10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Does Secondary Ice Processes in Mixed Phase Clouds More Important Than We Assume?
ASSAF ZIPORI, Naama Reicher, Yigal Erel, Daniel Rosenfeld, Amir Sandler, Daniel Knopf, Yinon Rudich, Weizmann Institute of Science
Abstract Number: 339 Working Group: Clouds and Climate
Abstract It is generally assumed that secondary ice processes (SIP) influence the ice content in clouds that form in areas with low aerosol and ice nucleation particles (INP) concentration. However, in continental clouds, with high aerosol and INP concentrations, SIP are thought to be minor, and ice nucleation is assumed to be dominated by the INP concentration.
Here we combine field observations, laboratory measurements and model results to investigate the possible role of primary ice nucleation in continental, mixed phase, orographic clouds. Primary ice nucleation was quantified using the WeIzmann Supercooled Droplets Observation on a Microchip (WISDOM). Particles collected from rain samples were used to estimate the ice nucleation efficiency in the immersion mode. The immersion freezing results were compared with satellite-retrieved glaciation temperatures (Tg) and with an immersion freezing model, and used to examine the relative role of primary and secondary ice processes on cloud glaciation. In addition, the INP efficiency was also related to the particles’ minerology.
We found that primary ice nucleation was controlled by K-Feldspar, although it was not the most abundant mineral. The role of SIP was assessed by comparing the observed Tg with the temperature where 50% of the drops froze in the WISDOM experiments (ΔT). This difference was minimal when the aerosols’ concentration in the rain water was 10mg L-1. ΔT increased at lower aerosol concentration, suggesting that SIP were more effective due to larger droplets. At higher dust concentrations, ΔT increased possibly due to higher surface area available in cloud droplets compared to the WISDOM experiments. In addition, giant cloud condensation nuclei could also have initiated SIP by forming large droplets at early cloud growth stages. Using a simple model of primary ice nucleation, the calculated glaciation temperature was compared with the observed Tg. Although the model design favored primary ice nucleation, it did not explain the observed Tg from the satellite.
Our results show a direct relation between the drops size and the importance of SIP. In addition, we show that SIP is not limited to clouds with low aerosols concentration, but exist also in continental clouds, which were assumed to be less prone to SIP. This is due to large dust aerosols that are carried to the clouds during severe dust storms. These large dust aerosols can act as giant cloud condensation nuclei that freeze at relatively high temperatures, and initiate SIP. Our results emphasize the importance of SIP on Earth’s radiation budget and its influence on global climate, as it is larger than usually assumed.