Are observed small rounded ice particles important in a climate model?

The importance of cirrus or ice crystal clouds in the tropical tropopause layer (TTL) in terms of their impact on the radiative balance of the layer through heating, spatial distribution, opacity, and composition is well known [1]. However, despite cirrus being important to the radiative balance of the TTL, there is little known about their ice crystal composition owing to their very high altitudes and low temperatures, making them difficult to access using research aircraft. This situation changed in 2014 when NASA flew its high-altitude UAV in the Western Pacific during the Airborne Tropical Tropopause Experiment (ATTREX) [2]. As part of the ATTREX campaign in 2015, the University of Hertfordshire flew its Aerosol Ice Interface Transition Spectrometer (AIITS) [3] on board NASA’s UAV, which provided 2D light scattering images of single ice particles in the size range 1–100 μm.
Analysis of the 2D light scattering images by Mathen [4] and subsequent modelling of the images using a physical–optics beam tracing method developed by Ballington and Hesse [5] found that at the colder temperatures in the TTL, the small ice crystals were dominated by rounded particles with some degree of surface roughening up to sizes of about 45 μm. At sizes between 45 and 100 μm, at the colidest temperatures, the particles were smooth thin hexagonal plates.
Here, the beam tracing method has been employed to calculate the total optical properties (extinction and scattering cross sections, single-scattering albedo, and asymmetry parameter) of the surface- roughened rounded ice particles and smooth plates, with sizes ranging between 5 and 45 and 55 to 95 μm, respectively. In the Met Office’s single-scattering database of the ensemble model (consisting of hexagonal columns, rosettes, and aggregates of hexagonal columns) developed by Baran and Labonnote [6], the columns and rosettes are replaced by the rounded particles and thin plates found in the TTL at sizes up to 100 μm.
In this presentation, we will explore two-stream radiative transfer simulations and climate model runs to assess the potential significance of rounded ice particles. The climate model runs will investigate any differences between the rounded particles and the ensemble model over a ten–year period, focusing on globally–averaged short–wave radiation fields and zonally–averaged temperatures. These experiments aim to determine whether rounded ice particles are important enough to be considered operationally in climate model radiation and temperature simulations. If so, this will have significant implications for climate modelling in the tropics, particularly in the TTL region, where currently rounded ice particles are precluded in all climate models.

[1] S. Lei, B-J. Sohn, J. Kim, and C. Liu, Atmos. Res. 293, 106919 (2023).
[2] S. Woods, R. P. Lawson, E. Jensen et al., J. Geophys. Res. 123, 6053 (2018).
[3] C. Stopford, P. Kaye, J. Ulanowski, and E. Hirst, AIITS: Preliminary light scattering data from tropical tropopause cirrus (Composition and Transport in the Tropical Troposphere and Lower Stratosphere, 2015).
[4] E. R. Mathen, E. Hesse, A. Baran, Analysis and modelling of TTL ice crystals based on in-situ measurements
of scattering patterns (19th Electromagnetic and light scattering conference, 2021).
[5] H. Ballington and E. Hesse, JQSRT, 323, 109054 (2024).
[6] A. J. Baran and L. Labonnote, Quart. Jour. Royal. Meteorol. Soc., 133, 1899 (2007).