Dear GCSS Intercomparison Participants, Attached is a description of a second GCSS-sponsored intercomparison (GCSS2) which we invite you to participate in. Based on discussions at the Aug 94 workshop, this is a smoke cloud case, in which a radiatively active smoke layer cools radiatively at a prescribed rate below an inversion. The layer thickness, smoke-top radiative flux divergence, and inversion strength are patterned on our previous case for ease of comparison. The UKMO and Univ. of Washington groups have tested this case already, with quite intriguing preliminary results. We anticipate writing a paper comparing 2D LES, 3D LES and experimental results for this case in which your contributions will be published. The following discussion is divided into four sections, Background, Run Setup, Data Format for Results, and Model Description. Enjoy! Please email results to Malcolm MacVean (mkmacvean@email.meto.govt.uk), who will synthesize them as Chin-Hoh and Steve did for the first GCSS intercomparison. If there is interest, as results become available, Chris can take graphs produced by Malcolm and make them available through Mosaic. Let Chris (breth@amath.washington.edu) know if you have access to Mosaic and are interested in this. If there is enough interest, we can have a discussion of this case at the Charlotte AMS Boundary Layer Conference. We plan to have a discussion of this case, along with ASTEX Lagrangians cases which you will hear about within a month or so, at a planned GCSS workshop in the Netherlands at the end of August...more details on this later. Chris Bretherton and Malcolm MacVean ******************************************************************************* Background: The A Dilemma, Stratocumulus Entrainment, and LES Modelling A proposed smoke-cloud simulation could help shed light on an important and currently paradoxical problem with entrainment closures for marine stratocumulus clouds. This problem was first raised by the work of Nicholls and Turton (1986,QJRMS,112,461-480), or NT, who compared entrainment calculated from aircraft measurements at the tops of stratocumulus over the North Sea with a laboratory experiment of McEwan & Paltridge (1976,JGR,81,1095-1102), or MP, which was very analogous to our proposed smoke plume simulation (more on this later). Both results were placed in the nondimensional form we/wstar = A/Ri where we is entrainment rate, wstar is a convective velocity scale such that wstar^3 = 2.5*(vertical integral of buoyancy flux over the convective layer), the bulk Richardson number Ri = zinv*{g*dthetav_inv/theta0}/wstar^2, where g is gravity, zinv is the inversion height, theta0 is a reference potl. temp., and (dthetav)inv is the virtual potential temp. jump across the inversion. For a dry layer heated from below, A = 0.2 is consistent with both lab experiments and atmospheric observations. For stratocu, however, typical values of we (0.005 m/s), wstar (0.6 m/s), dthetav_inv (7 K) and zinv (500 m) give Ri = 300 and A = 2.5, more than ten times as large. The experiment of McEwan and Paltridge studied entrainment into a purely radiatively driven mixed layer with no condensation. The strategy was to turn the geophysical system upside down, starting with two homogeneous layers of different densities and stimulating convection in the upper layer by radiatively heating its base. This was accomplished by putting a strongly light-absorbing dye in the upper layer, and beaming light up through the lower layer so that it would be absorbed in a thin skin at the bottom of the dyed upper layer. The entrainment of the upper layer into the lower layer could then be studied. This may be recognized as the smoke-cloud upside-down. MP drew some curious conclusions from their experiments due to the rather limited parameter regime that they studied. For instance, they found no clear dependence of entrainment rate on the amount of stratification as measured by Ri. However, NT took MP's typical values of we, wstar, etc. and deduced a typical A = 0.05 from their experiments...a factor of 50 smaller than the stratocumulus values!. They attributed the difference to evaporative enhancement of entrainment, which therefore should be a dominant influence on entrainment rate even for CTEI-stable cloud layers. One caveat in comparing MP's results with the atmosphere is that they were conducted in water, which has a different Prandtl number than air, and also that while the large-scale convective motions were turbulent, the entrainment processes may not have been fully turbulent. Nevertheless the difference from aircraft observations was striking. Bob Breidenthal, an experimental fluid dynamicist at the University of Washington, became intrigued by this problem after work he did with his student Shanqyang Shy on entrainment at a stably stratified interface between two fluids exhibiting buoyancy reversal (an analogue to evaporative cooling) upon mixing showed that if turbulence was mechanically stimulated in the lower layer, the entrainment rate was only weakly sensitive to the amount of buoyancy reversal, suggesting that the A's in stratocumulus should be almost identical (with 20%) to within smoke layers. Since MP's experiments left something to be desired in covering a large range of Ri, another student working with Breidenthal, Ben Sayler, repeated MP's experiments with a slightly more flexible arrangement for varying Ri. Their results (recently submitted to JFM) are consistent with Ri^-1 scaling for we/wstar, and generally corroborate MP's result in the overlap region, with A = 0.08. The paradox remains...why are clouds different? We call this the A dilemma. We are in a unique position to attack the A dilemma numerically. Can we reproduce MP and Sayler & Breidenthal's A = 0.05-0.08 for smoke-cloud LES simulations? Since the previous workshop showed that LES entrainment rates (0.7-1.5 cm/s) were at least in the ballpark for stratocu, we may be able to numerically capture and resolve the huge difference in A between real clouds and smoke clouds. This provides both a motivation for the smoke cloud intercomparison as our next case study, and also may provide a scientific forum for publishing our results, for if we can present results for the two cases from many LES's using different numerical schemes and which all show a large difference in A, we can better argue that our we's have physical meaning. ****************************************************************************** Run Setup The numerical setup is quite simple. We use a very similar domain size and vertical and horizontal resolution to that in our last intercomparison. However, we set mixing ratio qv to zero and start with a prescribed two-layer potential temperature distribution. We introduce a passive 'smoke' tracer S initially equal to 1 in the lower layer and 0 in the upper layer. At all later times, S will correspond to the fraction of lower layer fluid in any fluid parcel. We then let S be radiatively active by assuming a radiative flux Fr0 exiting upward out of the top of our numerical domain and then assuming that at the half-way heights between gridpoints (indexed by k, with spacing dz) Fr(k-.5) = Fr0*exp(-Ka*S_path(k-.5)) dtheta/dt (k) = -(Fr(k+.5) - Fr(k-.5))/(rho(k)*Cp*dz) S_path(k-.5) = sum from K = k to nz of rho(k)*S(k)*dz Fr0>0 corresponds to a negative downward flux of radiation, so 'absorption' or this negative flux causes cooling at the top of the lower layer. The absorption coefficient Ka = .02 m^2/kg, giving an optical depth of 44 m (realistic for a stratocumulus cloud). Taking Fr0= 60 W/m2 at a 7 K inversion at the top of a 700 m deep smoke layer, we get a pretty good radiative analogue of the stratocu with similar radiative flux divergence. ********************************************* Initial sounding ********************************************* Potential temperature (K) at height z (m): 0<= z <= 687.5 : 288 687.5< z < 712.5 : 288+0.28*(z-687.5) 712.5<= z <=1250 : 295+1x10^(-4)*(z-712.5) Total-water mixing ratio: 0 Smoke: 0<= z <= 687.5 : 1 687.5< z < 712.5 : 1-0.04*(z-687.5) 712.5<= z <=1250 : 0 If you use a prognostic equation for subgridscale turbulent kinetic energy or other turbulent quantities, initialize these to zero. Also, for such models choose the surface roughness length as 0 and von Karman's constant as 0.4. Mean surface pressure = 1000 mb Definition of potential temperature is theta = T*(pref/p)^(Rd/Cp), where pref = 1000 mb, Rd = 287 J/(kg-K) is the gas constant for dry air, and Cp = 1004 J/(kg-K) is the isobaric specific heat of dry air. If you are using a Boussinesq model, use as your reference density the density 1.1436 kg/m^3 of air of theta = 291.5 and pressure = 940 mb If you are using an anelastic model that is based upon an isentropic reference profile, use theta0 = 291.5 K to generate this profile. ********************************************* Domain ********************************************* Duration of run : 180 min Domain Height : 1250. m Domain Length (X) : 3200. m Domain Width (Y) : 3200. m Vertical Grid Points : 50 (dz = 25 m) X Grid Points : 64 (dx = 50 m) Y Grid Points : 64 (dy = 50 m) If your model uses variable vertical resolution, try to match dz = 25 m as best as you can, at least near the inversion. If you are doing a 2D run, our preliminary results suggest that you are likely to find large temporal variations of the smoke layer average potential temperature flux related to the domain size. We suggest that you also try and submit to us results from a wide domain run with a horizontal domain size of 25.6 km and otherwise identical to the simulation specified here. ******************************************** Other Details ******************************************** Free slip, rigid lid top and bottom BC's No scalar fluxes through top or bottom boundary Periodic lateral BC's No subsidence No Coriolis forcing No mean wind No sponge at the top of the domain Random Initialization : spatially uncorrelated random perturbation uniformly distributed between -0.1 and 0.1 K, applied to initial temperature field at all gridpoints with z < 700 m. *************************************************************************** Data Format for Presenting Results Our intercomparison results will be presented in five different sets. If you have a 1D model, you may choose to enter just the first four sets. The data format for presenting intercomparisons is modelled on Chin-Hoh's format for the first intercomparison, but because of the nature of the run, there are some differences, so read carefully. In what follows, f-bar refers to a horizontal average of f. The layer average of f is the vertical average of f-bar from the surface to the smoke top height zi (which is defined in A.2 below). Please precede each dataset below by a line of up to 80 characters including the letter identifying it, (for sets B-E) the middle of the averaging hour, the investigator first initial_last name and, optionally, any further distinguishing characteristics, separated by spaces, e. g. A M_MacVean 2D run in 25.6km domain ...data for set A... B 1.5 M_MacVean 2D run in 25.6km domain ...data for set B... . . . E 1.5 M_MacVean 2D run in 25.6km domain ...data for set E... B 1.5 M_MacVean 2D run in 25.6km domain ...data for set B... . . . E 1.5 M_MacVean 2D run in 25.6km domain ...data for set E... B 2.5 M_MacVean 2D run in 25.6km domain ...data for set B... . . . E 2.5 M_MacVean 2D run in 25.6km domain ...data for set E... Since models change, we are asking all participants to again fill out a model description section as was done last time. Since the purpose of an intercomparison is to understand how and why different models yield different results when applied to the same problem, please complete applicable parts of this section carefully. ---------------------------------------------------------------------- Set A of the intercomparison compares the time evolution, throughout the whole three hour simulation period, of: (1) simulation time (in min) (2) smoke top height zi (in meters), defined as the horizontal average of the column smoke top height zicol(i,j). The column smoke-top height is obtained by finding the highest grid level k at which S > 0.5, then linearly interpolating above this level to find the height at which S = 0.5: zicol(i,j) = (1-chi)*z(k) + chi*z(k+1), chi = (S(i,j,k)-0.5)/(S(i,j,k)-S(i,j,k+1)) (3) Layer averaged total (resolved + subgrid) turbulent kinetic energy (in m^2/s^2) (4) Layer averaged subgrid turbulent kinetic energy (in m^2/s^2) (5) Layer averaged total (resolved + subgrid) potential temperature flux rho*Cp*w'theta'-bar (in W/m^2) (6) Layer averaged subgrid potential temperature flux rho*Cp*w'theta'-bar (in W/m^2) (7) The horizontally averaged smoke path, defined as the integral from surface to top of rho(k)S(k)-bar (this is a check on smoke conservation). The data format for Set A is: (7F10.2). --------------------------------------------------------------------- Set B compares the horizontal mean profiles after 2.5 hours (1) height (in meters), where the following mean values locate, (2) U-bar (in m/s) (3) V-bar (in m/s) (4) potential temperature theta-bar (in K) (5) mean smoke mixing ratio S-bar (0-1) (6) Reference density profile rho(z) (kg/m^3) (use mean density for compressible models) The data format for Set B is (3F8.2, 3F8.3). Set C compares the flux profiles (resolved plus subgrid, except where explicitly mentioned), together with the skewness, eddy viscosity and eddy diffusivity, averaged over hours 2-3: (1) height (in meters), where the following fluxes locate, (2) u^2 + v^2-bar (in m^2/s^2) (3) w^2-bar (in m^2/s^2) (4) Potential temperature flux rho*Cp*w'theta'-bar (in W/m^2) (5) Subgrid potential temperature flux rho*Cp*w'theta'-bar (in W/m^2) (6) Radiation flux (in W/m^2) (7) Smoke flux w'S'-bar (in m/s) (8) Subgrid smoke flux w'S'-bar (in m/s) (9) The skewness profile w'^3-bar/w'^2-bar^(3/2) (10)Eddy viscosity-bar (11)Eddy diffusivity for heat, smoke-bar (in m^2/s) The data format for Set C is (F8.2, 2F8.4, 3F8.2, 3F10.6, 2F8.2). If your subgridscale scheme uses higher-order closure and cannot be formulated in terms of downgradient diffusion, write -1. for all levels in (10) and (11). Set D compares parameters relevant to entrainment closure: (1) Entrainment rate we = (zi(3 hrs) - zi(2 hrs))/3600 (in m/s) (2) wstar = (2.5*vertical integral from z=0 to domain top of the 2-3 hour average w'b'-bar)^(1/3) (in m/s) Here buoyancy flux w'b'-bar = g*w'theta'-bar/theta0 (units of m^2/s^3) and theta0 = 291.5 K is the reference potential temperature. (3) deltab = g*(theta(zi( 2.5 hrs)+100 m) - theta(zi(2.5 hrs)-100 m))/theta0 (m/s^2) (4) A = we*deltab*zi(2.5 hrs)/wstar^3 The data format for Set D is (3F8.5, F8.2) For set D, we encourage you to also submit statistics based on the 1.5 hour values and 1-2 hour averages, and the 3.5 hour values and 3-4 hour averages if you can carry out the run further than 3 hours. This will give some measure of the representativeness of the 2.5h values. --------------------------------------------------------------------- Set E compares the vertical profiles of the turbulent kinetic energy (in m^2/s^2) and its budget terms, each multiplied by 10**4 (in m^2/s^3), averaged over hour 2-3 for 2D and 3D runs: (1) height (in meters), where the following variables locate, (2) resolved turbulent kinetic energy q'-bar = 0.5*(u'^2 + v'^2 +