ICON Two-way grid-nesting in the ICON model G

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ICON Two-way grid-nesting in the ICON
modelGünther Zängl Deutscher
Wetterdienst, Offenbach

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Overview

• Introduction grid structure
• Implementation of two-way nesting
• One-way nesting mode and other options
• Selected test results (baroclinic wave and
  mountain-induced Rossby wave)
• Summary

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Grid structure (schematic view)
Triangles are used as primal cells Mass points
are in the circumcenter Velocity is defined at
the edge midpoints
Red cells refer to refined domain Boundary
interpolation is needed from parent to child mass
points and velocity points
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Implementation of two-way nesting

• Flow sequence 1 time step in parent domain,
  interpolation of lateral boundary
  fields/tendencies, 2 time steps in refined
  domain, feedback
• Boundary interpolation of scalars (dynamical and
  tracers)
• RBF reconstruction of 2D gradient at cell
  center
• Linear extrapolation of full fields and
  tendencies to child cell points
• Boundary interpolation of velocity tendencies
• RBF reconstruction of 2D vector at vertices
• Use to extrapolated to child edges
  lying on parent edge
• Direct RBF reconstruction of velocity
  tendencies at inner child edges
• Weak second-order boundary diffusion for velocity

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Implementation of two-way nesting

• Feedback
• Dynamical variables bilinear interpolation of
  time increments from child cells / main child
  edges to parent cells / edges
• Additive mass-conservation correction for
  density
• Tracers bilinear interpolation of full fields
  from child cells to parent cells, multiplicative
  mass-conservation correction
• Bilinear feedback is inverse operation of
  gradient-based interpolation
• For numerical stability, velocity feedback
  overlaps by one edge row with the interpolation
  zone
• Density and (virtual) potential temperature are
  used for boundary interpolation / feedback,
  rhotheta and Exner function are rediagnosed

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One-way nesting and other options

• One-way nesting option Feedback is turned off,
  but Davies nudging is performed near the nest
  boundaries (width and relaxation coefficients can
  be chosen via namelist variables)
• One-way and two-way nested grids can be
  arbitrarily combined
• An arbitrary number of nested domains per nesting
  level is allowed
• Multiple nested domains at the same nesting level
  can be combined into a logical domain to reduce
  parallelization overhead (exception one-way and
  two-way nested grids have to be assigned to
  different logical domains)
• An option to run computationally expensive
  physics parameterizations at reduced resolution
  is in preparation

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• Idealized tests
• Jablonowski-Williamson baroclinic wave test with
  nesting
• Are the disturbances induced at the nest
  boundaries small enough not to disturb the flow
  evolution?
• Modified Jablonowski-Williamson baroclinic wave
  test with moisture and cloud microphysics
  parameterization
• How well does tracer transport and physics
  coupling work in combination with nesting?
• Standard mountain-induced Rossby wave test with
  nesting
• Comparison of nested run with coarse-/high-resolu
  tion runs
• Example for multiple domains per nest level and
  comparison between one-way and two-way nesting

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Development of baroclinic waves

• Baroclinic wave case of Jablonowski-Williamson
  (2008) test suite
• Nonhydrostatic dynamical core
• Basic state geostrophically and hydrostatically
  balanced flow with very strong baroclinicity
  small initial perturbation in wind field
• Disturbance evolves very slowly during the first
  6 days, explosive cyclogenesis starts around day
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• Grid resolutions 140 km and 70 km, 35 vertical
  levels
• Results are shown after 10 days

location of nest
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Vertical velocity at 1.8 km AGL on day 10
70 km
140 km
140 km, nested
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Baroclinic wave test with moisture

• Modified baroclinic wave case of
  Jablonowski-Williamson (2008) test suite with
  moisture and Seifert-Beheng (2001) cloud
  microphysics parameterization (one-moment
  version QC, QI, QR, QS)
• Initial moisture field RH70 below 700 hPa, 60
  between 500 and 700 hPa, 25 above 500 hPa QV
  max. 17.5 g/kg to limit convective instability in
  tropics
• Transport schemes for moisture variables
• Horizontal Miura 2nd order with flux limiter
• Vertical 3rd-order PPM with slope limiter
• Grid resolutions 70 km and 35 km, 35 vertical
  levels
• Results are shown after 14 days

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Temperature at lowest model level on day 14
70 km
35 km
70 km, nested
nest, 35 km
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QV (g/kg) at 1.8 km AGL on day 14
70 km
35 km
70 km, nested
nest, 35 km
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Accumulated precipitation (mm WE) after 14 days
70 km
35 km
70 km, nested
nest, 35 km
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Rossby wave generation by a large-scale mountain

• Mountain-induced Rossby-wave case of
  Jablonowski-Williamson test suite
• Nonhydrostatic dynamical core
• Basic state isothermal atmosphere, zonal flow
  with max. 20 m/s
• Standard setup with 2000-m high circular mountain
  at 30N/90E
• High-resolution runs 35 km mesh size 35
  levels
• Coarse-resolution runs 140 km
• Nested runs 140 km globally, double nesting to
  35 km over mountain
• Results are shown after 20 days

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Vorticity (1/s) at surface level on day 20
high-resolution (35 km)
nested (140-km domain)
coarse-resolution (140 km)
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Vorticity at surface level on day 20 (mountain
region)
high-resolution
nested (35-km domain)
coarse-resolution
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Horizontal wind at surface level (barbs),
vertical wind at 2.5 km AGL on day 20
(colours)
high-resolution
nested (35-km domain)
coarse-resolution
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Different domain configuration

• Same basic setup as before, but
• both nested (logical) domains are composed of two
  non-contiguous physical domains
• at the finest nesting level (35 km mesh size),
  the boundary between the physical domains
  intersects the mountain approximately at its peak
• the nesting step from 70 km to 35 km is either
  one-way or two-way
• Results are shown for surface vorticity on day 20
  as before, but with different graphics software
  (GMT rather than NCL)

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Vorticity at lowest model level on day 20
35 km
70 km, two-way
70 km, one-way
35 km
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Vorticity at lowest model level on day 20
35 km, two-way
35 km, one-way
35 km globally
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• Summary
• Two-way grid nesting in ICON induces very small
  disturbances along nest boundaries even though no
  boundary nudging is used so far
• also works well with tracer transport and
  physics coupling
• also works well with nest boundaries over
  slopes
• One-way nesting (of course) needs boundary
  nudging but otherwise shows only the expected
  differences to two-way nesting
• Longer-term numerical stability has been tested
  up to 100 days
• boundary interpolation / feedback typically
  consume about 2 of total computing time of dry
  dynamical core
• MPIOpenMP parallelization implemented and
  validated except for boundary nudging

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QC (g/kg) at 1.8 km AGL on day 14
70 km
35 km
70 km, nested
nest, 35 km