Concept and Design for a Pilot Demonstration Ground-based Remote Icing Detection System

A remote sensing capability has been needed to identify clouds of large (drizzle-sized), supercooled droplets (SLD), a particularly severe icing hazard, and discriminate them from clouds with water-consuming, non-hazardous ice particles. Seven years of research with Ka-band (8.6 mm)-dual-polarization radar, and prior research with microwave radiometers, have led to development of a ground-based remote icing detection system (GRIDS) that addresses this need. It has been demonstrated that 1) a specialized Ka-band radar can be used to identify differing liquid and ice hydrometeors, 2) the estimation of the hydrometeor type can be accomplished deterministically by measuring the depolarization ratio (DR), and 3) hydrometeor discrimination is strongly influenced by the polarization state of the transmitted radiation. Scattering calculations and experimental measurements have been shown to agree. Both show that the use of standard horizontal polarization, with measurement of the linear depolarization ratio, LDR, is a very poor choice for this task, and either 45 -slant linear or circular polarization states (or these with slight ellipticities) work well and are much better choices. Thus, a radar of this type with the proper polarization state to identify clouds of SLD, combined with a microwave radiometer to measure the path-integrated cloud liquid water, offers a powerful remote sensing tool for detection of the aircraft icing hazard.

Given this foundation, the Federal Aviation Administration is supporting the design and construction of GRIDS as the initial, core component of the FAA Icing Sensors Testbed (FIRST). The design phase for GRIDS is presently underway. GRIDS is to be a pilot demonstration model of a robust system that will perform continuously and unattended with operational reliability. A capability for 365-day per year operation has already been demonstrated with ETL's millimeter cloud radar (MMCR), which was developed for the DOE Cloud and Radiation Testbed (CART). Hazardous clouds are normally very weakly reflective (Ze<0 dBZ), so GRIDS will include one of the most sensitive cloud radars ever built. The sensitivity will be achieved with a 3-m antenna, long dwell times (~60 sec), and pulse lengths up to ~2 s. The primary DR measurement will be made at a fixed low elevation angle in the 20 -50 range, as determined by scattering calculations and previous experiments, to offer the greatest separation of ice hydrometeors, which are shaped, from the spherical SLD. Assignment of the path liquid water to the cloud layers with lowest DR will provide an estimation of the liquid water content in the potentially hazardous zones. The fundamentals, the robust design, and continuous operations through seasons rather than brief field campaigns will make GRIDS a powerful and economical tool. The same factors will set the path to technology transfer to operational use. In this paper, the concept and design for GRIDS are discussed.