Long-Term Monitoring of the Cloudy Atmosphere with Microwave Sounders

A new era in microwave sounders began with the Advanced Microwave Sounding Unit (AMSU-A and AMSU-B), first deployed on the NOAA-15 satellite in 1998 and followed by four more similar satellites during the first decade of this century. Versions of the same system (AMSU-A and HSB – the Humidity Sounder for Brazil) were launched on the NASA Aqua platform in 2002 as part of the Atmospheric Infrared Sounder (AIRS) sounding suite. In 2006, the Eurepean EUMETSAT organizations initiated a series of Metop satellites that also carried AMSU sounders. This has provided continuous global coverage with AMSU sounders from 1998 until now. Then, in 2011 NASA launched the Advanced Technology Microwave Sounder (ATMS), a derivative of AMSU, on the Suomi National Polar Partnership (SNPP) satellite. NOAA later incorporated SNPP into its operational system and initiated a series of Joint Polar Satellite System (JPSS) follow-ons that also carry ATMS and is expected to continue well into the 2030s and perhaps beyond. ATMS has capabilities similar to those of AMSU, and therefore we can expect to have a continuous time series of high-quality microwave soundings that currently covers 25 years and is expected to extend to at least 40 years. (Microwave soundings extend much further back in time with the Microwave Sounding Unit, MSU, series that flew on pre-1998 NOAA satellites, but the quality of MSU data is considerably poorer than for AMSU and we are not considering MSU data in the present discussion.) The microwave sounders have always been paired with infrared sounders, initially with the High-resolution Infrared Radiation Sounder (HIRS) on the early NOAA and on the Metop satellites (which also carry the hyperspectral Infrared Atmospheric Sounding Interferometer, IASI), then with AIRS, the first hyperspectral infrared sounder, on the Aqua satellite, and with the Cross-track Infrared Sounder (CrIS, also hyperspectral) on SNPP and the JPSS satellites. Infrared radiation cannot penetrate clouds, but microwave sounders are unhindered by clouds and in fact measure cloud properties (liquid and ice content), and this has created a strong synergy between the two systems and is the basis for operating them together. Because of the ability to deal with clouds, microwave sounders cover weather and climate regimes that infrared sounders do not measure well, and that has made it possible to extend the range of conditions that can be characterized. As a result, microwave sounder data is widely used to support atmospheric research. We will give an overview of the capabilities and roles of microwave sounders in weather and climate research. The 21-year Aqua AMSU record can already be used to estimate both tropospheric and stratospheric temperature trends. We will discuss the importance of continuing these observations and the data products derived from them.

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