Properties of cirrus and subvisible cirrus from nighttime Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), related to atmospheric dynamics and water vapor

摘要

We map cirrus and subvisible cirrus clouds (SVC, optical depth <0.03) on a global scale, detecting optically thin clouds in 2.5 years of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar observations. Cirrus clouds are mostly concentrated around strong convection areas in the tropics (cloud fractions (CF) 50%-60%, up to 90%), while SVC spread over higher latitudes (CF 30%-40%). We document cloud properties (geometrical thickness, top altitude, and midlayer temperature) and ice crystal depolarization ratios. SVC are thin (<1 km), are 2°C-3°C colder than cirrus clouds, and produce depolarization ratio lower by 0.03 on average, suggesting that the shapes of their crystals deviate from other cirrus clouds. We investigate correlations between retrieved properties and vertical and horizontal wind speed from European Centre for Medium-Range Weather Forecasts reanalyses characterizing vertical air motions in the tropics and jet streams in midlatitudes. In the tropics, cloud occurrence is correlated with vertical motions: cirrus CF goes from 5%-15% in subsidence to 30%-50% in updraft conditions, where clouds are 0.6 km thicker, -1 km higher, and -3° colder than in subsidence. In updraft conditions, cirrus CF is double the SVC CF (15%-25%). Optical properties of ice crystals do not change with vertical motions. In midlatitudes, horizontal winds faster than 30 m/s lead to higher CF, clouds -8°C wanner (i.e., 1.8 km lower), and particulate depolarization ratio 0.1 lower. Changes in wind speeds affect SVC and cirrus clouds alike. Where CALIOP detects cirrus and SVC clouds, upper tropospheric water vapor concentrations from collocated MLS observations increase by 15-30 ppmv (cirrus) and 5-10 ppmv (SVC).