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

2 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Motivation: Reforecast Bias too warm too wet TemperaturePrecipitation

3 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Motivation: Reforecast Bias TemperaturePrecipitation  Calibrating with reforecast reduces the forecast bias  Ensemble forecasts become more reliable  Shown for up to date global and regional EPS

4 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Outline Status of the reforecast project New results Outlook and open issues

5 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser 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

6 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Reforecasts Data Set  reforecasts over a period of 30 years ( )  deterministic run of COSMO-LEPS v 4.0 convection and physics randomly switched  ERA40 reanalysis as initial/boundary fields  90h lead time, every 3 rd day Intention: statistically independent data for a long period LT: 90h retrieve, time=12:00:00, date= , stream=enfh, step=0/to/90, levtype=sfc, expver=2, type=cf, hdate= /to/ class=co, param=61.2 Available at ECMWF mars archive:

7 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Reforecast Usefulness TemperaturePrecipitation

8 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Gridded observations for Switzerland 24h total precipitation domain Switzerland COSMO-LEPS grid (SYMAP, C. Frei) >450 Observations

9 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser 24h precipitation error characteristic 80 th quantile95 th quantile Winter Summer relative error [%] too wet too dry

10 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Calibration Calibration method:- CDF mapping - each grid point calibrated individually - using gridded observations over Switzerland day 1 day 2 day 3 day 4 Brier Skill Score winter precip. summer precip.

11 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser However…  Reforecasts are very expensive  cost for 2 years reforecasts = cost for 1 forecast member (current setup, reforecast every 3 rd day, 90h lead-time)  What‘s the skill of COSMO-LEPS with 16-n forecast members, calibrated with 2 x n years of reforecasts?

12 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser 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

13 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Isocost analysis - Precipitation

14 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Isocost analysis - Precipitation 2AFC score Winter-Spring 2007/2008 2AFC sore Summer-Fall 2008

15 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Isocost analysis – T2m Based on 61 SwissMetNet stations Calibration using reforecasts and observations from No seasonal or lead-time stratification RPS Winter-Spring Dec 07-Nov 082AFC Winter-Spring Dec 07-Nov 08

16 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Isocost analysis – T2m 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

17 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser Using „out of date“ reforecasts (precip) forecasts: v 4.0 reforecasts: v 4.0 forecasts: v 4.0 reforecasts: v 3.20

18 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser 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

19 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser 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?

20 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser

21 Ensemble Calibration | COSMO-GM 2009, Offenbach André Walser PDF CDF x „return period“ OBSMODFCST x „calibrated return periods“ „raw return periods“ x „return period“ Calibration strategy Quantiles w.r.t. observations are not reliable Quantiles w.r.t model climatology are reliable