On Which Microphysical Time Scales to Use in Studies of Entrainment-Mixing Mechanisms in Clouds

Chunsong Lu, Yangang Liu, Bin Zhu, Seong Soo Yum, Steven K. Krueger, Yujun Qiu, Shengjie Niu, Shi Luo

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Abstract

The commonly used time scales in entrainment-mixing studies are examined to seek the most appropriate one, based on aircraft observations of cumulus clouds from the RACORO campaign and numerical simulations with the Explicit Mixing Parcel Model. The time scales include the following: τevap, the time for droplet complete evaporation; τphase, the time for saturation ratio deficit (S) to reach 1/e of its initial value; τsatu, the time for S to reach −0.5%; and τreact, the time for complete droplet evaporation or S to reach −0.5%. It is found that the proper time scale to use depends on the specific objectives of entrainment-mixing studies. First, if the focus is on the variations of liquid water content (LWC) and S, then τreact for saturation, τsatu and τphase are almost equivalently appropriate, because they all represent the rate of dry air reaching saturation or of LWC decrease. Second, if one focuses on the variations of droplet size and number concentration, τreact for complete evaporation and τevap are proper because they characterize how fast droplets evaporate and whether number concentration decreases. Moreover, τreact for complete evaporation and τevap are always positively correlated with homogeneous mixing degree (ψ); thus, the two time scales, especially τevap, are recommended for developing parameterizations. However, ψ and the other time scales can be negatively, positively, or not correlated, depending on the dominant factors of the entrained air (i.e., relative humidity or aerosols). Third, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.

Original languageEnglish
Pages (from-to)3740-3756
Number of pages17
JournalJournal of Geophysical Research: Atmospheres
Volume123
Issue number7
DOIs
Publication statusPublished - 2018 Apr 16

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All Science Journal Classification (ASJC) codes

  • Geophysics
  • Forestry
  • Oceanography
  • Aquatic Science
  • Ecology
  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth-Surface Processes
  • Atmospheric Science
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science
  • Palaeontology

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