Download - FIM basics
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FIM basics
Rainer Bleck, Shan Sun, Tanya Smirnova(and a host of other contributors too numerous to mention)
NOAA Earth System Research Laboratory, Boulder, Colorado
July 2012
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short for FFFVIM
Flow-followingFinite-volume
Icosahedral
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Overriding goal:add genetic diversity to atmo and
oceanic dycores
• Unique combination of two (not quite so unique) grid types:– Icosahedral horizontal grid– Adaptive (terrain-following/isentropic) vertical grid
• Tradeoffs: – Some drawbacks in traditional grids are avoided– must learn to cope with a few new problems
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FIM grid layout (G5 resolution)
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Perfectly shaped hexagons on the icosahedron
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Horizontal grid: Pros and Cons
• Pros:– No pole problems– Fairly uniform mesh size & cell shape on sphere
• Cons:– Indirect addressing (an efficiency issue)– Wavenumber 5 zonal grid irregularity– Line-integral based discretizations hard to
manipulate• Minor annoyances:
– Model diagnostics, graphics display more complex
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Adaptive vertical grid(primarily isentropic)
Coordinate surfaces and isotachs: solid. Pot.temperature: color
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9FIM hybrid-isentropic vertical coordinate
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How we maintain the vertical grid: Continuity Equation in generalized (“s”) coordinates
divergence
fluxmass
horizontal
integrated
vertically
surface
through
motion
vertical
surface
of
motion
vertical
ss
(zero in fixed
grids)
(zero in material coord.)
(known)
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Vertical grid: Pros and Cons
• Pros (in q domain):– Numerical dispersion acts along isentropes– Optimal resolution of fronts and associated shear
zones• Cons (in q domain):
– Poor resolution in unstratified columns• Positive side effect of hybrid design:
– Weak stratification due to surface heating leads to deep sigma domain (=> more uniform grid spacing at low latitudes)
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FIM• Hydrostatic; primitive eqns (solved explicitly)• Arakawa A-grid (i.e. no staggering)• 3-level Adams-Bashforth time differencing• Finite-volume discretization; no explicit mixing terms
required to suppress grid-scale noise• Variable vertical grid spacing requires transport eqns in
flux form for conservation– Use FCT for pos-definiteness, monotonicity
• Divergence, vorticity, gradients expressed as line integrals along grid cell perimeter
• GFS column physics• Horiz/vert. resolution specified at runtime
– highest resolution regularly used: ~15km (2.6M cells)
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See
http://fim.noaa.gov
for FIM documentation, skill scores, and twice-daily 10-day forecasts based on
GFS initial conditions