Fig. 2 Results: Timestep has a big impact (suggests sequencing problem)!

1. Office of Science The Case of the Disappearing Condensate:: Concern: Like most GCMs, process rates in CAM are calculated using variables updated to include the effect of processes computed earlier in that timestep. This improves numerical stability but mangles the balance between source and sink processes, resulting in potentially unrealistic solutions. • Clues: • substantial ql exists after macro (Fig. 4a) • this is consistent with Fig. 2 since macro sets cldfrac • ql is completely gone after micro (Fig. 4b) • ⇒ Problem related to macro/micro coupling! • Potential sources of error are outlined in Fig. 5. a) qlaftermacrophysics (g/kg) 0.2 0.15 0.1 0.05 0.0 Pres (mb) Investigation: MPACE-B single-column runs are used to test if sequential parameterization application is causing problems in CAM5. Setup: Time Convergence: Problems with applying timestepping one process at a time should be eliminated as the timestep (dt) goes to 0. Thus we explore timestep sensitivity. b). qlafter microphysics (g/kg) 0.08 0.06 0.04 0.02 0.0 max obs≈ 0.3 g/m3 Pres (mb) • MPACE-B = steady, well-mixed, mixed-phase stratocumulus over open ocean off the N Slope of Alaska on Oct 9, 2004 • Specifed SHF=136.5 W m-2, LHF=107.7W m-2, SST=274.01K, fixed winds (NE @~14m s-1in PBL), no evolution above 700mb • CAM tag = scamcpt12_cam5_0_12-r26229 (≈ release 5.0) • Aerosol fixed at observed values • SCAM5 assumes aerosol=0 by default, causing clouds to rain out! MPACE-B Initial Conditions: Days since 0Z, 10/9/99 Issues with Sequentially-Integrated CAM5 Parameterizations Peter Caldwell, Hugh Morrison, Sungsu Park, Steve Klein, Chris Bretherton, Phil Rasch, Steve Ghan, Xiaohong Liu, Andrew Gettelman, and Jim Boyle (from LLNL, NCAR, UW, PNNL; contact: caldwell19@llnl.gov) Fig 4: Time-height cross sections ofql from a 20 min timestep run after macro and after micro. Fig 5: Sequence of CAM5 parameterization calls with potential problems highlighted. Solutions: Two solutions are proposed to solve the issues identified in Fig. 5. These are found to substantially improve single- column, forecast, and global simulations. Potential Fixes: MPACE-B Results: Global Model Results: ISDAC CAPT Results: Pressure (mb) Current sequence: Fig 1: observed and initial model vapor and liquid mixing ratios (qv and ql; left panel), and potential temperature (θ; right panel). From Klein et al (2009 QJRMS). 2 substeps New ql applied but not drop number (nl) Fix 1: • Solves ql,nl inconsistency but not push/pull • Solves ql,nland push/pull problems New qland nlapplied Fix 2: Fix 1:move nlFix 2: move ql g/kg • Fig. 2 Results: • Timestep has a big impact (suggests sequencing problem)! • shallow convective cldfrac hits 0.2 upper limit @ dt=30s • BL stratifies because turbulence doesn’t see ice-induced cloudtop cooling(?) • The liquid:ice ratio decreases at finer resolution (bottom 2 rows) • Fig. 3 Results: • Radiation sees 0 condensate at default resolution even though cldfrac=1! • Condensate is present but still lower than obs at 30 sdt Default (20 min) dt Fine (30 s) dt Total Water Path (mean Δ = +9.13 g m-2) Boundary Layer Depth (mean Δ = +13 m) Total cldfrac 1.0 0.75 0.5 0.25 0.0 2 substeps no updates (g m-2) (m) 120 80 40 0 -40 -80 -120 400 200 0 -200 -400 Convective cldfrac 0.2 0.15 0.1 0.05 0.0 Fig 8: Effect of fix 1 on total water path (left) and boundary layer height (right) from atmosphere-only global runs with fixed Y2K boundary conditions. Fig 6: As in Fig. 4 but with fixes applied. • Conclusions: • SCAM5 requires fixed aerosol • Timestep sensitivity is severe • Issue with fine dt: TKE must see ice cloudtop cooling • ql, nl inconsistency and macro/micro push/pull result in unrealistic liquid depletion • Suggested fixes greatly improve simulation • Code modularity (e.g. macro/micro separation) can cause trouble if coupling isn’t carefully considered Liquid cldfrac 1.0 0.75 0.5 0.25 0.0 Pres (mb) (%) 20 min dt 30 sdt 100 80 60 40 20 0 200 400 600 800 1000 Fix 1 ARSCL Obs Total water Path Physics Sequencing: Problem 2 (push/pull): Ice cldfrac Problem 1: ql, nl inconsistency Pres (mb) 1.0 0.75 0.5 0.25 0.0 Apr 7 Apr 8 Apr 9 Apr 10 200 400 600 800 1000 Default CAM5 Fig 7: Cloudfrac for ISDAC “Golden Day” from 24-48 hr CAM5 forecasts forced by MERRA reanalysis. ISDAC was an experiment over sea ice off the N Slope Alaska in Apr 2008. obs suggest 0.13 to 0.23 kg/m2 Days since 10/9/99 Days since 10/9/99 Fig 2: Time-height cross sections for various model cloud fractions. Coarse dt (default) results are on left, fine dt results are on right. Fig 3: Timeseries of vertical integral of total condensate (TGCLDWP) as seen by radiatioDefaultn. Model state is updated after each process Apr 7 Apr 8 Apr 9 Apr 10 time in days since 1999-10-9 This work performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 with funding from the Earth System Modeling program at DOE Office of Science. LLNL-POST-487583