Recent Changes in the Cirrus Clouds over the Amazon Rainforest from CALIPSO Observations

Abstract

Clouds cover about 70 % of the surface of our planet and play a fundamental role in the terrestrial radiative balance. Cloudiness is higher in the tropics due to intense convective activity, and there is also a larger presence of cirrus clouds. Cirrus are made of ice crystals and are found at altitudes above 8 km. They are semi-transparent to visible radiation and opaque to infrared radiation, helping to warm the planet. Recent studies have shown that (1) the presence of cirrus in the Amazon is higher than in other tropical regions, but also that (2) convective activity in the region is decreasing, which could mean a decrease in high clouds. In that context, we used data from 2006 to 2019 from Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP), aboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), to study cirrus clouds over the Amazon. Here we report on an analysis of the frequency of occurrence, base and top altitude, thickness, and optical depth for the whole Amazon, as well as their spatial distribution. We also investigated medium-term trends.A total of 1,474,457 vertical profiles were analyzed containing 942,124 cirrus layers, being 38.1 % in the wet season and 19.4 % 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. Considering the entire period, the cirrus have a base at 13.4 ± 2.1 km altitude, top at 15.3 ± 1.8 km, thickness of 1.9 ± 1.3 km and optical depth of 0.3 ± 0.5. Both the bases and tops are higher in the wet season, when the tropopause is at higher altitudes. The distribution of cirrus layers thickness or optical depth are similar in both seasons. Most cirrus were optically thin (about 41.8 %), while subvisual (COD<0.03) and opaque (COD>0.3) corresponded to 28.9 and 29.3 %, respectively. The subvisuals were mainly concentrated between 15-16 km altitude, close to the tropopause, while thin and opaque clouds had a wider vertical distribution.As for the medium-term trends, we observed that the 95th percentile of the thickness distribution is decreasing by 14 m year-1 (196 m from 2006 to 2019), which agrees with previous studies indicating a reduction of the convective activity. At the same time, we found that optically thinner cirrus (5th percentile of the COD distribution) are becoming more opaque in the dry season since 2016. This may be associated with a reduction in CALIOP’s sensitivity, in this case, we believe this is related to well known laser degradation and the interference of the South Atlantic Magnetic Anomaly. The results also indicated a clear reduction in the frequency of cirrus occurrence of 0.7 % year-1 in the region (9.8 % from 2006 to 2019), which also corroborates the reduction of convection and high clouds in the region. These gradual changes can lead to significant changes in a few decades and, consequently, to important changes on the radiative balance of the Amazon.