1. The THORPEX Interactive Global Ensemble (TIGGE)
Multi-model ensembles and
Tropical cyclone forecasting
Richard Swinbank,
with thanks to
the GIFS-TIGGE working group,
THORPEX IPO and Met Office & other colleagues
Ensemble TC forecasting workshop, Nanjing, December 2011

2. TIGGE and ensemble forecast products
Objectives
TIGGE archive
Ensemble forecasting
Probabilistic forecasting
Use of multi-model ensembles
Forecast products
Plans for development of Global Interactive Forecast System
Developing links with CBS/SWFDP
Tropical cyclone forecast products

3. TIGGE THORPEX Interactive Grand Global Ensemble
A major component of THORPEX: a World Weather Research Programme to accelerate the improvements in the accuracy of 1-day to 2-week high-impact weather forecasts
GEO task WE – “TIGGE and the Development of a Global Interactive Forecast System for Weather”
Objectives:
Enhance collaboration on ensemble prediction, both internationally and between operational centres & universities.
Facilitate research on ensemble prediction methods, especially methods to combine ensembles and to correct systematic errors
Enable evolution towards a prototype operational system, the “Global Interactive Forecast System”

4. TIGGE data
Ten of the leading global forecast centres are providing regular ensemble predictions to support research on predictability, dynamical processes and the development of probabilistic forecasting methods.
TIGGE data is made available for research after a 48-hour delay. Near real-time access may be granted for specific projects through the THORPEX International Project Office.

5. Summary of TIGGE database (late 2010)
Centre
Ensemble
members
Output data
resolution
Forecast
length
Forecasts
per day
Fields
(out of 73)
Start date
BOM* 33
1.50º x 1.50º
10 day 2 55
3 Sep 07
CMA 15
0.56º x 0.56º 60
15 May 07
MSC 21
0.9º x 0.9º
16 day 56
3 Oct 07
CPTEC
0.94º x 0.94º
15 day
1 Feb 08
ECMWF 51
N200 (Reduced Gaussian)
N128 after day 10 70
1 Oct 06
JMA
9 day 1 61
KMA* 17
1.00º x 1.00º 46
28 Dec 07
Météo-France 35
4.5 day 62
25 Oct 07
NCEP 4 69
5 Mar 07
UKMO 24
0.83º x 0.55º 72
* Delivery of BoM data currently suspended; KMA resumed Aug 2011

6. TIGGE Archive Usage
(NCAR + ECMWF)

7. Information about TIGGE
Major Article in BAMS
New leaflet to publicise TIGGE to researchers
Contribution in GEO book “Crafting Geoinformation”
Tropical cyclone case study in WMO Bulletin
TIGGE website

8. TIGGE Research
Following the successful establishment of the TIGGE dataset, the main focus of the GIFS-TIGGE working group has shifted towards research on ensemble forecasting. Particular topics of interest include:
a posteriori calibration of ensemble forecasts (bias correction, downscaling, etc.);
combination of ensembles produced by multiple models;
research on and development of probabilistic forecast products.
TIGGE data is also invaluable as a resource for a wide range of research projects, for example on dynamical processes and predictability.
Up to the end of 2010, 43 articles related to TIGGE have been published in the scientific literature

9. Ensemble forecasting X time Deterministic Forecast
Initial Condition Uncertainty X
Analysis
Animation in this slide can help illustrate the ensemble concept. Time runs from left to right so we are forecasting from the analysis to some point in the future which must lie within the climatology. There is a region of uncertainty in the forecast but we do not know what it is. The standard deterministic forecast just gives a point within this region. However we know there is uncertainty in the analysis (click to introduce). By randomly sampling within this uncertainty we can produce a set of forecasts (click to animate) which gives us a sampling of the forecast uncertainty on the right – the only estimate we have of what this forecast uncertainty is.
Climatology
time
Forecast uncertainty

10. Ensemble forecasting - effect of Chaos
The atmosphere is a chaotic system: “… one flap of a seagull’s wing may forever change the future course of the weather”, (Lorenz, 1963)
Up to about 3 days ahead we can usually forecast the general pattern of the weather quite accurately
Beyond 3 days Chaos becomes a major factor
Tiny errors in how we analyse the current state of the atmosphere lead to large errors in the forecast – these are both equally valid 4-day forecasts!
Lorenz’s famous quote is probably an exaggeration for day-to-day forecasting, but nevertheless typifies the problem. We can never analyse the current state of the atmosphere perfectly, even on much larger scales than a seagull! The two charts shown are from quite a typical 4-day ECMWF ensemble forecast – a more extreme example, but certainly not a rare one. You may like to comment that the differences in initial conditions between them are too small for a forecaster to be able to analyse which is the better. The left chart predicts a deep area of low pressure near NW Ireland bringing strong winds and rain to much of the British Isles; the right chart predicts a much weaker low NE of Scotland, with a ridge of high pressure likely to bring fine weather to the southern half of the British Isles. Clearly in this situation a forecaster who has access to only a single model forecast is in danger of issuing a forecast which could go seriously wrong.
Fine details (eg rainfall) have shorter predictability
With acknowledgements to Ken Mylne for general Ensemble Forecasting slides

11. Lorenz Model – a simple chaotic system
Variations in predictability can be illustrated using the Lorenz (1963) model:
Simple non-linear system.

12. Ensemble Forecasting in the Lorenz Model
1. Predictable -
deterministic OK
2. Predictable at first -
probability OK
3. Unpredictable
climatology OK
Thanks to Tim Palmer, ECMWF

13. Ensemble Forecasting
The aim of ensemble prediction system is to represent the uncertainty in the state of the NWP model at all stages through the forecast range
Each ensemble member is designed to sample a PDF representing uncertainties in the model state. The PDF is usually represented by a control run plus many perturbed forecasts.
Initial condition perturbations are designed to represent uncertainties in the initial analysis – closely linked to data assimilation. These grow with time as a result of the chaotic nature of model dynamics.
The uncertainty in forecasts also grows as a result of model error. This effect may be represented by perturbing model physics – e.g. stochastic physics schemes.

14. Examples of comparison of ensemble spread & error (from 2010 MOGREPS upgrade)
Compare
Red – old
Blue − new
Note:
Ensemble spread is less than RMS errors
New system both reduces RMS error & increases spread
Under-spread in surface temperature exaggerated because of representivity errors (point observations not representative of grid-square averages)

15. Under-dispersive forecast
Real forecast uncertainty
Deterministic Forecast
Initial Condition Uncertainty
Estimated forecast uncertainty X
The ensemble may capture reality less often than it should.
Dangerous - false sense of security!
Analysis
time

16. What makes a good ensemble?
Resolution – ability to distinguish between different events.
Reliability – the forecast probabilities match the associated observed frequencies.
Calibration – apply post-processing so that the statistics match those of a reference dataset. This might include bias-correction and variance adjustment.
Observed
The observed track should lie within the ensemble.

17. Met Office Multi-Model Ensemble
One approach to improving the calibration is to use multi-model ensembles
NCEP
21 Member
3 variables:
MSLP
2m Temp
500mb Height
ECMWF
51 Member
Met Office
24 Member
Met Office MME results courtesy
Christine Johnson

18. Reliability Diagrams
ECMWF
UKMO
Probability that 2m temperature is greater than the climatological mean at T+240. Globally averaged.
Multimodel
NCEP
Multi-model gives better reliability and reduced sharpness.

19. Brier Skill Scores
1 day
5 day
Brier
Probability of 2m temperature greater than climatological mean.
Multi-model gives improvement in reliability and resolution at all lead times.
Reliability
Resolution

20. Dissimilarity of ensembles
D has large values (high dissimilarity, red) if the between-model variance is large compared to the mean-square-error of the multi-model mean.
mslp
temp
D+2
D+2
D+10
D+10

21. Multi-model ensemble forecast
real forecast uncertainty
Deterministic Forecast
Initial Condition Uncertainty
Estimated forecast uncertainty X
Use of multi-model ensembles can improve the sampling of forecast uncertainty
Analysis
time

22. Multi-model ensemble forecasts of T850
Demonstrates benefit of multi-model ensemble, provided that the most skilful models are used.
Renate Hagedorn, ECMWF

23. Multi-model ensemble compared with reforecast calibration
Reforecast calibration gives comparable benefit to multi-model ensemble
Choice of verification data set (in this case, ERA-Interim) could have subtle but significant effect on relative benefits
Calibration could further enhance benefit of multi-model ensemble
Renate Hagedorn

24. Precipitation forecasts over USA
24 hour accumulations, data from 1 July 2010 to 31 October 2010.
20 members each from ECMWF, NCEP, UK Met Office, Canadian Meteorological Centre.
80-member, equally weighted, multi-model ensemble verified as well.
Verification follows Hamill and Juras (QJ, Oct 2006) to avoid over-estimating skill due to variations in climatology.
Conclusions:
ECMWF generally most skillful.
Multi-model beats all.
Tom Hamill

25. Summary - EPS & multi-model ensembles
Ensembles are valuable for forecasting the risks of exceeding thresholds (e.g. for high-impact weather events).
But forecasts often need calibration to correct both biases and variability (e.g. to correct under-estimates of forecast spread).
The best approach is to address the systematic errors, i.e. reduce model biases and improve the representation of model errors in the EPSs.
That is a long-term goal. In the mean time….
Use of multi-model ensembles is a pragmatic approach that reduces calibration errors, especially where models have similar skill but different types of systematic error
Reforecasts (forecasts of past cases with current NWP models) can be used to estimate, and then correct, model biases

26. Towards the Global Interactive Forecast System (GIFS)
Many weather forecast situations are low probability but high risk – unlikely but potentially catastrophic. Probabilistic forecasting is a powerful tool to improve early warning of high-impact events.
The objective of the GIFS is to realise the benefits of THORPEX research by developing and evaluating probabilistic products to deliver improved forecasts of high-impact weather.
GIFS-TIGGE WG has initiated a GIFS development project to develop & evaluate products, focused on
Tropical cyclones
Heavy precipitation
Strong winds

27. Tropical cyclones
As a first step, the GIFS-TIGGE working group set up a pilot project for the exchange of real-time tropical cyclone predictions using “Cyclone XML” format.
Example of combined TC track forecasts (Met Office + ECMWF)

28. CXML track exchange

29. GIFS links with SWFDP
GIFS will collaborate with WMO Severe Weather Forecast Demonstration Project (SWFDP) and other FDPs and RDPs
to ensure that products address needs of operational forecasters and end users;
to provide an environment for the evaluation of prototype products.
GIFS will use global-regional-national cascade pioneered by the SWFDP. No single “GIFS centre”.
Use of web-enabled technology for generation and distribution of products.

30. Tropical cyclone products: Examples from Met Office
Uses Julian Heming’s code to identify and track tropical cyclones (TC), originally developed for the deterministic global Unified Model.
Tropical Cyclones (TCs) are identified where 850hPa relative vorticity (RV) maxima are greater than a threshold
For TC tracking, use search radius of 4 degrees for analysis and 5 degrees for forecast positions
Identified storms that do not match with a named storm or a TC identified at a previous time are counted as TC genesis
Met Office TC products courtesy
Pier Buchanan, Helen Titley and Julian Heming

31. MOGREPS-15 products for Ma-On, 12Z Friday 14th July 2011.
Left hand plot: 24 ensemble tracks
Middle plot: strike probability – probability that the storm will be within 75 miles within thhe next 15 days.
Right hand plot: MOGREPS ensemble mean (blue), control (cyan), previous observations (red) and deterministic track (green)

32. Products for named and potential storms on a basin by basin basis.

33. StormTracker Ma-On, 12Z Friday 14th July 2011.
Black – previous track
White – deterministic model forecast
Purple UKMO ensemble mean, mustard ECMWF ensemble mean and pink NCEP GEFS ensemble mean.

34. TOMAS: MOGREPS-15 12Z North Atlantic forecast on Wednesday 27th October 2010.

35. Yasi MOGREPS Forecasts

36. Comparison of deterministic forecast with MOGREPS-15 ensemble mean

37. Comparison of MOGREPS, ECMWF and multi-model ensemble.

38. Tropical cyclone products from MRI/JMA http://tparc. mri-jma. go

39. New tropical cyclone product: Strike probability time-series at a city
Tetsuo Nakazawa

40. Use of MOGREPS-15 products in SWFDDP

41. Warnings of heavy precipitation
Similar products also available for
hot & cold temperatures,
strong winds
Prototype product courtesy
Mio Matsueda

42. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 7-day forecast
TRMM daily
precipitation
exceeds the TRMM's 95th percentile.

43. Case 1: heavy precipitation by cyclone YASI
+ 6-day forecast

44. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 5-day forecast

45. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 4-day forecast

46. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 3-day forecast

47. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 2-day forecast

48. Case 1: heavy rainfall by cyclone YASI (Feb. 2011)
+ 1-day forecast

49. Other possible visualisation: combined warnings
Prototype product courtesy
Mio Matsueda

50. Website: http://icdm2012.csp.escience.cn
ICDM workshop
Workshop on Dynamics and Predictability of High-Impact Weather Events and Climate Extremes
Kunming, China 6-9 August 2012
Organised by IAMAS International Commission on Dynamical Meteorology
Sponsored by WWRP, WCRP, IUGG, IAMAS, Chinese IUGG committee, CAS, NSFC, LASG/IAP, Nanjing University & research projects.
Further information (first announcement) available from me.
Website:

51. TIGGE website: http://tigge.ecmwf.int
Summary
Since October 2006, the TIGGE archive has been accumulating regular ensemble forecasts from leading global NWP centres.
The archive is a tremendous resource for the research community at large, particularly on EPS – including use of multi-model & other approaches to reduce systematic errors.
As the basis of the development of the future Global Interactive Forecast System, products are being developed to enhance the prediction of high-impact weather, starting with tropical cyclones.
GIFS products will be developed & evaluated in conjunction with SWFDP and other regional projects.
TIGGE website: