An In-Cloud ICing and Large-drop Experiment (ICICLE) Case Study

Aircraft icing is a major concern to the aviation community due to the tendency of ice accretion on aircraft surfaces to decrease lift and increase drag, resulting in an increased safety risk. To develop and evaluate methods for accurate diagnosis and forecasting of aircraft icing conditions at the surface and aloft, a broad spectrum of in-situ measurements within icing and non-icing environments is needed. To this end, the Federal Aviation Administration (FAA) planned the In-Cloud ICing and Large-drop Experiment (ICICLE) flight campaign, which took place from January to March of 2019. During this project, the FAA, the National Center for Atmospheric Research, the National Research Council of Canada (NRC), Environment and Climate Change Canada, and others collaborated to capture data in icing conditions with the NRC Convair-580 research aircraft. The Convair-580 was equipped with a wide array of in-situ and remote sensing instrumentation providing measurements of atmospheric state parameters, aerosol and cloud microphysical properties, and cloud structure. The focus of this presentation is a case study of a particularly interesting ICICLE research flight that was conducted on February 26^th, 2019.

On February 26^th, a multi-layer, mixed phase cloud was predicted over Eastern Iowa around the solar terminator. The High-Resolution Rapid Refresh (HRRR) model predicted high liquid water contents (LWC) and the potential for non-classical supercooled large drops (SLD) to form within the lower cloud deck. Over time, the HRRR anticipated rapidly increasing LWC and a transition from Appendix C to Appendix O SLD conditions over the Cedar Rapids, IA (KCID) region. A gradient in the intensity of icing was also predicted to occur between KCID and Davenport, IA (KDVN), with weaker conditions near KDVN. The goal of this flight was to sample the forecasted rapid increase in liquid water and the transition from small to large drop icing. A secondary goal was to evaluate the performance of various icing tools, including surface observations, radar reflectivity, satellite returns, and the HRRR. Flight legs conducted between KCID and KDVN confirmed much of the spatial tendencies forecasted by the HRRR, with rapid intensification of icing over KCID, a strong gradient in icing between KCID and KDVN, and a significant spatial and temporal evolution in the drop size distribution across this area. An overview of this event will be discussed.

This research is in response to requirements and funding by the FAA. The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.