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Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin
- Source :
- Atmospheric Chemistry and Physics, Vol 17, Pp 14433-14456 (2017)
- Publication Year :
- 2017
- Publisher :
- Copernicus Publications, 2017.
-
Abstract
- We have investigated how aerosols affect the height above cloud base of rain and ice hydrometeor initiation and the subsequent vertical evolution of cloud droplet size and number concentrations in growing convective cumulus. For this purpose we used in situ data of hydrometeor size distributions measured with instruments mounted on HALO aircraft during the ACRIDICON–CHUVA campaign over the Amazon during September 2014. The results show that the height of rain initiation by collision and coalescence processes (Dr, in units of meters above cloud base) is linearly correlated with the number concentration of droplets (Nd in cm−3) nucleated at cloud base (Dr ≈ 5 ⋅ Nd). Additional cloud processes associated with Dr, such as GCCN, cloud, and mixing with ambient air and other processes, produce deviations of ∼ 21 % in the linear relationship, but it does not mask the clear relationship between Dr and Nd, which was also found at different regions around the globe (e.g., Israel and India). When Nd exceeded values of about 1000 cm−3, Dr became greater than 5000 m, and the first observed precipitation particles were ice hydrometeors. Therefore, no liquid water raindrops were observed within growing convective cumulus during polluted conditions. Furthermore, the formation of ice particles also took place at higher altitudes in the clouds in polluted conditions because the resulting smaller cloud droplets froze at colder temperatures compared to the larger drops in the unpolluted cases. The measured vertical profiles of droplet effective radius (re) were close to those estimated by assuming adiabatic conditions (rea), supporting the hypothesis that the entrainment and mixing of air into convective clouds is nearly inhomogeneous. Additional CCN activation on aerosol particles from biomass burning and air pollution reduced re below rea, which further inhibited the formation of raindrops and ice particles and resulted in even higher altitudes for rain and ice initiation.
- Subjects :
- Convection
Atmospheric Science
010504 meteorology & atmospheric sciences
010502 geochemistry & geophysics
Atmospheric sciences
01 natural sciences
lcsh:Chemistry
Cloud base
cloud
rain
Wolkenphysik
Aerosol
Physics::Atmospheric and Oceanic Physics
convection
0105 earth and related environmental sciences
Effective radius
Coalescence (physics)
15. Life on land
lcsh:QC1-999
Ambient air
lcsh:QD1-999
13. Climate action
Environmental science
Halo
lcsh:Physics
Amazon basin
Subjects
Details
- Language :
- English
- ISSN :
- 16807324 and 16807316
- Volume :
- 17
- Database :
- OpenAIRE
- Journal :
- Atmospheric Chemistry and Physics
- Accession number :
- edsair.doi.dedup.....71754433f8e22de31412d360ff021b6e