Precipitation Susceptibility of Marine Stratocumulus with Variable Above and Below-Cloud Aerosol Concentrations over the Southeast Atlantic

Abstract

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.

Intercomparison of airborne and surface-based measurements during the CLARIFY, ORACLES and LASIC field experiments

“…Nominal bin dimensions applicable to BAe-146 and P3 CDPs along with calibrated bin dimensions for BAe-146 are given in Table S1. Gupta et al (2021b) compared data from the P3 CDP against that collected by a cloud and aerosol spectrometer (CAS) also installed on the P3, concluding that the CDP provided data most consistent with bulk water contents measured by a King probe and less than calculated adiabatic water contents. Errors are comprised of Poisson counting uncertainties, true airspeed uncertainties assumed to be 5 %, and sample area uncertainty of 5 %, all combined in quadrature.…”

“…LWC derived from the Picarro L2120-i hygrometer (Sect 2.3) fitted downstream of the Counterflow Virtual Impactor inlet (CVI) was used to determine when the NASA P3 was in cloud free conditions by locating times when the bulk water content was determined to be zero. Closure tests between the LWC derived from the P3 cloud probe spectra and the King hot-wire were conducted for in-cloud measurements from each ORACLES deployment (Gupta et al, 2021b).…”