Parameteric origins of the negative correlation between cloud droplet number and liquid water path
Historical cooling due to anthropogenic aerosols is the largest source of uncertainty in inferring future warming from observations. The dominant source of uncertainty in historical aerosol cooling is how aerosols change cloud properties, specifically how changes in liquid microphysics change macrophysical cloud properties such and liquid amount and coverage – termed aerosol-cloud adjustments. The sign of forcing due to aerosol-cloud adjustments is uncertain due to competition between precipitation, sedimentation, and entrainment processes. Present-day observations of droplet number concentration (Nd) and liquid water path (LWP) show a broadly negative correlation between Nd and LWP characterized by a positive correlation at low Nd and a negative correlation at high Nd – termed the ‘upside-down V’. This behavior has been used to argue for the dominance of the entrainment and sedimentation terms and a positive net forcing due to aerosol-cloud adjustments. Here, using a perturbed parameter ensemble in a global Earth system model we show that this negative correlation can be explained by coalescence scavenging and that it is not strongly indicative of a historical warming due to aerosol-cloud adjustments.