Efficient In‐Cloud Removal of Aerosols by Deep Convection

Abstract

Convective systems dominate the vertical transport of aerosols and trace gases. The most recent in situ aerosol measurements presented here show that the concentrations of primary aerosols including sea salt and black carbon drop by factors of 10 to 10,000 from the surface to the upper troposphere. In this study we show that the default convective transport scheme in the National Science Foundation/Department of Energy Community Earth System Model results in a high bias of 10–1,000 times the measured aerosol mass for black carbon and sea salt in the middle and upper troposphere. A modified transport scheme, which considers aerosol activation from entrained air above the cloud base and aerosol‐cloud interaction associated with convection, dramatically improves model agreement with in situ measurements suggesting that deep convection can efficiently remove primary aerosols. We suggest that models that fail to consider secondary activation may overestimate black carbon's radiative forcing by a factor of 2.

Climate Forcing and Trends of Organic Aerosols in the Community Earth System Model (CESM2)

“…Additional future improvements for the model are planned to include the addition of the optical properties and radiative effects of brown carbon (Brown et al, ) and updates in below cloud top scavenging. These will likely improve the high bias of BC (and POM) in middle to upper troposphere as pointed out by Yu et al (), who used the same model code for cloud top scavenging. Both these updates are expected to further impact radiative effects in CESM2.…”

“…WACCM6‐VBSext shows a slight improvement in the high bias, and a significant improvement in BC in the Arctic lower troposphere (Figures and ). Yu et al () have shown that wet scavenging above the cloud top in all CESM2 configurations is largely underestimated, which results in the high bias. On the other hand, WACCM6‐VBSext shows much more realistic BC values in the lower troposphere (below 5 km) compared to WACCM6‐SOAG.…”

Characterization of Organic Aerosol across the Global Remote Troposphere: A comparison of ATom measurements and global chemistry models

“…Aerosol wet scavenging considers incloud scavenging (the removal of cloud-borne particles that were activated at the cloud base) and below-cloud scavenging for both convective and grid-scale clouds. CESM1-CARMA simulations use the configuration described in Yu et al (2019) which is based on CESM1 and the sectional Community Aerosol and Radiation Model for Atmospheres (CARMA v3.0). Anthropogenic emissions are those from the Greenhouse gas-Air pollution Interactions and Synergies (GAINS) model, and biomass burning emissions are from the Global Fire Emission Database (GFED v3, van der Werf et al, 2010).…”

“…The removal of OA occurs only by dry and wet deposition. Compared to the CESM2-SMP and CESM2-DYN simulations, the convective removal of aerosols uses the modified scheme described in Yu et al (2019) which accounts for aerosol secondary activation from the entrained air above the cloud base, and the scavenging of activated aerosols in convective updrafts. The default CESM can transport aerosols from the cloud base to the top of the cloud in strong convective updrafts in one time step without scavenging them, while the new scheme allows for a more efficient removal off all aerosols inside convective clouds.…”

Global aerosol simulations using NICAM.16 on a 14-km grid spacing for a climate study: Improved and remaining issues relative to a lower-resolution model

“…The overestimation of the BC 20 lifetime may be attributed to the overestimation of the convective mass flux above 500 hPa, which may be improved by increasing the number of vertical layers in the model (Allen and Landuyt, 2014). Another possibility is a lack of secondary aerosol activation by convective clouds and associated removal by precipitation (Yu et al, 2019).…”