CCAM Regional climate modelling Dr Marcus Thatcher Research Scientist December 2007

CMAR RCM Overview This presentation describes CCAM climate downscaling Specifically, we downscale from NCEP 2.5 deg reanalyses to 60 km resolution climate simulation The output can then be processed using NCO toolkit for netCDF files to create climate averages, etc. These results can then be compared to downscaled climate projections (e.g., CSIRO Mk3)

CMAR RCM Regional Climate Modelling with CCAM RCM is achieved with a multiple nesting procedure, by first downscaling to 60 km resolution and then downscaling to 15 km resolution Mk3 CCAM 60km CCAM 15km Comparison of average rainfall (mm/day) between Mk3, CCAM at 60 km resolution and CCAM at 15 km resolution for northern Queensland

CMAR RCM Regional Climate Modelling with CCAM The climate downscaling script can be found at  $HOME/ccam/scripts/clim/pd60_run This script will generate a ten year downscaled climate run for Indonesia (this takes a long time to run!) Today we will simply start the script running and examine the first month of output

CMAR RCM Host model – NCEP reanalyses Instead of preparing initial conditions, we must first prepare the host model from which to downscale. Usually we downscale from NCEP 2.5 degree reanalyses Alternatively we downscale from CSIRO Mk3 climate projections However, any model once prepared on the CCAM grid can in theory be used (e.g., ERA-40) Aerosols and CO 2 data Run 60 km CCAM for Indonesia Process output for climatology Host model Interpolated to C48 CCAM grid

CMAR RCM Aerosol input Next the aerosols for a particular year must be prepared for CCAM This is achieved using the aero_m20.sh script. The aerosol data files are located at  $HOME/ccam/stdin/sulfateA2.???? Note that the aerosol data files are valid from 1961 to 2100 Run 60 km CCAM for Indonesia CO 2 data Aerosols (aero_m20.sh)

CMAR RCM CO 2 input The CO 2 levels must also be specified for a CCAM climate run. These files also are valid from 1961 to 2100 and are located at  $HOME/ccam/stdin/co2_data.???? Note that there is no preprocessing of CO 2 files since (unlike aerosols), the CO 2 levels are independent of position However, the CO 2 files only work for CCAM with 18 vertical levels. For different vertical resolution, the CO 2 data files need to regenerated Run 60 km CCAM for Indonesia CO 2 data Aerosols (aero_m20.sh)

CMAR RCM CCAM namelist options for climate downscaling The CCAM namelist for climate simulations is also slightly different to the NWP example. CCAM switches used for climate simulations are listed as follows:  nbd = 0 (disable far field nudging)  mbd = 20 (spectral nudging)  kbotdav = 10 (nudge upper levels only)

CMAR RCM Sigma levels kbotdav=4 (typically NWP) kbotdav=10 (typically climate)

CMAR RCM CCAM climate simulation When performing a climate simulation, we run only one month at a time. Once the month simulation is complete, it creates a restart file which is used to initialise CCAM for the next month CCAM simulation (Jan) CCAM simulation (Feb) CCAM simulation (Mar) Restart file Restart file NCEP 2.5 degree reanalysis host model (nudging)

CMAR RCM CCAM restart files If CCAM stops during a long climate run (e.g, electricity black-out), then the climate simulation can be restarted from the last restart file Also, the restart files are usually archived so that CCAM can be restarted from any month during the climate simulation

CMAR RCM CCAM climate simulation CCAM restart files are called  $HOME/ccam/scripts/clim/RESTART/Restccam_NCEP_60.?????? Typically we tar the files from the same year The daily CCAM output can be found at  $HOME/ccam/scripts/clim/daily The monthly averaged CCAM output is  $HOME/ccam/scripts/clim/monthy_mean The raw CCAM output (from which the above files can be regenerated) is  $HOME/ccam/scripts/clim/OUTPUT

CMAR RCM CCAM output Below is an example cc2hist namelist &input kta=720 ktb= ktc=720 minlat = -20., maxlat = -10., minlon = 90., maxlon = 120. use_plevs = T plevs = 1000, 850, 700, 500, 300 &end &histnl htype=“ave" hnames= "all" hfreq = 0 &end Generate average Use and define Pressure levels (instead of sigma levels) Output domain Every 12 hours

CMAR RCM NCO toolkit for NetCDF files CCAM output can also be processed using the NCO toolkit for manipulating NetCDF files Specifically, the ncea program for averaging NetCDF files is a quick way to calculate the average climate (i.e., for study)

CMAR RCM Other types of climate run There are several different versions of CCAM climate runs, depending on the application NCEP 2.5 deg reanalyses:  Used to test CCAM downscaling with ‘observed’ present day or historical weather behaviour. CSIRO Mk3 control run:  Nesting CCAM in a simulated present day climate to test the performance of CCAM when there are biases in the host model CSIRO Mk3 climate projection:  Nesting CCAM in simulated future climate to predict regional changes arising from increasing global temperatures.

CMAR RCM Some RCM examples The following slides show some examples of RCM with CCAM In these examples we show Australia However, during the tutorial we can start to create examples for Indonesia

DJF JJA Obs Tmax CCAM-m 30-year run, downscaled from Mk3 CGCM

DJF JJA Obs Tmin CCAM-m

Obs Rainfall (mm/d) CCAM-m DJF JJA

CMAR RCM Obs – 30 yr climatology New CCAM – 30 yr climatology 10 ms -1 threshold Tropical cyclone days/year – observed vs model Latest 60 km simulations produce good climatology of “cyclone-like” vortices. Above shows example of C-CAM downscaled from Mk3 CGCM

CMAR RCM Multiple nesting for climate simulations Like the CCAM NWP forecasts, it is possible to downscale the 60 km climate run to 15 km resolution by multiple nesting of the forecast However, given the small time step for the 15 km simulation, this takes a very long time to simulate (multiprocessor CCAM is a necessity)

CMAR RCM Average maximum temperature (deg C) OBS - Jan CCAM - JanOBS - AprCCAM - Apr OBS - Jul CCAM - Jul OBS - Oct CCAM - Oct

CMAR RCM Average minimum temperature (deg C) OBS - Jan CCAM - JanOBS - AprCCAM - Apr OBS - Jul CCAM - JulOBS - OctCCAM - Oct

CMAR RCM Average rainfall (mm/day) OBS - Jan CCAM - Jan OBS - Apr CCAM - Apr OBS - Jul CCAM - Jul OBS - Oct CCAM - Oct

CMAR RCM CCAM technical notes Missing data in the highly stretched region is replaced with weather information from the previous, lower resolution forecast CCAM uses a multiple stretched grid technique to ‘step down’ or downscale the forecast 8km 1km 60km

Full nudging of far-field winds in the free atmosphere, with e-folding time ~24 h Partial nudging No nudging (only SSTs) Far-field nudging Model uses NCEP or GCM winds only outside the inner red boundary Model uses NCEP or GCM winds (and optionally other fields) at all points Global nudging Full nudging of fields in the free atmosphere, with e-folding time ~48 h For long, variable-resolution climate runs (only), some nudging is desirable to assist the climatology in the coarsest part of the globe

CMAR RCM 8 km resolution No hard boundaries at panel edges: Far-field nudging from 60 km model run on this panel Rest of world 8 km resolution throughout this panel About 50 km resolution at this edge

CMAR RCM Far field nudging

CMAR RCM Advanced CCAM configuration CCAM can now also use spectral downscaling, although it is slower than far field nudging Spectral downscaling modifies the large scale structure of the atmosphere to be the same as the host model everywhere 8km 60km The large scale structure of the atmosphere is nudged everywhere Small scale structure is not perturbed

CMAR RCM Spectral nudging

Thank You Marine and Atmospheric Research NameDr Marcus Thatcher TitleResearch Scientist Contact CSIRO Phone Webwww.csiro.au