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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-415
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
08 May 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
The Role of 1D and 3D Radiative Heating on the Organization of Shallow Cumulus Convection and the Formation of Cloud Streets
Fabian Jakub and Bernhard Mayer Meteorological Institute, Ludwig Maximilian Universität München, LMU
Abstract. The formation of shallow cumulus cloud streets was historically attributed primarily to dynamics. Here, we focus on the interaction between radiatively induced surface heterogeneities and the resulting patterns in the flow. Our results suggest that solar radiative heating has the potential to organize clouds perpendicular to the sun's incidence angle.

To quantify the extent of organization, we performed a high resolution LES parameter study. We varied the horizontal wind speed, the surface heat capacity, the solar zenith and azimuth angles, as well as radiative transfer parameterizations (1D and 3D). As a quantitative measure we introduce a simple algorithm that provides a scalar quantity for the degree of organization and the alignment. We find that, in the absence of a horizontal wind, 3D radiative transfer produces cloud streets perpendicular to the sun's incident direction, whereas the 1D approximation or constant surface fluxes produce circular, randomly positioned, clouds. Our reasoning for the enhancement or reduction of organization is the geometric position of the cloud's shadow and the corresponding surface fluxes. Furthermore, when increasing horizontal wind speeds to 5 or 10 m s−1, we observe the development of dynamically induced cloud streets. If in addition, solar radiation illuminates the surface beneath the cloud, i.e. when the sun is positioned orthogonal to the mean wind field and the solar zenith angle is larger than 20°, the cloud-radiative feedback has the potential to significantly enhance the tendency to organize in cloud streets. In contrast, in the case of the 1D approximation (or overhead sun), the tendency to organize is weaker or even prohibited because the shadow is cast directly beneath the cloud. The radiative feedback on surface heterogeneities is generally diminished for large surface heat capacities. We therefore expect radiative feedbacks to be strongest over land surfaces and weaker over the ocean. Given the results of this study we expect that simulations including shallow cumulus convection will have difficulties producing cloud streets if they employ 1D radiative transfer solvers or may need unrealistically high wind speeds to excite cloud street organization.


Citation: Jakub, F. and Mayer, B.: The Role of 1D and 3D Radiative Heating on the Organization of Shallow Cumulus Convection and the Formation of Cloud Streets, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-415, in review, 2017.
Fabian Jakub and Bernhard Mayer
Fabian Jakub and Bernhard Mayer
Fabian Jakub and Bernhard Mayer

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Short summary
The formation of shallow cumulus cloud streets was historically attributed primarily to dynamics. Here, we focus on the interaction between radiatively induced surface heterogeneities and the resulting patterns in the flow. Our results suggest that solar radiative heating has the potential to organize clouds perpendicular to the sun's incidence angle.
The formation of shallow cumulus cloud streets was historically attributed primarily to...
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