Factors affecting precipitation formation and precipitation susceptibility of marine stratocumulus with variable above- and below-cloud aerosol concentrations over the Southeast Atlantic

Aerosol-cloud-precipitation interactions (ACIs) provide the greatest source of uncertainties in predicting changes in Earth's energy budget due to poor representation of marine stratocumulus and the associated ACIs in climate models. Using in situ data from 329 cloud profiles across 24 research flights from the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign in September 2016, August 2017, and October 2018, it is shown that contact between above-cloud biomass burning aerosols and marine stratocumulus over the Southeast Atlantic Ocean was associated with precipitation suppression and a decrease in the precipitation susceptibility (S-o) to aerosols. The 173 contact profiles with aerosol concentration (N-a) greater than 500 cm(-3) within 100 m above cloud tops had a 50 % lower precipitation rate (R-p) and a 20 % lower S-o, on average, compared to 156 separated profiles with N-a less than 500 cm(-3) up to at least 100 m above cloud tops. Contact and separated profiles had statistically significant differences in droplet concentration (N-e) and effective radius (R-e) (95 % confidence intervals from a two-sample t test are reported). Contact profiles had 84 to 90 cm(-3) higher N-c and 1.4 to 1.6 mu m lower R-e compared to separated profiles. In clean boundary layers (below-cloud N-a less than 350 cm(-3)), contact profiles had 25 to 31 cm(-3) higher N-c and 0.2 to 0.5 mu m lower R-e. In polluted boundary layers (below-cloud N-a exceeding 350 cm(-3)), contact profiles had 98 to 108 cm(-3) higher N-c and 1.6 to 1.8 mu m lower R-e . On the other hand, contact and separated profiles had statistically insignificant differences between the average liquid water path, cloud thickness, and meteorological parameters like surface temperature, lower tropospheric stability, and estimated inversion strength. These results suggest the changes in cloud microphysical properties were driven by ACIs rather than meteorological effects, and adjustments to existing relationships between R-p and N-c in model parameterizations should be considered to account for the role of ACIs.

Automated summary

Aerosol-cloud-precipitation interactions (ACIs) contribute to uncertainties in climate models. Observations from the ORACLES field campaign showed that contact between biomass burning aerosols and marine stratocumulus resulted in precipitation suppression and reduced precipitation susceptibility. Different aerosol concentrations also influenced cloud microphysical properties.