Aerosol-Coud Indirect Effects in Midlatitude Cyclones—Insights from Remote Sensing Observations and Convection-Permitting Global Aquaplanet Simulations.

Aerosol-cloud interactions are a major source of uncertainty in inferring the climate sensitivity from the observational record of temperature. Here we examine how the cloud droplet number concentration (CDNC) observed from space covaries with different aerosol species and responds to long-term trends in volcanism and anthropogenic pollution. It is found that the majority of observed variability in CDNC may be explained by variability in sulfate mass concentration (Figure A). We find that the sensitivity of CDNC to sulfate implies an aerosol-cloud forcing between -0.75 and -1.25 Wm^-2. The results from this study also elucidate how cloud macrophysical properties adjust to changes in CDNC in the midlatitudes. Idealized experiments in high-resolution, convection-permitting global aquaplanet simulations (Figure B) are compared to thirteen years of remote-sensing observations. Observations and aquaplanet simulations agree that increased warm conveyor belt (WCB) moisture flux into cyclones is consistent with higher cyclone liquid water path (CLWP). Further, they agree that enhanced CDNC at a given WCB moisture flux enhances CLWP. We propose that the large-scale environment controls cyclone rain rate via the moisture flux into the cyclone; enhanced CDNC decreases precipitation efficiency; then the CLWP adjusts to maintain the rain rate dictated by the large-scale environment. If cyclones in the top and bottom tercile of CDNC are contrasted it is found that they not only have higher CLWP (Figure C), but also cloud cover, and albedo. The difference in cyclone albedo between the cyclones in the top and bottom third of CDNC is observed by CERES to be between 0.018 and 0.032, which is consistent with a 4.6-8.3 Wm^-2 in-cyclone enhancement in upwelling shortwave when scaled by annual-mean insolation. Based on a regression model to observed cyclone properties roughly 60% of the observed variability in CLWP can be explained by CDNC and WCB moisture flux.