Introducing the Current Icing Product (CIP) and Forecast Icing Product (FIP) Freezing Rain Potential

The Current Icing Product (CIP) and Forecast Icing Product (FIP) provide operational icing weather guidance over the contiguous United States by blending numerical weather prediction output with data from satellite, radar, voice pilot reports, and surface observations. Both produce three fields: Icing Probability, Icing Severity, and Supercooled Large Drop (SLD) Potential. Icing Probability is the probability of any supercooled liquid water, while SLD Potential is an uncalibrated estimate of the likelihood of supercooled liquid where the maximum drop diameter (DMax) is at least 100 μm. Neither CIP nor FIP currently distinguish between the two subcategories of SLD: freezing drizzle (FZDZ, 100 μm ≤ DMax ≤ 500 μm) and freezing rain (FZRA, DMax > 500 μm). A product distinguishing between FZDZ and FZRA is needed, however, because some aircraft are certified to operate in FZDZ, but not FZRA. Leveraging data and research from the In-Cloud ICing and Large-drop Experiment (ICICLE) in 2019, an experimental FZRA Potential product has been developed for both CIP and FIP.

Like existing CIP/FIP products, the method for computing FZRA Potential will depend on the “scenario,” or the meteorological and microphysical processes dominant in a given environment. The names of these scenarios, a description of each, and the methods for computing FZRA Potential are outlined in Table 1. This presentation will provide more details on the algorithm, a comparison to in situ aircraft observations, and maps of the gridded product.

Table 1: Names of FZRA Potential scenarios, their physical meaning, and the ingredients used to compute FZRA Potential.

Scenario Name	Physical Meaning	FZRA Potential Ingredients
Convection	Deep, moist convection	Model temperature, model vertical wind
Below Warm Nose	Snow/ice falls through an above-freezing layer, melts into rain, then falls into a below-freezing layer	Model temperature, model precipitable condensate, radar reflectivity (CIP only)
Recirculation	Snow/ice falls into above-freezing layer, melts into rain, and is then lifted back up to sub-freezing temperatures through updrafts	Model temperature, model vertical wind, model liquid condensate, model rain water content, radar reflectivity (CIP only)
Collision Coalescence	Growth of liquid droplets through collision-coalescence processes	Model temperature; cloud top temperature and depth below cloud top (cloud top inferred from satellite for CIP and from the model for FIP); cloud reflectivity at 0.64, 1.6, and 2.2 μm from satellite (CIP only); surface precipitation reports (CIP only)

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