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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Radar Observations of Fog Layers

R. Boers, H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman

18.10.2012


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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.


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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?


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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%)


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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]

Measure visibility locally

radar …….……………………………………………………

…….Establish local link between radar

reflectivity and visibility

Use local link to convert entire radar signal to visibility


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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


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The puzzling conversion ofradar reflectivity to visibility

Measure visibility with standard visibility detectors at the same time

begin

end


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The puzzling conversion ofradar reflectivity to visibility

Measure visibility with standard visibility detectors at the same time

begin

end


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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


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


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


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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

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


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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!

Documents

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