Climatology and trends of cirrus geometrical and optical properties in the Amazon region from 7-yr of CALIPSO observations

Abstract

Recent studies have shown that the presence of cirrus clouds in the Amazon is higher than in other tropical regions, but also that convective activity in the region is decreasing, which could mean a decrease in high clouds. We used data from 2009 to 2016 from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), aboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), to study cirrus over the Amazon. Here we report on an analysis of the frequency of occurrence, base and top altitude, geometric thickness, and optical depth for the whole Amazon, as well as their spatial distribution and medium-term trends. A total of 1,473,863 vertical profiles were analyzed containing 728,123 cirrus layers, being 37.0% in the wet and 21.2% in the dry season. They are evenly distributed throughout the region during the wet season and concentrated in the northwest of the Amazon during the dry season. In terms of cloud optical depth (COD), most cirrus were optically thin (0.03<COD<0.3), with a relative frequency of 43.9%, while subvisual (COD<0.03) and thick (COD>0.3) corresponded to 22.0 and 34.1%, respectively. The results indicate a significant reduction in the frequency of cirrus occurrence of 1.3 ± 0.3% per year in the wet season, accompanied by a reduction of the geometric thickness of the thickest cirrus by 57 ± 18 m. This corroborates previous reports of a reduction of convective activity and high cloud fraction in the region.