(Inter-)Calibration of Microwave and Infrared Sounders With the Moon

Abstract:

Serendipitous observations of the Moon provide a unique means to the calibration of instruments on meteorological research satellites, because the physical properties of its surface change very little, even on large time scales. Establishing the Moon as a flux reference for microwave and infrared sounders became a focus of attention in the last few years, but we investigated as well how certain instrumental effects can be characterised and corrected with lunar observations. For this we identified and analysed intrusions of the Moon in the deep space views of AMSU-A, AMSU-B, MHS, and ATMS and as well of HIRS/2, HIRS/3, and HIRS/4 on various satellites in polar orbits. Starting with the measured counts as a function of time we calculated the brightness temperature of the lunar disk at different wavelengths and phase angles and compared it to predictions of thermo-physical models of the Moon. We identified the phase angle of the highest disk-integrated brightness temperature at 89 and 183 GHz and found that this temperature changes by about 4 K between the Moon’s perihelion and aphelion. We measured the co-registration of the different channels of the microwave sounders both in the along- and across-track direction and their beam diameters. These results did not always agree with those from ground tests. Similar studies were carried out with HIRS, where we put in addition tight constraints on the non-linearity of the short-wavelength channels.

By analysing more than a hundred appearances of the Moon in the deep space view of microwave sounders, we can search for calibration drifts against a stable standard and perform inter-calibration between instruments operational at quite different times. With HIRS, the Moon is particularly useful for checking the photometric consistency of the sounding channels.