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Developing a new general circulation model for planetary atmospheres - how (and why!) Claire Newman Kliegel Planetary Science Seminar March 1st 2005.

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Presentation on theme: "Developing a new general circulation model for planetary atmospheres - how (and why!) Claire Newman Kliegel Planetary Science Seminar March 1st 2005."— Presentation transcript:

1 Developing a new general circulation model for planetary atmospheres - how (and why!) Claire Newman Kliegel Planetary Science Seminar March 1st 2005

2 Overview of the talk What is a general circulation model (GCM)? What is a general circulation model (GCM)? Why develop a new model for planetary atmospheres: what questions are we trying to answer? Why develop a new model for planetary atmospheres: what questions are we trying to answer? How is this new model being developed? How is this new model being developed? Description of the base model: the Earth-based, limited area Weather Research and Forecasting (WRF) model Description of the base model: the Earth-based, limited area Weather Research and Forecasting (WRF) model Description of the changes needed to globalize WRF Description of the changes needed to globalize WRF Description of the changes needed to make planetary WRF Description of the changes needed to make planetary WRF Recent results and future work: Earth, Mars and Titan Recent results and future work: Earth, Mars and Titan

3 What is a general circulation model (GCM)? dynamics physics Basically Newton II in 3 dimensions: force = mass x acceleration (subject to mass & energy conservation) write down You can actually write down the complete physics of how air parcels move in a rotating frame (ignoring relativity and quantum mechanics), even if to solve the problem you need to make approximations (like ignoring small terms, working with a finite number of points, etc.) Generally is conceptually (and practically) split into two components: Includes everything acting at a smaller scale to the dynamics, all of which is represented via parameterizations This includes: 1.Small scale turbulence 2.Friction at the surface 3.Absorption, emission and scattering of radiation

4 DU = 2 V sin - 2 W cos - UW + Uv tan -1 p + F x Dt a a X Material derivative = rate of change of U following an air parcel U = U(t, x, y, z) => U = U/ t t + U/ x x + U/ y y + U/ z z => U/ t = U/ t + U/ x x/ t + U/ y y/ t + U/ z z/ t By definition, U = x/ t, V = y/ t and W = z/ t => DU/Dt = U/ t + U/ x U + U/ y V + U/ z W Coriolis terms due to air parcel moving in a rotating (not inertial) frame Dynamics, e.g., the zonal (E-W) momentum equation: U, V, W = wind in E-W, N-S and vertical respectively, = latitude, p = pressure, = density, a = planet radius, t = time, X = E-W distance Terms due to the coordinate system rotating Pressure gradient force per unit mass Frictional force per unit mass - usually added in during physics, as must be parameterized Acceleration Force / mass

5 Examples of physics in a GCM Radiative transfer in a planetary atmosphere: Absorption and scattering in the atmosphere Atmospheric layer Surface Absorption and scattering at the surface Solar wavelengths Thermal wavelengths Surface emission (~T surf 4 ) Absorption and scattering in the atmosphere Atmospheric emission (~T 4 ) Absorption and scattering at the surface Temperature changes depend on heating rates, which are determined from net fluxes, which in turn depend on temperature => many interconnected equations and many methods of solving them to find T(z)

6 Why do we need a new GCM for planetary atmospheres? What questions do we want to answer? Why do we need a new GCM for planetary atmospheres? To understand this, you first need to understand: What questions do we want to answer?

7 EarthMarsTitan CO 2 atmosphere P surf ~ 610 Pa T surf ~ 210 K Very eccentric orbit Major topography Dust storms N 2 atmosphere P surf ~ 1.5x10 5 Pa T surf ~ 93 K Thick haze layers Methane hydrology Slowly rotating N 2 atmosphere P surf ~ 1x10 5 Pa T surf ~ 288 K Water cycle Oceans & land surfaces

8 Dust opacities for 2001 global storm from MGS TES website Onset and evolution of a Martian dust storm

9 Meridional circulation strengthens T increases inside dust cloud Surface Strong winds Wind stress lifting: +ve feedbacks 2 - global scale S pole N pole S pole N pole Fairly strong associated winds and dust lifting Very strong associated winds and much more dust lifting Wind stress lifting: +ve feedbacks 1 - local scale Storm onset & evolution: multiscale feedbacks

10 180° 120°W60°W120°E0°60°E 90°N 60°N 30°N 30°S 60°S 0° 90°S Tharsis: strong slope flows; Western boundary currents on the eastern edge Argyre and Hellas: slope flows in region of strong zonal winds and near cap edge Northern plains: relatively uninteresting Regions of interest on Mars

11 180° 120°W60°W120°E0°60°E 90°N 60°N 30°N 30°S 60°S 0° 90°S Potential areas of higher resolution Regions of interest on Mars

12 Must consider multi- scale feedbacks: look at local dust lifting and the effect on the local and global circulation, which in turn affects further lifting => Model with high resolution areas within global domain, and information passing both ways (2-way feedbacks) The big questions for Mars (I) How do dust storms begin and evolve, and why do some become global? How do dust storms begin and evolve, and why do some become global? How do flows associated with the large topography interact with the global circulation? How do flows associated with the large topography interact with the global circulation? Need higher resolution just in regions where local slopes and circulations may be crucial

13 Brightness temperature Dust opacity Areocentric longitude Ls Storm season From Liu et al. JGR 2003 Interannual variability in Marss atmosphere

14 The big questions for Mars (II) What determines the variability in the Martian dust cycle and hence climate? What determines the variability in the Martian dust cycle and hence climate? What was the climate like in the past, & does this help us understand present geological features? What was the climate like in the past, & does this help us understand present geological features? When and where was water stable at the surface, and where would subsurface water deposits be? When and where was water stable at the surface, and where would subsurface water deposits be? Need to look at interannual variability and/or changes over long timescales => Need efficient and accurate (mass and angular momentum conserving) model

15 Titan imaged over 9 days in the K filter (centered at 2.12 m) which sees down to the surface and troposphere, using the AO system at Keck. (Images scaled to the brightest point in each case.) Clouds on Titan

16 The big questions for Titan (I) What controls when and where methane clouds form? What controls when and where methane clouds form? Want to use higher resolution just over regions where clouds form now (and over other cloud-formation regions in other seasons) => Need a model capable of placing high resolution regions with the global domain

17 Spinning up Titans atmosphere Results from the LMD Titan GCM, from Hourdin et al., Icarus 1995 The atmosphere can gain/lose angular momentum from/to the surface When a GCM is spun up this transfer must average to zero over a year

18 The big questions for Titan (II) How much does interaction with the surface affect the atmospheric circulation? How much does interaction with the surface affect the atmospheric circulation? What determines the equatorial superrotation? What determines the equatorial superrotation? How variable is Titans circulation and albedo (at different wavelengths) over the long Titan year? How variable is Titans circulation and albedo (at different wavelengths) over the long Titan year? A long Titan year and thick atmosphere (high dynamical inertia) mean long spin-up times => Experiments to explore sensitivities and study variability take a long time => Need a model which is fast, and accurate over the integration times required

19 The Weather Research and Forecasting (WRF) model Mesoscale (limited area) model for weather research and forecasting on Earth Mesoscale (limited area) model for weather research and forecasting on Earth Developed by NCAR in collaboration with other agencies (NOAA, AFWA, etc.) Developed by NCAR in collaboration with other agencies (NOAA, AFWA, etc.) Aim: to produce a reliable mesoscale model, to be used for real-time forecasting and as a research tool, with improvements being worked into new releases Aim: to produce a reliable mesoscale model, to be used for real-time forecasting and as a research tool, with improvements being worked into new releases

20 Features of WRF Dynamics conserve mass and angular momentum - highly accurate Dynamics conserve mass and angular momentum - highly accurate Highly parallel code => efficient Highly parallel code => efficient Large suite of physics parameterizations and a modular form => flexible Large suite of physics parameterizations and a modular form => flexible Uses Arakawa C-grid Uses Arakawa C-grid V VV V V V VVV VVV UUUU UUUU UUUU TTT TTT TTT U = zonal (E-W) velocity point V = meridional (N-S) velocity point T = temperature / mass point Mass in kg (x10 18 ) 375 Days

21 Features of WRF (cont.) 2-way nesting capability: 2-way nesting capability: Nesting capability: Nesting capability: Mother domain Child Child 1 Child 2 siblings Grandchild 1-way nesting2-way nesting

22 The usual approach - how mesoscale WRF runs: a) place nests within a mesoscale model (WRF), with b) its initial and boundary conditions being provided by a separate global model Interface between global and mesoscale models is one-way => no feedbacks from small to larger scale Unless specially designed to match, often have different dynamics and/or physics - inconsistent Interface is also messy, e.g., must view output from the two models using different tools Drawbacks: WRF Separate global model

23 Globalising WRF gives a highly accurate & efficient global model, in which we can place 1- & 2-way nests So we are basically using WRFs nesting abilities to nest all the way down from global

24 Changes required for global WRF Allow use of a latitude-longitude grid Allow use of a latitude-longitude grid We still need a rectangular grid, but one which will reach from the south to the north pole => lat-lon grid WRF is set up for conformal rectangular grids (such as polar stereograhic) where the map to real world scaling factor is the same in the x as in the y direction

25 Original WRFGlobal WRF Conformal grid => for all map projections available (mercator, polar stereographic, etc.), m x = m y at all points => Only one map scale factor (m) used, and omitted altogether when m x and m y cancelled If dx = gap between grid points in map coordinates, and dX = actual distance (in meters), then dX = (1/m x ) dx and likewise dY = (1/m y ) dy Lat-lon grid => x = a, y = a, => dx = a d, dy = a d, whereas dX = a cos d, dY = a d => m x = dx/dX = sec, m y = dy/dY = 1 => m x m y => Needed to identify which map scale factor was required in all equations where m appeared, and reintroduce map scale factors where they previously cancelled (so were omitted)

26 Changes required for global WRF Deal with polar boundary conditions Deal with polar boundary conditions Place v points at poles, with v there = 0 Nothing is allowed to pass directly over the poles - atmospheric mass is pushed around the pole in longitude instead - and no fluxes can come from the polar points when calculating variables Deal with instabilities at the model top Deal with instabilities at the model top VV VV VV UUU UUUTT TT The basic mesoscale WRF model generally only reached a maximum of ~30km, plus was regularly (and frequently) forced by a separate GCM However, standalone global WRF will develop upper level instabilities due to spurious wave reflection at the model top if these are not damped in some way - we must therefore introduce a sponge layer

27 This is a problem due to the CFL (Courant Friedrichs Lewy) criterion: t < x / U where U is the fastest moving wave in the problem => As x -> 0, t must -> 0 also, which is very expensive => a) Use a small t (far less than needed to satisfy at the equator), OR b) Increase largest effective scale x by filtering out smaller wavelengths (e.g. retaining only wavenumber 1 at the pole itself) Usual method in GCMs is to use a polar Fourier filter Usual method in GCMs is to use a polar Fourier filter Avoid instabilities due to E-W distance between grid points becoming small near poles Avoid instabilities due to E-W distance between grid points becoming small near poles Changes required for global WRF

28 Changes for planetary WRF Remove hardwired planet-specific constants - instead use parameters which vary with planet Remove hardwired planet-specific constants - instead use parameters which vary with planet Change Earth time to general planet time Change Earth time to general planet time Allow orbital parameters to be specified Allow orbital parameters to be specified Add physics parameterizations where needed Add physics parameterizations where needed Models are generally very Earth-specific!

29 Results: for Earth (up to 3.) 1. Solid-body rotation test 1. Solid-body rotation test (for a non-rotating planet!) including solid body rotation over the poles 2. Held-Suarez standard test of a dynamical core: 2. Held-Suarez standard test of a dynamical core: Newtonian relaxation to typical tropospheric temperature profiles with Rayleigh friction (winds slowed towards zonal mean) increasing with height 3. Polvani-Kushner extension to Held-Suarez: 3. Polvani-Kushner extension to Held-Suarez: added a simple stratosphere with cooling over winter pole 4. Further testing 4. Further testing to look at wave propagation etc.

30 1. Initial wind pattern for solid body rotation over the poles North pole South pole North pole

31 Wind pattern after 1 1/2 days

32 Wind pattern after 4 1/2 days

33 2. The Held-Suarez test: a. Zonal mean T averaged over last 1000 days Global WRFExpected result

34 2. The Held-Suarez test: b. Zonal mean u averaged over last 1000 days Global WRFExpected result

35 3. Polvani-Kushner - in initial stages (up to 380 days, but need average over last 9000 days of day experiment) Zonal mean u in global WRF at 380 days Expected zonal mean u (average over last 9,000 days)

36 Results: for Mars (up to 3.) 1. No CO 2 condensation, no atmospheric dust, no topography, diurnally-averaged heating 2. Added topography, diurnal cycle 3. Mars with a realistic (but prescribed) atmospheric dust content and with a CO 2 cycle 4. Mars with interactive dust lifting and transport 5. High resolution nests over Hellas, Tharsis, etc.

37 Northern summer solstice: GFDL Mars GCM and WRF without dust

38 Northern summer solstice: Oxford Mars GCM and WRF without dust

39 Southern spring equinox: GFDL Mars GCM and WRF without dust

40 MGS TES zonal mean T Ls = 190°: global WRF zonal mean T, u & wind MGS TES zonal mean u

41

42 Results: for Titan (up to 1.) 1. Prescribed haze distribution 2. Include interactive haze production and transport using a microphysics scheme 3. Add methane cloud microphysics 4. Introduce photochemistry schemes

43 Northern winter solsticeNorthern spring equinox 1.Prescribed haze distribution: some results we will compare with: a. Meridional streamfunctions produced by the LMD Titan GCM b. Zonal mean zonal winds produced by the LMD Titan GCM

44 Conclusions Global, planetary WRF is a highly efficient and accurate global model in which high resolution 2-way nests can be embedded Global, planetary WRF is a highly efficient and accurate global model in which high resolution 2-way nests can be embedded It has performed (and is performing) well in general tests (e.g. mass conservation) and tests used for other Earth GCMs (e.g. Held- Suarez) It has performed (and is performing) well in general tests (e.g. mass conservation) and tests used for other Earth GCMs (e.g. Held- Suarez) Initial Mars results (no dust or CO 2 cycle) match those from other Mars GCMs Initial Mars results (no dust or CO 2 cycle) match those from other Mars GCMs

45 Conclusions Ongoing work includes Mars with realistic dust and a CO 2 cycle, and spinning up Titans atmosphere (including running in parallel on a beowulf cluster) Ongoing work includes Mars with realistic dust and a CO 2 cycle, and spinning up Titans atmosphere (including running in parallel on a beowulf cluster)


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