A Lagrangian perspective of the radiative effect of aerosol-cloud interactions

The atmosphere is cleaner, having fewer accumulation mode aerosols, after periods of precipitation. This is demonstrated using back trajectory Lagrangian analyses in satellite observations and Energy Exascale Earth System Model (E3SM) simulations at Graciosa Island in the Azores where a US Department of Energy Atmosphere Radiation Measurement (ARM) site is located. Significantly lower concentrations (55%) of cloud condensation nuclei (CCN) are measured when precipitation rates integrated along 48-hour trajectories are above 1 mm/d (the median value over the North Atlantic) as retrieved using the Integrated Multi-satellitE Retrievals for GPM (IMERG) product. The depletion of CCN is nearly twice as large in the observations compared to E3SM simulations. The model CCN bias remains even after modifying the autoconversion and accretion processes to promote more rapid occurrence of precipitation. When the airmass has relatively high CCN values (above the median CCN value), ARM and MODerate Resolution Imaging Spectroradiometer (MODIS) retrieved cloud droplet number concentration (Nd) is significantly larger (by 50%) than air with relatively lower CCN. As the clouds in trajectories with larger initial values of CCN advect away from Graciosa Island they maintain higher values of Nd over multiple days compared to trajectories that are initially starting with lower Nd and CCN values. The response remains robust after controlling for meteorological factors such as lower troposphere stability. Finally, the Twomey radiative effect of aerosols along forward trajectories are similar amongst autoconversion and accretion sensitivity experiments in the E3SM model and slightly larger than CERES retrievals. This result suggests that there may be a weak link between these warm rain processes and the Twomey radiative effect in the current version (EAMV1) of the E3SM model. We will discuss the liquid water path and cloud fraction adjustments and how these influence the overall aerosol indirect radiative effect from a Lagrangian framework.