A passive tracer perspective on the origin and evolution of tropical cirrus clouds

The processes controlling tropical cirrus clouds are poorly understood, contributing to significant uncertainty in estimating how clouds respond to global warming. Much of this uncertainty stems from a lack of knowledge about the cirrus life cycle. Not knowing how cirrus clouds evolve also makes it hard to determine the fraction of clouds that comes from deep convective outflow compared to those formed by in situ ice nucleation at temperatures colder than -40°C. These two types of clouds are controlled by different processes that may operate differently in a warmer climate, making it even more important to assess their origin.

We implement passive tracers in the cloud-resolving model SAM used in a tropical channel setup to track the 3D evolution of cloudy parcels from two different perspectives:

• A detrainment perspective, useful for tracking the evolution of anvil clouds.
• An ice nucleation perspective, useful for following the evolution of in situ cirrus.

Using the detrainment tracer, we can accurately determine how long it's been since an air parcel left a deep convective plume. Our analysis shows that freshly detrained air parcels consist mainly of many large ice crystals with radii of 30-80 μm. These quickly fall out of the atmosphere, resulting in aged anvils containing fewer and smaller ice crystals.

The ice nucleation tracer tracks the time after the onset of ice nucleation. This proves valuable for studying the evolution pathways of in situ cirrus ice crystals. Initially, small, freshly nucleated in situ cirrus mostly contain 20-200 ice crystals/liter, occasionally spiking due to relatively rare homogeneous nucleation events. However, the number of ice crystals decreases rapidly, likely because of sublimation, leading to concentrations of < 10/liter in aged clouds.

Tracers also help us understand the climatology of cirrus formation. On average, we find that in situ cirrus account for 20% (at T>-50°C) to 70% (at T<-70°C) of all tropical cirrus.

While tracers cannot follow individual cloud parcels and different realizations of the tropical atmosphere in global models and other idealized frameworks may affect their behavior and interpretation somewhat, our research shows that they can provide valuable insights into cloud evolution and microphysics. They also have the potential to improve our mechanistic understanding of how tropical cirrus respond to global warming.