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Orography filtering

Added by Stefan Lange over 7 years ago

Dear All,

These days I am about to set up a CCLM simulation over some mountaineous domain at 0.1° horizontal resolution. Doing so I started to wonder about the orography filtering. Reading COSMO Newsletter 1 (2001) by Almut Grassmann I understood that the latter is useful if not even mandatory. Yet I am still puzzled about the meaning of the different namelist parameters and the effects different values would have on the simulated climate.

I checked http://cosmo-model.org/content/tasks/operational/ and tried the INT2LM namelist setups DWD uses for COSMO-EU with

lfilter_oro             T
lxso_first F
l_topo_z F
norder_filter 5
eps_filter 0.1000
ilow_pass_oro 1
numfilt_oro 1
ilow_pass_xso 0
numfilt_xso 1
rxso_mask 0.0000
rfill_valley 0.0000
ifill_valley 0

and the setup Meteo Swiss uses for COSMO-7 and COSMO-2 with

lfilter_oro             T
lxso_first F
l_topo_z F
norder_filter 5
eps_filter 0.1000
ilow_pass_oro 4
numfilt_oro 1
ilow_pass_xso 5
numfilt_xso 1
rxso_mask 750.0000
rfill_valley 0.0000
ifill_valley 7

The orography produced with the latter setup is much smoother than that produced with the former.

I also saw that the COSMO-DE setup equals that of COSMO-7 and COSMO-2 except rxso_mask = 625.0000 and ifill_valley = 0. As my resolution is slightly coarser than those of COSMO-EU and COSMO-7 I thought I should choose between the two setups listed above but since the respective resulting orographies differ so much I don't know what to do. Any suggestions?

I'd like to know why there are all those different filtering setups in use and what kind of differences I can expect between the simulated weathers/climates with differently smoothed orographies. Also I wondered if one should filter differently for climate simulations than for weather forecasts...

With many thanks in advance,
Stefan