Tropical cirrus evolution in a km-scale model with improved ice microphysics

Abstract. Tropical cirrus clouds form via in situ ice nucleation below the homogeneous freezing temperature of water or detrainment from deep convection. Despite their importance, limited understanding of their evolution and formation pathways contributes to large uncertainty in climate projections. To address these challenges, we implement novel passive tracers in the cloud-resolving model SAM to track the three-dimensional development of cirrus clouds. One tracer tracks air parcels exiting convective updrafts, revealing a rapid decline in ice crystal size and number as anvils age. Another tracer focuses on in situ cirrus, capturing their formation in the cold upper atmosphere and the subsequent reduction in ice crystal number over time. We find that in situ cirrus dominate at colder temperatures and lower ice water contents, while anvil cirrus prevail at temperatures above -60 °C. Although in situ cirrus have a smaller radiative impact compared to anvil cirrus, their contribution must be considered when evaluating top-of-the-atmosphere radiative effects. These findings improve our ability to assess the distinct roles of convective and in situ cirrus in shaping tropical cirrus properties and their impacts on climate.

We also improve the model's representation of tropical cirrus through simple, computationally inexpensive microphysics modifications, achieving better agreement with tropical aircraft observations. We show that updrafts critical for tropical cirrus formation are only resolved at horizontal grid spacings finer than 250 m—much finer than those used in global storm-resolving models. To mitigate this limitation, we propose microphysics improvements that reduce biases without increasing computational costs.