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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-85
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-85
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  23 Apr 2020

23 Apr 2020

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A revised version of this preprint is currently under review for the journal AMT.

Microwave single scattering properties of non-spheroidal rain drops

Robin Ekelund1, Patrick Eriksson1, and Michael Kahnert1,2 Robin Ekelund et al.
  • 1Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
  • 2Research Department, Swedish Meteorological and Hydrological Institute, Folkborgsvägen 17, 601 76 Norrköping, Sweden

Abstract. Falling rain drops undergo a change in morphology as they grow in size and the fall-speed increases. This change can lead to significant effects in passive and active microwave remote sensing measurements, typically in the form of a polarization signal. Because previous studies generally only considered either passive or active measurements and a limited set of frequencies, there exist no general guidelines on how and when to consider such rain drop effects in scientific and meteorological remote sensing. In an attempt to provide an overview on this topic, this study considered passive and active remote sensing simultaneously and a wider set of frequencies than in previous studies. Single scattering properties (SSP) data of horizontally oriented rain drops were calculated using the T-matrix method at a large set of frequencies (34 in total). The shapes of the rain drops were calculated assuming an aerodynamic equilibrium model, resulting in drops with flattened bases. The SSP data are published in an open-access repository in order to promote the usage of realistic microphysical assumptions in the microwave remote sensing community. Furthermore, the SSP were employed in radiative transfer simulations of passive and active microwave rain observations, in order to investigate the impact of rain drop shape upon observations and to provide general guidelines on usage of the published database. Several instances of significant rain drop shape-induced effects could be identified. For instance, it was found that the flattened base of equilibrium drops can lead to an enhancement in back-scattering at 94.1 GHz. The passive simulations showed shape induced effects of over 1 K at brightness temperatures below 150 GHz.

Robin Ekelund et al.

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Robin Ekelund et al.

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Microwave single scattering properties of non-spheroidal rain drops R. Ekelund, P. Eriksson, and M. Kahnert https://doi.org/10.5281/zenodo.3700744

Robin Ekelund et al.

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Short summary
Rain drops become flattened due to aerodynamic drag, as they increase in mass and fall-speed. The shape of such rain drops was calculated, and the electromagnetic interaction between microwave radiation and the rain drops was calculated. The calculations are made publicly available to the scientific community, in order to promote accurate representations of rain drops in measurements. Tests show that the drop shape can indeed have a noticeable effect on microwave observations of heavy rainfall.
Rain drops become flattened due to aerodynamic drag, as they increase in mass and fall-speed....
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