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TECO, R. Boers 18 October 2012
1
Radar Observations of Fog Layers
R. Boers, H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman
18.10.2012
TECO, R. Boers 18 October 2012
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Purpose of the Project
• To assess the fog detection capabilities of ground based remote sensing instruments [in particular cloud radar, 35GHz].
• To interpret the remote sensing data in terms of the physical processes that are responsible for fog formation.
• To arrive at a visibility product based on remote sensing data.
TECO, R. Boers 18 October 2012
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Why do we do this project?
• Fog is a restricting factor in aircraft movements at airports: Which instruments have added –value in air traffic control?
• Fog is a restricting factor in road traffic: What new information
can remote sensing instruments bring to contribute to road safety?
TECO, R. Boers 18 October 2012
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Meteorological definition of fog is based on visibility only,
i.e. it is a definition based on ‘diffuse’ principles
Are we dealing with droplets, aerosols, spiders, anything?
Fog: visibility less than 1000 mDense fog: visibility less than 200 mVery dense fog: visibility less than 50 m Mist: visibility more than 1000 m, less than 5000mHaze: restriction of visibility by dry aerosols
(RH < 80%)
TECO, R. Boers 18 October 2012
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In cloud physics there is a strict discrimination between water droplets and wet aerosol.
Wet aerosol: Aerosol particles having attracted water vapor RH < 100%Water droplets: Only form when RH > 100%
So: for fog mist haze, we need to understand the physics of wet aerosol AND water droplets
TECO, R. Boers 18 October 2012
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Procedure to acquire a VIS-RAD product
Measure radar reflectivity [up to many km away from observer]
Measure visibility locally
radar …….……………………………………………………
…….Establish local link between radar
reflectivity and visibility
Use local link to convert entire radar signal to visibility
TECO, R. Boers 18 October 2012
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Cabauw
Cabauw Experimental Site for Atmospheric Research [CESAR]
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Fog detection configuration at the Cabauw Experimental Site for Atmospheric Research (CESAR)
Radar, lidar, microwave radiometer location
View angle adapted for fog configuration
Normal cloud radar configuration
Visibility sensors Aerosol size spectraThermodynamics
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Installatie van reflectorplaat op Cabauw
Fase 1 [December 2010]Fase 2 [Februari 2011]
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Interpretation of the next pictures
radar
reflector
fog
3.4 degrees
Radar signal path
Top of fog layer
TECO, R. Boers 18 October 2012
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TECO, R. Boers 18 October 2012
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The puzzling conversion ofradar reflectivity to visibility
Measure visibility with standard visibility detectors at the same time
begin
end
TECO, R. Boers 18 October 2012
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TECO, R. Boers 18 October 2012
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The puzzling conversion ofradar reflectivity to visibility
Measure visibility with standard visibility detectors at the same time
begin
end
TECO, R. Boers 18 October 2012
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Can we understand the characteristic signature of the radar – visibility link?
Modelling the onset of fog
Use aerosol data at tower at 60 m, and model the evolution of the particle size spectra.
Modelling done during 1 cycle of a fog event cooling - warming
TECO, R. Boers 18 October 2012
16(Hilding Köhler, 1888-1982; Professor for Meteorology, Uppsala, S)
What is droplet activation? Köhler curvesThe growth of every dry aerosol
particle when it takes up water is prescribed by a Köhler curve
Small particle
Bigger particle
Even bigger particle
The domain of wet aerosol
The domain of fog droplets
TECO, R. Boers 18 October 2012
17(Hilding Köhler, 1888-1982; Professor for Meteorology, Uppsala, S)
A movie of droplet activation
Ambient relative humidity (RH)
Equilibrium saturation relative humidity at the
surface of individual particle (Es)
Droplet growth is proportional to the difference between
RH and Es
TECO, R. Boers 18 October 2012
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Fog droplet growth
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Condensation and evaporation of fog are distinctly different
The onset and disappearance of fogs is very sudden
Clouds and fogs have distinct edges
TECO, R. Boers 18 October 2012
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Modelled droplet activation (12000 dry particles to start with)
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Very few aerosol particles are activated to become cloud droplets!
[About 1% of total]
Why?
Because fog is equivalent of a cloudy air parcel moving upward at very low speed
(< 4 cm/s!)
So, only very few droplets can be activated[And some will evaporate again before
reaching maturity]
TECO, R. Boers 18 October 2012
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The link between radar reflectivity and visibility
Model
condensation
evaporation
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Conclusions
1) Most visibility reduction down to 1 km is attributable to swelling / wetting of aerosol but only water droplet activation is responsible for dense fog.
2) The process of condensation is not symmetric to evaporation
3) For dense fog [Vis < 700m] a radar visibility product can be made
4) For less dense fogs [700m < Vis < 1500m] a lidar visibility product should be contemplated
5) Fogs have less water droplets than clouds
TECO, R. Boers 18 October 2012
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Prospects
1) Design specs for radar (TNO / TUDelft) [on way]
2) Business case KNMI – TNO – TUDelft – KLM – Schiphol [not yet]
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Thank you!
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