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Droplet growth in atmospheric turbulence: A direct numerical simulation study
Stockholms universitet, Naturvetenskapliga fakulteten, Meteorologiska institutionen (MISU).ORCID-id: 0000-0002-5722-0018
2018 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Meteorology, Stockholm University , 2018. , s. 26
Nyckelord [en]
cloud micro-physics, turbulence, inertial particles, DNS, condensation, collision, coalescence
Nationell ämneskategori
Klimatforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
URN: urn:nbn:se:su:diva-158537ISBN: 978-91-7797-358-4 (tryckt)ISBN: 978-91-7797-359-1 (digital)OAI: oai:DiVA.org:su-158537DiVA, id: diva2:1237428
Disputation
2018-09-21, Högbomsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Forskningsfinansiär
Norges forskningsråd, FRINATEK grant 231444
Anmärkning

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.

Tillgänglig från: 2018-08-29 Skapad: 2018-08-08 Senast uppdaterad: 2018-08-29Bibliografiskt granskad
Delarbeten
1. Eulerian and Lagrangian approaches to multidimensional condensation and collection
Öppna denna publikation i ny flik eller fönster >>Eulerian and Lagrangian approaches to multidimensional condensation and collection
2017 (Engelska)Ingår i: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 9, nr 2, s. 1116-1137Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
Eulerian Smoluchowski and Lagrangian superdroplet, collection, cloud droplets, turbulence, size spectra
Nationell ämneskategori
Geovetenskap och miljövetenskap
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-146001 (URN)10.1002/2017MS000930 (DOI)000406239300020 ()
Tillgänglig från: 2017-08-28 Skapad: 2017-08-28 Senast uppdaterad: 2019-12-17Bibliografiskt granskad
2. Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds
Öppna denna publikation i ny flik eller fönster >>Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds
(Engelska)Ingår i: Artikel i tidskrift (Refereegranskat) Submitted
Abstract [en]

Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations with droplets being modeled as Lagrangian particles. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as t^(1/2) . Also, we numerically confirm that the time evolution of the size distribution depends strongly on the Reynolds number and only weakly on the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increases with increasing Reynolds number, therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the updraft velocity is almost zero.

Nationell ämneskategori
Klimatforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-158536 (URN)
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, Dnr. KAW 2014.0048Norges forskningsråd, FRINATEK grant 231444Vetenskapsrådet, 2012-5797 and 2013-03992
Tillgänglig från: 2018-08-08 Skapad: 2018-08-08 Senast uppdaterad: 2019-12-12
3. Effect of turbulence on collisional growth of cloud droplets
Öppna denna publikation i ny flik eller fönster >>Effect of turbulence on collisional growth of cloud droplets
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2018 (Engelska)Ingår i: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 75, s. 3469-3487Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We investigate the effect of turbulence on the collisional growth of um-sized droplets through high- resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. This can be explained through the energy dissipation rate dependence of the mean collision rate described by the Saffman-Turner collision model. Consistent with the Saffman-Turner collision model and its extensions, the collision rate increases as the square root of the energy dissipation rate even when coalescence is invoked. The size distribution exhibits power law behavior with a slope of -3.7 between a maximum at approximately 10 um up to about 40 um. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is due to the fact that turbulence enhances the collisional growth between similar sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth.

Nationell ämneskategori
Klimatforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-158534 (URN)10.1175/JAS-D-18-0081.1 (DOI)000443931100002 ()
Forskningsfinansiär
Norges forskningsråd, FRINATEK grant 231444Vetenskapsrådet, 2012-5797; 2013-03992Knut och Alice Wallenbergs Stiftelse, Dnr. KAW 2014.0048
Tillgänglig från: 2018-08-08 Skapad: 2018-08-08 Senast uppdaterad: 2019-12-12Bibliografiskt granskad
4. Fluctuations and growth histories of cloud droplets: super-particle simulations of the collision-coalescence process
Öppna denna publikation i ny flik eller fönster >>Fluctuations and growth histories of cloud droplets: super-particle simulations of the collision-coalescence process
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(Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
Abstract [en]

Direct numerical simulations of collisional aggregation in turbulent aerosols are computationally demanding. Many authors therefore use an approximate model of the collision-coalescence process that is computationally more efficient: it relies on representing physical particles in terms of ‘superparticles’. One monitors collisions between superparticles and accounts for collisions between physical particles using a Monte-Carlo algorithm. It has been shown that this algorithm can faithfully represent mean particle growth in turbulent aerosols. Here we investigate how fluctuations are represented in this algorithm. We study particles of different sizes settling under gravity, assuming that the effect of turbulence is simply to mix the particles horizontally. We compute the statistics of growth histories and analyze their fluctuations in terms of the ‘lucky-droplet’ model. We discuss under which circumstances artefacts change the fluctuations of the growth histories, how these can be avoided, and which questions remain to be answered when turbulent fluctuations are explicitly incorporated.

Nationell ämneskategori
Klimatforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-158540 (URN)
Forskningsfinansiär
Norges forskningsråd, FRINATEK grant 231444Knut och Alice Wallenbergs Stiftelse, Dnr. KAW 2014.0048Vetenskapsrådet, 2012-5797, 2013-03992, and 2017-03865
Tillgänglig från: 2018-08-08 Skapad: 2018-08-08 Senast uppdaterad: 2019-12-12Bibliografiskt granskad
5. Condensational and collisional growth of cloud droplets in a turbulent environment
Öppna denna publikation i ny flik eller fönster >>Condensational and collisional growth of cloud droplets in a turbulent environment
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(Engelska)Ingår i: Artikel i tidskrift (Refereegranskat) Submitted
Abstract [en]

The effect of turbulence on combined condensational and collisional growth of cloud droplets is investigated using high-resolution direct numerical simulations. The motion of droplets is subjected to both turbulence and gravity. We solve the thermodynamic equations that govern the supersaturation field together with the hydrodynamic equations describing the turbulence. The collision-coalescence process is approximated by a superparticle approach assuming unit collision and coalescence efficiency, i.e., droplet coalesce upon collision. Condensational growth of cloud droplets due to supersaturation fluctuations depends on the Reynolds number, while the collisional growth was previously found to depend on the mean energy dissipation rate. Here we show that the combined processes depend on both Reynolds number and the mean energy dissipation rate. Droplet size distributions broaden either with increasing Reynolds number or mean energy dissipation rate in the range explored here. Even though collisional growth alone is insensitive to Reynolds number, it is indirectly affected by the large scales of turbulence through condensation. This is argued to be due to the fact that condensational growth results in wider droplet-size distributions, which triggers collisional growth. Since turbulence in warm clouds has a relatively small mean energy dissipation rate, but a large Reynolds number, turbulence mainly affects the condensational growth and thus influences the collisional growth indirectly through condensation. Thus, the combined condensational and collisional growth of cloud droplets is mostly dominated by Reynolds number. This work, for the first time, numerically demonstrates that supersaturation fluctuations enhance the collisional growth. It supports the findings from laboratory experiments and the observations that supersaturation fluctuations are important for precipitation.

Nationell ämneskategori
Klimatforskning
Forskningsämne
atmosfärvetenskap och oceanografi
Identifikatorer
urn:nbn:se:su:diva-158535 (URN)
Forskningsfinansiär
Knut och Alice Wallenbergs Stiftelse, Dnr. KAW 2014.0048Vetenskapsrådet, 017-03865 and 2014-585Norges forskningsråd, FRINATEK grant 231444
Tillgänglig från: 2018-08-08 Skapad: 2018-08-08 Senast uppdaterad: 2019-12-12

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