U002-02
A Synthesis of Observations of Aerosol-Cloud Interactions over the Pristine, Biologically Active Southern Ocean and the Implications for Global Climate Model Predictions
Monday, 7 December 2020: 16:08
Isabel Louise McCoy1, Daniel McCoy2, Robert Wood3, Leighton Anunda Regayre4, Duncan Watson-Parris5, Dan Grosvenor6, Christopher S. Bretherton7, Cynthia H Twohy8, Andrew Gettelman9, Charles Bardeen10, Jane Patricia Mulcahy11, Yongxiang Hu12, Frida Bender13, Paul Field14, Ken S Carslaw6, Hamish Gordon15 and Darin W Toohey16, (1)University of Miami, RSMAS, Miami, FL, United States, (2)University of Wyoming, Laramie, United States, (3)University of Washington, Atmospheric Sciences, Seattle, WA, United States, (4)University of Leeds, Leeds, LS2, United Kingdom, (5)University of Oxford, Oxford, United Kingdom, (6)University of Leeds, Leeds, United Kingdom, (7)Vulcan, Inc., Climate Modeling, Seattle, WA, United States, (8)NorthWest Research Associates Boulder, Boulder, CO, United States, (9)NCAR, Boulder, CO, United States, (10)National Center for Atmospheric Research, Boulder, CO, United States, (11)Met Office Hadley Centre for Climate Change, Exeter, United Kingdom, (12)NASA Langley Research Center, Hampton, VA, United States, (13)Stockholm University, Stockholm, Sweden, (14)United Kingdom Met Office, Exeter, United Kingdom, (15)College of Engineering, Carnegie Mellon University, Pittsburgh, PA, United States, (16)Univ Colorado, Boulder, CO, United States
Abstract:
The change in planetary albedo due to aerosol-cloud interactions (aci) during the industrial era is the leading source of uncertainty in inferring Earth's climate sensitivity to increased greenhouse gases from the historical record. Examining pristine environments such as the Southern Ocean (SO) helps us to understand the pre-industrial state and constrain the change in cloud brightness over the industrial period associated with aci. This study presents two methods of utilizing observations of pristine environments to examine climate models and our understanding of the pre-industrial state.
First, cloud droplet number concentration (Nd) is used as an aci indicator variable. Global climate models (GCMs) show that the hemispheric contrast in liquid cloud Nd between the pristine SO and the polluted Northern Hemisphere observed in the present-day can be used as a proxy for the increase in Nd from the pre-industrial. A hemispheric difference constraint developed from MODIS satellite observations indicates that pre-industrial Nd may have been higher than previously thought and provides an estimate of the radiative forcing associated with aci (RFaci) between -1.2 and -0.6 Wm-2. Comparisons with MODIS Nd highlight significant GCM discrepancies in pristine, biologically active regions.
Second, aerosol and cloud microphysical observations from a recent SO aircraft campaign are used to identify two potentially important mechanisms missing in GCMs: i) production of new aerosol particles through synoptic uplift and ii) buffering of Nd against precipitation removal by small, free tropospheric particles. The latter may significantly contribute to the high, summertime SO Nd levels which persist despite precipitation depletion associated with mid-latitude storm systems. Observational comparisons with nudged GCM hindcasts highlight large aerosol number and composition discrepancies that may significantly and negatively impact the ability of current GCMs to capture aci in pristine environments.
