## 4 Discussion  
### Discussion of MISTiC Winds Requirements and Their Relation to Other Instrument Requirements  
MISTiC Winds provides an opportunity to develop the requirements and design for a remote sensing capability that are fundamentally driven by the specific needs of the vertically resolved wind observation. This situation differs from the historical case with GOES or especially for AIRS, where the instrument requirements did not explicitly address the needs of wind observation, and where the capabilities of the resulting instrument were adapted to wind observations after the fact. Two questions that arise in the present case relate to requirements sufficiency and requirements feasibility. The driving requirements of wind speed accuracy are considered from both the sufficiency and feasibility perspectives. The LAWS Program previously identified wind speed uncertainty requirements for a future Doppler Wind LIDAR as 2 m/s for the lower troposphere (0-3 km) and 3 m/s for the upper troposphere (3-15 km) [4]. These are also the values more recently identified by the World Meteorological Organization [27] (requirement IDs 383 and 385 in the OSCAR database) as breakthrough values for the horizontal wind uncertainties for the lower and upper troposphere, respectively, based on weather forecast impact for high-resolution regional and global NWP. More recently, the NASA Weather Focus Area Workshop in 2015 [28] identified a wind speed observation requirement of 2 m/s rms uncertainty for the remote sensing of vertically resolved wind field during its deliberations, as sufficient for advancements in weather remote sensing research and weather forecast improvement (although specific numerical requirements of this type were not included in the final report of this workshop). The wind speed uncertainty requirement adopted for MISTiC Winds, shown in Table 2, is informed by these requirements. The MISTiC OSSE [23] has shown that vertically resolved observations provide significant weather forecast impact. OSSEs can provide constraints on the accuracy that must be provided in order for these new observations to have the desired impact on forecast accuracy. Within this MISTiC OSSE, the total wind speed uncertainty employed was approximately 5 m/s rms. However, this included both the observation errors and errors associated with the 3 DVAR data assimilation, so the allowed contribution from observation errors would again be in the 2-3 m/s range, consistent with Table 2. An updated MISTiC OSSE may provide more specific constraints on observation error requirements. Finally, a \(<\)2 m/s wind speed observation uncertainty was reported to be an important aspect of more recent DWL OSSEs [19; 29]. Figure 16: AIRS water vapor retrievals for a cloud-free sounding closest to the Edwards Air Force Base radiosonde and MISTiC airborne data. The area of high water vapor concentration peaking near 560 hPa (4546 m) is consistent with the Edwards radiosonde data in Figure 15 and the presence of a higher-altitude region of higher water vapor concentration apparent in the images in Figure 14.  
Requirements feasibility can be assessed by considering the implications of the requirements in light of physical or cost constraints. Under the MISTIC Winds IIP, an initial partitioning of the 2 m/s wind speed error requirement was performed, and this partitioning includes flow-down requirements for the error due to tracked feature height assignment (<1.5 m/s) and errors in horizontal feature tracking (<1 m/s) as leading terms in the error budget. These errors are resumed to be un-correlated. The AMV height assignment error contribution to wind speed error is only significant in the presence of significant wind-shear. To support an initial assessment, a wind speed difference of 20 m/s, between the 850 and 200 hPa pressure levels is assumed as a reference case, with the further assumption of linear wind speed change with pressure height. Under these assumptions, a 30 hPa feature height assignment error (see Table 2) would yield a 1.5 m/s wind speed error.