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GESTAR II Seminar Series, February 8th at 11:00am

Join us for a virtual seminar by Dr. Greg McFarquhar, Director of the Cooperative Institute for Severe and High Impact Weather Research and Operations and faculty member of the School of Meteorology at University of Oklahoma. His talk is titled "Use of Airborne Cloud Measurements and Model Simulations to Inform About Ice Multiplication."

Date and Time: Thursday, February 8, 2024 at 11:00am
Join us via Teams.

"Primary ice nucleation mechanisms are rarely sufficient for producing observed concentrations of ice crystals, meaning secondary ice production (SIP) processes are common. Two recent field campaigns offer a great opportunity for studying SIP: the 2018 Southern Ocean Cloud Radiation Transport Experimental Study (SOCRATES) conducted over the Australasia sector of the Southern Ocean (SO), which is the most pristine area on Earth with persistent supercooled liquid dominated mixed phase stratocumulus clouds, and the 2015-2016 High Altitude Ice Crystals-High Ice Water Content (HAIC-HIWC) field campaigns which measured high ice water content (HIWC) clouds consisting of many small crystals off the coasts of Darwin Australia and Cayenne French Guiana.

In-situ SOCRATES data show observed ice crystal concentrations (Ni) were 1 to 2 orders of magnitude higher than simultaneously measured ice nucleating particle (INP) concentrations for temperatures less than -10 degrees C, and 5 orders of magnitude for temperatures greater than -10 degrees C, suggesting SIP was common. Evidence of rime splintering included relatively high concentrations of liquid drops with diameters greater than 25 mm associated with high ice-containing phase frequencies at temperatures greater than -8 degrees C, but the SIP mechanism acting at lower temperatures was less clear. Concentrations of cloud condensation nuclei (CCN) were negatively correlated with those of larger liquid drops with diameters greater than 25 mm, suggesting SIP is inhibited in environments having high CCN. Although size distributions (SDs) modeled with the NCAR Community Atmosphere Model version 6 (CAM6) represented the dual peaked shape and form of the observed SDs, CAM6 simulations were not able to simulate the observed Ni and exhibited limited sensitivity to the representation of the rime-splintering process, inconsistent with observations and highlighting uncertainties in SIP in CAM6.

For HAIC-HIWC, measurements of total water content by an isokinetic evaporator probe (IKP) and of SDs were used with a unique multi-modal gamma fitting software routine to characterize where HIWC conditions occur and the dependence of SDs on temperature, IWC, and convective and meteorological characteristics. Comparison of regime-specific observations against properties predicted from WRF simulations using existing parameterization schemes show that although the coverage and evolution of the convection is well predicted, the simulations overestimate the intensity and spatial extent of the observed X-band reflectivity and do not well depict the measured SDs. Sensitivity tests with the P3 two-ice parameterization scheme showed that without including SIP processes in the model, Ni was 2 orders of magnitude less than observed at -10C and three orders of magnitude less at -30C, but slightly larger at -45C. These tests also showed that including one of three SIP mechanisms separately (i.e., Hallett-Mossop mechanism, fragmentation during ice-ice collisions, and shattering of freezing droplets) cannot replicate observed Ni, but simulations including all three SIP processes better replicated the observations. Implications for understanding SIP processes, and their representations in models are discussed.

Greg McFarquhar received his B.Sc. in mathematics and physics from the University of Toronto, Canada in 1987. Thereafter, he changed his field of study to atmospheric sciences and received his M.Sc. (1989) and Ph.D. (1993) also from the University of Toronto, but specializing in cloud physics. Greg spent two years as a postdoctoral fellow at the Scripps Institute of Oceanography in La Jolla, California (1993-94) and worked at the National Center for Atmospheric Research in Boulder, Colorado (1995-2001; 2015-16), the University of Illinois (2001-17) and the University Blaise Pascal in Clermont, France (2012) before joining the faculty at the University of Oklahoma in Aug. 2017 where he also serves as Director of the Cooperative Institute for Severe and High Impact Weather Research and Operations (CIWRO). He is a former chair of the American Meteorological Society Committee on Cloud Physics and currently President of the International Commission on Clouds and Precipitation. He is the chief editor for the American Meteorological Society's Monographs Collection, an associate editor for the Quarterly Journal of the Royal Meteorological Society, and has active research grants from the National Science Foundation (NSF), the Department of Energy (DOE), the National Aeronautics and Space Administration (NASA), and the National Oceanic and Atmospheric Administration (NOAA). He has formerly served as the Chief Scientist for the Atmospheric Radiation Measurement (ARM) Program's Aerial Facilities (AAF). He has 237 publications in the refereed literature, an h-index of 65, and he and his group have made over 750 presentations at conferences and working group meetings. He has participated in or led 37 different air- or ship-based cloud measurement field campaigns, and is a Fellow of the American Meteorological Society and of the American Geophysical Union.

For more information on the GESTAR II Seminar Series, click here.

Posted: February 2, 2024, 10:13 AM