Cabauw dag , R. Boers 1 Radar Observations of Fog Layers R. Boers contributions from H. Klein Baltink, J. Hemink, F. Bosveld, and M. Moerman

Cabauw dag , R. Boers 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.

Cabauw dag , R. Boers 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?

Cabauw dag , R. Boers 4 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 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%)

Cabauw dag , R. Boers 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

Cabauw dag , R. Boers 6 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

Cabauw dag , R. Boers 7 Cabauw Cabauw Experimental Site for Atmospheric Research [CESAR]

Cabauw dag , R. Boers 8 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 spectra Thermodynamics

Cabauw dag , R. Boers 9 Installatie van reflectorplaat op Cabauw Fase 1 [December 2010] Fase 2 [Februari 2011]

Cabauw dag , R. Boers 10 Interpretation of the next pictures radar reflector fog 3.4 degrees Radar signal path Top of fog layer

Cabauw dag , R. Boers 11

Cabauw dag , R. Boers 12 The puzzling conversion of radar reflectivity to visibility Measure visibility with standard visibility detectors at the same time begin end

Cabauw dag , R. Boers 13

Cabauw dag , R. Boers 14 The puzzling conversion of radar reflectivity to visibility Measure visibility with standard visibility detectors at the same time begin end

Cabauw dag , R. Boers 15 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

Cabauw dag , R. Boers 16 (Hilding Köhler, ; Professor for Meteorology, Uppsala, S) What is droplet activation? Köhler curves The growth of every dry aerosol particle when it takes up water is prescribed by a K ö hler curve Small particle Bigger particle Even bigger particle The domain of wet aerosol The domain of fog droplets

Cabauw dag , R. Boers 17 (Hilding Köhler, ; 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

Cabauw dag , R. Boers 18 Fog droplet growth

Cabauw dag , R. Boers 19 Condensation and evaporation of fog are distinctly different The onset and disappearance of fogs is very sudden Clouds and fogs have distinct edges

Cabauw dag , R. Boers 20 Modelled droplet activation (12000 dry particles to start with)

Cabauw dag , R. Boers 21 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]

Cabauw dag , R. Boers 22 The link between radar reflectivity and visibility Model condensation evaporation

Cabauw dag , R. Boers 23 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

Cabauw dag , R. Boers 24

Cabauw dag , R. Boers 25 Thank you!