Diurnal Cycle and Seasonality of Cirrus Clouds over the Amazon from a Seven-Year Ground-Based Lidar Record

Abstract

Cirrus clouds strongly influence Earth’s radiation balance, yet their varied optical traits and formation pathways add uncertainty to climate models. Satellites offer a global view of cirrus but have key limitations: passive sensors struggle to detect thin layers, and active instruments in polar orbits miss the diurnal cycle. Consequently, long-term records above tropical forests are scarce. We analyzed 18 915 h of lidar data gathered near Manaus, Brazil (July 2011–December 2017), to characterize the cirrus clouds over the Amazon rainforest. An automated routine set cloud boundaries and multiple-scattering-corrected retrievals yielded cloud optical depth (COD) and lidar ratio. Cirrus were found to occur with a frequency of 73.2%. Thin layers (COD=0.03–0.30) are the most prevalent (33.0%), followed by sub-visual (<0.03; 20.5%) and opaque (>0.30; 15.9%). The mean base and top altitudes were 12.8±2.2 km and 14.4±1.9 km, and the lidar ratio averaged 26.1±8.3 sr, reaching a peak for opaque cirrus. Thinner clouds clustered near the tropopause, while higher tops tracked the tropopause variability. A clear diurnal cycle shows a noon minimum frequency of occurrence and late-afternoon maximum, strongest for opaque cirrus and consistent with convective-anvil outflow. Optical properties such as COD, lidar ratio, and geometric properties follow a well-defined daily rhythm. Seasonally, cirrus are more frequent in the wet season (82.8%) than in the dry (54.5%). Bases, tops, and thickness are likewise larger in the wet season, whereas lidar ratios peak in the dry season. This long-term record benchmarks satellite retrievals, sharpens radiative-impact calculations and clarifies cirrus formation over tropical forests.