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Discussion papers
https://doi.org/10.5194/acp-2018-1204
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1204
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 Nov 2018

Research article | 30 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Cloud Droplet Growth in Shallow Cumulus Clouds Considering 1D and 3D Thermal Radiative Effects

Carolin Klinger1,2, Graham Feingold1, and Takanobu Yamaguchi1,3 Carolin Klinger et al.
  • 1Chemical Sciences Division, NOAA Earth System Research Laboratory (ESRL), Boulder, Colorado, USA
  • 2Ludwig-Maximilians-Universität München, Lehrstuhl für Experimentelle Meteorologie
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Boulder, CO, USA

Abstract. The effect of 1D and 3D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size resolved cloud microphysics. A two step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model with bin resolved microphysics is used where cloud droplets are grown along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and rain production. Droplets grow to larger size bins in the 10–30μm radius range. The main effect in terms of rain production arises from recirculating parcels, where a small number of droplets is exposed to strong thermal cooling at cloud edge. These recirculating parcels, comprising about 6–7% of all parcels investigated, make up 45% of the accumulated rain rate for the no radiation simulation and up to 60% when 3D radiative effects are considered. The effect of 3D thermal radiation on rain production is stronger than that of 1D thermal radiation. 3D thermal radiation can enhance the rain rate up to 40% compared to standard droplet growth without radiative effects in this idealized framework.

In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. 3D thermal radiative effects again exceed 1D thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling, and stronger updraft velocities in the cloud center. It is shown that an evaporation-circulation feedback reduces the amount of rain produced in simulations where 3D thermal radiation is applied to microphysics and dynamics, in comparison where 3D thermal radiation is only applied to dynamics.

Carolin Klinger et al.
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Short summary
The effect of 1D and 3D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size resolved cloud microphysics. A two step approach is used for separating microphysical effects from dynamical feedbacks. In a parcel framework the main effect on rain production arises from recirculating parcels. Large eddy simulations show that radiative effects on dynamics are stronger than on microphysics, as far as rain production is concerned.
The effect of 1D and 3D thermal radiation on cloud droplet growth in shallow cumulus clouds is...
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