# Multi-LEO Satellite Stereo Winds
## 5 Conclusions
The Compact Midwave Imaging System (CMIS) [74] is another sensor with multi-angle IR capability with potential future flight opportunities, flying in tandem pairs or in a train with another series of satellites (e.g., JPSS or MetOp-Second Generation). In most of the work presented here, stereo coverage is limited by the narrow swath of the SLSTR oblique view, which is still larger than that of MISR (740 km versus 380 km). A two-satellite constellation will only cover a small portion of each pole in each orbit (e.g., Figures A1-A4). This could be sufficient for research purposes, but a larger constellation of low-cost imagers could serve as a cost-effective solution for operational wind products serving NWP objectives. Such constellation concepts are under formulation to meet future weather sensing needs (e.g., as a follow-on to JPSS [75]). We have validated our stereo-wind retrievals using fixed clear-sky ground points, collocated CALIOP cloud-top heights, and ERA5 reanalysis winds. All validation methods have some nuance to their interpretations. The ground points show the smallest retrieval errors, ~200 m and ~0.25 m/s, but ground points are not clouds borne in the wind and are expected to underestimate wind retrieval errors. Moreover, we use the ground- point disparities to remove biases in the retrievals (~400 m and ~0.25 m/s) that are due to small relative geo-registration errors between MODIS and SLSTR with a second retrieval processing pass; therefore, ground points are not entirely independent measurements of retrieval accuracy. The ground-point disparities are modeled as constant for each polar overpass but may have a more complicated temporal structural to be modeled to improve upon the compensation. More work is needed to fully understand these errors, which can be related directly to height and velocity retrieval errors. Cloud optical depth and inhomogeneity can play a significant role in the cloud-top heights observed by LiDAR and IR sensors [58]. The LiDAR measurements only exist along a narrow path cutting through the retrieval swath that may not representatively sample the cloud tops of a tracking template. Moreover, the LiDAR signal return may be from an optical depth above the effective height of clouds in passive imagery or even from a different layer. It is therefore not surprising to see generally positive biases of several hundred meters between CALIOP layer heights and the stereo retrievals, with ~500 m standard errors. The reanalysis winds are expected to show some differences with observational wind fields, since they are modeled winds on discrete layers and not always simultaneous with the observational winds. Our comparisons with ERA5 are generally unbiased, with ~3 m/s standard errors, which are similar to other comparisons between reanalysis and observational winds [60]. Stereo-wind observations in the polar regions not only facilitate the study of polar atmospheric dynamics but also provide a reliable technique for mapping polar clouds in darkness or when cloud layers are close to the ground. Large uncertainty about cloud cover is a challenging problem in polar regions, with a direct bearing on climate. Consistent polar cloud detection from stereo-wind methods during daytime and nighttime is critical for model evaluation and understanding of the underlying dynamical and radiative processes. We show, as has also been demonstrated with ATSR-2 [65], that stereo methods have potential to contribute to consistent polar day-night cloud detection. Data supplement is available for download as [PERSON]; [PERSON]; [PERSON], [PERSON]; [PERSON] (2023), Polar Stereo Winds, Dryad, Dataset, [[https://doi.org/10.5061/dryad.rfj6q5](https://doi.org/10.5061/dryad.rfj6q5)]([https://doi.org/10.5061/dryad.rfj6q5](https://doi.org/10.5061/dryad.rfj6q5))\({}^{\prime}\)g5 (accessed on 19 February 2023). Conceptualization, J.L.C. and D.L.W. ; methodology, J.L.C. and D.L.W. ; software, J.L.C., M.D.F., and T.C.S. ; validation, J.L.C. ; formal analysis, J.L.C. and D.L.W. ; investigation, J.L.C. and D.L.W.