# MODIS Cloud Detection Evaluation Using CALIOP over Polluted Eastern China
###### Abstract  
Haze pollution has frequently occurred in winter over Eastern China in recent years. Over Eastern China, Moderate Resolution Imaging Spectroradiometer (MODIS) cloud detection data were compared with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) for three years (2013-2016) for three kinds of underlying surface types (dark, bright, and water). We found that MODIS and CALIOP agree most of the time (82% on average), but discrepancies occurred at low CALIOP cloud optical thickness (COT \(<\) 0.4) and low MODIS cloud top height (CTH \(<\) 1.5 km). In spring and summer, the CALIOP cloud fraction was higher by more than 0.1 than MODIS due to MODIS's incapability of observing clouds with a lower COT. The discrepancy increased significantly with a decrease in MODIS CTH and an increase in aerosol optical depth (AOD, about 2-4 times), and MODIS observed more clouds that were undetected by CALIOP over PM\({}_{2.5}\)\(>\) 75 \(\upmu\)g m\({}^{-3}\) regions in autumn and particularly in winter, suggesting that polluted weather over Eastern China may contaminate MODIS cloud detections because MODIS will misclassify a heavy aerosol layer as cloudy under intense haze conditions. Besides aerosols, the high solar zenith angle (SZA) in winter also affects MODIS cloud detection, and the ratio of MODIS cloud pixel numbers to CALIOP cloud-free pixel numbers at a high SZA increased a great deal (about 4-21 times) relative to that at low SZA for the three surfaces. As a result of the effects of aerosol and SZA, MODIS cloud fraction was 0.08 higher than CALIOP, and MODIS CTH was more than 2 km lower than CALIOP CTH in winter. As for the cloud phases and types, the results showed that most of the discrepancies could be attributed to water clouds and low clouds (cumulus and stratocumulus), which is consistent with most of the discrepancies at low MODIS CTH. cloud detection; aerosol; solar zenith angle; MODIS; CALIPSO; Eastern China 12220191033330/ATmos1006033330/ATmos100603330/ATmos10060330/ATmos10060330  
(cloud fraction, cloud height, cloud droplet concentration, etc.) could have a significant effect on the Earth's energy balance [7; 8; 9; 10]. For example, a doubling of cloud droplet concentration in global marine stratiform clouds (corresponding to a 6% increase in cloud-top albedo) would roughly offset the warming from a CO\({}_{2}\) doubling [7]. [PERSON] et al. [9] deduced that a 9-18 W m\({}^{-2}\) per decade increase in downwelling global solar irradiance for the dry season (May-October) during the period 1988-2012 in Hawaii was most likely the result of cloud cover, which has shown a 5% to 11% decrease trend per decade. [PERSON] et al. [8] found that the increase in surface net shortwave radiation over the Mediterranean Basin during the period 1979-2012 was mostly associated with a decrease in cloud optical thickness (COT). [PERSON] et al. [10] demonstrated that the increases in aerosol optical depth and COT contributed to the decrease in surface net shortwave radiation over the urban region of Hyderabad, India, during the period 1979-2005. Therefore, observing and understanding the global distribution of clouds is one of the most important factors for understanding climate change. Satellite remote sensing is a very important approach for observing global cloud properties. The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua satellite and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite are two important sensors for atmospheric observations. Both CALIPSO and Aqua satellites belong to the \"A-Train\" satellite constellation [11]. MODIS retrieves cloud properties using visible and infrared radiances and provides long-term and global cloud observations [12; 13; 14].