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

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

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) UKA2 - 1.5km 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. 2017 (GMD)

UK Domain (1.5km) Atmosphere (UKA): 950 x 1025 cells with 0.0135° 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

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 ... http://raanz.org.nz/wiki/pmwiki.php?n=TM.Meteorology http://coastalcare.org/2012/12/mercury-in-coastal-fog-a-study/ http://www.discoverceredigion.co.uk/English/where/Coast/wildlifeandnaturalfeatures/Pages/River-estuaries.aspx

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

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 [..]

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

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 -1 2.5 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

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

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] 10 14 18 22 26 0 0.2 0.4 0.6 0.8 1.0 -0.2 0.2 Cloud fraction Tpot Vert. Velocity [ms-2]

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

Evaluating feedbacks ΔSST Δ Atmosphere Coupling: Coupled - Uncoupled 1 2 -3 -1 Fallmann et al. 2017 (GRL)

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