In situ and satellite-based estimates of aerosol-cloud interactions between biomass burning aerosols and marine stratocumulus clouds over the southeast Atlantic Ocean
Ubiquitous low-level, marine stratocumulus clouds provide the largest contribution of all cloud types to the shortwave cloud radiative forcing. A cooling effect from small changes in low-level cloud properties due to aerosol-cloud interactions (ACIs) could partially offset the global warming due to increasing greenhouse gas concentrations in the atmosphere. A large marine stratocumulus cloud deck exists over the southeast Atlantic Ocean where the clouds are overlaid by biomass burning aerosols with instances of contact and separation between the aerosol and cloud layers. Biases in satellite retrievals of aerosol and cloud properties and the vertical distance between the aerosol and cloud layers have led to uncertainties in the regional estimates of ACIs and the effective radiative forcing due to ACIs (ERFaci). ERFaci remains the largest source of uncertainty in climate model estimates of Earth’s energy budget in future climate scenarios.
In this study, in situ data are used to quantify aerosol-induced changes in stratocumulus cloud properties and to evaluate satellite-based estimates of the aerosol-induced changes. Size distributions of aerosols and cloud droplets were sampled during the three phases of the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign using in situ probes onboard the NASA P-3B aircraft. Size distributions from vertical profiles of aerosol and cloud layers over the southeast Atlantic were used to estimate aerosol concentration (Na) along with cloud microphysical properties like droplet concentration (Nc), effective radius (Re), and liquid water content (LWC), optical properties like cloud optical thickness (), and macrophysical properties like liquid water path (LWP), cloud geometric thickness (H) and precipitation rate (Rp).
Across the ORACLES campaigns in September 2016, August 2017, and October 2018, 173 “contact” profiles had Na > 500 cm-3 within 100 m above cloud tops and 156 “separated” profiles had Na < 500 cm-3 up to 100 m above cloud tops. The average Nc, LWC, and for contact profiles were 87 cm-3, 0.02 g m-3, and 1.8 higher and Re was 1.5 m lower compared to separated profiles. These differences were associated with higher below-cloud Na and weaker droplet evaporation near cloud top in the presence of high Na immediately above cloud tops. Larger differences were observed between Nc and Re for contact and separated profiles in high Na boundary layers (108 cm-3 and 1.8 m) compared to low Na boundary layers (31 cm-3 and 0.5 m). A smaller decrease in humidity across cloud top during contact profiles led to a smaller decrease in median Nc and LWC near cloud top (25% and 12%) compared to separated profiles (33% and 18%).
Higher Nc and lower Re for contact profiles resulted in precipitation suppression with 50% lower Rp compared to separated profiles along with 20% lower precipitation susceptibility to aerosols (So). So depends on both Nc and Rp, and differences between So for contact and separated profiles varied with H due to the co-variability between changes in Nc and Rp due to droplet growth with height and increasing Na. Based on reanalysis data, contact and separated profiles had statistically similar meteorological conditions like surface temperature (To), lower tropospheric stability (LTS), and estimated inversion strength (EIS), on average.
For 67 contact and 82 separated profiles, in situ data were co-located with a retrieval from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra or Aqua satellite with a time gap of less than 1 hour. On average, the MODIS Re, , and Nc (11.4 m, 11.7, and 150.3 cm-3) were 1.7 m, 2.4, and less than 1 cm-3 higher than the in situ Re, , and Nc with Pearson’s correlation coefficient (R) = 0.78, 0.72, and 0.90, respectively. The 67 contact profiles had 103 cm-3 and 2.8 higher in situ Nc and with 2.2 m lower in situ Re compared to the 82 separated profiles. MODIS estimates of the differences in Re, , and Nc between contact and separated profiles were within 0.5 m, 0.7, and 5 cm-3 of the in situ estimates when profiles with MODIS Re > 15 m and MODIS > 25 were removed. Agreement between MODIS and in situ estimates of Re, , and Nc and the aerosol-induced changes in Re, , and Nc was observed due to low biases in MODIS retrievals which were consistent for contact and separated profiles.
The aerosol-induced changes in cloud properties quantified in this study could impact the stratocumulus-to-cumulus or closed-to-open cell transitions in the region. Future work should examine in-cloud aerosol samples from the counterflow virtual impactor inlet to examine the extent of entrainment mixing of aerosols into the cloud layer. Modeling studies should examine the impact of precipitation suppression on cloud lifetime and boundary layer dynamics. Model parameterizations of Rp should be adjusted to account for changes in the relationship between Nc, Rp, and H under different aerosol conditions. Future work should also be aimed at improving satellite-based estimates of the vertical displacement between the aerosol and cloud layers. Combined with MODIS retrievals, this would allow studies of ACIs in marine stratocumulus over longer timescales and larger domains than possible using in situ data alone
