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:
1Developing a new general circulation model for planetary atmospheres - how (and why!) Claire Newman Kliegel Planetary Science Seminar March 1st 2005
2Overview of the talk What is a general circulation model (GCM)? Why develop a new model for planetary atmospheres: what questions are we trying to answer?How is this new model being developed?Description of the base model: the Earth-based, limited area “Weather Research and Forecasting” (WRF) modelDescription of the changes needed to ‘globalize’ WRFDescription of the changes needed to make ‘planetary’ WRFRecent results and future work: Earth, Mars and Titan
3What is a general circulation model (GCM)? Generally is conceptually (and practically) split into two components:physicsdynamicsBasically Newton II in 3 dimensions:force = mass x acceleration(subject to mass & energy conservation)Includes everything acting at a smaller scale to the dynamics, all of which is represented viaparameterizationsYou 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.)(Discrete problem rather than continuous)This includes:Small scale turbulenceFriction at the surfaceAbsorption, emission and scattering of radiation
4Dynamics, 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 distanceForce / massAccelerationDU = 2V sin - 2W cos - UW + Uv tan -1 p + FxDt a a XFrictional force per unit mass - usually added in during physics, as must be parameterized‘Coriolis’ terms due to air parcel moving in a rotating (not inertial) frameTerms due to the coordinate system rotatingPressure gradient force per unit mass‘Material derivative’ = rate of change of U following an air parcelU = 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/tBy definition, U = x/t, V = y/t and W = z/t=> DU/Dt = U/t + U/x U + U/y V + U/z W
5Examples of physics in a GCM Radiative transfer in a planetary atmosphere: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)SolarwavelengthsAtmosphericlayerAbsorption and scatteringin the atmosphereAtmosphericemission (~T4)Absorption and scatteringin the atmosphereThermalwavelengthsAbsorption and scatteringat the surfaceAbsorption and scatteringat the surfaceSurfaceSurface emission(~Tsurf4)
6Why do we need a new GCM for planetary atmospheres 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?
8Onset and evolution of a Martian dust storm Dust opacities for 2001 global storm from MGS TES website
9Storm onset & evolution: multiscale feedbacks Wind stress lifting: +ve feedbacks 1 - local scaleT increases inside dust cloudStrongwindsSurfaceWind stress lifting: +ve feedbacks 2 - global scale1 - why circ strengthens2 - why can’t put high resn everywhereMeridional circulation strengthensVery strong associatedwinds and much moredust liftingFairly strong associated winds and dust liftingS poleN poleS poleN pole
10Regions of interest on Mars 0°30°S60°S90°S180°120°W60°W0°60°E120°E180°Tharsis: strong slope flows; Western boundary currents on the eastern edgeArgyre and Hellas: slope flows in region of strong zonal winds and near cap edgeNorthern plains: relatively uninteresting
11Regions of interest on Mars 0°30°SSAY RE RESN HERE!!60°S90°S180°120°W60°W0°60°E120°E180°Potential areas of higher resolution
12The big questions for Mars (I) 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?Need higher resolution just in regions where local slopes and circulations may be crucialMust 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)
13Interannual variability in Mars’s atmosphere ‘Storm season’Dust opacityBrightness temperatureBrightness temp = T for sig fraction of lower atm, with info content centred on ~0.5mbar (~25km)‘Storm season’Areocentric longitude LsAreocentric longitude LsFrom Liu et al. JGR 2003
14The big questions for Mars (II) 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?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
15Clouds on TitanTitan 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.)
16The big questions for Titan (I) 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” Titan’s atmosphere The atmosphere can gain/lose angular momentum from/to the surfaceWhen a GCM is ‘spun up’ this transfer must average to zero over a yearResults from the LMD Titan GCM, from Hourdin et al., Icarus 1995
18The big questions for Titan (II) How much does interaction with the surface affect the atmospheric circulation?What determines the equatorial superrotation?How variable is Titan’s circulation and albedo (at different wavelengths) over the long Titan year?A long Titan year and thick atmosphere (high dynamical inertia) meanlong spin-uptimes=> Need a model which is fast, and accurate over the integration times required=> Experiments to explore sensitivities and study variability take a long time
19The Weather Research and Forecasting (WRF) model Mesoscale (limited area) model for weather research and forecasting on EarthDeveloped 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
20Features of WRFMass in kg (x1018)Dynamics conserve mass and angular momentum highly accurateHighly parallel code => efficientLarge suite of physics parameterizations and a modular form => flexibleUses Arakawa C-grid125250375500DaysVVVTTTUUUUBetter than 4 parts in e5, and no trendVVVTTTUUUUVVVU = zonal (E-W) velocity pointV = meridional (N-S) velocity pointTTTUUUUT = temperature / mass pointVVV
22The usual approach - how mesoscale WRF runs: b) its initial and boundary conditions being provided by a separate global modela) place nests within a mesoscale model (WRF), withSeparate global modelWRFDrawbacks:Interface between global and mesoscale models is one-way => no feedbacks from small to larger scaleUnless specially designed to match, often have different dynamics and/or physics - inconsistentInterface is also ‘messy’, e.g., must view output from the two models using different tools
23Globalising WRF gives a highly accurate & efficient global model, in which we can place 1- & 2-way nestsSo we are basically using WRF’s nesting abilities to nest all the way down from global
24Changes required for global WRF Allow use of a latitude-longitude gridWRF 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 directionWe still need a rectangular grid, but one which will reach from the south to the north pole=> lat-lon gridNeed rectangular and pole to pole => need lat lonNB - mercator will never reach pole; polar stereographic can’t meet at both polesREAD!!
25Original WRF Global WRF => mx ≠ my mx = my at all points If dx = gap between grid points in map coordinates,and dX = actual distance (in meters),then dX = (1/mx) dx and likewise dY = (1/my) dyOriginal WRFGlobal WRFLat-lon grid => x = a, y = a,=> dx = a d, dy = a d,whereas dX = a cos d, dY = a d=> mx = dx/dX = sec, my = dy/dY = 1=> mx ≠ my=> 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)Conformal grid=> for all map projections available (mercator, polar stereographic, etc.),mx = my at all points=> Only one map scale factor (m) used, and omitted altogether when mx and my cancelledMORE?? NO!
26Changes required for global WRF Deal with polar boundary conditionsVVPlace v points at poles, with v there = 0Nothing 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 variablesUTUTUVVTTUUUVVDeal with instabilities at the model topCHANGE 2nd!!The basic mesoscale WRF model generally only reached a maximum of ~30km, plus was regularly (and frequently) forced by a separate GCMHowever, ‘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’
27Changes required for global WRF Avoid instabilities due to E-W distance between grid points becoming small near polesThis 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), ORb) Increase largest effective scale ∆ x by filtering out smallerwavelengths (e.g. retaining only wavenumber 1 at the pole itself)Usual method in GCMs is to use a polar Fourier filter
28Changes for planetary WRF Models are generally very Earth-specific!Remove ‘hardwired’ planet-specific constants - instead use parameters which vary with planetChange ‘Earth time’ to ‘general planet time’Allow orbital parameters to be specifiedAdd physics parameterizations where needed
29Results: for Earth (up to 3.) Solid-body rotation test (for a non-rotating planet!) including solid body rotation over the polesHeld-Suarez standard test of a dynamical core: Newtonian relaxation to typical tropospheric temperature profiles with Rayleigh friction (winds slowed towards zonal mean) increasing with heightPolvani-Kushner extension to Held-Suarez: added a simple stratosphere with cooling over winter poleFurther testing to look at wave propagation etc.
30North pole North pole South pole South pole 1. Initial wind pattern for solid body rotation over the polesNorth poleNorth poleOverpole test resultsSouth poleSouth pole
33a. Zonal mean T averaged over last 1000 days 2. The Held-Suarez test:a. Zonal mean T averaged over last 1000 daysGlobal WRFExpected result
34b. Zonal mean u averaged over last 1000 days 2. The Held-Suarez test:b. Zonal mean u averaged over last 1000 daysGlobal WRFExpected result
353. 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 daysExpected zonal mean u (average over last 9,000 days)
36Results: for Mars (up to 3.) No CO2 condensation, no atmospheric dust, no topography, diurnally-averaged heatingAdded topography, diurnal cycleMars with a realistic (but prescribed) atmospheric dust content and with a CO2 cycleMars with interactive dust lifting and transportHigh resolution nests over Hellas, Tharsis, etc.
37Northern summer solstice: GFDL Mars GCM and WRF without dust
38Northern summer solstice: Oxford Mars GCM and WRF without dust
39Southern spring equinox: GFDL Mars GCM and WRF without dust
40Ls = 190°:global WRF zonal mean T, u & windMGS TES zonal mean TMGS TES zonal mean u
42Results: for Titan (up to 1.) Prescribed haze distributionInclude interactive haze production and transport using a microphysics schemeAdd methane cloud microphysicsIntroduce photochemistry schemes
43Prescribed haze distribution: some results we will compare with: Northern winter solsticeNorthern spring equinoxa. Meridional streamfunctions produced by the LMD Titan GCMb. Zonal mean zonal winds produced by the LMD Titan GCM
44ConclusionsGlobal, planetary WRF is a highly efficient and accurate global model in which high resolution 2-way nests can be embeddedIt 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 CO2 cycle) match those from other Mars GCMs
45ConclusionsOngoing work includes Mars with realistic dust and a CO2 cycle, and spinning up Titan’s atmosphere (including running in parallel on a beowulf cluster)