An airborne submillimeter-wavelength radiometer, expected to be built and tested in the near future, is designed primarily to yield measurement data that can be processed to quantify the ice contents and mean sizes (and, to some extent, the shapes) of ice crystals in cirrus clouds that range from optically thin to opaque. Secondarily, this radiometer is also designed to enable the characterization of watervapor profiles in the presence of optically thick clouds. The ice and water-vapor data are needed to improve understanding of processes that affect weather and climate.

Submillimeter-wave cloud-ice radiometry is a relatively new technique that originated in two theoretical studies published in 1995. These studies showed that (1) cirrus ice particles scatter upwelling radiation emitted by water vapor in the lower troposphere; (2) this effect makes the clouds look radiatively cold against a warm emission background; and (3) the ability of cirrus ice to scatter radiation is primarily a function of the ice content and the distribution of crystal sizes. Accordingly, submillimeter- wavelength cloud-ice radiometry is based on the proposition that by measuring submillimeter-wavelength radiation at two widely separated frequencies, it should be possible to distinguish between changes in scattering of radiation induced by changes in median crystal size and changes in scattering induced by changes in the total ice content.

The radiometer now under development will be used to verify the theoretical studies and demonstrate the principle of cloud-ice radiometry. A notable part of the development has been the design of a 325- and a 448-GHz receiver, both capable of taking measurements to within 1 GHz of their local-oscillator frequencies, as needed to optimize retrieval algorithms. Earlier proof-of-concept measurements by use of other radiometers did not provide corrections for water vapor. This instrument is designed to provide much higher accuracy, including, when applicable, providing the data needed to correct for water vapor.

This work was done by Erich Schlecht, Imran Mehdi, Lorene Samoska, Paul Batelaan, Peter Siegel, Steven Walter, Robert Ivlev, Robert Losey, Trong-Huang Lee, Kent Evans, and Jose Guerrero of Caltech for NASA’s Jet Propulsion Laboratory.