Enhanced ice generation and suppressed drizzle formation by dust particles in stratiform clouds observed from CALIPSO and CloudSat measurements

Heterogeneous ice generation in atmospheric clouds is still poorly understood and parameterized because of the complicated three phase interactions of water and the largely unknown properties of ice nuclei (IN). Although dust particles are widely regarded as effective IN, there still are large uncertainties on dust effectiveness as IN at relatively warm temperatures as highlighted by recent laboratory experiments and field observations. Two years collocated CALIPSO and CloudSat measurements were used to investigate the impacts of dust particles on the ice generation in mid-level water topped stratiform clouds (cloud top higher than 2.5 km above the surface and cloud top temperature–CTT warmer than -40 ̊C) and drizzle formation in warm stratiform clouds (CTT warmer than 0 ̊C) globally. The results showed that the dusty mid-level stratiform clouds not only have higher mixed-phase fraction but also have larger layer maximum radar reflectivity (which indicate higher ice particle concentration) than non-dusty similar cases at given CTT (below than -6 ̊C) in the same geographical region. Dust particles originated from different source regions also showed different ice nucleation abilities, which is related to their different chemical components. Recent in situ observations show that large dust particles coated with small fraction of soluble material can act as effective cloud condensation nuclei (CCN). Our studies showed that dusty particles can noticeably suppress drizzle formation in warm stratiform clouds at give CTT by comparing similar clouds under dusty and non-dusty conditions. These global results will significantly improve our capability to better represent dust aerosol impacts on climate and weather models.