Andrew L. Reehorst

NASA Icing Remote Sensing System comparisons from AIRS II

NASA has an on-going activity to develop remote sensing technologies for the detection and measurement of icing conditions aloft. A multiple instrument approach is the current emphasis of this activity. Utilizing radar, radiometry, and lidar, a region of supercooled liquid is identified. If the liquid water content (LWC) is sufficiently high, then the region of supercooled liquid cloud is flagged as being an aviation hazard. The instruments utilized for the current effort are an X-band vertical staring radar, a radiometer that measures twelve frequencies between 22 and 59 GHz, and a lidar ceilometer. The radar data determine cloud boundaries, the radiometer determines the sub-freezing temperature heights and total liquid water content, and the ceilometer refines the lower cloud boundary. Data is post-processed with a LabVIEW program with a resultant supercooled LWC profile and aircraft hazard identification. Individual remotely sensed measurements gathered during the 2003-2004 Alliance Icing Research Study (AIRS II) were compared to aircraft in-situ measurements. Comparisons between the remote sensing systems fused icing product and in-situ measurements from the research aircraft are reviewed here. While there are areas where improvement can be made, the cases examined indicate that the fused sensor remote sensing technique appears to be a valid approach.

Preliminary Analysis of X-band and Ka-band Radar for Use in the Detection of Icing Conditions Aloft

ABSTRACT NASA and the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) have an on-going activity to develop remote sensing technologies for the detection and measurement of icing conditions aloft. Radar has been identified as a strong tool for this work. However, since the remote detection of icing conditions with the intent to identify areas of icing hazard is a new and evolving capability, there are no set requirements for radar sensitivity. This work is an initial attempt to quantify, through analysis, the sensitivity requirements for an icing remote sensing radar. The primary radar of interest for cloud measurements is Ka-band, however, since NASA is currently using an X-band unit, this frequency is also examined. Several aspects of radar signal analysis were examined. Cloud reflectivity was calculated for several forms of cloud using two different techniques. The Air Force Geophysical Laboratory (AFGL) cloud models, with different drop spectra represented by a modified gamma distribution, were utilized to examine several categories of cloud formation. Also a fundamental methods approach was used to allow manipulation of the cloud droplet size spectra. And an analytical icing radar simulator was developed to examine the complete radar system response to a configurable multi-layer cloud environment. Also discussed is the NASA vertical pointing X-band radar. The radar and its data system are described, and several summer weather events are reviewed.

CLOSE-UP ANALYSIS OF INFLIGHT ICE ACCRETION.

The objective of this effort was to validate in flight, data that has been gathered in the NASA Lewis Research Centers Icing Research Tunnel (IRT) over the past several years. All data was acquired in flight on the NASA Lewis Research Centers Twin Otter Icing Research Aircraft. A faired 3.5 in. diameter metal-clad cylinder exposed to the natural icing environment was observed by a close-up video camera. The grazing angle video footage was recorded to S-VHS video tape and after the icing encounter, the resultant ice shape was documented by 35 mm photography and pencil tracings. The feather growth area was of primary interest; however, all regions of the ice accretion, from the stagnation line to the aft edge of run back were observed and recorded. After analysis of the recorded data several interesting points became evident: (1) the measured flight feather growth rate is consistent with IRT values, (2) the feather growth rate appears to be influenced by droplet size, (3) the feathers were straighter in the lower, spottier LWC of flight in comparison to those observed in the IRT, (4) feather shedding and ice sublimation may be significant to the final ice shape, and (5) the snow encountered on these flights appeared to have little influence on ice growth.

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.

Spatial Analysis of Great Lakes Regional Icing Cloud Liquid Water Content

Abstract Clustering of cloud microphysical conditions, such as liquid water content (LWC) and drop size, can affect the rate and shape of ice accretion and the airworthiness of aircraft. Clustering may also degrade the accuracy of cloud LWC measurements from radars and microwave radiometers being developed by the government for remotely mapping icing conditions ahead of aircraft in flight. This paper evaluates spatial clustering of LWC in icing clouds using measurements collected during NASA research flights in the Great Lakes region. We used graphical and analytical approaches to describe clustering. The analytical approach involves determining the average size of clusters and computing a clustering intensity parameter. We analyzed flight data composed of 1-s-frequency LWC measurements for 12 periods ranging from 17.4 minutes (73 km) to 45.3 minutes (190 km) in duration. Graphically some flight segments showed evidence of consistency with regard to clustering patterns. Cluster intensity varied from 0.06, indicating little clustering, to a high of 2.42. Cluster lengths ranged from 0.1 minutes (0.6 km) to 4.1 minutes (17.3 km). Additional analyses will allow us to determine if clustering climatologies can be developed to characterize cluster conditions by region, time period, or weather condition. Introduction

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.