High Ice Water Content and Airborne Temperature Measurement Anomalies in Tropical Convection

Since the mid-1990s observations of jet engine powerloss in areas with high ice particle concentrations have generated interest from the aviation industry, regulatory agencies, and the general public. Numerous cases of powerloss or engine rollback (loss of engine control) involving commuter and large transport aircraft have been documented.

Air Data System performance disruptions, such as anomalies in the measurement of Total Air Temperature (TAT), have also been noted in these conditions. The majority of these events occurred in flight conditions where conventional icing by supercooled liquid was not suspected, and where ice particles were prevalent. In 2003 the Engine Harmonization Working Group (EHWG), an international committee of regulators and engine aircraft manufacturers, concluded that these events were likely caused by ingest of high mass concentrations of ice particles. Investigation of these events has provided a basic understanding of the meteorological conditions that support areas of HIWC. Observations indicate that many HIWC regions occur near convective systems in tropical air masses at high altitudes. Clouds are generally glaciated and radar reflectivity is low, suggesting the presence of small ice particles. Details of the cloud micro- and macro-physical processes that lead to HIWC conditions, however, are still under investigation.

An experiment conducted in 2010 using the National Science Foundation/National Center for Atmospheric Research Gulfstream V aircraft provided comprehensive measurements in HIWC conditions near tropical convection. The Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) explored tropical wave disturbances that promote development of tropical depressions. In support of this objective, the NSF/NCAR GV payload 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. During 26 research flights in the upper levels of developing tropical depressions, areas of HIWC were frequently encountered. Anomalous spikes in the research temperature probe data (referred to hereafter as “TAT anomalies”) and performance degradation in other sensors were often an indication of HIWC at flight level.

In this study we examine the cloud micro- and macro-physical properties associated with 22 confirmed TAT anomalies that occurred during the experiment. Specifically we investigate the ice water content (IWC) threshold required for inducement of a TAT anomaly and the duration of exposure to HIWC conditions prior to the event. We also characterize the habits and size distribution of ice crystals associated with TAT anomalies. Associated bulk cloud properties such as cloud top height and temperature are derived from satellite data, and tropopause height is estimated from a microwave radiometer on the aircraft. Results to be presented at the conference will provide further insight on the frequency of occurrence, location within convective cloud systems, and absolute magnitude of HIWC conditions.