Preprints
https://doi.org/10.5194/amt-2021-185
https://doi.org/10.5194/amt-2021-185

  30 Jul 2021

30 Jul 2021

Review status: a revised version of this preprint is currently under review for the journal AMT.

Post-flight analysis of detailed size distributions of warm cloud droplets, as determined in situ by cloud and aerosol spectrometers

Sorin Nicolae Vâjâiac1, Andreea Calcan1, Robert Oscar David3, Denisa-Elena Moacă1,2, Gabriela Iorga2,4, Trude Storelvmo3, Viorel Vulturescu5, and Valeriu Filip1,6,7 Sorin Nicolae Vâjâiac et al.
  • 1National Institute for Aerospace Research "Elie Carafoli", Bucharest, Romania
  • 2University of Bucharest, Faculty of Physics, P.O.Box MG-11, Magurele, 077125, Bucharest, Romania
  • 3Department of Geosciences, University of Oslo, Oslo, Norway
  • 4University of Bucharest, Faculty of Chemistry, Department of Physical Chemistry (Physics Group), Bd. Regina Elisabeta 4-12, 030018, Bucharest, Romania
  • 5Politehnica University of Bucharest, Faculty of Industrial Engineering and Robotics, Theory of Mechanisms and Robots Department, Bucharest, Romania
  • 6Graduate School of Physics, University of Bucharest, Romania
  • 7Research Center for Surface Science and Nanotechnology, University POLITEHNICA of Bucharest, Romania

Abstract. Warm clouds, consisting of liquid cloud droplets, play an important role in modulating the amount of incoming solar radiation to Earth’s surface and thus, the climate. The size and number concentration of these cloud droplets control the reflectance of the cloud, the formation of precipitation and ultimately, the lifetime of the cloud. Therefore, in situ observations of the number and diameter of cloud droplets are frequently performed with cloud and aerosol spectrometers, which determine the optical diameters of cloud particles (in the range of up to a few tens of microns) by measuring their forward scattering cross sections in visible light and comparing these values with Mie-theoretical computations. The use of such instruments must rely on a fast working scheme consisting of a limited pre-defined uneven grid of cross section values that corresponds to a theoretically derived uneven set of size intervals (bins). However, as more detailed structural analyses of warm clouds are needed to improve future climate projects, we present a new numerical post-flight methodology using recorded particle-by-particle sample files. The Mie formalism produces a complicated relationship between a particle’s diameter and its forward scattering cross section. This relationship cannot be expressed in an analytically closed form and it should be numerically computed point by point, over a certain grid of diameter values. The optimal resolution required for constructing the diagram of this relationship is therefore analysed. Cloud particle statistics are further assessed using a fine grid of particle diameters in order to capture the finest details of the cloud particle size distributions. The possibility and the usefulness of using coarser size grids, with either uneven or equal intervals is also discussed. For coarse equidistant size grids, the general expressions of cloud microphysical parameters are calculated and the ensuing relative errors are discussed in detail. The proposed methodology is further applied to a subset of measured data and it is shown that the overall uncertainties in computing various cloud parameters are mainly driven by the measurement errors of the forward scattering cross section for each particle. Finally, the influence of the relatively large imprecision in the real and imaginary parts of the refractive index of cloud droplets on the size distributions and on the ensuing cloud parameters is analysed. It is concluded that, in the presence of high atmospheric loads of hydrophilic and light absorbing aerosols, such imprecisions may drastically affect the reliability of the cloud data obtained with cloud and aerosol spectrometers. Some complementary measurements for improving the quality of the cloud droplet size distributions obtained in post-flight analyses are suggested.

Sorin Nicolae Vâjâiac et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'cloud droplet size spectra', Anonymous Referee #2, 19 Aug 2021
  • RC2: 'Comment on amt-2021-185', Darrel Baumgardner, 21 Aug 2021

Sorin Nicolae Vâjâiac et al.

Sorin Nicolae Vâjâiac et al.

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Short summary
Warm clouds (with liquid droplets) play an important role in modulating the amount of incoming solar radiation to Earth’s surface and thus, the climate. The most efficient way to study them is by in situ optical measurements. The paper proposes a new methodology for providing more detailed and reliable structural analyses of warm clouds through post-flight processing of collected data. The impact of fine aerosol incorporation in water droplets might have on such measurements is also discussed.