Abstract
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 (So) to aerosols. The 173 “contact” profiles with aerosol concentration (Na) greater than 500 cm−3 within 100 m above cloud tops had 50 % lower precipitation rate (Rp) and 20 % lower So, on average, compared to 156 “separated” profiles with Na 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 (Nc) and effective radius (Re) (95 % confidence intervals from a two-sample t-test are reported). Contact profiles had 84 to 90 cm−3 higher Nc and 1.4 to 1.6 μm lower Re compared to separated profiles. In clean boundary layers (below-cloud Na less than 350 cm−3), contact profiles had 25 to 31 cm−3 higher Nc and 0.2 to 0.5 μm lower Re. In polluted boundary layers (below-cloud Na exceeding 350 cm−3), contact profiles had 98 to 108 cm−3 higher Nc and 1.6 to 1.8 μm lower Re. 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 properties were driven by ACIs rather than meteorological effects, and the existing relationships between Rp and Nc must be adjusted to account for the role of ACIs.
Highlights
Clouds drive the global hydrological cycle with an annual average precipitation rate of 3 mm day-1 over the oceans (Behrangi et al, 2014)
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 properties were driven by Aerosol-cloud-precipitation interactions (ACIs) rather than 35 meteorological effects, and the existing relationships between Rp and Nc must be adjusted to account for the role of ACIs. 1 Introduction
When all profiles were considered, there were insignificant differences between the average ERA5 RWP, sea surface temperature (SST), To, estimated inversion strength (EIS), and LTS for contact and separated profiles. This suggests the differences between contact and separated 610 profiles found during the ORACLES Intensive Observation Periods (IOPs) were primarily associated with ACIs instead of meteorological effects
Summary
Clouds drive the global hydrological cycle with an annual average precipitation rate of 3 mm day-1 over the oceans (Behrangi et al, 2014). The response of the MSC to above- and below-cloud aerosols is further examined using data from all three ORACLES IOPs, and precipitation formation and So are evaluated as a function of H. The N(D) from the merged droplet size distribution was integrated to calculate Nc. The 1 Hz data samples with Nc > 10 cm-3 and King LWC > 0.05 g m-3 were defined as in-cloud measurements (G21). For 304 cloud profiles with LWPad > 5 g m-2, the average was 0.72 ± 0.31 (0.85 ± 0.41 if the King hot-wire was used to represent LWC) This was consistent with for MSC over the northeast Pacific (0.77 ± 0.13) (Braun et al, 2018) and the southeast Pacific The inverse relationship between and H is consistent with previous MSC observations (Braun et al, 2018)
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