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A new approach to nowcasting at the Met Office Brian Golding Head of Forecasting Research

A new approach to nowcasting at the Met Office Brian Golding Head of Forecasting Research Met Office. 8 th September 2005. Acknowledgements. Clive Pierce Stephen Moseley Humphrey Lean Nigel Roberts Malcolm Kitchen Roger Saunders Peter Clark Roderick Smith Bob Moore Vicky Bell

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A new approach to nowcasting at the Met Office Brian Golding Head of Forecasting Research

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  1. A new approach to nowcasting at the Met Office Brian Golding Head of Forecasting Research Met Office 8th September 2005

  2. Acknowledgements • Clive Pierce • Stephen Moseley • Humphrey Lean • Nigel Roberts • Malcolm Kitchen • Roger Saunders • Peter Clark • Roderick Smith • Bob Moore • Vicky Bell • & many others

  3. Outline • History: Nowcasts from combined extrapolation & NWP • Improved observational & NWP forecast data • Future: Nowcasts from post-processed convective scale NWP

  4. Scales of Motion & Predictability 1000km 100km 10km 1km ExtratropicalCyclone Space Scale MCS* Front Thunderstorm Hail shaft Lifetime Predictability Nowcast 10mins 1 hr 12hrs 3 days 30mins 3 hrs 36hrs 9 days 5mins 30min 6hrs 36hrs * Mesoscale Convective System

  5. Log Scale Nowcast (Extrapolation Forecast) Limit (“Perfect Forecast”) Information Content NWP (Model Forecast) 10 0.01 0.1 1 Log Scale (days) Forecast Length Basis for nowcasting

  6. Log Scale Nowcast (Extrapolation Forecast) Limit (“Perfect Forecast”) Nimrod Information Content NWP (Model Forecast) 10 0.01 0.1 1 Log Scale (days) Forecast Length Basis for Nimrod

  7. Nimrod • Nimrod implemented 1996 • 6 hour nowcasts of: • Precipitation rate, accumulation & type • Lightning • Visibility, low cloud, temperature & humidity • Wind & pressure • Based on • Fusion of observation sources including radar & satellite • Extrapolation of analysis • Merged with mesoscale NWP output • 5/15km grid – now 2km for precipitation • Hourly output - now 5mins for precipitation • Updated hourly - now 15 mins for precipitation

  8. Log Scale Nowcast (Extrapolation Forecast) Limit (“Perfect Forecast”) Hi Res NWP Information Content NWP (Model Forecast) 10 0.01 0.1 1 Log Scale (days) Forecast Length Basis for using convective scale NWP

  9. Improved observational & NWP forecast data • Radar: Radarnet IV, implemented summer 2005 • Satellite: Autosat IV, Meteosat 8, implemented summer 2004 • Convective Scale NWP, implemented 2005

  10. Improved radar processing • Centralised processing system • Processing in polar coordinates • Vertical profile correction for bright band, beam filling, attenuation & range • Clutter/anaprop removal using advanced signal processing & complementary data sources (satellite, lightning, surface) • Main benefit – • improved infilling in areas of clutter contamination

  11. Improved radar clutter handling Radarnet III Radarnet IV Frequency of detection (Jersey) Rain rate (Corse Hill)

  12. Meteosat-8 Cloud Processing • Cloud mask - combination of single channel threshold tests: • Meteosat-7: Vis, IR • Meteosat-8: Vis0.6, Vis 0.8, IR3.9, IR8.7, IR10.8, IR12.0 • Cloud top height - • Meteosat-7: match of satellite BT to NWP forecast temperature allowing for boundary layer stability. (IR only) • Meteosat-8: variational fit of satellite BTs to NWP forecast BTs allowing for boundary layer stability (IR6.2, IR7.3, IR8.7, IR10.8, IR12.0, IR13.4 ) • Main benefit – • Low cloud detection at night, especially for use in fog nowcasting and spurious radar echo removal

  13. Improved Cloud Top Height Visible Meteosat 7 Meteosat-8 IR

  14. Convective Scale NWP • 3-hourly 4km UK NWP system, available within 1 hour of data time, implemented 2005 • On-demand 1km sub-area model on trial • Hourly 1km UK NWP system, available within 1 hour of data time, planned for implementation by 2010

  15. Boscastle flash flood (16/8/2004) accumulations using 1km grid convective scale NWP 1km Radar

  16. Visibility prediction with High Resolution NWP Visibility (m)

  17. Scale dependent forecast skill for heavy rain location in 16 forecasts from 4 cases in summer 2003

  18. Future: Nowcasts from post-processed NWP • Integral part of NWP suite – all model output processed – minimise changes, consistent with benefit. • Products transparent to different models at different lead times (initially T+36 with Rapid Update to T+6 but will be extended to T+144 or longer) • Regrid and downscale to 2km: • Model currently on 4km grid, but only resolves >~20km scales & orography • Rapid Update Cycle to adjust key variables to latest observations • Hourly (15min for precipitation), regardless of model update frequency • Update precipitation to Radar analysis every 15 minutes • Update visibility, cloud (& T, RH) to satellite/surface observations every hour • Update wind/pressure to surface observations every hour • Adjust early part of each forecast towards extrapolated analysis • Integrate uncertainty estimates for all variables • Diagnose required products from updated variables

  19. UK NWP model & post-processing domains • New 4km grid, ~20km resolution, UK NWP model, implemented 2005 • Products downscaled to 2km grid & topography on standard projection and domain

  20. Rapid Update : Precipitation Rate • Generate analysis using radar where it has good visibility, and elsewhere using variational blending of: • Meteosat visible and infrared imagery calibrated against radar • Lightning fixes (minimum rain rate) • A very short period forecast • Surface in situ weather reports • Generate adjusted NWP forecast using STEPS: • Compute advection field using optic flow algorithm, • Decompose analysis into scale cascade, • Merge model, advected analysis, autocorrelated noise, orographic enhancement, • Recompose forecast.

  21. Precipitation analysis & forecast

  22. Precipitation analysis & forecast

  23. Precipitation analysis & forecast

  24. Precipitation analysis & forecast

  25. Precipitation analysis & forecast

  26. Precipitation analysis & forecast

  27. Precipitation analysis & forecast

  28. Precipitation analysis & forecast

  29. Precipitation analysis & forecast

  30. Precipitation analysis & forecast

  31. Precipitation analysis & forecast

  32. Precipitation analysis & forecast

  33. Satellite images define highest cloud layer Model forecast & surface visual cloud type observations define intermediate cloud layers Surface visual & instrumental observations define lower cloud layers Rapid Update: Cloud 29 horizontal levels, focussed near ground Analysis uses METEOSAT imagery & surface observations with forecast 1st guess Precipitating cloud moved with precipitation vectors Non-precipitating cloud moved with model layer wind Merged with model cloud forecast

  34. Observations Satellite area of fog/low cloud x x x x x x x x x Rapid Update: Visibility • 2km grid; liquid water temperature and total water variables • analysis based on METEOSAT-8 imagery and surface observations • extrapolation forecast is persistence • merged with mesoscale model forecast • hill fog added from cloud forecast

  35. Visibility analysis

  36. Uncertainty estimates Presenting NWP positional forecast uncertainty for Boscastle flash flood (a) exceedance probability (b) probable maximum for warning areas • Precipitation – STEPS noise cascade • Fog – humidity pdf • Snow – land height / fractional melting • Lightning – flash rate expressed as return period • Severe weather – Bayesian predictor combination • NWP positional uncertainty • Short Range EPS • ECMWF EPS

  37. Diagnostic outputs • Precipitation type • Snow & hail probability, freezing rain & drizzle • Severe Weather indicators • Large hail, frequent lightning & tornadoes • Road State • MOSES-PDM-RFM • Soil moisture, run-off & river flow • Fire risk • Structural icing risk • Strong wind risk to power transmission • Distinguish normal impact risk from probability of extreme impact beyond normal planned responses.

  38. Simulated river flow for Carlisle flood (8/1/2005) <85% 85-95% 95-100% >100%

  39. Summary • Expect 1km grid NWP to be operational by 2010 • Improved NWP resolution restricts value of extrapolation to shorter lead times • Extrapolation is one of many techniques for enhancing the value of NWP output • Incorporate nowcasting in NWP post-processing: • downscaling to standard grid • frequent rapid update to latest radar/satellite/in situ observations • incorporation of uncertainty • diagnosis of impacts.

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