Long-resident droplets at the stratocumulus top

Lozar, Alberto de; Muessle, Lukas

doi:10.5194/acp-16-6563-2016

Cited by 16 publications

(14 citation statements)

References 43 publications

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“…But in this paper we focus on mechanisms involving the condensation process. For example, results from Lagrangian tracking studies suggest that large droplets from condensation growth within parcels having favored trajectories can trigger collisions and drizzle formation in warm clouds (Lasher-Trapp et al, 2005;Cooper et al, 2013;Magaritz-Ronen et al, 2014Naumann and Seifert, 2015;de Lozar and Muessle, 2016). Korolev et al (2013) proposed that the droplet size distribution can be broadened through diffusion growth due to cloud base mixing and vertical fluctuation.…”

Section: Introductionmentioning

confidence: 99%

Conditions for super-adiabatic droplet growth after entrainment mixing

2016

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Abstract. Cloud droplet response to entrainment and mixing between a cloud and its environment is considered, accounting for subsequent droplet growth during adiabatic ascent following a mixing event. The vertical profile for liquid water mixing ratio after a mixing event is derived analytically, allowing the reduction to be predicted from the mixing fraction and from the temperature and humidity for both the cloud and environment. It is derived for the limit of homogeneous mixing. The expression leads to a critical height above the mixing level: at the critical height the cloud droplet radius is the same for both mixed and unmixed parcels, and the critical height is independent of the updraft velocity and mixing fraction. Cloud droplets in a mixed parcel are larger than in an unmixed parcel above the critical height, which we refer to as the “super-adiabatic” growth region. Analytical results are confirmed with a bin microphysics cloud model. Using the model, we explore the effects of updraft velocity, aerosol source in the environmental air, and polydisperse cloud droplets. Results show that the mixed parcel is more likely to reach the super-adiabatic growth region when the environmental air is humid and clean. It is also confirmed that the analytical predictions are matched by the volume-mean cloud droplet radius for polydisperse size distributions. The findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.

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Section: Introductionmentioning

confidence: 99%

Conditions for super-adiabatic droplet growth after entrainment mixing

2016

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“…An interesting question is to explain why the CDSD is wider than predicted and the presence of the large droplet sizes in the tail of the distribution (e.g., Siebert and Shaw, 2017), which might be related to the fast-rain process in the atmosphere (e.g., Göke et al, 2007). Several pos- 15 sible mechanisms have been proposed, including the existence of giant cloud condensational nuclei (GCCN, usually defined as aerosols with dry diameter larger than few µm) (e.g., Feingold et al, 1999;Yin et al, 2000;Jensen and Lee, 2008;Cheng et al, 2009;Jensen and Nugent, 2017), lucky cloud droplets (e.g., Kostinski and Shaw, 2005;Naumann and Seifert, 2015;Lozar and Muessle, 2016), mixing with environmental air (e.g., Lasher-Trapp et al, 2005;Cooper et al, 2013;Korolev et al, 2013;Yang et al, 2016), supersaturation fluctuations (e.g., Chandrakar et al, 2016;Siebert and Shaw, 2017), and enhancement of collision 20 efficiency due to turbulence or charge (e.g., Paluch, 1970;Grabowski and Wang, 2013;Falkovich and Pumir, 2015;Lu and Shaw, 2015). Recently, Jensen and Nugent (2017) investigated the effect of GCCN on droplet growth and rain formation using a cloud parcel model.…”

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confidence: 99%

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

Yang

Kollias

Shaw

et al. 2017

Preprint

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Abstract. Cloud droplet size distributions (CDSDs), which are related to cloud albedo and lifetime, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a Lagrangian bin-microphysics cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, sub-micrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with Korolev (1995). The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of 10 the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are: (1) droplets form on polydisperse aerosols of varying hygroscopicity and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest 15 it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in-situ measurements and 3-D dynamic models.

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“…Turbulence can also modulate the condensational growth of cloud droplets through mixing and entrainment (e.g., Lasher-Trapp et al, 2005;Cooper et al, 2013;Korolev et al, 2013;Yang et al, 2016). In addition, turbulence can enhance the collision efficiency between droplets and produce "lucky" cloud droplets through stochastic collisions, which has been confirmed by direct numerical simulations and Lagrangian drop models (e.g., Paluch, 1970;Kostinski and Shaw, 2005;Falkovich and Pumir, 2007;Grabowski and Wang, 2013;Naumann and Seifert, 2015;de Lozar and Muessle, 2016).…”

mentioning

confidence: 69%

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

et al. 2018

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Abstract. Cloud droplet size distributions (CDSDs), which are related to cloud albedo and rain formation, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a moving-size-grid cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, submicrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with early work. The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by numerous smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are as follows: (1) droplets form on aerosols of different sizes, and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in situ measurements and 3-D dynamic models.

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Long-resident droplets at the stratocumulus top

Cited by 16 publications

References 43 publications

Conditions for super-adiabatic droplet growth after entrainment mixing

Conditions for super-adiabatic droplet growth after entrainment mixing

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

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