Comparing simulated polar stratospheric clouds in the UKESM with CALIOP satellite data

Polar stratospheric clouds (PSCs) play a fundamental role in depleting stratospheric ozone. Heterogeneous reactions on their surfaces increase the concentration of active chlorine, which can catalytically destroy ozone and prolong ozone depletion by denitrifying and dehydrating the stratosphere. However, parametrisations of PSC formation is poorly included in global chemistry-climate models due to the complexity of the microphysical processes involved in PSC particle formation. This limits our ability to project the future recovery of the stratospheric ozone and the resulting climate impacts.

In this work, the representation of PSCs in the UK Earth System Model (UKESM) has been improved by refining the particle formation schemes to 1) kinetically determine the growth of nitric acid trihydrate particles rather than using a thermodynamic assumption, and 2) include the growth of supercooled ternary solution particles through the uptake of nitric acid rather than using a sulphate aerosol climatology. To validate these changes, the simulated PSCs are converted into optical properties and evaluated against satellite data. Here, a comparison of the results from the new PSC scheme in the UKESM with the observations from the satellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is presented. Whether the changes to the formation parameterisation improve the model’s ability to accurately simulate PSCs both temporally and spatially in the Northern and Southern hemisphere is assessed and the effect on stratospheric denitrification in the model is examined.