On the origin of subvisible cirrus clouds in the tropical upper troposphere

Abstract

Abstract. Spaceborne lidar observations have recently revealed a previously undetected significant population of Subvisible Cirrus (SVC). We show them to be colder than −74 • , with an optical depth below 0.0015 on average. The formation and persistence over time of this new cloud population could be related to several atmospheric phenomena. In this paper, we investigate if these clouds follow the same formation mechanisms as the general tropical cirrus population (including convection and in-situ ice nucleation), or if specific nucleation sites and trace species play a role in their formation. The importance of three scenarios in the formation of the global SVC population is investigated through different approaches that include comparisons with data imaging from several spaceborne instruments and back-trajectories that document the history and behavior of air masses leading to the point in time and space where subvisible cirrus were detected. In order to simplify the study of their formation, we singled out SVC with coherent temperature histories (mean variance lower than 4 K) according to back-trajectories along 5, 10 or 15 days (respectively 58, 25 and 11 % of SVC). Our results suggest that external processes, including local increases in liquid and hygroscopic aerosol concentration (either through biomass burning or volcanic injection forming sulfate-based aerosols in the troposphere or the stratosphere) have very limited short-term or mid-term impact on the SVC population. On the other hand, we find that ∼ 20 % of air masses leading to SVC formation interacted with convective activity 5 days before they led to cloud formation and detection, a number that climbs to 60 % over 15 days. SVC formation appears especially linked to convection over Africa and Central America, more so during JJA than DJF. These results support the view that the SVC population observed by CALIOP is an extension of the general upper tropospheric ice clouds population with its extreme thinness as its only differentiating factor.

Statistical downscaling of water vapour satellite measurements from profiles of tropical ice clouds

“…Overall, these results suggest that, at the instantaneous scale of cloud measurements, the water vapour response along the whole troposphere in correspondence to ice cloud profiles is well predicted only accounting for their capability to backscatter radiation (given by the observed SR profile). While the large-scale link between relative humidity and the cloud properties (vertical distribution, phase, and opacity) has been well documented in previous studies (Martins et al, 2011;Reverdy et al, 2012), this work represents the evidence that this relationship can also be detected at much smaller spatio-temporal scales. The emergence of a clear signal at these fine scales also highlights the limitations of SAPHIR measurements: although SAPHIR observes the water vapour field at a much finer horizontal resolution than what is currently available in reanalysis products, in order to explain physical processes, downscaled observations are needed.…”

Cirrus feedback on interannual climate fluctuations

“…The cloud optical depth (τ) is not provided in the data set we used, so we calculate the optical depth of single-layer cirrus clouds using the formulation of Reverdy et al [2012]:…”

Assessment of the accuracy of (re)analyses in the equatorial lower stratosphere

“…Such processes include cirrus cloud formation, dehydration, and transport of gaseous chemical species and aerosols to the stratosphere. Because they strongly depend on the history of air parcels, i.e., on the Lagrangian evolution of temperature, humidity, and diabatic heating, these processes are often investigated using trajectories calculated with (re)analysis products [e.g., Fueglistaler et al , ; Reverdy et al , ; Liu et al , ]. In these studies, errors in the analyzed wind fields have been identified as a potential source of uncertainty in the results, and, for instance, Hasebe et al [] reported on differences that can reach a few m/s between analyzed and observed winds in the TTL over the Maritime Continent.…”