Further evidence for CCN aerosol concentrations determining the height of warm rain and ice initiation in convective clouds over the Amazon basin
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...
| Main Authors: | , , , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/14433/2017/acp-17-14433-2017.pdf |
| Summary: | 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 (<i>D</i><sub>r</sub>, in units of meters above cloud base) is linearly
correlated with the number concentration of droplets (<i>N</i><sub>d</sub> in
cm<sup>−3</sup>) nucleated at cloud base (<i>D</i><sub>r</sub> ≈ 5 ⋅ <i>N</i><sub>d</sub>). Additional cloud processes associated with <i>D</i><sub>r</sub>, 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 <i>D</i><sub>r</sub> and <i>N</i><sub>d</sub>, which was also found at different
regions around the globe (e.g., Israel and India). When <i>N</i><sub>d</sub> exceeded
values of about 1000 cm<sup>−3</sup>, <i>D</i><sub>r</sub> 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 (<i>r</i><sub>e</sub>) were close to those estimated by assuming
adiabatic conditions (<i>r</i><sub>ea</sub>), 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 <i>r</i><sub>e</sub> below <i>r</i><sub>ea</sub>, which further inhibited the
formation of raindrops and ice particles and resulted in even higher
altitudes for rain and ice initiation. |
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| ISSN: | 1680-7316 1680-7324 |