Clouds CPT/GCSS WG4 RCE Intercomparison Specification
Goals:
(I) 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).
(II)Examine the sensitivity of the clouds, radiation, water vapor and
circulation, to a uniform 2K increase in the specified SST, as a
primitive but straightforward intercomparison of climate sensitivity.
Case author:
Chris Bretherton (breth@atmos.washington.edu)
7 October 2004
with Peter Blossey (bloss@atmos.washington.edu) contributing
considerable assistance and testing.
Case coordinator (to whom you should send output):
Brian Mapes (Brian.Mapes@noaa.gov) with a cc to Peter Blossey (email above)
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 31 Mar 2004.
Other GCSS WG4 participants are also welcome to participate; we ask
for results by 31 Mar 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 our U of Washington linux
cluster, it takes us a day on two dual processor nodes to do a
150-day 2D simulation following the specs below and about a week on
eight dual processor nodes to do a 150-day 'bowling alley' simulation.
References:
This is similar to a simulation published by
Grabowski, W. W., J.-I. Yano, and M. W. Moncrieff, 2000:
Cloud-resolving modeling of tropical circulations driven by
large-scale SST gradients. J. Atmos. Sci., 57, 2022-2039.
Peter Blossey and I are currently drafting a paper about our CRM
simulations of related Walker circulation cases, including with
underlying slab ocean, for submission to J. Climate. When our paper
is submitted, a hyperlink to it will be noted here.
Physical specs
1) Domain 0 < x < L, L = 4096 km, with periodic lateral BCs in x (and
y if you use a 'bowling alley' configuration.)
2) Specified SST = 299.15 - 2 cos(2*pi*x/L) [K] with a maximum at x =
L/2. This 'control' run will have a run name of '299'.
3) We would also like to have results from an identical run with
SST everywhere increased by 2K, if you have the computational
resources to do it. This 'SST+2' run will have a run name of '301'
4) No ambient rotation
5) Radiation
Cloud-interactive shortwave and longwave radiation
Insolation: Solar constant of 685 W/m2, constant zenith angle of
51.7 deg, CO2 = 355 ppm, no aerosol.
6) 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.
7) 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.
8) Run length
150 d. The first 50 days are for approach to equilibrium. All
average quantities are to be computed over the equilibrium period
50-150 days.
9) 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 256 km, or a doubly periodic 3D simulation with 16x16
columns and dx = dy = 256 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.
10) 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.
11) 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 = 299 or 301 depending on domain-mean SST)
x-time sections, daily averages for days 1-150, with arrays
indexed (t, x), of:
PS (Surface pressure, Pa, to check for mass conservation)
PCP (Precipitation, mm/day)
PW (Precipitable water, kg/m2)
FLNT (TOA net upward longwave radiative flux, a.k.a. OLR, W/m2)
SAV (dry static energy Cp*T+g*z, mass-weighted from 100 hPa to
surface, J/kg, to check for tropospheric temperature drift)
(ii) runname-xz.nc:
51-150 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)
51-150 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.)