 50% 25% 5% Jan-Mar 1998 Jan-Mar 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Improvements in predictive skill in atmospheric flow patterns since FGGE NH extra-tropics has improved by more than three days SH extra-tropics is as good as the NH Five day forecasts in the tropics are as good as a one day forecast 25 years ago But There are large differences in skill from day to day The time it takes for a forecast at 90% correlation to reach 60% correlation in the extra-tropics is about 3.5 days. This has not changed. Forecast improvements is due to more accurate initial states.

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 The impact of observations on forecast skill Experiments with different observing systems DJF 1990/91 using ERA40 observations The control system (using all observations) A terrestrial based system ( radio-sondes and aircraft obs.) A satellite based system ( satellites and surface pressure) A surface based system ( surface observations)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Global forecasts DJF 90/91 7- day forecasts, every 6hr. Later ECMWF model T159/L60 Extra-tropics 20-90N and 20-90S 500 hPa Z, normalized SD for the period Tropics 20N-20S Wind vector field 850 and 250hPa

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Observing systems and predictive skill Northern Hemisphere extra-tropics Bengtsson and Hodges, 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Observing systems and predictive skill Southern Hemisphere extra-tropics Bengtsson and Hodges, 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Observing systems and predictive skill Tropics V 250 hPa Bengtsson and Hodges, 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Observing systems and predictive skill Northern Hemisphere Z 500 hPa DJF 1990/91 (full line) and DJF 2000/01 (dashed))

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Observing systems and predictive skill Southern Hemisphere Z 500 hPa DJF 1990/91 (full line) and DJF 2000/01 (dashed))

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Forecast skill and observations The terrestrial system is the best at the NH extra-tropics The satellite system is crucial for the SH extra-tropics In the tropics the terrestrial system and the satellite system are equally useful and highly complementary Between 1990 and 2000 the satellite system has increased its information content and the terrestrial system has decreased it

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 The impact of observations on forecast skill Experiment with ERA40 The impact of observations of humidity on NWP 360 global forecasts DJF 1990/91

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Impact of humidity observations NH Z 500 hPa Full observing system No humidity observations in data-assimilation

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Impact of humidity observations SH Z 500 hPa Full observing system No humidity observations in data-assimilation

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Impact of humidity observations Tropics wind at 850 hPa Full observing system No humidity observations in data-assimilation

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Impact of humidity observations Tropics wind at 850 hPa (mean error at day 5) Full observing system No humidity observations in data-assimilation Control No humidity

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Humidity observations in present data-assimilation Moisture observations have no detectable influence on the overall large scale predictive skill During the cause of the data-assimilation the large scale moisture field is mainly controlled by the model dynamics There is an urgent need to develop techniques for a better assimilation of humidity observations

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Assessment of predictability Growth of small perturbations inserted in a model Comparing how consecutive forecasts separate from each other ( Lorenz, 1982)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Estimation of predictability ( Lorenz, 1982) The error between consecutive forecasts separated by a day ( or shorter) is a suitable expression for predictability. The initial error is then the difference between the analysis an day 1 and the forecast from the day before

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictive skill ( Z 500 hPa) for the NH and predictability estimates ( for 6 ( red) and 24 hr (blue) increments)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictive skill ( Z 500 hPa) for the SH and predictability estimates ( for 6 ( red) and 24 hr (blue) increments)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictive skill ( V 850 hPa) for the Tropics and predictability estimates ( for 6 ( red) and 24 hr (blue) increments)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 What are the predictive skill and predictability of storm-tracks? Storm- tracks as a proxy for predicting the transient weather Predictive skill of storm-tracks as a function of the observing system Estimation of predictability following Lorenz (1982)

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 NH, MSLP, Cyclones Tracks Intensities

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 NCEP Ensemble Storm Tracks Courtesy Z Todt, NCEP and L Froude, ESSC

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictive skill and predictability of storm tracks for different observing systems NH SH

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Conclusions Major progress have been achieved following the FGGE in 1979 but significant improvements are still feasible Predictability estimates indicate a possible gain of several days compared to the present best forecasts The largest potential improvements are in the tropics The Thorpex program is a timely initiative and should be strongly supported

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 What is needed? Weather independent observations of wind and temperature profiles Research to better identify priorities of the observing system Higher resolution in model integrations to reduce numerical errors and representation problems and to simplify the parameterization of physical processes Continue dedicated process studies and associated field experiments

Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 END