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Title: Conditions for super-adiabatic droplet growth after entrainment mixing

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 analyticalmore » predictions are matched by the volume-mean cloud droplet radius for polydisperse size distributions. Lastly, these findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.« less

Authors:
 [1];  [1];  [2]
  1. Michigan Technological Univ., Houghton, MI (United States)
  2. Peking Univ., Beijing (China)
Publication Date:
Research Org.:
Michigan Technological Univ., Houghton, MI (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1275972
Grant/Contract Number:
SC0011690
Resource Type:
Journal Article: Published Article
Journal Name:
Atmospheric Chemistry and Physics (Online)
Additional Journal Information:
Journal Name: Atmospheric Chemistry and Physics (Online); Journal Volume: 16; Journal Issue: 14; Journal ID: ISSN 1680-7324
Publisher:
European Geosciences Union
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Yang, Fan, Shaw, Raymond, and Xue, Huiwen. Conditions for super-adiabatic droplet growth after entrainment mixing. United States: N. p., 2016. Web. doi:10.5194/acp-16-9421-2016.
Yang, Fan, Shaw, Raymond, & Xue, Huiwen. Conditions for super-adiabatic droplet growth after entrainment mixing. United States. doi:10.5194/acp-16-9421-2016.
Yang, Fan, Shaw, Raymond, and Xue, Huiwen. 2016. "Conditions for super-adiabatic droplet growth after entrainment mixing". United States. doi:10.5194/acp-16-9421-2016.
@article{osti_1275972,
title = {Conditions for super-adiabatic droplet growth after entrainment mixing},
author = {Yang, Fan and Shaw, Raymond and Xue, Huiwen},
abstractNote = {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. Lastly, these findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.},
doi = {10.5194/acp-16-9421-2016},
journal = {Atmospheric Chemistry and Physics (Online)},
number = 14,
volume = 16,
place = {United States},
year = 2016,
month = 7
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.5194/acp-16-9421-2016

Citation Metrics:
Cited by: 2works
Citation information provided by
Web of Science

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  • 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 unmixedmore » 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. Lastly, these findings have implications for the origin of large cloud droplets that may contribute to onset of collision–coalescence in warm clouds.« less
  • In this paper the entrainment rate of liquid droplets into the core region of a two-phase annular mist flow is investigated in tubes with helically ribbed and wired inner surfaces, with the aim of improving evaporative heat transfer in various steam generators of once-through type. The results show that the entrainment rate in the tubes depends on the height, helical pitch, and number of ribs and wires, the gas velocity and the liquid flow rate, and that it decreases to about 40 percent of a smooth tube. So-called large disturbance waves on the surface of the liquid film flow aremore » reduced in number and the surface is smoothed in the tubes. In addition, the waves change their original annular shape to the three-dimensional one. These characteristics could be the cause of the entrainment suppression described above. The suppression effect can be correlated with three nondimensional parameters. The pressure drop of the tow-phase flow through the tubes is influenced by some other factors as well as by the hydraulic diameter.« less
  • In order to determine the values of droplet entrainment, investigations have been carried out on an experimental unit shown in this article. The unit included a dust collector with a disk atomizer, a system of air ducts, a recirculating water supply, a centrifugal blower, a control device, and measuring devices. In carrying out the investigation, use was made of a matrix design in which the main controlling factors were taken to be the air velocity, the irrigation density, and the rate of rotation of the atomizer. As a result of the experimental investigations, a relationship was obtained which makes itmore » possible to determine the value of the droplet entrainment in dust collectors with disk atomizers, and subsequently to evaluate the effect of droplet entrainment on the effectiveness of dust collection.« less
  • The influence of clear air entrainment on the droplet effective radius of cloudy air parcels is investigated theoretically and experimentally by using data collected in 16 warm maritime tropical cumuli during the Joint Hawaii Warm Rain Project (1985). The theoretical study consists of calculations of the droplet spectrum, droplet effective radius, and liquid water content performed by an entraining cloud parcel model for different entrainment-mixing scenarios. The numerical simulation results are interpreted by means of an analytic equation of the droplet effective radius expressed as a function of both the liquid water content and the droplet concentration. In the experimentmore » study, the behavior of the effective radius is examined at all scales as a function of the liquid water content, used as a dilution degree indicator. At a given cloud level, in the abscence of secondary droplet activation, the effective radius of the droplet spectrum of small-scale parcels (10-Hz data) is roughly independent of the liquid water content and appears unaffected by entrainment. In contrast, if secondary droplet activation occurs in diluted ascending cloud parcels, a wide range of effective radius values is observed for a given liquid water content as a result of the induced variation of the droplet concentration. Further, mean cloud pass effective radii increase with increasing mean pass liquid water contents and mean pass height above cloud base. The results limit the validity of the classical cloud effective radius parameterizations used in the radiative transfer calculations in climate models and some suggestions to improve these parameterizations are presented.« less