## 1 Introduction
### Historical Background for IR Sounding and Atmospheric Motion Vector Winds
Cloud motion vectors are observed using atmospheric window channels. In the thermal infrared, cloud brightness temperature could indicate the cloud pressure height, but only if the vertical temperature profile was also known. In the absence of temperature profile knowledge, various approaches to height assignment have been developed, but these have shortcomings. Channelbrightness temperature could also be an indicator of height for water vapor motion-vector wind tracers, but determining the height from the radiance requires knowledge of both the temperature and water vapor concentration vertical profiles. As a result of this missing information, the AMV wind speed error associated with current multispectral imager observations remains relatively large. For the GOES 16 imager, errors near 5-6 m/s rms relative to radiosondes (near 500 hPa) are reported [18], which is approximately 2-3\(\times\) larger than the wind speed error needed by NWP models [19]. An additional concern presented by the multi-spectral imaging approach for AMVs is that these wind observations are not well distributed vertically through the troposphere. Cloud motion vector winds (which, of course, require the presence of clouds for an observation) are primarily located in the lower troposphere, well below 600 hPa, whereas moisture-motion-vector winds are primarily located in the upper troposphere, above 400 hPa [18]. Very few wind observations are available near 500 hPa, where observing system simulation experiments (OSSEs) show that wind observations would have the greatest impact on weather forecast accuracy [19]. Hyperspectral infrared sounding has an inherent capacity to provide pressure-height information for the image-features observed (through the retrieved vertical profiles) that has long been recognized as beneficial for AMVs, and wind retrieval approaches employing hyperspectral techniques have been developed [20; 21]. The challenge has been to provide the hyperspectral sounding observations themselves at a temporal and spatial resolution that would support wind observations-affordably. In the US, NASA initially selected the geosynchronous imaging Fourier transform spectrometer (GIFTS), a GEO hyperspectral IR sounder, under its New Millennium Program, with plans for higher temporal rate IR soundings as well as hyperspectral AMV winds [7]. However, GIFTS was canceled, with the projected total program cost relative to the mission cost cap cited as an important factor. NOAA thoroughly evaluated the possibility of including a hyperspectral IR sounder (that would have supported hyperspectral AMVs) in the GOES-R series, supporting instrument concept development, trade studies, and technology risk reduction efforts through the Formulation Phase Program. However, NOAA determined that the cost and risk were too high in light of GOES-R budget constraints [22]. The indefinite deferral by NOAA's GOES Program of hyperspectral observation methods, even though they have been shown to provide significant weather forecast impact, was one of the primary motivations for the development of MISTic Winds. MISTic Winds addresses both of these AMV wind velocity error terms, (height assignment error and tracking error) in mission implementation that is much lower in cost, more flexible, and more resilient than providing these observations from a GOES-R class IR sounder. MISTic Winds does this while providing a measurement of the vertical temperature and moisture profile of the troposphere at unprecedented spatial resolution and refresh rate. The approach to vertically resolved wind observations has long been envisioned as a Doppler Wind LIDAR. However, MISTic Winds may provide tropospheric wind measurements with vertical resolution sufficient for many purposes, at a substantially lower cost and under a wide range of conditions.