Boase Hydrologic Sciences and Water Resources Engineering Seminar Series

Speaker: Assistant Professor Ivy Tan, Physics Dept., University of Colorado Boulder

Title: T_he Role of Secondary Ice Production in Clouds in Earth’s Changing Climate_

Abstract: 

Secondary ice production (SIP) is the generation of ice particles from pre-existing ice particles in clouds.  SIP contrasts with primary ice production, i.e. ice nucleation.  While ice nucleation remains poorly understood, even less is known about SIP, and few climate models represent one or more SIP mechanisms.  We implemented three SIP mechanisms into the CESM2 model: (i) droplet shattering during rain freezing and (ii) ice-ice fragmentation in addition to the (iii) Hallett-Mossop rime splintering process that is included in the standard version of the model and evaluate its influence on Earth’s present and future climate.  We find that the inclusion of SIP processes decreases the climate sensitivity of the CESM2 model by ~0.7˚C and is largely driven by an expected cloud-phase feedback whereby an increased mass and number concentration of ice crystals is replaced by liquid droplets in Earth’s extratropical latitudes, however, cloud ice number concentration also plays an important role in influencing the opacity of clouds.  We also identify a contrasting state-dependent tropical microphysics-dynamics coupled response in the tropics that further dampens the Walker circulation in a perturbed warmed state.  This response is driven by SIP-induced weakened updrafts in the Tropical West Pacific and weakened downdrafts in the Tropical East Pacific by altering the vertical distribution of diabatic heating.  In the west, SIP enhances melting-layer cooling and depositional heating aloft, whereas in the east it increases upper-tropospheric sublimative cooling through a greater concentration of suspended ice particles and enhances lower-tropospheric warming by reducing ice sedimentation. Together, these changes modify the tropical heating gradient and further dampen the Walker circulation in a warmer climate.  Our results emphasize the need for global observational constraints on SIP and its implementation into climate models used to inform climate policy.

Bio: 

Ivy Tan is an Assistant Professor in the Physics Department at CU Boulder.  She obtained her B. Sc. in Physics and Mathematics at the University of Toronto and Ph.D. at Yale University before joining NASA Goddard Space Flight Center's Climate and Radiation Laboratory as a NASA Postdoctoral Program Fellow and McGill University as an Assistant Professor.  She is a Senior Editor of Atmospheric Chemistry and Physics and an Associate Editor of Journal of Climate.  Her favourite pastimes include playing Chopin on the piano and figure skating.