Downslope windstorm in Iceland – WRF/MM5 model comparison

Downslope windstorm in Iceland – WRF/MM5 model comparison

Abstract. A severe windstorm downstream of Mt. Öræfajökull
in Southeast Iceland is simulated on a grid of 1 km
horizontal resolution by using the PSU/NCAR MM5 model
and the Advanced Research WRF model. Both models are
run with a new, two equation planetary boundary layer (PBL)
scheme as well as the ETA/MYJ PBL schemes. The storm
is also simulated using six different micro-physics schemes
in combination with the MYJ PBL scheme in WRF, as well
as one “dry” run. Output from a 3 km MM5 domain simulation
is used to initialise and drive both the 1 km MM5
and WRF simulations. Both models capture gravity-wave
breaking over Mt. Öræfajökull, while the vertical structure
of the lee wave differs between the two models and the PBL
schemes. The WRF simulated downslope winds, using both
the MYJ and 2EQ PBL schemes, are in good agreement
with the strength of the observed downslope windstorm. The
MM5 simulated surface winds, with the new two equation
model, are in better agreement to observations than when using
the ETA scheme. Micro-physics processes are shown to
play an important role in the formation of downslope windstorms
and a correctly simulated moisture distribution is decisive
for a successful windstorm prediction. Of the microphysics
schemes tested, only the Thompson scheme captures
the downslope windstorm.