David J. Serke

Supercooled large drop detection with NASA's Icing Remote Sensing System

In-flight icing occurs when aircraft impact supercooled liquid drops. The supercooled liquid freezes on contact and the accreted ice changes a planes aerodynamic characteristics, which can lead to dangerous loss of control. NASAs Icing Remote Sensing System consists of a multi-channel radiometer, a laser ceilometer and a vertically-pointing Kaband radar, whos fields are merged with internal software logic to arrive at a hazard classification for in-flight icing. The radiometer is used to derive atmospheric temperature soundings and integrated liquid water and the ceilometer and radar are used to define cloud boundaries. The integrated liquid is then distributed within the determined cloud boundaries and layers to arrive at liquid water content profiles, which if present below freezing are categorized as icing hazards. This work outlines how the derived liquid water content and measured Ka-band reflectivity factor profiles can be used to derive a vertical profile of radar-estimated particle size. This is only possible because NASAs system arrives at independent and non-correlated measures of liquid water and reflectivity factor for a given range volume. The size of the drops significantly effect the drop collection efficiency and the location that icing accretion occurs on the crafts superstructure and thus how a vehicles performance is altered. Large drops, generally defined as over 50 μm in diameter, tend to accrete behind the normal ice protected areas of the leading edge of the wing and other control surfaces. The NASA Icing Remote Sensing System was operated near Montreal, Canada for the Alliance Icing Research Study II in 2003 and near Cleveland, Ohio from 2006 onward. In this study, we present case studies to show how NASAs Icing Remote Sensing System can detect and differentiate between no icing, small drop and large drop in-flight icing hazards to aircraft. This new product provides crucial realtime hazard detection capabilities which improve avaiation safety in the near-airport environment with cost-effective, existing instrumentation technologies.

Use of X-band radars to support the detection of in-flight icing hazards

The NASA Icing Remote Sensing System was operated for the Alliance Icing Research Study II field program during the winter of 2003 around Montreal, Canada and around Cleveland, Ohio during the winter of 2005. Icing research aircraft flights from these field programs provided verification data on liquid water content, air temperature and also cloud particle imagery and distributions. The purpose of this work is to show that the NASA Icing Remote Sensing System X-band radar reflectivity profiles could be used beyond merely defining vertical cloud boundaries, by operationally deriving a qualitative small drop icing hazard warning flag. Several case studies are presented which depict a variety of synoptic weather scenarios. These cases demonstrate that X-band reflectivities below -10 dBZ and above the minimum detectable are uniquely indicative of a particle population dominated by small, liquid droplets. A discussion is included for each case on how an in-flight icing hazard flag from the radar reflectivity profile would improve the operational hazard detection system. Comparison of the NASA Icing Remote Sensing Systems X-band radar data to a nearby similar X-band from McGill University is done to ensure data quality and consistency.

The use of x-band radar to support the detection of in-flight icing hazards by the NASA Icing Remote Sensing System

In-flight icing hazards from supercooled small drops, drizzle and freezing rain pose a threat to all aircraft. Several products have been developed to provide hazard warning of in-flight icing to the aviation community. NCARs Current Icing Product1 (CIP) was developed to provide a near-realtime assessment of the hazard presented by supercooled liquid water (SLW) aloft in an algorithm that combines data from satellites, the Rapid Update Cycle (RUC) model, the national 2-D composite of S-band NEXRAD radar reflectivity, surface observations and pilot reports (PIREPs). NIRSS2 (Fig. 1) was developed by NASA to provide a ground-based, qualitative in-flight icing hazard assessment in the airport environment with commercially available instrumentation. The system utilizes a multichannel radiometer3, built by Radiometrics Corporation, to derive the temperature profile and integrated liquid water (ILW). NIRSSs radar is a modified airborne X-band model WU-870 made by Honeywell. The ceilometer used is a standard Vaisala CT25K Laser Ceilometer. The data from the vertically pointing ceilometer and X-band radar are only used to define the cloud bases and tops. The liquid water content (LWC) is then distributed within the cloud layers by the system software. A qualitative icing hazard profile is produced where the vertical temperature is between 0 and -20°C and there is measurable LWC.