Proceedings - C-SRNWP Project

Overview
COSMO ensemble systems
Pierre Eckert, MeteoSwiss Geneva
Cosmo develops different kinds of ensembles systems. One of them is the COSMO LEPS
developed at ARPA SIM (Bologna), representing mainly a downscaling of the ECMWF EPS.
Multimodel ensembles have also been investigated at ARPA Piemonte and also have been
used at the 2006 Olympic games in Torino. A project SREPS for a short range ensemble,
including perturbations in the initial state and in the physics is also under way and is reported
on by C. Marsigli in the same conference. Plans to run ensembles at very small scale (2-3 km)
like the DWD EELMK also exist but will not be presented here.
The COSMO LEPS
The system is designed so as to provide a 10 km resolution ensemble forecast targeted on
days 3 to 5. Two consecutive ECMWF (00z and 12z) EPS forecasts provide an ensemble of
102 members. These members are clustered on the days 4 and 5 and on the variables
geopotential, u component of the wind, v component of the wind and specific humidity at
levels 500, 700 and 850 hPa. In the present configuration, 16 representative members (RMs)
are chosen from the clustering, and are used as initial and boundary conditions for 16 LM runs
covering central Europe (cf. next figure) with a resolution of 10 km and 40 model levels. No
proper analysis is done. For the integration two convection schemes (Tiedke or Kain-Fritsch)
are randomly chosen
COSMO-
LEPS
clustering
area
COSMO-
LEPS
Integration
Domain
Various probabilities, meteograms and guidance for warnings are generated. Probabilities are
normally computed by weighting the values given by one member be the size of the cluster it
represents. Some studies show that especially in extreme cases non weighting (each member
counts for 1/16th) gives better results, respectively higher probabilities for the given event.
But the results are not conclusive. An example of meteogram is shown on the next page.
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