Transcript Met Office forecasting methods for fog and low cloud

Met Office forecasting
methods for fog and low cloud
B.Golding
Head of Forecasting Research
Met Office, UK
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Outline
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NWP
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Site-specific model
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Nowcast system
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Manual techniques
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Plans
Unified Model NWP
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UK mesoscale model
– ~12.5km grid, 38 levels
– v5 (June 2002): non-hydrostatic, semi-implicit, semi-Lagrangian
– conserved thermodynamic variables, two phase cloud, CAPE closure
convection
– predicted aerosol
– visibility recently upgraded to allow for rain and snow as well as cloud water
– verification shows skill limited to large scale fog events
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SSFM
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Single column UM with 70 levels
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Uses local upwind topography/land use
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Verification shows skill limited by lack of local data input
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Used as input trial product generation in AutoTAF and Autotext
High resolution land use and
orography data
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SSFM graphical output
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Autotext example
Predicción para MADRID 320016 válida desde 00:00 el
21/02/2002 hasta 12:00 el 22/02/2002
Madrugada
Cielo: despejado
Temperatura: temperaturas sin cambio a 4 grados C
Viento: oeste girando al noroeste 3
Tiempo: seco
Visibilidad: buena
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Nimrod
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Precipitation
– 2km England & Wales, updated every 15 mins
– 5km Europe, updated hourly
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Cloud: 15km, NW Europe, updated hourly
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Visibility: 5km NW Europe, updated hourly
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Others: lightning, wind, pressure
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All nowcasts to 6 hours ahead
Nimrod - Cloud
– 15km grid, 29 horizontal levels
– analysis uses METEOSAT imagery and surface observations with forecast
first guess
– precipitating cloud moved with precipitation vectors
– non-precipitating cloud moved with model layer wind
– merged with mesoscale model cloud forecast
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Nimrod cloud data
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
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Nimrod cloud base forecast
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Nimrod - visibility
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5km grid; liquid water temperature and total water variables
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analysis based on METEOSAT / AVHRR imagery and surface observations
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nowcast is persistence
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merged with mesoscale model forecast
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hill fog added from cloud forecast
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used as input to Time & Place product
Visibility Analysis
Observations
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Satellite area
of fog/low cloud
Nimrod visibility forecast
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Manual techniques
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Saunders or Craddock & Prichard used to forecast fog point, and hence fog
formation with a cooling curve
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Jefferson or Barthrum used to predict fog clearance temperature, and hence fog
clearance with a warming curve
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Both temperatures better predicted by SSFM
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Problem is with cooling/warming curve shape, especially in presence of cloud
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Local experience can be a major benefit
Summary
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Guidance provided by NWP, SSFM, Nimrod
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Experimental automated services on trial
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Manual forecast techniques unhelpful
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High impact forecasts (e.g. TAFs) require human experience and
judgement
Plans
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MSG – fog product, cloud mask, cloud top height
– Improved analysis
– Extrapolation of temporal trends
– Fog simulation studies with RTTOV
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High resolution NWP