Opinion: Tropical cirrus – from micro-scale processes to climate-scale impacts

Abstract

Abstract. Tropical cirrus clouds, i.e., any type of ice cloud with tops above 400 hPa, play a critical role in the climate system and are a major source of uncertainty in our understanding of global warming. Tropical cirrus clouds involve processes spanning a wide range of spatial and temporal scales, from ice microphysics on cloud scales to mesoscale convective organization and planetary wave dynamics. This complexity makes tropical cirrus clouds notoriously difficult to model and has left many important questions stubbornly unanswered. At the same time, their multi-scale nature makes them well-positioned to benefit from the rise of global, high-resolution simulations of Earth's atmosphere and a growing abundance of remotely sensed and in situ observations. Rapid progress on our understanding of tropical cirrus requires coordinated efforts to take advantage of these modern computational and observational abilities. In this opinion paper, we review recent progress in cirrus studies, highlight important unanswered questions, and discuss promising paths forward. Significant progress has been made in understanding the life cycle of convectively generated “anvil” cirrus and the response of their macrophysical properties to large-scale controls. On the other hand, much work remains to be done to fully understand how small-scale anvil processes and the climatological anvil radiative effect will respond to global warming. Thin, in situ formed cirrus clouds are now known to be closely tied to the thermal structure and humidity of the tropical tropopause layer, but microphysical uncertainties prevent a full understanding of this link, as well as the precise amount of water vapor entering the stratosphere. Model representation of ice-nucleating particles, water vapor supersaturation, and ice depositional growth continue to pose great challenges to cirrus modeling. We believe that major advances in the understanding of tropical cirrus can be made through a combination of cross-tool synthesis and cross-scale studies conducted by cross-disciplinary research teams.

Evaluating Cloud Feedback Components in Observations and Their Representation in Climate Models

“…More work is needed to better understand the processes governing high-cloud optical depth in nature (including those that determine cloud phase, total water content, particle sizes, and other cloud microphysical properties) and its changes in response to interannual temperature variations. This echoes recent studies arguing for the importance of examining high-cloud optical depth feedbacks (Gasparini et al, 2023;Li et al, 2019;McKim et al, 2024;Zelinka et al, 2022). The consistent underestimation in high cloud feedbacks motivates further work to analyze how high clouds respond to their cloud controlling factors.…”

The First Global Insight of Cirrus Clouds Characterized by Hollow Ice Crystals From Space‐Borne Lidar

“…This is particularly noticeable in the thin cirrus samples with COD ≤ 0.1 (Figure 2f). This phenomenon is associated with the abundance of tropical tropopause layer (TTL) cirrus clouds generated by circulation and gravity waves in the tropics (Gasparini et al, 2023;Sassen et al, 2009).…”

“…In higher cirrus clouds, particularly near the tropopause, crystals tend to become nearly quasi-spherical (droxtals), which is attributed to the sublimation and the decrease in atmospheric water vapor content, leading to slower vapor deposition rates (Woods et al, 2018). Following the growth of ice crystals through vapor deposition, variations in falling or rising speed result in collisions and adhesion, leading to the formation of aggregates (Gasparini et al, 2023;Sölch & Kärcher, 2011). The newly introduced Voronoi model has demonstrated greater suitability for retrieving microphysical properties and conducting • Hollow characteristic cirrus clouds radiative assessments of ice clouds compared to the plates and columns aggregation models employed in the past (Letu et al, 2016;Li et al, 2022).…”

Factors determining tropical upper-level cloud radiative effect in the radiative-convective equilibrium framework

“…Moreover, regional studies in the Pacific have revealed complex patterns of CRE of upper-level clouds, with positive CREs observed for single-layered thin clouds colder than 260 K, peaking for very cold clouds which has their optical thickness around 1, and decreasing with increased thickness 9 . In summary, despite numerous studies investigating the radiative effect of TUCs, findings have shown variability due to differences in the target region, observational instruments, and the methodologies used to define TUCs 10 .…”