MW-Luna-2 – Establishing the Moon as reference for climate data from microwave sounders at low frequencies

• Applicant: Professor Dr. Stefan Bühler
• Subject Area: Atmospheric Science ; Astrophysics and Astronomy
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 421761264

In a nutshell, the aim of the proposed project is to set the definitive value for the trend in tropospheric temperature and thus to put an end to the controversies surrounding the various microwave satellite observations. It is the intention to achieve this by establishing a flux reference with an unchangeable surface that has been observed by all microwave sounders many times over their operational lifetime: our Moon. In a precursor project it was demonstrated that the observed lunar brightness temperatures at a frequency of 89 GHz are in very good agreement with the predictions of a new model. The observations were obtained with AMSU-B (Advanced Microwave Sounding Unit - B) and MHS (Microwave Humidity Sounder). With AMSU-A and ATMS (Advanced Technology Microwave Sounder) it is possible to extend this investigation to lower frequencies, in particular the 54 GHz band. Published trends of the tropical tropospheric temperature based on this band range from 0.09° to 0.20° per decade, and these inconsistencies arose mainly from imperfect instrument calibration. In contrast stands the brightness temperature of the Moon, which depends only on its distance from the Sun and the viewing angle. This relationship can be characterized very accurately with the great number of observations that happened, when the Moon appeared serendipitously, i. e. during routine operations, in the deep space view of a microwave sounder. The focus of this study is on detecting changes, and the absolute value for the radiance of the Moon is obtained as a by-product. It would nevertheless be useful for future microwave sounders, in particular when the internal calibration source is dropped as a cost saving measure. Besides, the careful characterization of the disk-integrated brightness temperature of the Moon under various conditions of solar illumination would improve our understanding of the thermo-physical properties of its regolith. The observations to be analyzed were made with AMSU-A on nine different satellites and ATMS on two satellites. Together they cover all phase angles within some 80° around full Moon. Usually the Moon approaches the deep space view several times per year and shows then up in several orbits in a row. As most of the meteorological research satellites are more than ten years in service, one gets hundreds of observations of the Moon from each of them, betraying every possible instrumental anomaly and thus leading to its correction and the measurement of atmospheric temperatures with unprecedented accuracy.

• DFG Programme: Research Grants