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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)

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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

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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

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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 andCO2 data

Run 60 km CCAM for Indonesia

Process output for climatology

Host modelInterpolated to

C48 CCAM grid

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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

CO2 dataAerosols(aero_m20.sh)

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CO2 input

The CO2 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 CO2 files since (unlike aerosols), the CO2 levels are independent of position

However, the CO2 files only work for CCAM with 18 vertical levels. For different vertical resolution, the CO2 data files need to regenerated

Run 60 km CCAM for Indonesia

CO2 dataAerosols(aero_m20.sh)

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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)

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Sigma levels

kbotdav=4 (typically NWP)

kbotdav=10 (typically climate)

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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)

Restartfile

Restartfile

NCEP 2.5 degree reanalysis host model

(nudging)

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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

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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

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CCAM output

Below is an example cc2hist namelist

&input

kta=720 ktb=999999 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 definePressure levels(instead of sigmalevels)

Output domain

Every 12 hours

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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)

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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.

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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

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DJF

JJA

Obs Tmax CCAM-m

30-year run, downscaled from Mk3 CGCM

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DJF

JJA

Obs Tmin CCAM-m

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Obs Rainfall (mm/d) CCAM-m

DJF

JJA

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Obs – 30 yr climatologyNew CCAM – 30 yr climatology10 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

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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)

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Average maximum temperature (deg C)

OBS - Jan CCAM - Jan OBS - Apr CCAM - Apr

OBS - Jul CCAM - Jul OBS - Oct CCAM - Oct

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Average minimum temperature (deg C)

OBS - Jan CCAM - Jan OBS - Apr CCAM - Apr

OBS - Jul CCAM - Jul OBS - Oct CCAM - Oct

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Average rainfall (mm/day)

OBS - Jan CCAM - Jan OBS - Apr CCAM - Apr

OBS - Jul CCAM - Jul OBS - Oct CCAM - Oct

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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

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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

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8 km resolution

No hard boundaries at panel edges:Far-field nudging from 60 km model run on this panel

Rest of world8 km resolution throughoutthis panel About 50 km resolution

at this edge

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Far field nudging

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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 atmosphereis nudged everywhere

Small scale structure is not perturbed

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Spectral nudging

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Thank You

Marine and Atmospheric Research

Name Dr Marcus Thatcher

Title Research Scientist

Email Marcus.Thatcher@csiro.au

Contact CSIRO

Phone 1300 363 400

+61 3 9545 2176

Email enquiries@csiro.au

Web www.csiro.au