Federal Department of Home Affairs FDHA
Federal Office of Meteorology and Climatology MeteoSwiss
Cost Efficient Use of
COSMO-LEPS Reforecasts
Felix Fundel, André Walser, Mark Liniger, Christof Appenzeller
Motivation: Reforecast Bias 1971-2000
Temperature
Precipitation
too warm
too wet
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
2
Motivation: Reforecast Bias 1971-2000
Temperature
Precipitation
 Calibrating with reforecast reduces the forecast bias
 Ensemble forecasts become more reliable
 Shown for up to date global and regional EPS
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
3
Outline
• Status of the reforecast project
• New results
• Outlook and open issues
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
4
Status of the reforecast project
• Research-Program has finished
• Work in progress to make calibrated COSMO-LEPS
products operational
• Large reforecast data set available, also used in other
research projects
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
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Reforecasts Data Set
Intention:
statistically independent data for a long period




reforecasts over a period of 30 years (1971-2000)
deterministic run of COSMO-LEPS v 4.0
convection and physics randomly switched
ERA40 reanalysis as initial/boundary fields
90h lead time, every 3rd day
1971-01-01
1971-01-03
1971-01-06
LT: 90h
Available at ECMWF mars archive:
retrieve,
time=12:00:00,
date=2008-03-10,
stream=enfh,
step=0/to/90,
levtype=sfc,
expver=2,
type=cf,
hdate=1971-03-10/to/2000-03-10
class=co,
param=61.2
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
2000-12-27
2000-12-29
2000-12-31
6
Gridded observations for Switzerland
•
•
•
•
24h total precipitation
domain Switzerland
COSMO-LEPS grid (SYMAP, C. Frei)
>450 Observations
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
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Calibration
Calibration method: - CDF mapping
- each grid point calibrated individually
- using gridded observations over Switzerland
winter precip.
Brier Skill Score
summer precip.
day 1
day 2
day 3
day 4
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
day 1
day 2
day 3
day 4
10
However…
 Reforecasts are very expensive
 cost for 2 years reforecasts = cost for 1 forecast member
(current setup, reforecast every 3rd day, 90h lead-time)
 What‘s the skill of COSMO-LEPS with 16-n forecast
members, calibrated with 2xn years of reforecasts?
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
11
Isocost analysis - Precipitation
RPS
Winter-Spring 2007/2008
RPS
Summer-Fall 2008
Best setup:
16 member & 30y reforecasts
Best isocost setup:
11 member & 10y reforecasts
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
12
Isocost analysis - Precipitation
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
13
Isocost analysis - Precipitation
2AFC score
Winter-Spring 2007/2008
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
2AFC sore
Summer-Fall 2008
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Isocost analysis – T2m
Based on 61 SwissMetNet stations
Calibration using reforecasts and observations from 1981-2000
No seasonal or lead-time stratification
RPS Winter-Spring Dec 07-Nov 08
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
2AFC Winter-Spring Dec 07-Nov 08
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Isocost analysis – T2m
spread   sd ( f d (m))
m  1,..,16, d  1,..365
d
error
 sd ( f d (m)  od ) d  1,..365
16 member DMO
DMO calibr. w. 1y refcst
DMO calibr. w. 20y refcst
Calibrating T2m improves ensemble spread
Most efficient around noon
Still not enough to correct under-dispersiveness
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
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Conclusion
 Forecast skill can be improved without additional (CPU) cost
improved reliability, small loss in resolution
 Optimal COSMO-LEPS setup ~12 members / 8 reforecast
years (for 24h precipitation and temperature)
But:
 Some applications require large ensembles
 Corresponding observations needed for calibrated values
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
18
Outlook & open issues
• Adapt reforecast suite for 7 km COSMO-LEPS suite, but...
...how should we initialize the soil of these reforecasts?
• How perform alternative (and cheaper) post-processing
methods vs. our calibrated reforecasts?  CONSENS
• Should the reforecast project become part of the operational
COSMO-LEPS suite of COSMO?
Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
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Ensemble Calibration | COSMO-GM 2009, Offenbach
André Walser
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