# Construction of Nighttime Cloud Layer Height and Classification of Cloud Types
## 4 Results and Discussion
### Daytime Verification of the Cloud Classification Results  
To compare the classification based on the NSRM method with the other classification method, we applied the NSRM method to the daytime dataset of the same period as in the earlier sections. In the daytime, the combined measurements in the visible and the infrared bands from MODIS allow retrievals of cloud optical thickness (COT) and multilayer flag (MLF). The ISCCP provides a simple and efficient method, using CTP and COT to separate clouds into nine types, which basically match the cloud types from CloudSat (except that the cirrus and the cirrostratus are combined as high clouds in CloudSat product). It is noted that MODIS only retrieves column COT, which means if there is more than one cloud layer, the ISCCP method could not work properly. Therefore, in the daytime study, we only compared the results from pixels with MLF = 1, which indicates a single cloud layer. This comparison in the daytime was used as an approximation of the reliability of cloud classification in the nighttime. However, it is noted that results from single layer clouds could be different from all cloud layers, as shown in Figure 9. Other methods such as the constrained spectral radiance matching (CSRM) algorithm [22] and the cloud-type matching algorithm [19] have been designed and performed  
Figure 9: Fractional distribution of the eight types of clouds identified by the NSRM algorithm during the nighttime over the tested period. Each panel presents the distribution of one of the eight cloud types, in terms of all cloud layers (all), the uppermost cloud layer (top), and the single layers (single). The panels are arranged from high, thin clouds to low, thick clouds.  
for daytime observations, which might give better classification; however, these methods could not be applied to the nighttime condition.  
Figure 11 shows the distribution of each cloud type identified by the MODIS measurements using the ISCCP method and from the RXS-expand results using the NSRM method. The \(-400\) km indicates 400 km to the west of the track, and 400 km indicates 400 km to the east of the ground track. The figure illustrates that a generally good agreement was found between the two methods for most cloud types, showing a promising trend over the entire expanded range. The fact that the numbers of clouds classified by the ISCCP method (by MODIS) were almost consistently lower than those identified by the NSRM method (by construction based on active sensors) could be explained by the focus on pixels with MODIS MLF = 1. MODIS only detects one cloud layer at these pixels, but since the active sensor is more sensitive to thin and high clouds, the constructed profiles might have had more than one cloud layer, causing the numbers to be generally higher. The disagreement was relatively large for Cu, which might have been due to the problem that thin, low level clouds could be undetected by CPR and CALIOP [35]. The gradual decrease of the total number of cloud layers as the expanded range  
Figure 10: Summary of comparison between the clouds identified from the original profiles (ori) and the reconstruction profiles (rec) with a dead zone of 100 km. Each panel presents one of the eight cloud types, arranged from high, thin clouds to low, thick clouds.  
increased on the both sides partly resulted from the removal of multi-layer cloud pixels and pixels with CTH retrievals that significantly contradicted each other.