Observations using slant angle passive radiometry during in-flight icing cases

For the past number of years, the NASA Icing Remote Sensing System has been in development with the purpose of providing accurate detection of icing hazards in the airport environment by merging inputs from existing, off-the -shelf sensor technologies. The system currently consists of a ceilometer, vertically pointing Doppler Ka-band radar and a multichannel radiometer. To fulfill its previously stated purpose, the system will have to be evolved from a proof-of-concept vertically pointing system to a system that successfully covers the airport volume– including takeoff and approach areas. Beginning in early 2012, researchers began a continuous scan routine for the radiometer which included 15 degree elevation scans at 45, 135, 225 and 315 degrees azimuth as well as a zenith scan. At 15 degrees elevation, the radiometer is effectively viewing four atmospheres as opposed to one atmosphere at zenith. The radiometer's channel weighting functions become maximized in the lowest few kilometers and do so much closer in altitude to each other, which acts to increase the sensor's resolution in the lowest atmospheric levels. Another benefit to low elevation slant viewing is that liquid or solid precipitation is much less likely to exist on the nearly vertical sides of the hydrophobic radome material, thus less chance of signal contamination than from zenith readings. The purpose of this study is to identify benefits and disadvantages of utilizing slant angle radiometry for a future volumetric ground-based icing remote sensing system. Questions to be examined include: 1)is there a a lead-time warning afforded by slant angle scanning, 2) can the four slant angle scans track transient variations in integrated liquid water across the airport domain and 3) how do the azimuthally varying radiometer outputs compare to pilot reported location and severities of icing in the airport domain.