Studies carried out in the late 1970s suggest that a simple linear relationshipexists in practice between the optical extinction in the thermal IR and theliquid water content (LWC) in fogs. Such a relationship opens thepossibility to monitor the vertical profile of the LWC in fogs with a rathersimple backscatter lidar. Little is known on how the LWC varies as afunction of height and during the fog life cycle, so the new measurementtechnique would help understand fog physics and provide valuable data forimproving the quality of fog forecasts. In this paper, the validityof the linear relationship is revisited in the light of recent observationsof fog droplet size distributions measured with a combination of sensorscovering a large range of droplet radii. In particular, large droplets(radius above 15 μm) are now detected, which was not the case in the late1970s. The results confirm that the linear relationship still holds, at least forthe mostly radiative fogs observed during the campaign. The impact of theprecise value of the real and imaginary parts of the refractive index on thecoefficient of the linear relationship is also studied. The usual practiceconsiders that droplets are made of pure water. This assumption is probably validfor big drops, but it may be questioned for small ones since droplets are formedfrom condensation nuclei of highly variable chemical composition. The studysuggests that the precise nature of condensation nuclei will primarily affectrather light fogs with small droplets and light liquid water contents.
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