Diurnal Variations of Global Clouds Observed from the CATS Spaceborne Lidar and Their Links to Large-Scale Meteorological Factors

Diurnal cycle of cloud (DCC), referring to the diurnal variation of cloud macro- and micro-physical properties, thus largely determining the strength of net cloud radiative forcing (CRF), is a critical feature of clouds’ variation and is important for weather and climate evolutions. Nevertheless, neither the DCC vertical structures and their links to meteorology are well understood, nor the DCCs for different cloud type are accurately represented in current climate models. With unique orbit of the international space station, Cloud-Aerosol Transport System (CATS) lidar onboard the international space station (ISS) can sample cloud profiles at different local times and provide DCC vertical structures. In this study, we analyzed 2-year CATS data and found that the amplitude of diurnal cycle is significantly correlated with the mean frequency of occurrence. High clouds and oceanic low clouds have strong vertical development during nighttime, and continental low clouds tend to develop in daytime. These DCC features can impact the strength and the direction of CRF. Overall, large cloud cover and amplitude can amplify net cloud cooling effects, and high cloud nighttime (18:00 PM–06:00 AM) occurrence frequency can strengthen the cloud warming effects. To explain the DCC phenomenon, the instantaneous links between cloud vertical structure and lower-tropospheric stability (LTS), vertical velocity and cold point temperature (CPT) are discussed individually to show the evidence of their controls on cloud properties from tropics to midlatitude. Our results confirm that tropical water clouds and cirrus are more affected by LTS and CPT, respectively. Towards midlatitude from tropics, vertical velocity gradually plays a more important role in cloud development and dissipation. According to the diurnal cycles of these factors, temperature and static stability have the largest daily amplitude in the boundary layer of tropics and subtropics, which can explain the diurnal cycle of relative humidity and low clouds evolution, whereas vertical velocity has the largest daily amplitude in midlatitude, which is more related to the diurnal cycle of relative humidity and clouds in upper level of troposphere.