# MODIS Cloud Detection Evaluation Using CALIOP over Polluted Eastern China
## 3 Results and Discussion  
### Cloud Detection Differences between MODIS and CALIOP as a Function of Cloud Properties  
To compare and validate the MODIS and CALIOP cloud detection data over Eastern China and the surrounding marginal seas, the variation of the observed cloud pixel numbers along with different cloud properties were compared over dark, bright, and water surfaces (Figure 2). In Figure 2, consistent observation means that both MODIS and CALIOP observed clouds in each MODIS 1 \(\times\) 1 km\({}^{2}\) pixel, while non-consistent observation means that only MODIS or only CALIOP observed cloud in each MODIS 1 \(\times\) 1 km\({}^{2}\) pixel. The consistent ratio is the ratio of consistent pixel numbers to the sum of consistent and non-consistent pixel numbers. Over Eastern China and the surrounding seas, MODIS and CALIOP observed clouds with CALIOP CBH ranging within 0-18 km and MODIS/CALIOP CTH ranging within 0-20 km. MODIS and CALIOP observed consistent two-peak distribution of clouds along with MODIS CTH, CALIOP CTH, and CALIOP CBH (Figure 2a,c,d). One peak appeared at low CBH (1-1.5 km) and CTH altitude (1.5-2 km) and another peak was located at high CBH (8-8.5 km) and CTH altitude (9.5-11 km). The cloud thickness ranged from several hundred meters to as thick as 5 km. Most clouds have a cloud thickness within 400-600 m. The consistency between MODIS and CALIOP was comparable for dark and water surfaces, and slightly lower for desert surfaces, with a consistent ratio over desert surfaces of 5% to 22% lower than that over dark surfaces. On average, the cloud detection from MODIS and CALIOP agreed 84%, 97%, 62%, and 83% of the time over three surfaces for MODIS CTH, MODIS. COT, CALIOP CTH, and CALIOP COT, respectively, with an average of 82%. That is comparable to the global fractional agreement ratio 87% for August 2006 and February 2007 [17]. When CALIOP did not detect clouds, MODIS mostly observed clouds at low altitudes (Figure 2a). Among the clouds observed by MODIS and undetected by CALIOP, 80%, 64%, and 85% of them were lower than 1 km, and 88%, 94%, and 91% of them were lower than 1.5 km over the dark,  
Figure 2: The histogram of total pixel numbers of consistent and non-consistent Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) cloud detection over dark, bright, and water surfaces along with (**a**) MODIS cloud top height (CTH); (**b**) MODIS cloud optical thickness (COT); (**c**) CALIOP CTH; (**d**) CALIOP cloud base height (CBH); (**e**) CALIOP cloud thickness; and (**f**) CALIOP COT. Gray and purple groups are consistent and non-consistent observations, respectively. Numbers shown on each panel are total pixel numbers. bright, and water surfaces, respectively (Figure 2a). About 75%, 87%, and 72% of clouds observed by MODIS and undetected by CALOP had a COT less than 10 over dark, bright, and water surfaces, respectively (Figure 2b). Previous studies showed that MODIS misclassified heavy aerosols as clouds at low altitudes [21; 22; 23], which induced MODIS to observe low CTH. When MODIS did not observe clouds, CALIOP observed clouds at all CTH and CBH values (Figure 2c,d). The number of clouds undetected by MODIS was mostly less than the consistent clouds observed by both MODIS and CALIOP; however, for high clouds with CBH \(>\) 12 km, 9 km, 12.5 km, and CTH \(>\) 15 km, 13.5 km, and 15.5 km over dark, bright, and water surfaces, respectively, MODIS missed more clouds. Most clouds observed by CALIOP but undetected by MODIS had a small cloud thickness (Figure 2e) and small COT (Figure 2f). About 74%, 65%, and 72% of the clouds undetected by MODIS had a cloud thickness less than 1.05 km over dark, bright, and water surfaces, respectively. About 69%, 74%, and 42% of the clouds undetected by MODIS had a COT less than 0.4 over dark, bright, and water surfaces, respectively, with an average of 62%. That suggested that MODIS may miss high or optically thin clouds.