Proceedings - C-SRNWP Project

is partly due to the double penalty problem inherent to the ETS still needs to be evaluated.
The mean FBI curve presented in fig. 1b indicates that LM-K forecasts starting from an
assimilation without LHN (blue dashed lines) generally underestimate the precipitation. On
the contrary, forecasts starting from an assimilation with LHN (green solid lines) begin with
almost ideal values (1.0) of FBI, rapidly drop off towards the graph of the control experiment
after 3-4 hours, and afterwards still stay slightly below this one for several hours. It seems that
there exists a general model deficiency in generating precipitation, and LHN, however, is only
able to help this problem for a short time period directly after the assimilation. More
precisely, LHN does not cure the problem but it just rearranges the occurrence of precipitation
during the first several hours. A higher FBI during the first four forecast hours is compensated
by lower FBI-values for several hours thereafter.
4. A CASE STUDY WITH A NESTED ENSEMBLE SYSTEM
a) The role of the convective environment
In order to quantify the role of the convective environment, a nested ensemble simulation of a
severe convective event in southern Germany has been performed at MeteoSwiss. The setup
of the experiment is depicted in Fig. 2. Each of the 10 members of a 7km COSMO-LEPS
(Marsigli et al., 2005) simulation, driven by an ECMWF EPS run, provides the initial and
boundary conditions for a 2.2km COSMO model ensemble and thus a different convective
environment in which the storm develops. Each fine-scale ensemble member starts from the
interpolated corresponding 7km member at 06UTC. In addition, a deterministic run
interpolated from an operational MeteoSwiss 7km analysis at 06UTC is performed.
Conventional and radar data are assimilated during the first 10 hours in all 2.2km simulations,
followed by a free forecast of 8 hours.
Figure 3a) shows the mean precipitation over the QPF validation area shown in Fig. 2
(black) as simulated by the fine-scale 2.2km ensemble (member 1-10, color lines) and
observed by the radar (bold black line). A large variation of the simulated precipitation
amount is evident among the different ensemble members. This variation results only from
the different convective environment each COSMO-LEPS member provides for its nested
fine-scale member. The timing of the maximum convective activity is captured by all
members due to the radar assimilation which ends at t=0 at the beginning of the convective
event. But the amount of precipitation among the members differs by a factor of up to 3. The
best member supports convection and reaches 78% of the observed peak, while the worst
member does not support the storm and reaches only 21%.
-28h
-10h
0
+8h
12UTC
06UTC
16UTC
ECMWF EPS
COSMO LEPS (7km)
HIRES LEPS (2.2km) ANA FC
00UTC
QPF
Validation Area
Figure 2. Setup of the nested ensemble experiment. An ECMWF EPS run (green) drives the COSMO-LEPS (7km) (red)
which drives the 2.2km (HIRES) ensemble (blue). The yellow flashes mark the time of the convection. The domains of the
COSMO-LEPS and the 2.2km ensemble are depicted in the respective colors and the QPF validation area is depicted in
black.
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