1. Weather forecasting in a coupled world
From Ocean to
Atmosphere
Weather forecasting in a coupled world
Joachim Fallmann, Huw Lewis, Juan Castillo, Alex Arnold
Met Office, Exeter
‘Serving Society with better Weather and Climate Information.’
EMS Annual Conference 2017 Dublin, 4 – 8 September 2017
© Crown Copyright

2. Challenging the complexity in a coupled world
Physical
Biological
Chemical
+ …

3. UK Environmental Prediction
SST
currents 2
A ‚coupled view‘
Coastal Shelf Sea (NEMO)
UKO2
1.5km
Regional BCs
Coupled u,v, T….
solar radn
non-solar rad
winds
rain; snow
evaporation
surface stress
msl pressure
river discharge 4
Land surface (JULES)
River flow
Atmosphere
(Unified Model)
UKA km
Implicit coupling
Global BCs
Coupled SST 5
wave height
energy
currents
depth 3
UK Environmental Prediction
Waves
(WaveWatchIII)
UKW1 1.5km
Regional BCs
Coupled u,v, p….
winds 6
Charnock = Roughness 1
Lewis et al (GMD)

4. UK Domain (1.5km) Atmosphere (UKA):
950 x 1025 cells with ° resolution (1.5km in the centre)
70 levels terrain following up to 40 km
16 levels within lowest 1 km
BC: global weather forecast at 17 km
Ocean (UKO):
1458 x 1345 regular 1.5 km grid
central region matching atm. grid
51 vertical levels, terrain following
ocean is initialised from a one-year long simulation forced by ERA-Interim at the surface
Boundary conditions from operational global seasonal prediction system GloSea5 ¼ ° resolution
Forcing by high res UKA run
Waves (UKW)
Forced by atm. Winds, ocean currents and water levels

5. frontal propagation and precipitation coastal fog sea breeze
Sensitivity analysis – Atmosphere, Waves, Ocean
frontal propagation and precipitation
coastal fog
sea breeze
wind speed and waves during winter storm
surface to boundary layer coupling – low level clouds
sea surface salinity in estuaries
...

6. Precipitation rate (mm/h)
Sensitivity analysis – Precipitation
August 2014
Precipitation [mm/h] – SW-England
Accumulated grid cells over land (SW-England)
‚frontogenesis‘
Coupled
Precipitation [mm/12h] at 06:00
Weather station ‚Chivenor ‘
OBS: 17 mm/12h
UM-forecast: 6.2 mm/12h
Coupled: 15.2mm/12h
Uncoupled
Precipitation rate (mm/h)
Uncoupled
Coupled
OBS
Chivenor

7. Sensitivity analysis – coastal fog (July 2013)
Visibility - Fog [m] – Southern North Sea
1.5m VIS
Coupled
Uncoupled
Meteosat Fog temperature and thickness
Coupled
Uncoupled 24 10
VIS [km]
Average visibility over southern North Sea
289
288
SST [K]
Coupled
Uncoupled
Increase of domain average SST by 1 K over 5 days
Met Office Weather Summary 17 July 2013 evening:
[...]main exception to the fine and sunny weather was north-west Scotland. It was much cloudier there, with mist and fog [..]

8. Experiment - Atmospheric feedback to 1 Kelvin warmer sea surface
Reduction of initial global OSTIA SST by 1K
T_init – 1 K
T_init + 1 K
No Fog
Fog

9. Sensitivity analysis – Sea Breeze (July 2013)
Coupled - Uncoupled
SST
Coupled
1100 h
302
286
[K]
1600 h
2200 h
Tpot and wind direction 5 July 2013
Wind direction and location of sea breeze fronts (1100h)
Effect of SST increase on Tair by coupling (2000h) 1 -1
Coupled - Uncoupled
Displacement ~ 15 km
+ 11 cm/s
-9 cm/s
height [m]
Coupled - Uncoupled
Δ Vertical velocity [cm/s]
Hovmoller plot for vertical velocity shows the diurnal movement of the sea breeze front – differnces indicate displacement

10. Sensitivity analysis – wave/wind feedback (October 1014)
Roughness relative to uncoupled
Rougher surface than control constant (UKV)
Smoother surface than control constant
0.01
-0.01
Wave Height [m] – North Atlantic
16 m
1.5
-1.5
Wind Speed - [m/s]
Reduction of the mean wind speed when introducing a rougher surface through Atmosphere – Wave coupling
Coupled
Uncoupled 14 7

11. Towards process-based research...
Atmospheric boundary layer characteristics through horizontal and vertical profiles
cloud to boundary layer coupling
0.2
0.0
-0.2
2000
1500
1000
500
0.6
0.0
-0.6
Δ cloud fraction [-]
Δ TKE [m-2s-2]
Coupled
Uncoupled
2000
1500
1000
500
[m]
Cloud fraction
Tpot
Vert.
Velocity
[ms-2]

12. Evaluating feedbacks Δ SST Δ Atmosphere Coupling: Heat flux Cloud
Coupled - Uncoupled
Coupling:
SST
Heat flux
Cloud
Shortwave reflexion
PBL height
400
0.5 50
200
-400
-0.5
-50
Coupled
Uncoupled
Meteosat

13. Evaluating feedbacks ΔSST Δ Atmosphere Coupling: Coupled - Uncoupled 12 -3 -1
Fallmann et al (GRL)

14. Thank you