Ice Formation in an Atmospheric River Event During ACAPEX

Ice formation processes in mixed phase clouds are crucial to understanding cloud phase, the formation of precipitation, and its temporal and spatial evolution. Rosenfeld et al. (2013) found a large amount of supercooled rain and drizzle present in clouds sampled in the CalWater campaign over California’s Sierra Mountains in 2011. However, variations in cloud condensation nuclei (CCN) and ice nucleating particle (INP) concentrations can greatly affect the distribution of supercooled water, ice and the amount of precipitation formed (Fan et al., 2017). This paper examines ice formation during an atmospheric river (AR) event from the ACAPEX (ARM Cloud Aerosol Precipitation Experiment) campaign in Central California in winter 2015. Data from the Two-Dimensional Stereo (2D-S) optical array probe (OAP), on board the DOE G-1 aircraft, is processed using the University of Illinois/Oklahoma OAP Processing Software (UIOOPS). Here, numerous morphological properties from ice particle images are used in conjunction with number distribution functions and ice number concentrations to help determine cloud phase. The vertical and thermal structure and location of the clouds help characterize how much ice formation can be explained by primary nucleation versus regions where secondary ice formation dominates. INP concentrations were measured with a Continuous Flow Diffusion Chamber (CFDC) and from immersion freezing analyses of aerosol filter collections using the CSU Ice Spectrometer (IS). CCN concentrations were measured at approximately 0.3 and 0.6 percent supersaturation. Additional IS measurements of INP concentrations were made from filters collected at a coastal site (UC Davis Bodega Marine Laboratory), and on the NOAA Ronald H. Brown research ship to gain a regional perspective as part of the overall CalWater-2015 project. INP temperature spectra and aerosol composition data indicate the strong role of INPs from marine sources in the 2015 campaign and little evidence for the long range impacts of mineral dust INPs that were evident in the 2011 campaign. Comparisons between the cloud properties and aerosol measurements are used to conceptually describe ice formation processes during the AR event.

Fan, J., L. R. Leung, D. Rosenfeld, and P.J. DeMott, 2017: Effects of Cloud Condensation Nuclei and Ice Nucleating Particles on Precipitation Processes and Supercooled Liquid in Mixed-phase Orographic Clouds, Atmos. Chem. Phys., 17, 1017–1035, doi:10.5194/acp-17-1017-2017.

Rosenfeld, D. R. et al., 2013: The Common Occurrence of Highly Supercooled Drizzle and Rain near the Coastal Regions of the Western United States, J. Geophys. Res. - Atmos., 118, 9819–9833, doi:10.1002/jgrd.50529.