Cloud droplet formation at the base of tropical convective clouds: closure between modeling and measurement results of ACRIDICON–CHUVA
<p>Aerosol–cloud interactions contribute to the large uncertainties in current estimates of climate forcing. We investigated the effect of aerosol particles on cloud droplet formation by model calculations and aircraft measurements over the Amazon and over the western tropical Atlantic during...
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| Autores principales: | , , , , , , , , , , , , , , , , , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
Copernicus Publications
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/e69351355908468d8bd527b1865d8724 |
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| Sumario: | <p>Aerosol–cloud interactions contribute to the large uncertainties in current
estimates of climate forcing. We investigated the effect of aerosol particles
on cloud droplet formation by model calculations and aircraft measurements
over the Amazon and over the western tropical Atlantic during the
ACRIDICON–CHUVA campaign in September 2014. On the HALO (High
Altitude Long Range Research) research aircraft,
cloud droplet number concentrations (<span class="inline-formula"><i>N</i><sub>d</sub></span>) were measured near the
base of clean and polluted growing convective cumuli using a cloud combination
probe (CCP) and a cloud and aerosol spectrometer (CAS-DPOL). An adiabatic
parcel model was used to perform cloud droplet number closure studies for
flights in differently polluted air masses. Model input parameters included
aerosol size distributions measured with an ultra-high sensitive aerosol
spectrometer (UHSAS), in combination with a condensation particle counter
(CPC). Updraft velocities (<span class="inline-formula"><i>w</i></span>) were measured with a boom-mounted Rosemount
probe. Over the continent, the aerosol size distributions were dominated by
accumulation mode particles, and good agreement between measured and modeled
<span class="inline-formula"><i>N</i><sub>d</sub></span> values was obtained (deviations <span class="inline-formula"><i>≲</i></span> 10 <span class="inline-formula">%</span>)
assuming an average hygroscopicity of <span class="inline-formula"><i>κ</i>∼0.1</span>, which is consistent
with Amazonian biomass burning and secondary organic aerosol. Above the
ocean, fair agreement was obtained assuming an average hygroscopicity of
<span class="inline-formula"><i>κ</i>∼0.2</span> (deviations <span class="inline-formula"><i>≲</i></span> 16 <span class="inline-formula">%</span>) and further
improvement was achieved assuming different hygroscopicities for Aitken and
accumulation mode particles (<span class="inline-formula"><i>κ</i><sub>Ait</sub>=0.8</span>, <span class="inline-formula"><i>κ</i><sub>acc</sub>=0.2</span>; deviations <span class="inline-formula"><i>≲</i></span> 10 <span class="inline-formula">%</span>), which may reflect
secondary marine sulfate particles. Our results indicate that Aitken mode
particles and their hygroscopicity can be important for droplet formation at
low pollution levels and high updraft velocities in tropical convective
clouds.</p> |
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