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

[摘要] Aerosol–cloud–precipitation interactions (ACIs) provide thegreatest source of uncertainties in predicting changes in Earth's energybudget due to poor representation of marine stratocumulus and the associatedACIs in climate models. Using in situ data from 329 cloud profiles across 24research flights from the NASA ObseRvations of Aerosols above CLouds andtheir intEractionS (ORACLES) field campaign in September 2016, August 2017,and October 2018, it is shown that contact between above-cloudbiomass burning aerosols and marine stratocumulus over the Southeast AtlanticOcean was associated with precipitation suppression and a decrease in theprecipitation 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 abovecloud tops. Contact and separated profiles had statistically significant differences indroplet concentration ( N c ) and effective radius ( R e ) (95 %confidence intervals from a two-sample t test are reported). Contactprofiles had 84 to 90 cm −3 higher N c and 1.4 to1.6  µ 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  µ m lower R e . In polluted boundarylayers (below-cloud N a exceeding 350 cm −3 ), contact profiles had 98to 108 cm −3 higher N c and 1.6 to 1.8  µ m lower R e . On theother hand, contact and separated profiles had statistically insignificantdifferences between the average liquid water path, cloud thickness, andmeteorological parameters like surface temperature, lower troposphericstability, and estimated inversion strength. These results suggest thechanges in cloud microphysical properties were driven by ACIs rather thanmeteorological effects, and adjustments to existing relationships between R p and N c in model parameterizations should be considered to accountfor the role of ACIs.

[学科分类] 医学（综合）