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

Abstract

The commonly used time scales in entrainment-mixing studies are examined in this paper 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: τ evap, the time for droplet complete evaporation; τ phase, the time for saturation ratio deficit (S) to reach 1/e of its initial value; τ satu, the time for S to reach -0.5%; τ react, 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 τ react for saturation, τ satu and τ phase 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, τ react for complete evaporation and τ evap are proper because they characterize how fast droplets evaporate and whether number concentration decreases. Moreover, τ react for complete evaporation and τ evap are always positively correlated withmore » homogeneous mixing degree (ψ), thus the two time scales, especially τ evap, 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 and finally, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [4]; ORCiD logo [5];  [3];  [3];  [3]
  1. Nanjing Univ. of Information Science and Technology (China). Key Lab. of Meteorological Disaster. Joint International Research Lab. of Climate and Environment Change (ILCEC). Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD). Key Lab. for Aerosol-Cloud-Precipitation of China Meteorological Administration; Tsinghua Univ., Beijing (China). Ministry of Education Key Lab. for Earth System Modeling. Dept. for Earth System Science
  2. Brookhaven National Lab. (BNL), Upton, NY (United States). Environmental and Climate Sciences Dept.
  3. Nanjing Univ. of Information Science and Technology (China). Key Lab. of Meteorological Disaster. Joint International Research Lab. of Climate and Environment Change (ILCEC). Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD). Key Lab. for Aerosol-Cloud-Precipitation of China Meteorological Administration
  4. Yonsei Univ., Seoul (Korea, Republic of). Dept. of Atmospheric Sciences
  5. Univ. of Utah, Salt Lake City, UT (United States). Dept. of Atmospheric Sciences
Publication Date:
Research Org.:
Brookhaven National Lab. (BNL), Upton, NY (United States); Nanjing Univ. of Information Science and Technology (China); Yonsei Univ., Seoul (Korea, Republic of)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23); National Key Research and Development Program of China; National Natural Science Foundation of China (NNSFC); China Meteorological Administration; Natural Science Foundation of Jiangsu Province (China); Six Talent Peak Project in Jiangsu (China); 333 High-level Talents Training Project in Jiangsu (China); Korea Meteorological Administration (KMA)
OSTI Identifier:
1430845
Report Number(s):
BNL-203380-2018-JAAM
Journal ID: ISSN 2169-897X
Grant/Contract Number:  
SC0012704; 2017YFA0604000; 91537108; 41475035; 41305120; GYHY201406001; BK20160041; 2015-JY-011; BRA2016424; KMIPA2015-1061
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Geophysical Research: Atmospheres
Additional Journal Information:
Journal Volume: 123; Journal Issue: 7; Journal ID: ISSN 2169-897X
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; 58 GEOSCIENCES; entrainment-mixing; clouds; time scales; observation; simulation

Citation Formats

Lu, Chunsong, Liu, Yangang, Zhu, Bin, Yum, Seong Soo, Krueger, Steven K., Qiu, Yujun, Niu, Shengjie, and Luo, Shi. On Which Microphysical Time Scales to Use in Studies of Entrainment-Mixing Mechanisms in Clouds. United States: N. p., 2018. Web. doi:10.1002/2017JD027985.
Lu, Chunsong, Liu, Yangang, Zhu, Bin, Yum, Seong Soo, Krueger, Steven K., Qiu, Yujun, Niu, Shengjie, & Luo, Shi. On Which Microphysical Time Scales to Use in Studies of Entrainment-Mixing Mechanisms in Clouds. United States. doi:10.1002/2017JD027985.
Lu, Chunsong, Liu, Yangang, Zhu, Bin, Yum, Seong Soo, Krueger, Steven K., Qiu, Yujun, Niu, Shengjie, and Luo, Shi. Fri . "On Which Microphysical Time Scales to Use in Studies of Entrainment-Mixing Mechanisms in Clouds". United States. doi:10.1002/2017JD027985. https://www.osti.gov/servlets/purl/1430845.
@article{osti_1430845,
title = {On Which Microphysical Time Scales to Use in Studies of Entrainment-Mixing Mechanisms in Clouds},
author = {Lu, Chunsong and Liu, Yangang and Zhu, Bin and Yum, Seong Soo and Krueger, Steven K. and Qiu, Yujun and Niu, Shengjie and Luo, Shi},
abstractNote = {The commonly used time scales in entrainment-mixing studies are examined in this paper 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: 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%; 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 and finally, all time scales are proportional to each other under certain microphysical and thermodynamic conditions.},
doi = {10.1002/2017JD027985},
journal = {Journal of Geophysical Research: Atmospheres},
number = 7,
volume = 123,
place = {United States},
year = {2018},
month = {3}
}

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Figures / Tables:

Table 1 Table 1: Summary of different time scales, their applicability for different scientific objectives and their unification.

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