This study investigates the relationship between aerosol particles and cloud droplet formation at the base of tropical convective clouds during the ACRIDICON-CHUVA campaign in September 2014. Researchers used a combination of model calculations and aircraft measurements over the Amazon and western tropical Atlantic.
Key Findings:
1. Aerosol Size Distributions: Over the continent, aerosol size distributions were dominated by accumulation mode particles, with good agreement between measured and modeled cloud droplet number concentrations (Nd) assuming an average hygroscopicity of κ ∼0.1. Above the ocean, a better match was achieved assuming different hygroscopicities for Aitken and accumulation mode particles (κAit = 0.8, κacc = 0.2).
2. Cloud Droplet Number Closure: The study performed cloud droplet number closure studies using an adiabatic parcel model. Model input parameters included aerosol size distributions and updraft velocities measured during flights in differently polluted air masses.
3. Importance of Aitken Mode Particles: The results suggest that Aitken mode particles (smaller particles) and their hygroscopicity can significantly influence droplet formation at low pollution levels and high updraft velocities in tropical convective clouds.
4. Uncertainties in Updraft Velocity: Similar to previous studies, updraft velocity was identified as one of the most uncertain parameters affecting predictions of cloud droplet number concentrations.
Methodology:
- Measurements: Cloud droplet number concentrations (Nd) 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).
- Modeling: An adiabatic parcel model was used to calculate Nd,p based on the hygroscopic growth of CCN particles with prescribed κ and updraft velocities. Input parameters included aerosol size distributions measured with an ultra-high sensitive aerosol spectrometer (UHSAS) and a condensation particle counter (CPC).
Significance:
This study contributes to our understanding of aerosol-cloud interactions, particularly in shallow and deep convective clouds over the Amazon Basin. The findings highlight the importance of Aitken mode particles and their hygroscopicity in cloud droplet formation processes under specific atmospheric conditions.
