Characterization of High Ice Water Content Conditions that Impact Air Data System Performance

Disruptions to Air Data System (ADS) performance, such as anomalies in the measurement of Total Air Temperature (TAT) and True Air Speed (TAS), have been noted by various researchers during flight in glaciated clouds. Such behavior has been attributed to High Ice Water Content (HIWC) in clouds produced by convective activity associated with tropical cyclones, Mesoscale Convective Systems (MCS), and other storm types. Ingest of ice particles into aircraft inlets can restrict airflow, and accumulated ice can melt upon contact with a heated inlet thereby compromising the measurements. Performance issues with various other types of research sensors may also occur in response to HIWC in these environments. Data obtained from experiments using the NASA DC-8 Airborne Science Laboratory and the National Science Foundation/National Center for Atmospheric Research Gulfstream V research aircraft provide numerous examples of sensor performance degradation due to HIWC conditions.

Data for this analysis are derived from two field experiments in which tropical and sub-tropical convective clouds were sampled. The HIWC Radar Experiment in 2015 conducted 10 flights over the Gulf of Mexico, Caribbean and western Atlantic Ocean in MCSs and tropical storms. The Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment explored tropical wave disturbances in the Atlantic and Caribbean during 26 research flights. PREDICT did not specifically target HIWC conditions, but contains multiple examples of anomalous behavior in ADS and research sensor performance caused by encounters with large concentrations of ice crystals. During flights in both experiments, areas of HIWC were frequently encountered. Anomalous spikes in the TAT measurements (referred to hereafter as “TAT anomalies”), similar irregularities in TAS and other measurements derived from the pitot-static system (“pitot anomalies”), and performance degradation in other research sensors were often an indication of HIWC at flight level during these experiments.

The research payloads for both experiments included cloud microphysics sensors that provide ice particle images, size distributions, and total water content at altitudes up to 45 kft and vertical profiles of these properties during ascents and descents. Satellite products provide bulk characteristics of observed cloud systems. Using this data set, we characterize the cloud micro- and macro-physical conditions that produce sufficient Ice Water Content (IWC) to induce TAT and pitot anomalies and other disruptions to sensor performance.

The payloads also included multiple TAT sensors, some of which were used for research measurements and others within the aircraft ADS. The suite included heated temperature sensors manufactured by Rosemount and Harco. All were found to be susceptible to the effects of ice particle ingest at various times. Analysis of approximately 50 TAT and pitot anomalies reveals the conditions that degraded performance of the various sensors. IWC at the times of the TAT anomalies consistently exceeded 0.5 g m^-3, and were often above 1.0 g m^-3. Durations of elevated IWC prior to the events were between 1 and 8 minutes. Ice particle number concentrations measured by a SPEC Inc. 2D-S probe were 3000-8000 L^-1. Satellite-derived cloud top heights of 13-16 km were typical, and some overshooting cloud tops (i.e., cloud top heights above the tropopause) were present. Ice water path (IWP) derived from satellite data varied from 1000-4000 g m^-2 during the TAT and pitot anomaly events. Additional analysis focuses on flight segments where ice microphysical conditions were similar but ADS anomalies did not occur. A set of these “null events” has been identified and attributes are being compared to understand differences between conditions that induced ADS anomalies and those that did not. Finally, the ADS anomaly events from each aircraft will be reviewed for similarities and differences.

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.