Clouds CPT/GCSS WG4 RCE Intercomparison Specification
Goal: Use an idealized Walker circulation over a sinusoidal SST maximum as
an intercomparison of tropical clouds and climate sensitivity in
different CRMs (we encourage both 2D and 3D 'bowling alley'
simulations (i.e. a long, narrow 3D domains with forcing varying only
in x) and 'dynamically coupled' SCMs (i.e. a 2D or 3D regional model
built with the same column physics and transport algorithms as your
full GCM, but with periodic lateral BCs and an adjustable horizontal
grid spacing)
Case author:
Chris Bretherton (breth@atmos.washington.edu)
20 Aug 2004
with guinea-pigging assistance from Peter Blossey (bloss@atmos.washington.edu)
Case coordinator (to whom you should send output):
Brian Mapes (Brian.Mapes@noaa.gov)
Brian will set up a web site linked to this one for case results.
Contribution deadline
We would like CPT CRM/SCM groups to contribute simulations by 15 Oct 2004 so
that we quickly learn about possible problems in the case specs.
Other GCSS WG4 participants are also welcome to participate; we ask
for results by 31 Dec 2004, though there may be some iteration or
deadline-stretching if that seems necessary. Please email to Brian
Mapes that you intend to participate so we know who will be
involved.
How big a computation is this?
Using the Khairoutdinov-Randall SAM CRM on eight dual-processor nodes
of one of our U of Washington linux clusters, it takes us 2 days to do a
300-day 2D simulation following the specs below, and O(2 weeks) to do
a 'bowling alley' simulation.
References:
Peter Blossey and I are currently drafting a paper about our
CRM simulations of this case for submission to J. Climate in autumn
2004. When our paper is submitted, a hyperlink to it will be noted here.
Physical specs
1) Domain 0 < x < L, L = 1024 km, with periodic lateral BCs in x (and
y if you use a 'bowling alley' configuration.)
2) Specified SST = 298 - 2 cos(2*pi*x/L) with a maximum at x = L/2
3) No ambient rotation
4) Radiation
Cloud-interactive shortwave and longwave radiation
Insolation: Solar constant of 685 W/m2, constant zenith angle of 51.7 deg
Two radiative forcing cases: (1x) CO2 = 350 ppm, (2x) CO2 = 700 ppm
In the output files these should be labeled with runname = 1x or
2x respectively.
5) Initial sounding
Surface pressure of 1005.5 hPa.
Horizontally uniform initial atmospheric sounding
http://www.atmos.washington.edu/~breth/CPT-public/Walker-RCE-init-snd.txt
We used 0.1K white noise added to temperature in lowest five layers
to initiate convection, but you probably don't need any
perturbation at all if you don't want it.
Your model's default ozone/trace gases/aerosol profiles.
6) Surface exchange
Use your model's default Monin-Obukhov-like scheme for computing
surface fluxes. Use a over-ocean specification of surface
roughness, z0 = 10^-4 m if your CRM doesn't have its own
Charnock-like scheme.
7) Run length
200 d. The first 50 days are for approach to equilibrium. All
average quantities are to be computed over the equilibrium period
50-200 days.
8) Horizontal resolution
CRMs: 2 km in x. If you do a bowling alley, use 32
gridpoints and periodic BCs in y with dy = 2 km, and for output
files average all outputs in y to retain only the x-z-t structure.
SCMs: Try a 2D simulation with 16 columns with a nominal x grid
spacing of 64 km, or a doubly periodic 3D simulation with 16x16
columns and dx = dy = 64 km. If something else seems better, try
it and tell us what you did and why. Forcing and other specs are
the same as for CRMs.
9) Vertical domain size/resolution
CRMs:
Lz approximately 27.5 km, including a sponge layer of 8.75 km thickness
You can use a vertical resolution of your choosing, but we suggest
64 or more vertical levels, with dz = O(100 m) near the surface,
asymptoting to uniform 400 m dz in the upper
troposphere and a 1 km dz in the sponge. A recommended stretching algorithm
is to calculate layer midpoints z(n) as follows (matlab format):
nz = 64
dz0 = 75
dztrop = 400
dzsponge = 1000
ztropbase = 2000
zspongebase = 20000
zspongetrans = 1500
dz(1) = dz0;
z(1) = dz0/2;
for n = 2:64
dz(n) = dz0 + (dztrop-dz0)*tanh(z(n-1)/ztropbase)...
+(dzsponge-dztrop)*0.5*(1+tanh((z(n-1)-zspongebase)/zspongetrans));
z(n) = z(n-1) + dz(n);
end
Here dz0 = 75 m is the grid spacing at the surface, which transitions to
the tropospheric grid spacing dztrop = 400 m as we move through the
level ztropbase = 2 km. As we move through the layer zspongebase =
20 km, the grid spacing again smoothly transitions to dzsponge = 1 km.
Newtonial damping timescale tau in the sponge (roughly upper 1/3 of domain):
tau = 120sec* 60^((ztop-z)/(0.3*ztop)) for 0.7*ztop < z < ztop
where ztop = z(nz).
SCMs:
Use operational resolution Optionally do sensitivity studies with
different choices of vertical levels.
9) Desired output (2 netcdf files, with time units of days). Note
that if some outputs are impossible or too painful to produce, we
would be happy to see what you can conveniently provide. Just
put missing values in the fields you can't provide.
(i) runname-xt.nc: (runname = 1x or 2x)
x-time sections, daily averages for days 1-300, with arrays
indexed (t, x), of:
PCP (Precipitation, mm/day)
PW (Precipitable water, kg/m2)
SAV (dry static energy Cp*T+g*z, mass-weighted from 100 hPa to
surface, J/kg)
(ii) runname-xz.nc:
50-200 day mean x profiles of
SST (K...just as a check and for plotting convenience)
E (Evaporation, mm/day)
FSNT (TOA net downward shortwave radiative flux, W/m2)
FLNT (TOA net upward longwave radiative flux, W/m2)
FSNS (surface net downward shortwave radiative flux, W/m2)
FLNS (surface net upward longwave radiative flux, W/m2)
FSNTC(TOA net downward clear-sky shortwave radiative flux, W/m2)
FLNTC(TOA net upward clear-sky longwave radiative flux, W/m2)
FSNSC(surface net downward clear-sky shortwave radiative flux, W/m2)
FLNSC(surface net upward clear-sky longwave radiative flux, W/m2)
PW (Precipitable water, kg/m2)
LHF (Latent heat flux, W/m2)
SHF (Sensible heat flux, W/m2)
50-200 day mean x-vertical model level sections indexed (x,level) of
Z (level height, m)
P (pressure, Pa)
RHO (density, kg/m3)
T (temperature, K)
Q (water vapor mixing ratio, kg/kg)
RH (relative humidity with respect to water saturation, 0-1)
U (x-velocity, m/s)
W (vertical velocity, m/s)
QR (net radiative heating rate, K/s)
QC (cloud water, kg/kg)
QI (cloud ice, kg/kg)
QR (rain mixing ratio, kg/kg)
QS (snow+graupel+hail mixing ratio, kg/kg)
CF (cloud fraction, defined for CRMs as the fraction of
gridpoint columns where radiatively active condensate
exceeds 0.005 g/kg, and for SCMs however it is done in your model.)