In-flight icing occurs when aircraft impact supercooled liquid drops. The supercooled liquid freezes on contact
and the accreted ice changes a plane's aerodynamic characteristics, which can lead to dangerous loss of control. NASA's
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 NASA's 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 craft's
superstructure and thus how a vehicle's 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
NASA's 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.
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 System's X-band radar data to a nearby similar X-band from McGill University is done to ensure data quality and consistency.
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. NCAR's
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).
NIRSS's 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.
The multi-agency Flight in Icing Remote Sensing Team (FIRST), a consortium of the National Aeronautics and
Space Administration (NASA), the Federal Aviation Administration (FAA), the National Center for Atmospheric
Research (NCAR), the National Oceanographic and Atmospheric Administration (NOAA), and the Army Corps of
Engineers (USACE), has developed technologies for remotely detecting hazardous inflight icing conditions. The
USACE Cold Regions Research and Engineering Laboratory (CRREL) assessed the potential of onboard passive
microwave radiometers for remotely detecting icing conditions ahead of aircraft. The dual wavelength system
differences the brightness temperature of Space and clouds, with greater differences potentially indicating closer and
higher magnitude Cloud Liquid Water Content (CLWC). The Air Force RADiative TRANsfer model (RADTRAN)
was enhanced to assess the flight track sensing concept, and a "flying" RADTRAN was developed to simulate a
radiometer system flying through simulated clouds. Neural network techniques were developed to invert brightness
temperatures and obtain integrated cloud liquid water. In addition, a dual wavelength Direct-Detection Polarimeter
Radiometer (DDPR) system was built for detecting hazardous drizzle drops. This paper reviews technology
development to date and addresses initial polarimeter performance.
NASA has teamed with the FAA, DoD, industry, and academia for research into the remote detection and measurement
of atmospheric conditions leading to aircraft icing hazards. The ultimate goal of this effort is to provide pilots,
controllers, and dispatchers sufficient information to allow aircraft to avoid or minimize their exposure to the hazards of
in-flight icing. Since the hazard of in-flight icing is the outcome of aircraft flight through clouds containing supercooled
liquid water and strongly influenced by the aircraft's speed and configuration and by the length of exposure, the hazard
can't be directly detected, but must be inferred based upon the measurement of conducive atmospheric conditions.
Therefore, icing hazard detection is accomplished through the detection and measurement of liquid water in regions of
measured sub-freezing air temperatures. The icing environment is currently remotely measured from the ground with a
system fusing radar, lidar, and multi-frequency microwave radiometer sensors. Based upon expected ice accretion
severity for the measured environment, a resultant aircraft hazard is then calculated. Because of the power, size, weight,
and view angle constraints of airborne platforms, the current ground-based solution is not applicable for flight. Two
current airborne concepts are the use of either multi-frequency radiometers or multi-frequency radar. Both ground-based
and airborne solutions are required for the future since ground-based systems can provide hazard detection for all aircraft
in airport terminal regions while airborne systems will be needed to provide equipped aircraft with flight path coverage
between terminal regions.