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Eulerian and Lagrangian approaches to multidimensional condensation and collection
Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado, USA.
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Colorado, USA.
Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden; National Center for Atmospheric Research, USA.
Number of Authors: 42017 (English)In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 9, no 2, p. 1116-1137Article in journal (Refereed) Published
Abstract [en]

Turbulence is argued to play a crucial role in cloud droplet growth. The combined problem of turbulence and cloud droplet growth is numerically challenging. Here an Eulerian scheme based on the Smoluchowski equation is compared with two Lagrangian superparticle (or superdroplet) schemes in the presence of condensation and collection. The growth processes are studied either separately or in combination using either two-dimensional turbulence, a steady flow or just gravitational acceleration without gas flow. Good agreement between the different schemes for the time evolution of the size spectra is observed in the presence of gravity or turbulence. The Lagrangian superparticle schemes are found to be superior over the Eulerian one in terms of computational performance. However, it is shown that the use of interpolation schemes such as the cloud-in-cell algorithm is detrimental in connection with superparticle or superdroplet approaches. Furthermore, the use of symmetric over asymmetric collection schemes is shown to reduce the amount of scatter in the results. For the Eulerian scheme, gravitational collection is rather sensitive to the mass bin resolution, but not so in the case with turbulence. Plain Language Summary The bottleneck problem of cloud droplet growth is one of the most challenging problems in cloud physics. Cloud droplet growth is neither dominated by condensation nor gravitational collision in the size range of 15 mu m similar to 40 mu m [1]. Turbulence-generated collection has been thought to be the mechanism to bridge the size gap, i.e., the bottleneck problem. This study compares the Lagrangian and Eulerian schemes in detail to tackle with the turbulence-generated collection.

Place, publisher, year, edition, pages
2017. Vol. 9, no 2, p. 1116-1137
Keywords [en]
Eulerian Smoluchowski and Lagrangian superdroplet, collection, cloud droplets, turbulence, size spectra
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-146001DOI: 10.1002/2017MS000930ISI: 000406239300020OAI: oai:DiVA.org:su-146001DiVA, id: diva2:1136556
Available from: 2017-08-28 Created: 2017-08-28 Last updated: 2018-08-15Bibliographically approved
In thesis
1. Droplet growth in atmospheric turbulence: A direct numerical simulation study
Open this publication in new window or tab >>Droplet growth in atmospheric turbulence: A direct numerical simulation study
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This Ph.D. thesis examines the challenging problem of how turbulence affects the growth of cloud droplets in warm clouds. Droplets grow by either condensation or collision. Without turbulence, the condensation process driven by a uniform supersaturation field is only efficient when droplets are smaller than 15 μm (radius). Gravitational collision becomes effective when the radius of droplets is larger than 50 μm. The size gap of 15–50 μm, in which neither condensation nor collision processes dominate droplet growth, has puzzled the cloud microphysics community for around 70 years. It is the key to explaining the rapid warm rain formation with a timescale of about 20 minutes. Turbulence has been proposed to bridge this size gap by enhancing droplet growth processes, and thereby, to explain rapid warm rain formation. Since cloud–climate interaction is one of the greatest uncertainties for climate models, the fundamental understanding of rapid warm rain formation may help improve climate models.

The condensational and collisional growth of cloud droplets in atmospheric turbulence is essentially the problem of turbulence-droplet interaction. However, turbulence alone is one of the unresolved and most challenging problems in classical physics. The turbulence–droplet interaction is even more difficult due to its strong nonlinearity and multi-scale properties in both time and space. In this thesis, Eulerian and Lagrangian models are developed and compared to tackle turbulence–droplet interactions. It is found that the Lagrangian superparticle model is computationally less demanding than the Eulerian Smoluchowski model.

The condensation process is then investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations with droplets modeled as Lagrangian particles. With turbulence included, the droplet size distribution is found to broaden, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower droplet size distributions. Furthermore, the time evolution of droplet size distributions is observed to strongly depend on the Reynolds number and only weakly on the mean energy dissipation rate. Subsequently, the effect of turbulence on the collision process driven by both turbulence and gravity is explored. It is found that the droplet size distribution depends moderately on the mean energy dissipation rate, but is insensitive to the Reynolds number. Finally, the effect of turbulence on the combined condensational and collisional growth is investigated. In this case, the droplet size distribution is found to depend on both the Reynolds number and the mean energy dissipation rate. Considering small values of the mean energy dissipation rate and high Reynolds numbers in warm clouds, it is concluded that turbulence enhances the condensational growth with increasing Reynolds number, while the collision process is indirectly affected by turbulence through the condensation process. Therefore, turbulence facilitates warm rain formation by enhancing the condensation process, which predominantly depends on the Reynolds number.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2018. p. 26
Keywords
cloud micro-physics, turbulence, inertial particles, DNS, condensation, collision, coalescence
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-158537 (URN)978-91-7797-358-4 (ISBN)978-91-7797-359-1 (ISBN)
Public defence
2018-09-21, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
The Research Council of Norway, FRINATEK grant 231444
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: Manuscript. Paper 5: Submitted.

Available from: 2018-08-29 Created: 2018-08-08 Last updated: 2018-08-29Bibliographically approved

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