Marine Shallow Cloud Adjustments to the Presence of Shortwave-Absorbing Aerosols: Advancing understanding through a combined analysis of LASIC datasets and process modeling

Abstract

The global majority of the shortwave-absorbing aerosols located above low clouds occur over the southeast Atlantic, with regional model and satellite estimates of the direct, indirect, and semi-direct aerosol-cloud-radiation effects questionable. The Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign is an observational strategy focused on using ARM Mobile Facility 1 datasets gathered from July 1, 2016 through October 31, 2017 over Ascension Island to improve our understanding of both the shortwave-absorbing aerosol characteristics and the fast cloud adjustments to the aerosol presence somewhere within the vertical column. The LASIC deployment encompasses two full biomass-burning aerosol seasons. The remote Ascension Island (14W, 8S) lies within the trade-wind cumulus regime, underneath the main outflow zone of biomass-burning aerosols from the continental African fires. The new measurements are already revealing that black carbon is frequently but not consistently present within the boundary layer, and is associated with a range of aerosol-cloud vertical structures.
This proposal focuses on understanding the low cloud response to the presence of the shortwave-absorbing aerosol. A priority is to understand the mechanism(s) by which the top-of-atmosphere reflectance is observed to increase when smoke is present in the atmosphere, indicating the low cloud deck must be overcompensating for the darkening of the scene by the smoke itself (the direct radiative effect). This low cloud response may be an adjustment to the aerosol radiative heating of the atmosphere, a semi-direct effect, but may also incorporate direct aerosol-cloud microphysical interactions, and can also reflect co-associated meteorological changes, for example in the free-tropospheric stability. A thorough observational analysis of LASIC datasets is proposed to address this.
Further process understanding and attribution will build on the observational analysis with large-eddy-scale simulations and cloud-resolving regional modeling. The former will be used for sensitivity experimentation, and to examine potential processes by which heating occurring 1-3 km above the low cloud deck could be transferred down to the cloud-top inversion. Regional modeling using the Weather Research and Forecasting Model with and without smoke emissions included will distinguish aerosol from meteorological influences. Sensitivities to the model aerosol treatment will be evaluated. The precipitation susceptibilities in observations and both models will be assessed, towards refining model microphysical parameterizations. A further satellite-perceived and modeling characterization of the stratocumulus-to-cumulus transition, evaluating the boundary-layer flow ending at Ascension, will seek to further identify the influence of the absorbing aerosol.