Journal of the Atmospheric Sciences

Hilario, Miguel Ricardo A.; Barth, Mary; Bennett, Ryan; Crosbie, Ewan; DiGangi, Joshua P.; Diskin, Glenn S.; Lorenzo, Genevieve Rose; Rutledge, Steven; Martin, Melissa Yang; Ziemba, Luke; Sorooshian, Armin

2025 Journal of the Atmospheric Sciences

doi: 10.1175/jas-d-24-0064.1

AbstractIn-cloud aerosol scavenging remains a large source of model uncertainty, affecting capabilities to capture the aerosol lifetime and impacts on air quality and climate. While past work quantified aerosol scavenging efficiencies (SEs) in midlatitude mixed-phase deep convection, SEs are less well-known for shallower convection. We used aircraft data over the tropical West Pacific to calculate SEs for three marine cumuli of different top heights (3 – 7 km above sea level) using a simple entrainment model and measurements of the cloud outflow and nearby clear air. Across cases, efficient scavenging was observed for sulfate (>86%) and black carbon (70 – 80%), while organic aerosols (53 – 60%) and nitrate (61.5%) were moderately scavenged. Ammonium had a wide SE range (53 – 87%). SEs of aerosol volume concentration showed near-total removal of aerosols with diameters greater than 100 nm (>92%) and inefficient removal for aerosols with diameters less than 100 nm (30 – 50%), associated with the preferential activation and removal of larger particles. Mass-based SEs did not differ substantially between tropical cumuli and midlatitude deep convection, attributed to the negligible mass activated at higher supersaturations. The efficient scavenging of BC can be explained by an enhanced hygroscopic fraction of BC based on model results from the CESMv2 Community Atmosphere Model with Chemistry, suggesting the internal mixing of BC with more soluble species during long-range transport through the marine atmosphere. The estimates of BC SEs provide direct evidence of substantial BC removal in convection as inferred by previous work and should motivate improvements in chemical transport models.

Shaaban, Ahmed; Roundy, Paul

2025 Journal of the Atmospheric Sciences

doi: 10.1175/jas-d-23-0237.1

AbstractThe variability of the phase speed of the Madden Julian oscillation (MJO) is poorly understood. The authors assess how the phase speed of the convective signal of the MJO associates with the background states over Eastern Africa and the Indian Ocean. Relaxation of the coupling between tropical modes and their circulation has been previously linked to faster propagation; for example, MJO speeds up over Eastern Pacific where its convective signal decouples from the circulation. In contrast, our results show that fast MJO events happen to exist during periods of wetter background states (> 90 days) from East Africa across the Indian Ocean, whereas slow MJO is associated with dry background states. We found that fast MJO exhibits strong active and inactive phases with structure suggesting more hierarchical convection. Results indicate that the association of the phase speed of the MJO as seen in the integrated filtered moist static energy with the its tendency is stronger than the association of the phase speed as observed in the dry static energy with its tendency which is consistent with the acceleration of the MJO during wet background states. Also, our results indicate the MJO may be faster during periods of enhanced low-level moisture because these periods have anomalously weak upper tropospheric easterly background wind, which reduces the westward advection of the MJO by the background easterly wind, resulting in higher eastward phase speed of the MJO. The acceleration of the MJO by the background zonal wind overwhelms the deceleration associated with the moist wave dynamics.