TECO, R. Boers 18 October 2012
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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:
Dense fog:
Very dense fog:
Mist:
Haze:
visibility less than 1000 m
visibility less than 200 m
visibility less than 50 m
visibility more than 1000 m, less than 5000m
restriction of visibility by dry aerosols
(RH < 80%)
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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
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Procedure to acquire a VIS-RAD product
Measure radar reflectivity [up to many km away from observer]
radar
…….………………………………………………………….
Measure visibility locally
Establish local link between radar
reflectivity and visibility
Use local link to convert entire
radar signal to visibility
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Cabauw Experimental Site for Atmospheric Research [CESAR]
Cabauw
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Fog detection configuration at the Cabauw Experimental Site
for Atmospheric Research (CESAR)
Normal cloud radar configuration
View angle
adapted for fog
configuration
Radar, lidar, microwave
radiometer location
Visibility sensors
Aerosol size
spectra
Thermodynamics
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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
Top of fog layer
reflector
3.4 degrees
radar
fog
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The puzzling conversion of
radar reflectivity to visibility
Measure visibility with standard visibility detectors
at the same time
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The puzzling conversion of
radar reflectivity to visibility
Measure visibility with standard visibility detectors
at the same time
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
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What is droplet activation? Köhler curves
The growth
of every dry
The domain
ofaerosol
fog
particle when it takes up water is
prescribeddroplets
by a Köhler curve
Small particle
Bigger particle
Even bigger particle
The domain of wet aerosol
(Hilding Köhler, 1888-1982; Professor for Meteorology, Uppsala, S)
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A movie of droplet activation
Droplet growth is proportional
to the difference between
RH and Es
Ambient relative humidity (RH)
Equilibrium saturation relative
humidity at the
surface of individual particle (Es)
(Hilding Köhler, 1888-1982; Professor for Meteorology, Uppsala, S)
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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
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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]
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The link between radar reflectivity and visibility
Model
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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
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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!