Radar fog detection R. Boers

Cabauw dag 18.06.2014, R. Boers1

Radar Observations of Fog Layers

R. Boers

contributions from H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman

18.06.2014

Cabauw dag 18.06.2014, R. Boers2 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. Cabauw dag 18.06.2014, R. Boers3Why 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?

Cabauw dag 18.06.2014, R. Boers4

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%)

Cabauw dag 18.06.2014, R. Boers5In 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

Cabauw dag 18.06.2014, R. Boers6 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 visibilityUse local link to convert entire radar signal to visibilityCabauw dag 18.06.2014, R. Boers7

Cabauw

Cabauw Experimental Site for Atmospheric Research [CESAR]Cabauw dag 18.06.2014, R. Boers8

Fog detection configuration at the Cabauw Experimental Site for Atmospheric Research (CESAR)Radar, lidar, microwave radiometer location

View angle adapted for fog configurationNormal cloud radar configurationVisibility sensors Aerosol size spectraThermodynamics

Cabauw dag 18.06.2014, R. Boers9Installatie van reflectorplaat op Cabauw

Fase 1 [December 2010]Fase 2 [Februari 2011]

Cabauw dag 18.06.2014, R. Boers10 Interpretation of the next picturesradarreflectorfog3.4 degreesRadar signal pathTop of fog layer

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The puzzling conversion ofradar reflectivity to visibilityMeasure visibility with standard visibility detectors at the same time

beginendCabauw dag 18.06.2014, R. Boers13

Cabauw dag 18.06.2014, R. Boers14

The puzzling conversion ofradar reflectivity to visibilityMeasure visibility with standard visibility detectors at the same time

beginendCabauw dag 18.06.2014, R. Boers15Can we understand the characteristic signature of the radar visibility link?

Modelling the onset of fogUse 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

Cabauw dag 18.06.2014, R. Boers16(Hilding Khler, 1888-1982; Professor for Meteorology, Uppsala, S)What is droplet activation? Khler curves

The growth of every dry aerosol particle when it takes up water is prescribed by a Khler curve Small particleBigger particleEven bigger particleThe domain of wet aerosolThe domain of fog dropletsCabauw dag 18.06.2014, R. Boers17(Hilding Khler, 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 EsCabauw dag 18.06.2014, R. Boers18Fog droplet growth

Cabauw dag 18.06.2014, R. Boers19Condensation and evaporation of fog are distinctly different

The onset and disappearance of fogs is very sudden

Clouds and fogs have distinct edgesCabauw dag 18.06.2014, R. Boers20

Modelled droplet activation (12000 dry particles to start with)Cabauw dag 18.06.2014, R. Boers21Very 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]Cabauw dag 18.06.2014, R. Boers22The link between radar reflectivity and visibility

ModelcondensationevaporationCabauw dag 18.06.2014, R. Boers23Conclusions1) 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 cloudsCabauw dag 18.06.2014, R. Boers24

Cabauw dag 18.06.2014, R. Boers25

Thank you!