22 May 2023
 | 22 May 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Technical note: Bimodal Parameterizations of in situ Ice Cloud Particle Size Distributions

Irene Bartolomé García, Odran Sourdeval, Reinhold Spang, and Martina Krämer

Abstract. The cloud particle size distribution (PSD) is a key parameter for the retrieval of micro-physical and optical properties from remote sensing instruments, which in turn are necessary for determining the radiative effect of clouds. Current representations of PSDs for ice clouds rely on parameterizations that were largely based on in situ measurements where the distribution of small ice crystals were uncertain. This makes current parameterizations deficient to simulate remote sensing observations sensitive to small ice, such as from lidar and thermal infrared instruments. In this study we fit the in situ PSDs of ice crystals from the JULIA (JÜLich In situ Aircraft data set) database, which consists of 11 campaigns covering the tropics, mid-latitudes and the Arctic, consistently processes and considered more robust in their measurements of small ice. For the fitting, we implement an established approach to PSD parameterizations, which consists in finding an adequate set of parameters for a modified gamma function after normalization of both PSD axes. These parameters are constrained to match in-situ mea surements when predicting microphysical properties from the PSDs, via a cost function minimization method. We selected the ice water content and the ice crystal number concentration, which are currently key parameters for modern satellite retrievals and model microphysics schemes. We found that a bimodal parameterization yields better results than a monomodal one. The bimodal parameterization has a lower spread for almost all ice crystal sizes over the entire range of analyzed temperatures and fits better the observations, especially for particles between 20 and about 110 μm at temperatures between −60 and −20 °C. For this temperature range, the root mean square error for the retrieved Nice is reduced from 0.36 to 0.20. This demonstrates a clear advantage to considering the bimodality of PSDs, e.g. for satellite retrievals.

Irene Bartolomé García et al.

Status: open (until 03 Jul 2023)

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Irene Bartolomé García et al.

Irene Bartolomé García et al.


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Short summary
How many ice crystals of each size are in a cloud is a key parameter for the retrieval of cloud properties. The distribution of ice crystals is obtained from in situ measurements and used to create parameterizations that can be used when analyzing the remote sensing data. Current parameterizations are based in datasets that do not include realible measurements of small crystals, but in our study we use a dataset that includes very small ice crystals to improve these parameterizations.