Impact of High Ice Water Content on Airborne Measurements from the NSF/NCAR Gulfstream V Aircraft

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, 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 National Science Foundation/National Center for Atmospheric Research Gulfstream V aircraft provide numerous examples of sensor performance degradation due to HIWC conditions.

A dataset from the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment that explored tropical wave disturbances contains multiple examples of anomalous behavior in ADS and research sensor performance. During 26 research flights in the upper levels of developing tropical depressions, areas of HIWC were frequently encountered. Anomalous spikes in the total air temperature measurements (referred to hereafter as “TAT anomalies”) and performance degradation in other sensors were often an indication of HIWC at flight level. The research payload for this experiment 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 IWC to induce TAT anomalies and other disruptions to sensor performance.

The PREDICT experiment payload included five total air temperature sensors, four of which were used for research measurements and one that was associated with 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 30 TAT anomalies reveals the conditions that degraded performance of the various temperature sensors. Ice water content (IWC) at the times of the TAT anomalies ranged from 0.5 – 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) varied from 1000-4000 g m^-2 during the TAT anomaly events. Additional analysis focuses on flight segments where ice microphysical conditions were similar but TAT 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 TAT anomalies and those that did not. Finally, the TAT anomaly data set will be compared with observations by other researchers who have documented such events on different aircraft.