Numerical weather prediction: current state and perspectives

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Numerical weather prediction current state and
perspectives

• M.A.Tolstykh
• Institute of Numerical Mathematics RAS, and
• Hydrometcentre of Russia

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What is the global atmospheric model

• Atmospheric equations averaged Navier-Stokes
  equations on the rotating sphere.
• Processes on unresolved scales  are
  parameterized. Currently, numerical solution of
  the equations for resolved dynamics accounts for
  30 of total computations time, the rest is for
  parameterizations

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Main ways to increase an accuracy of numerical
weather prediction

• 1) Increasing the horizontal and vertical
  resolution of atmospheric models
• Requires masssively parallel computations
• gt development of new dynamical cores (new
  governing equations, new numerical techniques)
•  2) Development of new parameterizations of
  subgrid-scale processes
• 3) Improvement of initial conditions

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RMS error of 3-day H500 forecast
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Current state of global NWP models

• Typical horizontal resolution at the end of 2009
  20-30 km
• Japan is the leader with 20 km, next year ECMWF
  will be the leader with 15 km

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The increase of the processor number necessary
for operational implementation of the SL-AV model

• 70 km, 28 levels 4 processors
• 37 km, 50 levels 40 processors
• 20 km, 50 levels - about 350 processors
• 10 km , 100 levels supposedly 6000 processors

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Development of new dynamical cores for global NWP
models

• Currently, a half of global NWP models us based
  on spectral techniques
• It scales up to0.5N_harm N_openmp(N_lev)
  processors, 5000 for ?1279.

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New dynamical cores of atmospheric models

• High parallel efficiency, locality of data
• A grid on the sphere with quasiconstant
  resolution
• Computational efficiency of numerical algorithm
  (sufficiently long time-step)
• Nonhydrostatic formulation (includes sound waves)

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Choice of the grid

• Traditional lat-lon grids have condensed
  meridians near the poles (from presentation by
  W.Skamarock, NCAR)

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Evolution of ps, day 9 (Jablonowski test)
CAM-FV-isen
BQ (GISS)
CAM-EUL
GEOS-FV
GEOS-FVCUBE
GME
HOMME
ICON
OLAM
hPa
with ?0, resolution 1?1L26
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Reduced latitude-longitude grid

• Routinely used in models based on spectral
  approach. It is possible to use it in
  finite-difference/finite volume models with
  specific formulation
• Advantages
• - High-order approximations are easily possible
• - Easy to code and parallelize

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Shallow-water model
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Developments in parameterizations of
subgrid-scale processes

• Parameterizations depend on horizontal resolution
  (examples deep convection, microphysics)
• Taking into account exchanges with adjacent
  horizontal grid cells (currenly, most of
  parameterizations are 1D in vertical)

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New and advanced parameterizations of
subgrid-scale processes

• Advanced land surface parameterization accounting
  for hydrology, evolution of snow cover,
  freezing/melting, bogs,
• Deep convection parameterization for partially
  resolved case
• Explicit description of microphysical processes
  in clouds
• Lake parameterizations
• Boundary layer parameterizations in the case of
  strongly stable stratification

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Land surface parameterization

• Tile approach (subcells describing water, low
  and high vegetation, etc)
• New directions
• Soil hydrology taking into account adjacent grid
  cells
• Biogeochemistry (carbon cycle, dynamical leaf
  area index )

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H-TESSEL surface parameterization scheme (ECMWF)

• The revised hydrology includes spatial
  variability related to topography (runoff) and
  soil texture (drainage)

Slide 19
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ECMWF New microphysics parameterization
Current Cloud Scheme
New Cloud Scheme
Slide 20
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Impact of initial data on model forecasts
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Data assimilation

• Weight optimally observations and short-range
  forecast from previous initial conditions to
  create initial conditions for the model
• Current approaches 4D-Var and ensemble Kalman
  filter

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Some directions of development for the global
semi-Lagrangian model SL-AV

• Increasing the scalability of the code from 300
  to 5000 processors
• Replacement of 3D solvers by divide-and conquer
  algorithms
• Nonhydrostatic dynamical core
• More advanced land surface parameterization
• (bogs, carbon cycle, multilayer soil, soil
  hydrology)

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Conclusions

• Challenges of the nearest decade development
  and implementation of global atmospheric models
  with the horizontal resolution 1-10 km.
• New approaches to develop new dynamical cores and
  parameterizations
• This requires efficient parallel implementation
  on 10000 processors
  
• We shorten the distance with leading centres in
  the field of global NWP

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Thank you for attention!