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Page 3© Crown copyright 2007SRNWP 8-11 October 2007, Dubrovnik NWP Model Orography 12 km 4 km 1 km Height of model orography (m)
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© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 1
Variable resolution or
lateral boundary conditionsTerry Davies Dynamics Research
Yongming Tang, Junichi Ishida
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 2
Convective scale NWP
One-way nestingrequires lateral boundary conditions from larger-scale model.
Variable resolution LAM – still requires lbcs but high resolution region can be moved away from lateral boundaries.
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 3
NWP Model Orography
12 km 4 km 1 km
Height of model orography (m)
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 4
Variable resolution costs (relative to 4km)
Area/ 2 km 1.85km 1.5 km 1 km
100% 8 10 19 64
80% 6.7 8.3 15 49
25% 5 - 10 25
10% - - - 10
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 5
May 3 2002 Case ----- Nested model
1 km high resolution nested model and radar rainfall at 14 UTC
1 km4 km12 km Radar
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May 3 2002 Case ----- variable resolution model
Rainfall at 15 UTC. The three regions of the variable resolution domain are also shown
Radar1km
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Variable Resolution
Grid StructureGrid varies from coarse resolution at the outer boundaries smoothly to a uniform fine resolution in the interior of the domain
UniformHigh Res
zoneVar-Res 2Var-Res 1
UniformCoarse Res 1
UniformCoarse Res 2
Typically, there are 3 regions, and inflation ratio R1 = R2 = 5~10%e.g. = 1 km, R1 = R2 = 10 % N_vr = 34 / 24 / 15 points = 25 / 10 / 4 km
R2R1
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 8
Variable Resolution
Grid StructureGrid varies from coarse resolution at the outer boundaries smoothly to a uniform fine resolution in the interior of the domain
UniformHigh Res
zoneVar-Res 2Var-Res 1
UniformCoarse Res 1
UniformCoarse Res 2
Typically, there are 3 regions, and inflation ratio R1 = R2 = 5~10%
e.g. = 1 km, R1 = R2 = 10 % N_vr = 34 / 24 / 15 points = 25 / 10 / 4 km
R2R1
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 9
Variable resolution
Being built into latest version of UM (switch for variable resolution).
Uses look-up tables for semi-Lagrangian searches (departure points and trajectories).
Other optimisations to code and changes to lateral boundaries for variable grids.
Work towards 1.5km variable resolution model for UK LAM for 2009 (new supercomputer).
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 10
Forecast Domain and orography
Regular UM (12km) Var-Res UM (1.5-4km)
Global : 432x325L38Regular UM (12km) : 250x250L38Var-Res UM (1.5-4km): 484x557L38Regular UM (4km) : 288x360L38Regular UM (1.5km) : 300x300L38
Domain of Var-Res UMDomain of 4km UM
High Resolution ZoneDomain of 1.5km UM
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 11
Typhoon track
Domain of Var-Res UM (1.5-4km)Domain of 4km UM
High Resolution ZoneDomain of 1.5km UM
12UTC 05th
00UTC 05th
00UTC 06th
The red lines and boxes show the observed track and position of the typhoon Nabi.
The initial time isRegular 12km UM : 12UTC on 5th Var-Res UM : 13UTC on 5th Regular 4km UM : 13UTC on 5th Regular 1.5km UM : 14UTC on 5th
The eye of the typhoon at initial time is located in the outside of the high resolution area and the typhoon moves into the area through the transition area (Var-Res UM) or lateral boundary (1.5km UM).
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 12
Global UM 12km UM Observation
4km UM Var-Res UM
Precipitation forecast
The 12kmUM, 4kmUM and Var-Res
UM well reproduce the rain caused
by the orographic forcing over the
Kyushu island.
Hourly precipitation at 21 UTC on 5th September
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 13
Precipitation forecast
4km UM 1.5km UM
Var-Res UM Observation
Hourly precipitation at 21 UTC on 5th September
•The rain band round the eye of the
typhoon in the1.5km and Var-Res
UM are better than that in 4km UM.
•The rain band in the Var-Res UM
is slightly better than that in the
1.5km UM. The typhoon in the
1.5km UM moved from outside of
the domain through the lateral
boundary and the 1.5km UM seems
not to have enough time to develop
the rain band (spin up problem).
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 14
Lateral boundaries
To run variable resolution LAM will still need lbcs.
Current lbcs use standard merging technique and wrong lateral boundary condition for solving for the pressure increment
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 15
Lateral boundaries
Semi-Lagrangian advection applies lbcs naturally.
Apply appropriate lbcs to Helmholtz equation instead of zero pressure increment at lateral boundaries.
Reorganise lbc updating during time step.
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 16
Lateral boundaries
For mpp, lateral boundary files do not need external halos – lbc files can be made 30% smaller.
It should also be possible to reduce the number of fields in the lbc file leading to further size reduction.
If interpolation to LAM is made on input, files could be even smaller.
© Crown copyright 2007 SRNWP 8-11 October 2007, Dubrovnik Page 17
The End
