Correlations among the optical properties of cirrus‐cloud particles: Microphysical interpretation

Abstract

[1] Cirrus measurements obtained with a ground-based polarization Raman lidar at 67.9°N in January 1997 reveal a strong positive correlation between the particle optical properties, specifically depolarization ratio d par and extinction-to-backscatter (lidar) ratio S par , for d par < $40%, and an anticorrelation for d par > $40%. Over the duration of the measurements, both particle properties vary systematically. This effect is particularly pronounced in the case of d par , which decreases significantly with time. The analysis of lidar humidity and radiosonde temperature data shows that the measured optical properties stem from scattering by dry solid ice particles, while scattering by supercooled droplets, or by wetted or subliming ice particles, can be excluded. For the microphysical interpretation of the lidar measurements, ray-tracing computations of particle scattering properties have been used. The comparison with the theoretical data suggests that the observed cirrus data can be interpreted in terms of size, shape, and growth of the cirrus particles, the latter under the assumption that the lidar measurements of consecutive cloud segments can be mapped on the temporal development of a single cloud parcel moving along its trajectory: Near the cloud top in the early stage of cirrus development, light scattering by nearly isometric particles that have the optical characteristics of hexagonal columns (short, column-like particles) is dominant. Over time, the ice particles grow, and as the cloud base height extends to lower altitudes characterized by warmer temperatures they become morphologically diverse. For large S par and depolarization values of $40%, the scattering contributions of column-and plate-like particles are roughly the same. In the lower ranges of the cirrus clouds, light scattering is predominantly by plate-like ice particles. This interpretation assumes random orientation of the cirrus particles. Simulations with a simple model suggest, however, that the positive correlation between S par and d par , which is observed for depolarization ratios <40% mainly at low cloud altitudes, can be alternatively explained by horizontal alignment of a fraction of the cirrus particle population.

Potential of polarization/Raman lidar to separate fine dust, coarse dust, maritime, and anthropogenic aerosol profiles

“…The technique has been used for a long time to monitor and investigate cirrus cloud systems (e.g., Sassen, 1991Sassen, , 2005Reichardt et al, 2002Reichardt et al, , 2008 and polar stratospheric cloud evolution (see, e.g., Browell et al, 1990;Achtert and Tesche, 2014). The method is well suited to study heterogeneous ice formation in mixed-phased clouds (e.g., Sassen et al, 2003;Ansmann et al, 2005Ansmann et al, , 2008Ansmann et al, 2009a;Seifert et al, , 2011 and liquidwater cloud developments (e.g., Bissonnette, 2005;Donovan et al, 2015).…”

Lidar characterization of the Arctic atmosphere during ASTAR 2007: four cases studies of boundary layer, mixed-phase and multi-layer clouds

“…They found lidar ratios as low as 3 sr, albeit frequently in conjunction with very high depolarization. The values of lidar ratio and depolarization of case C seem to best match with plates of high aspect ratio (Reichardt et al, 2002, their Fig. 7).…”

“…Our observations of the LR at 532 nm agree with these values in the given standard deviation. Reichardt et al (2002) calculated LR and depolarization of (almost) hexagonal particles in random orientation with a ray tracing code. They found lidar ratios as low as 3 sr, albeit frequently in conjunction with very high depolarization.…”

Mountain wave PSC dynamics and microphysics from ground-based lidar measurements and meteorological modeling

“…Theoretical optical data of irregular and hexagonal particles, spheroids (aspect ratios =1), and spheres were obtained with, respectively, finite-difference time-domain, T-matrix, and Mie computations. Results of geometrical-optics calculations for hexagonal ice columns are also shown (gray marks, center, right; aspect ratios between 1 and 2.5, slightly distorted shape, see Hess et al, 1998;Reichardt et al, 2002b). The wavelength is 355 nm.…”

“…Lidar-ratio (S par ) data have proven valuable in microphysical studies of tropospheric clouds (Reichardt et al, 2002b), and show great potential in PSC investigations (Reichardt et al, 2002c). In order to have S par information available for the PSC under investigation, we calculate the PSC-layer mean value of S par , S par , as PSC optical depth τ divided by integrated PSC backscatter coefficient (lidar-ratio profiles cannot be determined because of insufficient Raman signal strength above ∼20 km).…”