Siddhant Gupta-April 11

Observations of Aerosol-Cloud Interactions with Varying Vertical Separation between Biomass-Burning Aerosols and Stratocumulus Clouds over the South-East Atlantic

The ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) field campaign provided in‐situ measurements and remotely sensed retrievals of aerosol and cloud properties over the South-East Atlantic during August-September 2016. Biomass burning aerosol from Southern Africa is advected toward the South-East Atlantic at elevated altitudes and overlies the ubiquitous stratocumulus cloud deck over the ocean. The aerosols subside farther from the coast so that the vertical displacement between the clouds and aerosols varies, and whose effect on aerosol-cloud interaction is poorly known.

For measuring the droplet number distribution function (n(D)), the NASA P‐3 aircraft was equipped with a Cloud Droplet Probe (CDP) sizing particles between 2 and 50μm, a Cloud and Aerosol Spectrometer (CAS) for 0.51 < D < 50 μm, a 2D‐stereo probe (2DS), nominally sizing between 10 and 1280 μm, a Cloud Imaging Probe (CIP) for 25 < D < 1600 μm, and a High Volume Precipitation Sampler (HVPS‐3) for 150 μm < D < 1.92 cm. A King hot-wire measured the liquid water content (LWC) and a Passive Cavity Aerosol Spectrometer Probe (PCASP) measured aerosol concentration in the accumulation mode (N_a) for 0.1 < D < 3 μm.

Forty cloud profiles flown during six ORACLES research flights were classified into two regimes, Mixing and Separated, according to whether the plume of densest aerosols was mixing into or separated from cloud tops. During the Mixing regime, a prominent aerosol layer with N_a > 100 cm^-3 was within 100 m above cloud top, and an increase of 100 to 150 cm^-3 in droplet concentration (N_c) was observed relative to cases without such a layer above cloud top. Along with an increase in N_c, a decrease of 1.5 to 2 µm in effective radius was observed throughout the cloud deck with little difference observed in the average liquid water content (LWC) between cases. Precipitation suppression was observed during the mixing regime as a lower probability of high drizzle (D > 50 µm) concentration (> 0.1 cm^-3) was observed during the mixing regime (0.2) compared to the separated regime (0.24).