Comment on acp-2022-146

Abstract

The occurrence of cirrus clouds in the tropics (24 °S–24 °N) is analyzed using the 2007–2015 monthly data 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. In most cirrus clouds, the specific humidity (SPH) is larger than in cloud-free air and/or the temperature is smaller than in cloud-free air. Both positive SPH perturbations and negative temperature perturbations increase the relative humidity, resulting in favorable conditions for the formation and maintenance of clouds. The clouds in which there are positive SPH perturbations are considered to originate from convection. This is because, in the free troposphere, positive SPH anomalies are largely produced by the upward transport of moisture by convection followed by detrainment of the convective plumes. The remaining clouds that are not directly influenced by convection are driven by negative temperature perturbations. These temperature-driven clouds are formed and maintained in the cold phases of gravity waves and/or by the adiabatic cooling associated with the upwelling branch of the Brewer–Dobson circulation. Averaged over all altitudes of the tropical atmosphere, there are about three times more convective cirrus than non-convective ones. The level of maximum convective cirrus occurrence is at 14 km, i.e., the bottom of the tropical tropopause layer (TTL). Non-convective cirrus obtain their maximum frequency of occurrence at about 16 km, which is below the cold point tropopause (CPT). The seasonal cycle of convective cirrus is consistent with that of tropical convection, while the seasonal cycle of non-convective cirrus in the TTL is consistent with that of the CPT. There are two maxima in the frequency of occurrence of convective cirrus, one at around 10 °S in the austral summer, and the other at around 10 °N in the boreal summer. In contrast, non-convective cirrus occur most frequently near the equator in the boreal winter. The ice water content (IWC) in both convective and non-convective cirrus increases with increasing temperature (decreasing altitude). Thus, non-convective cirrus—which on average occur at lower temperatures (higher altitudes)—tend to have lower IWCs than convective cirrus.