Satellites Reveal Aerosols’ Major Impact on Cloud Freezing over Arctic Summertime Sea Ice

Aerosols have potentially large impacts on the surface radiative budget in the Arctic through their impacts on cloud properties. Our aim is to better understand how aerosols affect clouds in the Arctic during summer. To do so, we obtained CloudSat satellite data, plus aerosol and meteorological reanalysis products (MERRA-2, ECMWF). Although there is too much variation in the data to interpret what happens to individual clouds, having seven years of summertime CloudSat data over the entire Arctic allows us to apply previously published, advanced statistical methods (Zamora and Kahn, J. Climate 2020) to isolate aerosol effects from those of covarying meteorology. We find that on average, different aerosol types (dust, sulfate, black carbon, organic carbon) all increase cloud freezing by ~20-40% over sea ice. Given that these effects appear to be relatively independent of aerosol type and temperature (among other meteorological variables), we hypothesize that the large observed differences in cloud phase when aerosols are present may be due to any of several cloud condensation nuclei (CCN)-related mechanisms. These effects could include reducing cloud droplet size and increasing droplet number, which in turn may change riming, increase contact freezing, and reduce precipitation, ice nuclei losses, or ice particle sizes and deposition. To verify these hypotheses, further in situ data are needed. For example, NASA’s upcoming ARCSIX mission over Northern Greenland in summer 2024, in which we are participants, could provide such an opportunity. We will also discuss how we hope to use our satellite findings to help target and interpret upcoming field measurements to better understand the mechanisms driving aerosols effects on cloud lifetime, precipitation, radiation, and climate over the sensitive polar region.