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.)