Characterizing Arctic Ice and Mixed-phase Clouds Using ARM Ground-based Measurement

Ice clouds occur more than 20% of the time over Arctic region and they have significant role in reflecting solar radiation as well as regulating terrestrial emitted radiance. Detection of ice clouds and the retrievals of their vertical structures are the fundament for investigation of cloud-radiation interactions as well as aerosol-cloud interactions. Among the ground-based instruments, ceilometer and micropulse lidar (MPL) are most commonly used for deriving cloud base height (CBH) for their better ability to distinguish precipitation below cloud base than radar data. Compared with the ceilometer, the MPL laser is more sensitive to small particles and it has higher detection range limit. In this study, we found that ice clouds detected by ceilometer over the Arctic region are 3.1% lower than that detected by MPL (12.3% Vs. 15.4%), and this difference can be as large as 4—7% during March to July. The underestimation of ice cloud from ceilometer is mainly due to two reasons: the underestimation of high cloud that are beyond the instrument upper limit (contributes 1.2 %); and the lower sensitivity towards small particles for ceilometer (contributes 1.9 %). In addition to the underestimation of cloud fraction for ice cloud from ceilometer, it also overestimates the CBH and retrieves the cloud base in the middle of the cloud. The annual mean CBH for ice cloud from ceilometer and MPL are 3.3 km and 4.1 km, respectively, and the difference is larger during the period May-September. This study also found that when liquid is contained in the cloud, both ceilometer and MPL could detect the cloud well. But these two instruments will give two cloud bases for mixed-phase cloud: ceilometer defines the liquid layer and MPL defines the ice layer. The annual mean CBH from MPL and ceilometer for mixed-phase clouds are 0.7km and 1.0 km, respectively, with the largest difference in March of ~ 1.0 km. Therefore, we suggest that for mixed-phase clouds, both instruments should be used: the base of liquid cloud layer should be defined by the ceilometer, while the base for the ice dominant layer should be defined by MPL.