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Purpose of the Project

Radar Observations of Fog Layers R. Boers contributions from H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman 18.06.2014. Purpose of the Project. To assess the fog detection capabilities of ground based remote sensing instruments [in particular cloud radar, 35GHz].

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Purpose of the Project

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  1. Radar Observations of Fog LayersR. Boerscontributions from H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman18.06.2014

  2. 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.

  3. 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?

  4. Meteorological definition of fog is based on visibility only,i.e. it is a definition based on ‘diffuse’ principlesAre we dealing with droplets, aerosols, spiders, anything? Fog: visibility less than 1000 m Dense fog: visibility less than 200 m Very dense fog: visibility less than 50 m Mist: visibility more than 1000 m, less than 5000m Haze: restriction of visibility by dry aerosols (RH < 80%)

  5. 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

  6. Procedure to acquire a VIS-RAD product Measure radar reflectivity [up to many km away from observer] …….…………………………………………………………. radar Establish local link between radar reflectivity and visibility Measure visibility locally Use local link to convert entire radar signal to visibility

  7. Cabauw Experimental Site for Atmospheric Research [CESAR] Cabauw

  8. Fog detection configuration at the Cabauw Experimental Site for Atmospheric Research (CESAR) Radar, lidar, microwave radiometer location Normal cloud radar configuration View angle adapted for fog configuration Visibility sensors Aerosol size spectra Thermodynamics

  9. Installatie van reflectorplaat op CabauwFase 1 [December 2010]Fase 2 [Februari 2011]

  10. Interpretation of the next pictures Top of fog layer Radar signal path reflector 3.4 degrees radar fog

  11. The puzzling conversion ofradar reflectivity to visibility end begin Measure visibility with standard visibility detectors at the same time

  12. The puzzling conversion ofradar reflectivity to visibility end begin Measure visibility with standard visibility detectors at the same time

  13. 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

  14. What is droplet activation? Köhler curves The growth of every dry aerosol particle when it takes up water is prescribed by a Köhler curve The domain of fog droplets Small particle Bigger particle Even bigger particle The domain of wet aerosol (Hilding Köhler, 1888-1982; Professor for Meteorology, Uppsala, S)

  15. 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)

  16. Fog droplet growth

  17. Condensation and evaporation of fog are distinctly different The onset and disappearance of fogs is very sudden Clouds and fogs have distinct edges

  18. Modelled droplet activation (12000 dry particles to start with)

  19. 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]

  20. The link between radar reflectivity and visibility Model evaporation condensation

  21. 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

  22. Thank you!

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