An Evaluation of Model-based Icing Forecasts of a Supercooled Large Drop Icing Event Using Detailed In Situ Measurements

An atmospheric river event impacting the Pacific Northwest coast on 12 February 2014 brought abundant moisture as far inland as Boise, Idaho. During this event, supercooled liquid water (SLW) in excess of 1 mm was observed by a microwave radiometer near Boise. In this case, in situ measurements from the University of Wyoming King Air (UWKA) were also collected; however, this flight had to be cut short due to severe supercooled large drop (SLD) icing encountered by the UWKA aircraft.

Recent changes to the Federal Aviation Administration (FAA) aircraft certification process related to aircraft icing have an explicit focus on SLD icing conditions.  As such, the development of new predictive and diagnostic tools to address SLD icing conditions is needed.  Moreover, new numerical weather prediction (NWP) capabilities allow for the explicit prediction of drop size. As a result, the application of these new NWP capabilities for predicting SLD icing conditions is currently being evaluated.

This paper will present a suite of observations to evaluate NWP model forecasts of this SLD icing event.  In particular, in situ probe data from the UWKA flight and measurements from a W-band cloud radar onboard the UWKA will be combined with available ground-based measurements (radiometer, soundings, METAR, etc) to assess if the NWP forecasts predicted the appropriate SLW content, drop size, and hydrometeor phase.  Moreover, the explicit NWP forecast of these parameters will be compared against Forecast Icing Product (FIP) and Current Icing Product (CIP) outputs, in order to diagnose ways to improve both the FIP and CIP algorithms and NWP models to better predict SLD icing events.

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.