Properties and Temporal Evolution of an Elevated Arctic Liquid Fog by Lidar

In this work, we present a case study mainly from Raman lidar observations and radiosonde data for a site in the European Arctic to analyse how aerosol grows into a purely water-containing fog. We use a relatively high resolution (85 s, 7.5m) for the lidar evaluation and use an inversion technique to derive aerosol size distributions before fog formation, in the fog and directly after fog disintegration. We find predominantly accumulation-size particles before and after the fog event. In the fog layer, this mode splits up into two branches: one is hydrophobic, and the other roughly doubles its size within 85 s but does not grow into typical cloud droplet sizes. We find very high lidar ratios > 100 sr for 532 nm. We use Mie calculations to confirm that such high lidar ratios are to be expected for micron-sized droplets if the imaginary part of the refractive index is not negligible. Further, we analyse how lidar ratio and colour ratio change as a function of cloud optical depth and derive that the formation and disintegration of several short-lived fog events at the same altitude seem to be symmetrical. Finally, we employ Koehler theory to show that the droplets should have grown into super-micron sizes. As this has not been observed clearly, the time for particle growth into cloud regime under low turbulence conditions takes longer than 85 s.