Using WRF-ARW to Distinguish between Glaze and Rime Icing at the Air Force Weather Agency

Recent research at the U.S. Air Force Weather Agency (AFWA) has focused on using the operational Advanced Research Weather Research and Forecasting (WRF-ARW) model output fields to forecast “glaze” and “rime” ice separately. In general, “small” supercooled water droplets (of diameter smaller than 50 microns) are producers of rime ice, and “large” drops (diameter larger than 50 microns) are producers of glaze ice. This distinction between glaze and rime ice is important; glaze ice is typically a more severe hazard. The larger drops freeze more slowly, and are able to flow farther along the airfoil, forming a solid sheet.

Within AFWA's 4-km resolution ensemble WRF-ARW output, the only supercooled water information available is within the cloud droplet and rain drop fields. Through previous evaluation of the cloud droplet size distribution assumptions made within each of several single-moment microphysics schemes, empirical relations were established between cloud droplet diameter and mixing ratio. This information was then used to predict the location and amount of “small” and “large” supercooled water drops, and by extension, location and severity of glaze and rime ice. In this study, this method is extended to the full suite of microphysics schemes used with the AFWA ensemble, including double-moment schemes. Effects of explicitly perturbing cloud condensation nuclei and cloud droplet concentrations are examined, particularly in the context of the droplet spectra broadening required to produce freezing drizzle.

Predicted glaze and rime ice fields are evaluated using dual-polarized radar, pilot reports, and observations from the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field campaign.