## 3 Results
### MISTiC Airborne Instrument Observations from the NASA ER2  
Airborne versions hyperspectral IR sounding instruments have been used many times as part of validation campaigns for AIRS [26] and for other IR sounders. In anticipation of a NASA GIFTS Program, the NPOESS atmospheric sounder testbed instrument (NAST) was used to make hyperspectral observations from an ER2 during repeat-pass observations of a moisture field from which motion vector wind observations were derived [20], with a repeat-period of approximately 45 min. However, the specific use of repeat-pass flight trajectories with an air platform to simulate space-borne acquisition of a sequence of images from which to derive AMVs with image repeat periods of 10-20 min appropriate for changing cloud formations do not appear to have been reported previously. A key benefit of repeat-pass imagery on this time scale is that the primary changes between images are due to the translational motion of both clouds and moisture features as they move with the wind (advection), rather than due to changes in the spatial distribution (shape) of the features themselves. A second practical benefit is that the probability of observing an atmospheric feature on multiple passes from the relatively low vantage point of an aircraft platform is much higher if the time-separations used are not too large. Some atmospheric observations using this airborne instrument on the ER2 are described below. The path shown for this flight is shown in Figure 10. More details on the airborne observation demonstration flight on 4 December, including methods, weather conditions, and a comparison with an IASI satellite observation are provided in Appendix B. An example of the types of airborne MISTiC observations made using the repeat-pass hyperspectral imaging/sounding method for cloud AMV identification, together with approximately co-located observations from a radiosonde launched from the Edwards AFB during the ER2 overflight on 4 December are shown in Figures 11-15. In Figure 11, two MISTiC \"window-band\" radiance images taken at the latitude and longitude positions listed in the text in Figure 11 at times separated by approximately 1000 s. The images shown in Figure 11 have brightness temperature contrast selected to highlight the separation of the thermal emission of clouds, which is much lower than that from that of the surface. Common points in the cloud field in the two images were identified using a manual process for this airborne test. An involved but straight forward geometric construction is used to determine the absolute position of the common point of the selected cloud in each image, using the platform (3D) position, velocity, and heading, relative (angular) position of the cloud within each image, and airborne instrument design parameters (pixel size, focal length). The specific position and angle data for each image are listed in the image text at left. The AMV angle and speed were determined from the difference in absolution cloud positions, and the time at which the cloud was seen in each image. Additional information on this airborne observation approach is provided in Appendix B. One practical challenge in the airborne repeat-pass imaging is that at the timing of collection of each image was not synchronized with the platform position as it moved around the orbit several times, as shown in Figure 10. In this case, even a stationary feature appears in different relative positions in the two images. Another practical challenge is that the wind direction and speed were far from ideal for observing a feature twice. The cloud features were at the northern-most edge in the first image, and the southern-most edge of the second. Figure 11: Cloud positions observed from the MISTiC airborne instrument over Edwards Air Force Base during two over-passes, allowing determination of the cloud motion vector wind velocity at cloud level.