Formation Mechanisms of Tropical Cirrus Clouds Inferred from CALIPSO

The formation mechanisms of cirrus clouds in the tropics (24S24N) are investigated using the monthly data in 20072015 from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and the fifth generation reanalysis product (ERA5) of the European Centre for Medium-Range Weather Forecasts. The data show that the relative humidity with respect to ice (RHi) in cloudy air samples is larger than in cloud-free air samples. Based on the contributions of the specific humidity (SH) and temperature anomalies to the RHi anomalies in cloudy air samples relative to cloud-free air samples, we obtain two categories of clouds: moisture-driven (MD) cirrus, and temperature-driven (TD) cirrus. MD cirrus are formed predominantly by positive SH anomalies, while TD cirrus are formed predominantly by negative temperature anomalies. Since moisture must be brought up to the upper troposphere and tropopause layer by convection, all positive SH anomalies and therefore all MD cirrus are driven by convection. TD cirrus are classified into two sub-categories: those that experience positive SH anomalies and are therefore influenced by convection, and those that experience negative SH anomalies and are therefore not influenced by convection. Averaged over all altitudes of the tropical atmosphere, there are four to five times more convective cirrus (including MD and TD-convective cirrus) than non-convective ones. The level of maximum convective cirrus occurrence is at 1112 km, coincided with the level of maximum temperature lapse rate and convective outflow. Non-convective cirrus obtain their maximum frequency of occurrence at 16.3 km, which is just below the cold point tropopause at 16.8 km. The seasonal cycle of convective cirrus is consistent with that of tropical convection, while the seasonal cycle of non-convective cirrus is consistent with that of the cold point tropopause. There are two maxima in the frequency of occurrence of convective cirrus, one at 812 S in the austral summer, and the other at 812 N in the boreal summer. In contrast, non-convective cirrus occur most frequently near the equator in the boreal winter.