## 4 Discussion
### Significance of MISTiC Winds for Weather Research  
While tremendous advances have been made during the last two decades on space-based observations of the mass field with infrared and microwave sounders, the tropospheric wind vertical profile remains the number one un-met observation needed to advance weather forecasting ([PERSON], 2016). MISTiC Winds will observe the vertical wind profile in the troposphere, as well as the moisture and temperature vertical profiles simultaneously, with unprecedented spatial coverage, spatial resolution, and temporal refresh rate. The most recent NAS Decadal Survey ([PERSON], 2016) (p. 11) identified atmospheric winds as a targeted observable, specifically citing the need for _\"3D Winds in the troposphere/PBL for transport of pollutants/carbon/aerosol and water vapor, wind energy, cloud dynamics and convection, and large scale circulation.\"_ The MISTiC spectral range also includes channels that provide observation of nitrous oxide and carbon monoxide pollutants in the free troposphere, as well as some highly transparent (super-window) channels useful in observing aerosols. When combined with MISTiC's vertically resolved wind observations, the atmospheric transport of these materials can also be observed. The planetary boundary layer (PBL) has also been identified in the Decadal Survey as a targeted observable. PBL observations are recognized as challenging, and are placed in an incubation stage by the NRC. However, hyperspectral infrared sounding observations are noted as among the essential techniques that will be needed for PBL studies. While the vertical resolution needed for PBL characterization is a challenge for traditional vertical profile retrieval approaches, differential approaches (on-line/off-line methods) together with infrared skin-temperature observations should be effective in important situations, including over-ocean observations, where the surface emissivity is well characterized ([PERSON], 2016). The substantially higher spatial resolution of MISTiC observations relative to other infrared sounders would be particularly valuable in supporting studies of the lower troposphere and PBL in partly cloudy fields. The relatively higher frequency of observation, and potentially even the short-burst hyperspectral snap-shot sequence (10-min sampling) of the scene observed by each element in a wind triplet could provide valuable insight into PBL evolution. The MISTiC Winds combination of observations is expected to be particularly advantageous for improved NWP. The differential equations solved numerically to provide a weather forecast constitute an initial-value problem, which requires initial conditions provided by atmospheric observations for solution. Assimilated MISTiC Winds observations would provide valuable constraints on both the initial atmospheric state, and its first time-derivative. The value of this approach for weather forecast accuracy improvement has been demonstrated through an OSSE performed by NASA's Global Modeling and Assimilation Office evaluating the MISTiC observations on a global weather forecast ([PERSON], 2016). This simulation showed significant potential for weather forecast improvement from the new MISTiC observations, with the vertically resolved wind observations being most significant, but with significant improvement also provided by assimilating IR radiances. Of particular note are the Forecast Sensitivity of Observation Impact (FSOI), where the MISTiC vertically resolved water vapor motion and cloud motion vector wind observations of a MISTiC constellation showed the largest impact on the 24-h forecast of any other observation. When Himawari-sized errors are included for the wind observations in the GMAO OSSE system's 3D-Var assimilation, the MISTiC FSOI impact remains very high-comparable with the impact of the full microwave sounder constellation. An update to this MISTIC OSSE is currently under way at the Jet Propulsion Laboratory, using a more representative error model for hyperspectral AMV winds.