## 1 Introduction
### Historical Background for IR Sounding and Atmospheric Motion Vector Winds  
The concept for using infrared radiance spectra to measure the vertical profile of atmospheric temperature and water vapor (infrared sounding) was first advanced by [PERSON] et al. (1959) in 1959. Hyperspectral infrared sounding of the temperature and moisture vertical profiles have been performed from space since the NASA atmospheric infrared sounder (AIRS) instrument was launched aboard Aqua in 2002. NASA's AIRS instrument ([PERSON] et al., 2003), built by BAE Systems, provided the first on-orbit demonstration of [PERSON]'s method implemented with hyperspectral IR radiances. Weather and climate research based on AIRS data have made significant contributions to our understanding of the earth system ([PERSON] et al., 2016). Observations from AIRS and other LEO hyperspectral sounders, the infrared atmospheric interferometer sounder (IASI) and cross-track infrared sounder (CrIS), have since become some of the most important inputs into global operational numerical weather models ([PERSON] et al., 2003; [PERSON] et al., 2004; [PERSON] et al., 2005). A recent Forecast Sensitivity Observation Impact assessment by the UK Met Office ([PERSON] et al., 2016) (p.307) ranks IR sounders as having the most significant beneficial impact of any observation in reducing the 24-h global forecast error. However, while continued incremental improvements are likely, the relative infrequency of these LEO sounding observations and the relatively large size of IR sounder spatial IFOV (now comparable to the grid spacing of global NWP models) will constrain further improvements. Horizontal wind fields using AMVs have been derived from observations of geostationary meteorological multispectral imaging instruments (GEO MET Imagers) since the introduction of the operational geostationary weather satellite in the 1970s and '80s ([PERSON] et al., 2003; [PERSON] et al., 2005), (now the Geostationary Operational Satellite System) and are currently being provided for the US, operationally, by the GOES 16 and GOES 17 imagers and by the nearly identical Himawari instruments. GEO MET imager-based wind observations provide a positive impact to the numerical weather prediction (NWP) models, although not with as significant an impact as those provided by IR and microwave sounding. AMVs are determined from observing the horizontal displacement of a feature in either the 2D water vapor field, or the cloud field, over a known time interval. This feature velocity, identified with the wind velocity, must be assigned a height in the atmosphere to be used in NWP, and different operational centers deriving AMVs have different methods for this height assignment. The two leading wind-velocity error terms for GEO MET Imagers are the errors in the vertical height assignment (in the presence of wind shear) and errors in the relative position change of the tracked feature, also referred to as a tracking error. The GOES ABI (advanced baseline imager) improves the accuracy of the feature tracking term relative to prior geostationary imagers, by moving from the 4 km resolution of prior GOES imagers to 2 km for the IR channels in ABI at nadir. However, the height assignment error term has a basic cause linked to a fundamental design attribute of multi-spectral imagers, the spectral channel bandwidth generally employed, which varies from 0.5 to 1.0 micrometers for IR window and water vapor channels. For water vapor channels, these bandwidths are broad enough that they include many atmospheric absorption or emission lines for water vapor, with a broad range of line strengths. As a consequence, the vertical contribution functions for such channels are very broad, extending over much of the troposphere, and accurate feature height assignment is difficult. In current instruments, these bands were chosen to emphasize higher-level water vapor and avoid surface emission. One consequence of this choice is that these wind observations are primarily weighted high in the troposphere.