A comparative analysis of in situ measurements of high-altitude cirrus in the tropics

Francesco Cairo (1) , Martina Krämer (2, 3) , Armin Afchine (2) , Luca Di Liberto (1) , Sergey Khaykin (4) , Lorenza Lucaferri (5) , Valentin Mitev (6) , Max Port (3, 7) , Christian Rolf (2) , Marcel Snels (1) , Nicole Spelten (2) , Ralf Weigel (3) , Stephan Borrmann (3, 8)

1 ISAC - Istituto di Scienze dell'Atmosfera e del Clima
2 IEK-7 - Institut für Energie- und Klimaforschung - Stratosphäre
3 IPA - Institute for Atmospheric Physics [Mainz]
4 TROPO - LATMOS
5 Dipartimento di Fisica [Roma]
6 CSEM - Centre Suisse d'Electronique et de Microtechnique SA [Neuchatel]
7 Montessori-Zentrum Hofheim e.V.
8 MPIC - Max-Planck-Institut für Chemie

We analyze cirrus cloud measurements from two dual-instrument cloud spectrometers, two hygrometers and a backscattersonde in view to connect cirrus optical parameters usually accessible by remote sensing with microphysical size resolved and bulk properties accessible in situ. Specifically, we compare the particle backscattering coefficient and depolarization ratio to the particle size distribution, effective and mean radius, surface area density, particle aspherical fraction and ice water content. Data have been acquired by instruments on board the M55 Geophysica research aircraft during July and August 2017 during the Asian Monsoon campaign based in Kathmandu, Nepal, in the framework of the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) project. Cirrus have been observed over the Hymalaian region between 10 km and the tropopause, situated at 17–18 km. The observed particle number densities varied between 10 and 10-4 cm-3 in the dimensional range from 1.5 to 468.5 μm in radius. Correspondingly, backscatter ratios from one tenth up to 50 have been observed. Optical scattering theory has been used to compare the backscattering coefficient computed from measured particle size distribution with those directly observed by the backscattersonde. The aspect ratio of the particles, modeled as spheroids for the T-matrix approach, was left as a free parameter to match the calculations to the optical measures. The computed backscattering coefficient can be set in good agreement with the observed one, but the match between simulated and determined depolarization ratios is insufficient, however. Relationships between ice particle concentration, mean and effective radius, surface area density and ice water content with the measured backscattering coefficient are investigated for an estimate of the bulk microphysical parameters of cirrus clouds from remote sensing lidar data. The comparison between particle depolarization and aspherical fraction as measured by one of the cloud spectrometers equipped with a detector for polarization, represents a novelty since it was the first time the two instruments are operated simultaneously on aircraft. The analysis shows the difficulty of establishing an univocal link between depolarization values and the presence and amount of aspherical scatterers. This suggests the need of further investigation that could take into consideration not only the fraction of aspheric particles but also their predominant morphology.

Domaines: Météorologie

Francesco Cairo, Martina Krämer, Armin Afchine, Luca Di Liberto, Sergey Khaykin, et al.. A comparative analysis of in situ measurements of high-altitude cirrus in the tropics. Atmospheric Measurement Techniques, 2023, 16, pp.4899-4925. ⟨10.5194/amt-16-4899-2023⟩. ⟨insu-04008870v2⟩