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ECMWF User Meeting 14-16 June 2006 The use of ECMWF ensemble and lagged deterministic forecasts for...
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Transcript of ECMWF User Meeting 14-16 June 2006 The use of ECMWF ensemble and lagged deterministic forecasts for...
ECMWF User Meeting 14-16 June 2006
The use of ECMWF ensemble and lagged deterministic forecasts for 3-30 day outlooks in Sweden
1. Monthly instead of seasonal forecasting2. The used of lagged forecasts (as a complement to the EPS)3. Problems with weighting together different forecast systems
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
ECMWF User Meeting 14-16 June 2006
1.The seasonal forecasts
Not used, partly because the forecasts seem to repeat themselves
ECMWF User Meeting 14-16 June 2006
Warmerthan normal
2003 2004
2005 2006
The last four years’ ECMWF summer forecasts (issued in April)
Warmerthan normal
Warmerthan normal
Warmerthan normal
Colder than normal
Not warmerthan normal
Colder than normal
Colder than normal
ECMWF User Meeting 14-16 June 2006
2. The monthly forecast
Used and found skilful, but tendencies of “jumpiness” in transitional periods
ECMWF User Meeting 14-16 June 2006
Max +5 to 10
Max +10 to 15
”Jumpiness” experiencedat a specific location
850 hPa temperatureplume for Norrköping,southern Sweden
clim
clim
ECMWF User Meeting 14-16 June 2006
3. The 21 day forecast (+9 days) Using the last three days 21d forecasts enables us to inder the trends beyond day 10, even beyond day 15
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
ECMWF User Meeting 14-16 June 2006
10 d 20 d 30 d
normal
temperature
statistics
Last days ECMWF fcand EPS
Last days Control +21 d forecasts
Main method since summer 2003
ECMWF monthly forecast
ECMWF User Meeting 14-16 June 2006
4. The problem of verification
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
ECMWF User Meeting 14-16 June 2006
Level of useful
forecasts
Introduction of more
ECMWF data
ECMWF User Meeting 14-16 June 2006
What to do?Two ways to go:
1.Political (cover up, play illusionist tricks or change the norms)
2.Scientific (go to the roots of the problem)
ECMWF User Meeting 14-16 June 2006
ACC=98%Slope=0.8
“I just happen to have some fresh verifications here,
depicting the results during the first half of this year...”
Political trick: Selective sampling
ECMWF User Meeting 14-16 June 2006
Verifikation Månadsprognos Svealand Jan 2000 t.o.m Feb 2006
-6-5,5
-5-4,5
-4-3,5
-3-2,5
-2-1,5
-1-0,5
00,5
11,5
22,5
33,5
44,5
55,5
6
Jan-
00 Apr Jul
Okt
Jan-
01 Apr Jul
Okt
Jan-
02 Apr Jul
Okt
Jan-
03 Apr Jul
Okt
Jan-
04 Apr Jul
Okt
Jan-
05 Apr Jul
Okt
Jan-
06 Apr Jul
Okt
Månad
Te
mp
av
vik
els
e
Obs
Prognos
Anomaly correlation of monthly forecast for Stockholm (2 m temperature)
More ECMWF input
Verif
Prog
But it didn’t look that bad….
ECMWF User Meeting 14-16 June 2006
Scientific approach:The conventional verification disregarded three factors:
1. Variable range of variation between 2002 and 2003
2.More than one verification method should be used
3.Twelve forecasts per year is a too small sample
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
ECMWF User Meeting 14-16 June 2006
2003 Lower correlationSmaller errors
2002 Higher correlationLarger errors
ECMWF User Meeting 14-16 June 2006
Introduction of more
ECMWF datareduced
the errors!
Another verification method
RMSE
MABSE
ECMWF User Meeting 14-16 June 2006
Verifying two years at
a time
(Lagged) verification over 24 monthscompared to over 12 months
12 months
24 months
ECMWF User Meeting 14-16 June 2006
5. Swedish concerns about the quality of the centre’s EPS
1. Forecasters at SMHI and the Air Force do not find much use of the deterministic EPS compared to an elaborate use of the deterministic model
2. The scientists at SMHI and the MISU (Univ. Stockholm) are critical about the perturbations + (recently) the stochastic physics
3. My impression is not that the EPS is bad or has become worse, but has had problems to keep pace with the improvements of the deterministic model
ECMWF User Meeting 14-16 June 2006
511255
159
42
The size of the T42 EPS perturbations is very large
The picture depicts the status before 1 February 2006. Since then the resolution of the deterministic system has increased by 50%, but the EPS perturbations which remain at their 1995 level of T42
ECMWF User Meeting 14-16 June 2006
Before 2001 there was little quality difference between perturbed and non-perturbed forecasts, amounting beyond D+5 to an ACC
difference. Since then it has increased to 10-15%
?
Difference in ACC between the unperturbed Control and a randomly selected EPS member
ECMWF User Meeting 14-16 June 2006
1.Over spreading in during the first 24-48 hours made it difficult to use the EPS as BC for the HIRLAM
2.In cases of extreme or interesting events the signals often come 1-2 earlier in the T799 lagged system
3.In cases of consistent and skilful T799 performance the EPS keep the forecaster uncertain too long
For more details see presentation at the OD Workshop November 2005
ECMWF User Meeting 14-16 June 2006
The RMSE of individual EPS members The 2 m temperature forecasts for London Feb-April 2006
perturbed m
embers
1 day
EPS mean
”Lagged”
Unperturbed Control
ECMWF User Meeting 14-16 June 2006
Figure 2.1: Schematic image of the RMS error of the ensemble members, ensemble mean, and control forecast as a function of lead-time. The asymptotic predictability range is defined as the average difference between two randomly chosen atmospheric states. In a perfect ensemble system the RMS error of the ensemble members is a factor larger than the RMS error of the ensemble
mean.Courtesy, L. Bengtsson, MISU
climate
RMSE(pert member)= 1.414 (=sqrt2) RMSE (ensemble mean)
Perturbed member
Ensemble mean
Control
“Tim Palmer’sLaw”
ECMWF User Meeting 14-16 June 2006
6. Use of the last T799 runs forming lagged ensembles (work under development)
ECMWF User Meeting 14-16 June 2006
EPS Mean and “Lagged Mean” 24 March 00 UTC + 84h
ECMWF User Meeting 14-16 June 2006
Valid Sun 9 April 12 UTC
EPS Mean and lagged ECMWF T799 4 April 00 UTC + 132h
ECMWF User Meeting 14-16 June 2006
7. The public 6-10 day forecasts
Once a week, “four out of five forecasts verify”
ECMWF User Meeting 14-16 June 2006
6-10 day forecast presented on TV 26 January 2006
ECMWF User Meeting 14-16 June 2006
ECMWF User Meeting 14-16 June 2006
Epsogram forStockholm
ECMWF User Meeting 14-16 June 2006
8. When does it pay to weight together forecasts?
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
),( 212
1 EEcorrE
E
E1< E2
ECMWF User Meeting 14-16 June 2006
9. How should x and y, the weights, be calculated taking the forecast error correlation into account?
For details see :http://www.ecmwf.int/newsevents/meetings/forecast_products_user/Presentations2006/index.htm
),(
),(
21212
2
21212
1
EEcorrEEE
EEcorrEEE
y
x
E1< E2
ECMWF User Meeting 14-16 June 2006
Certain combinations of forecasts will not yield an improved weighted mean
),(0 212
1 EEcorrE
Eforx
E1< E2
ECMWF User Meeting 14-16 June 2006
Hypothetical error correlations
50%
100%
0%D+0 D+15
T799 vs T399
T799(T399) vs UKMOor an arbitrary eps-member
If all three models have the sameerror magnitude and correlation then the weights are 33.3%
But if the errors of T799 and T399 are more correlated than the errors of T799 (T399) versus UKMO the UKMO should be weighted the most
Extension to three models??
ECMWF User Meeting 14-16 June 2006
10. Future challenges
Extending the monthly forecasts by including precipitation and provide forecasts separately form week1, week2 and week34 - and much more….