Extended Abstract

3. MethodThe non-hydrostatic version of the Weather Research and Forecast model (AWR-WRF)includes variational data assimilation (VAR) capabilities (Barker et al. 2004) including afacility for the assimilation of radar reflectivity (Xiao et al. 2007) and radial Dopplervelocity observations (Xiao et al. 2005). The details of the variational assimilationscheme, which is not modified for this work, are not repeated here, but it is noted thatthe rain model used in the assimilation includes only warm rain processes, which maynot be suitable for NZ winter situations. For this reason only Doppler radial velocityobservations are assimilated in this example.WRF is run in - outer nest with 15 km grid spacingcovering NZ and the Tasman sea which feeds boundary conditions to a nest with 3 kmgrid spacing over the North Island of NZ (Figure 3). The nested domain is integratedwithout a cumulus parameterisation, as the grid spacing should be small enough toresolve some (but clearly not all) convective features. (The model is run on a modestcuster of some 15 dual-core Intel nodes which is maintained as a learning and researchresource by and for graduate students.)Both nests are run as ensembles with 10 members. The outer nest receives boundaryconditions from the NCEP global run with radiance and insitu observations assimilatedwith 3DVAR every 6 hours. Radar data from the 3 North Island MetService C-bandweather radars that were operational during the 2008 and 2009 winter UoAAPGdeployments is assimilated was the nested domain every 15 minutes, although due tocomputational limitations the model error statistics are not updated for everyassimilation cycle. The inner nest is also integrated without assimilation as a control.Figure 3: WRF domains. Left: 15km grid point spacing outer domain and Right: 3km gridpoint spacing nest.An example of the proposed observation experiment is given for a case on 2009/09/2718:00 to 2009/09/27 05:00 UTC, when a complex frontal system advanced over thecountry from the north west. The inner nest ensemble is run with Doppler radial velocity-239-