lennart bengtsson essc, uni. reading thorpex conference december 2004 predictability and predictive...
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![Page 1: Lennart Bengtsson ESSC, Uni. Reading THORPEX Conference December 2004 Predictability and predictive skill of weather systems and atmospheric flow patterns](https://reader035.vdocument.in/reader035/viewer/2022062713/56649f525503460f94c76894/html5/thumbnails/1.jpg)
Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Improvements in NWP from Miyakoda (1972) to 2002. Courtesy ECMWF
How long to get to D+10 in winter?
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
What are the causes of the improvements?
• More accurate models
• Advanced data-assimilation
• Better observational coverage
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
The principle of error reduction in data assimilation
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Same model and data- assimilation but different observations. Results from ERA40 Courtesy ECMWF
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, 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?
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
There are large variations in forecast skillECMWF 1000 hPa height anomaly correlation
Cumulative frequency distribution for Europe. Winter 1988 and 1998
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
0
10
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1 2 3 4 5 6 7 8 9 10
Forecast days
Anomaly correlation
There are large variations in forecast skillECMWF 500 hPa height anomaly correlation, Europe. Winter 1998 and 2004. Courtesy H Böttger, ECMWF
95% 75%
50%
25%
5%
Jan-Mar 1998
Jan-Mar 2004
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Lennart BengtssonESSC, 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.
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
The impact of observations on forecast skillExperiments 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)
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Observing systems and predictive skillNorthern Hemisphere extra-tropics
Bengtsson and Hodges, 2004
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Observing systems and predictive skillSouthern Hemisphere extra-tropics
Bengtsson and Hodges, 2004
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Observing systems and predictive skillTropics V 250 hPa
Bengtsson and Hodges, 2004
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Observing systems and predictive skillNorthern Hemisphere Z 500 hPa
DJF 1990/91 (full line) and DJF 2000/01 (dashed))
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Observing systems and predictive skillSouthern Hemisphere Z 500 hPa
DJF 1990/91 (full line) and DJF 2000/01 (dashed))
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
The impact of observations on forecast skillExperiment with ERA40
The impact of observations of humidity on NWP
360 global forecasts DJF 1990/91
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Impact of humidity observationsNH Z 500 hPa
Full observing systemNo humidity observations in data-assimilation
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Impact of humidity observationsSH Z 500 hPa
Full observing systemNo humidity observations in data-assimilation
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Impact of humidity observationsTropics wind at 850 hPa
Full observing systemNo humidity observations in data-assimilation
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Impact of humidity observationsTropics wind at 850 hPa (mean error at day 5)
Full observing systemNo humidity observations in data-assimilation
Control
No humidity
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Humidity observationsin 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
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Lennart BengtssonESSC, 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)
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Predictive skill ( Z 500 hPa) for the NHand predictability estimates ( for 6 ( red)
and 24 hr (blue) increments)
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Predictive skill ( Z 500 hPa) for the SHand predictability estimates ( for 6 ( red)
and 24 hr (blue) increments)
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Predictive skill ( V 850 hPa) for the Tropicsand predictability estimates ( for 6 ( red)
and 24 hr (blue) increments)
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
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Lennart BengtssonESSC, 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)
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
NH, MSLP, Cyclones
Tracks Intensities
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
NCEP Ensemble Storm TracksCourtesy Z Todt, NCEP and L Froude, ESSC
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
Predictive skill and predictability of storm tracksfor different observing systems
NH
SH
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, 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
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Lennart BengtssonESSC, Uni. Reading
THORPEX Conference December 2004
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