# Construction of Nighttime Cloud Layer Height and Classification of Cloud Types
## 5 Conclusions  
This work proposed and evaluated a cloud structure construction algorithm adapted for nighttime expansion of vertical information inferred from nadir-pointing cloud radar and lidar to cross-track locations next to the ground track. Based on matching and attributing nadir pixels (donors) into off-nadir pixels (recipients) with similar infrared radiances and passive retrieved cloud properties, the cloud vertical structure was expanded up to 400 km on both sides of the ground track. The constructed cloud structure was utilized for nighttime cloud classification and compared to the daytime ISCCP cloud classification based on the collocated MODIS measurements. Reconstruction of nadir profiles during the tested days verified the overall performance of the NSRM methods, which is related to the minimum distance between the donor and the recipient. By mimicking the off-nadir distance with a dead zone along the ground track, the reconstruction of nadir profiles shows that, at 200 km from the ground track, the CTH and the CBH reconstructed by the NSRM method were within 1.49 km and 1.81 km of the original measurements, respectively. The RMSEs of CTH and CBH were 3.26 and 3.6, respectively. At 400 km, the MD of CTH increased to  
Figure 11: Comparison between cloud identification from MODIS and from the RXS-expand results up to 400 km on both sides of the daytime ground track. Each panel presents one of the eight cloud types, arranged from high, thin clouds to low, thick clouds. 1.83 km, and CBH increased to 2.02 km, while the RMSE of CTH increased to 3.76 and CBH increased to 3.95. These values were calculated when the tolerance factor \(\alpha\) was set to 0.3, and \(\beta\) was set to 1.5, which could be changed for either narrower or looser constraints. The reliability of using a constructed regional structure for cloud classification was tested through both reconstruction and daytime comparison with passive wide-swath observation. The comparison between distributions summarized from reconstructed profiles at 100 km and original profiles shows that the eight identified cloud types matched well averaged by latitude. The comparison with classification results inferred using the ISCCP standards and the MODIS measurements during daytime shows general good agreement, except for thin, low level clouds (Cu). The construction of a cloud structure based on the NSRM method provides reliable estimates of regional cloud layer heights during nighttime. It can efficiently construct and provide vertical information almost simultaneously with radar and lidar overpass. It also has the potential to provide assessment of vertical information and classification for other clouds related studies, such as the cloud-aerosol interaction, over a broader range than the lidar ground track. In the future, reduction of the disagreement between active and passive retrieved cloud top properties will certainly benefit the application of constraints for selecting better donors. In addition, launching more satellites with active and passive sensors would help to increase the chance and the quality of selecting matching pixels. The contribution of each author to this research article is specified as follows: conceptualization: D.L. and X.Z. ; methodology, S.C. and D.L. ; software, C.C. ; validation, S.C. and C.C. ; formal analysis, S.C.; investigation, C.D. ; resources, F.W. ; data curation, B.T. ; writing--original draft preparation, S.C.; writing--review and editing, D.L. ; supervision, W.C.; project administration, B.C. ; funding acquisition, L.S. and D.L. All authors have read and agreed to the published version of the manuscript.