A simplified microwave radiometric apparatus has been proposed for use in quantifying the potential danger of accretion of ice onto an aircraft flying through a cloud. The quantity of interest, called the "icing hazard potential" (IHP), is the product of (1) the liquid-water content (LWC) of the cloud at a given altitude and (2) some function of the degree of supercooling of the liquid water at that altitude.

In 1983, it was demonstrated that reliable values of IHP could be calculated from readings of a ground-based instrumentation system that comprised (1) a water-vapor radiometer (for measuring LWC), (2) a microwave temperature profiler (MTP) (to measure the vertical temperature profile), (3) a hand-held infrared radiometer (to measure the cloud-base temperature and thus, in conjunction with the MTP, the cloud-base altitude), and (4) a surface meteorological subsystem (to measure surface values of temperature, barometric pressure, and humidity). The proposed system would share some features in common with the 1983 system, but would not include a water-vapor radiometer. Inasmuch as the radiometers embodied most of the cost the 1983 system, eliminating one radiometer would reduce the cost significantly.

The Absorption Spectrum of Oxygen in the absence of cloud liquid water depends on frequency and pressure altitude; it also depends on temperature and water-vapor density, nominal values of which were assumed in computing these curves.

To be able to determine the IHP, one needs to know the temperature and the LWC as a function of altitude. For the purpose of measuring these quantities, the proposed system would exploit the radiometric characteristics of oxygen and liquid water in the frequency range from about 50 to about 70 GHz. The absorption spectrum of oxygen molecules in this frequency range (see figure) depends on temperature, pressure, and water-vapor density. The brightness temperature observed by an MTP is an exponentially weighted average of all oxygen emission along its line of sight and closely approximates the air temperature at an effective distance from the radiometer equal to the reciprocal of the absorption coefficient.

Consider two observation frequencies - one on the high-frequency and one on the low-frequency side of the peak in the oxygen absorption spectrum- for which the absorption coefficient is equal. In the absence of cloud liquid water along the line of sight within the effective distance, the brightness temperatures observed at both frequencies would be the same, but in the presence of cloud liquid water, the brightness temperatures observed at the higher frequency would exceed that observed at the lower frequency, because cloud liquid water contributes to brightness temperature by an amount approximately proportional to the square of frequency. Thus, in principle, one could infer both the air temperature and the LWC from the brightness temperatures at the two frequencies.

The main radiometer in the proposed apparatus would be an extended version of an airborne MTP now in use. As such, it would include a scanning mirror used to scan the line of sight in elevation angle while measurements were taken at several pairs of frequencies (and thus several effective distances), the net effect being to acquire data from different effective altitudes. The apparatus would also include an infrared gun operating in the wavelength range of 8 to 14µm to determine the cloud-base altitude, plus simple sensors to measure the ambient temperature, pressure, and relative humidity.

The proposed apparatus would cost less than airport and aircraft radar systems do. Eventually, taking advantage of monolithic microwave integrated circuitry, it should be possible to mass-produce the apparatus and install it in aircraft at relatively low cost. Because icing is of particular concern during long, slow descents and approaches, prior to miniaturization and mass production, it could be desirable to install copies of the apparatus on the approach paths of major airports to warn approaching aircraft of potential icing hazards.

This work was done by Michael J. Mahoney of Caltech for NASA's Jet Propulsion Laboratory.

NPO-20291


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This article first appeared in the May, 2000 issue of Photonics Tech Briefs Magazine.

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