PhysicsDynamics Test Strategies Bridging the Gap with Simplified

Physics-Dynamics Test Strategies: Bridging the Gap with Simplified Moist Test Cases Christiane Jablonowski and Diana Thatcher University of Michigan, Ann Arbor, USA Physics-Dynamics Coupling Workshop (PDC 14), Ensenada, Mexico, 12/3/2014

The Talk at its Crossroads Effective Resolution: What should the scales be that the dynamical core passes to the physics (grid-point value, area-averaged, sub-sampled)? What are the believable scales in the dynamics? Test Strategies: Can we understand some aspects of the complex physics-dynamics coupling with simplified moist test cases? Partly covered by Peter Lauritzen’s talk Topic of this talk

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Physics-Dynamics Coupling: Session Announcements • Test strategies: It is important to be able to identify good coupling schemes from inferior ones early on in the development cycle. Once theoretical analysis of the scheme is complete, how can further evidence be collected to ensure the chosen scheme performs as anticipated? The full NWP trial stage usually only offers limited scope for (costly) change. The difficulty is to design tests with sufficient signal and validity, without being too complex such that they are useful in the early development/evaluation phase.

Test Cases: Hierarchy with Increasing Complexity

Some Desirable Design Criteria Test cases should • be designed for hydrostatic and non-hydrostatic dynamical cores on the sphere, ideally: for both shallow and deep atmosphere models • be easy to apply: analytic initial data suitable for all grids formulated for different vertical coordinates • deal with moisture in a simple way • reveal information about the physics-dynamics coupling • be as easy as possible, but as complex as necessary • be cheap and easy to evaluate • be relevant to atmospheric phenomena • have a converged reference solution • find broad acceptance in the modeling community

Overview of the Approaches • Short-term deterministic assessments (15 days) – Moist baroclinic waves with large-scale condensation – Moist baroclinic waves coupled to the `simple-physics’ package by Reed and Jablonowski (James, 2012) • Long-term ‘climate’ assessments (multiple years) – Moist version of the Held-Suarez test with elements of the `simple-physics’ package This talk’s goal: • Convince you that idealized physics processes lead to reasonable atmospheric circulations. Long-term goal (partly covered in this talk): • Evaluate whether idealized physics processes mimic the behavior of complex physics to aid our understanding.

Questions to Ask • What is our motivation to pursue idealized approaches? • Is it reasonable: How does a moist Held-Suarez (HS) aquaplanet simulation compare to a full-physics CAM 5 aquaplanet simulation? • Intercomparison: How do the different CAM 5 dynamical cores compare in moist HS and complex aqua-planet experiments? • Unit testing: How does the moist HS configuration compare to aqua-planet simulations that omit some processes (like the deep convection parameterization)? • Can we replicate some aspects of the complex physicsdynamics interactions with the moist HS setup? • What do we learn about the physics-dynamics coupling?

Motivation: Results from the Aqua-Planet Experiment (APE) • Aqua-planet model intercomparison revealed a huge spread in the GCM circulations and precipitation characteristics • Impossible to tell whether the APE differences are due to physics parameterizations or the dynamical cores or both? • Our test approaches level the playing field (identical physics). Zonal-mean time-mean total precipitation rates (hemispherically averaged) in 16 GCMs in aqua-planet mode, see Blackburn at al. (2013)

Adding Simple Large-Scale Condensation to the Dynamical Core Dynamics Process Variable PBL mixing Interaction Physics

Adding Simple Large-Scale Condensation • Add a specific humidity field q and transport it as a tracer • Compute condensation C tendencies to force q and the temperature T whenever the relative humidity (RH) at a grid point exceeds a threshold (e. g. RH > 100%): • The large-scale precipitation Pls removes the water instantaneously without a cloud stage Reed and Jablonowski (James, 2012)

Baroclinic Wave: Moisture and Large-Scale Condensation Dynamical Core Model Intercomparison Project (DCMIP) 42 Large-scale condensation in a moist version of the Jablonowski-Williamson (2006) baroclinic wave leads to an intensification of the baroclinic wave here at day 9 • It rains in the right spots (updraft areas associated with frontal zones) • Provides a first glimpse at the non-linear physics-dynamics interactions in the presence of moisture CAM-FV 1°x 1° L 30, dx = 110 km

Adding a Simple-Physics Package to the Dynamical Core Dynamics Physics Process Variable PBL mixing Interaction Reed and Jablonowski (James, 2012)

Simple-Physics Package: Basic Ideas • Replace the full-physics with a simple-physics package • The simple-physics tendencies are Reed and Jablonowski (James, 2012) • The fluxes are either – the bulk aerodynamic surface fluxes (latent and sensible heat, friction) or – mimic the turbulence in the boundary layer via a first-order closure (K-theory with surface wind-speed dependent eddy diffusivities) • C is large-scale condensation (no re-evaporation)

Moist Interactions: Baroclinic Wave Idealized moist baroclinic wave tests expose the behavior of simulations with complex physical parameterizations (here CAM 5) without radiation Surface pressure, day 9, CAM-FV 1°L 30 Dry Large-scale condensation Simple-Physics, no surface friction Complex CAM 5 physics no surface friction Simple-Physics, with surface friction Complex CAM 5 physics with surface friction h. Pa Tests based on Jablonowski and Williamson (2006), Simple-physics: Reed and Jablonowski (2012)

Moist Version of the Held-Suarez Test on an Aqua-Planet (prescribed SST) Color coding: Dynamics Held-Suarez (modified): • Radiation: Newtonian Temperature relaxation H • Rayleigh friction (PBL momentum mixing and surface friction) Variable Simple-Physics: • Surface fluxes of latent sensible heat • PBL mixing of moisture Interaction and temperature • Large-scale condensation Thatcher and Jablonowski (in prep. ) Reed and Jablonowski (James, 2012) PBL mixing Physics

Moist Held-Suarez and Complex Aqua-Planet CAM-SE 1° L 30: Reasonable - Moist Held-Suarez mimics Aqua-Planet Moist Held-Suarez with simple-physics Aqua-Planet with complex CAM 5 physics Temperature Zonal wind Thatcher and Jablonowski, in preparation

Moist Held-Suarez and Complex Aqua-Planet CAM-SE 1° L 30: Reasonable - Moist Held-Suarez mimics Aqua-Planet Moist Held-Suarez with simple-physics Aqua-Planet with complex CAM 5 physics Specific humidity Less efficient upward moisture transport, but distributions are similar Relative Humidity Thatcher and Jablonowski, in preparation

Moist Held-Suarez and Complex Aqua-Planet CAM-SE 1° L 30: Reasonable – Eddy transports are comparable Moist Held-Suarez with simple-physics Eddy heat flux Eddy kinetic energy Aqua-Planet with complex CAM 5 physics

Moist Held-Suarez and Complex Aqua-Planet CAM-SE 1° L 30: Reasonable – Physics forcing magnitudes comparable Moist Held-Suarez with simple-physics Aqua-Planet with complex CAM 5 physics Temperature tendency Deep convection peaks higher up Large-scale condensation Focus on the tropics Moisture tendency

Moist Held-Suarez and Complex Aqua-Planet CAM-SE 1° L 30: Similar tropical waves are apparent in the total precipitation rate (averaged between 5 S-5 N) in moist Held-Suarez (top) and Aqua-Planet (bottom) runs (here eastward traveling Kelvin waves) Same Kelvin. CAM 5 wave physics phase speeds Aqua-Planet with complex Moist Held-Suarez with simple-physics Precipitation is less organized in the moist HS experiment due to simplicity of precipitation Thatcher and Jablonowski, in preparation mm/day

Moist HS, Complex Aqua-Planet & Unit Testing • CAM-SE experiments with and without simple Betts-Miller (BM) and complex Zhang-Mc. Farlane (ZM) deep convection • Moist HS replicates complex Aqua-Planet (AP) behavior With deep No deep AP ZM deep AP Moist HS BM deep Moist HS Total precipitation rate AP HS

Intercomparisons & Unit Testing • Easier unit testing: How does CLUBB (new CAM PBL mixing, shallow convection, macrophysics) interact with the SE and SLD dycores and diffusion in CAM 5 aqua-planet experiments? SE SE SLD More diffusion Double versus single ITCZ Double ITCZ

Intercomparisons: CAM 5 dynamical cores The Community Atmosphere Model (CAM) provides four different dynamical cores (based on the primitive equations): 1. Semi-Lagrangian (SLD): two-time-level, semi-implicit semi. Lagrangian spectral transform model, Gaussian grid 2. Eulerian (EUL): three-time-level, semi-implicit Eulerian spectral transform dycore, Gaussian grid 3. Finite-Volume (FV): default dynamical core in CAM 5 & CAM 5. 1, grid-point-based finite-volume discretization, explicit time -stepping scheme, latitude-longitude grid 4. Spectral Element (SE): new default dynamical core (CAM 5. 3), based on continuous Galerkin spectral finite element method, designed for fully unstructured quadrilateral meshes (cubedsphere grid), locally energy- and mass-conserving, explicit time -stepping scheme

Intercomparisons: CAM 5 dynamical cores • The kinetic energy (KE) spectra of the moist HS experiments replicate the KE spectra of the complex CAM 5 aqua-planet runs (here with 110 -150 km grid spacing)

Intercomparisons: CAM 5 dynamical cores • Moist HS experiments can partly replicate the tropical precipitation rate characteristics of complex CAM 5 aquaplanet runs CAM 5 Aqua-Planet, Moist HS Increased precip. Increased convergence no deep convection CAM 5 Aqua-Planet

Intercomparisons: CAM 5 dynamical cores • Meridional Eddy moisture transport: v’q’ • Indication that the spectral dynamical cores EUL and SLD show systematic tropical differences in comparison to grid point models FV and SE in both moist HS and aqua-planet Moist HS SE FV EUL SLD Aqua-Planet

Conclusions • The interactions between the dynamical core and moisture processes can already be simulated with very simple model configurations, like large-scale condensation, simple-physics, or the moist HS test • Some aspects of the complex GCM behaviors can be replicated with the simplified physics setups • Tests give access to an easier understanding of the physicsdynamics coupling • Using identical physics with various dynamical cores levels the playing field • Approach allows unit testing of selected parameterizations or tests of the physics-dynamics coupling technique • Test cases hold promise to be useful for community use

References • Reed, K. A. , and C. Jablonowski (2012), Idealized tropical cyclone simulations of intermediate complexity: a test case for AGCMs, J. Adv. Model. Earth Syst. , Vol. 4, M 04001, doi: 10. 1029/2011 MS 000099 • Jablonowski, C. , and D. L. Williamson (2006), A Baroclinic Instability Test Case for Atmospheric Model Dynamical Cores, Quart. J. Roy. Met. Soc. , Vol. 132, 2943 -2975 • DCMIP shared workspace and DCMIP test case document: https: //www. earthsystemcog. org/projects/dcmip-2012/test_cases • Thatcher, D. R. and C. Jablonowski, A moist variant of the Held- Suarez test for atmospheric model dynamical cores: Aquaplanet comparison and sensitivity analysis, manuscript in preparation