Evaluation of polar stratospheric clouds in the global chemistry-climate model SOCOLv3.1 by comparison with CALIPSO spaceborne lidar measurements

• Main Authors: Steiner, Michael, Luo, Beiping, Peter, Thomas, Pitts, Michael C., Stenke, Andrea
• Format: Text
• Language: English
• Published: 2020
• Subjects: Antarctic; Antarctic Peninsula; The Antarctic; Antarc*
• Online Access: https://doi.org/10.5194/gmd-2020-102; https://gmd.copernicus.org/preprints/gmd-2020-102/

Polar Stratospheric Clouds (PSCs) contribute to catalytic ozone destruction by providing surfaces for the conversion of inert chlorine species into active forms and by denitrification of the stratosphere. Therefore, an accurate representation of PSCs in chemistry-climate models (CCMs) is of great importance to correctly simulate polar ozone concentrations. Here, we evaluate PSCs as simulated by the CCM SOCOLv3.1 for the Antarctic winter 2007 by comparison with backscatter measurements by CALIOP onboard the CALIPSO satellite. The model considers supercooled ternary solution (STS) droplets, nitric acid trihydrate (NAT) particles, water ice particles, and mixtures thereof. PSCs are parametrized in terms of temperature and partial pressures of HNO 3 and H 2 O, assuming equilibrium between gas and particulate phase. We use the CALIOP measurements to optimize three prescribed microphysical parameters of the PSC scheme, namely ice number density, NAT particle radius and maximum NAT number density. The choice of the prescribed value of the ice number density affects simulated optical properties and dehydration, while modifying the maximum NAT number density or the NAT particle radius impacts stratospheric composition by enhancing the HNO3-uptake and denitrification. Best agreement with the CALIOP optical properties and observed denitrification was for this case study found with the ice number density increased from the hitherto used value of 0.01 to 0.05 cm −3 and the maximum NAT number density from 5×10 −4 to 1×10 −3 cm −3 . The NAT radius was kept at the original value of 5 µm. The new parametrization reflects the higher importance attributed to heterogeneous nucleation of ice and NAT particles, e.g. on meteoric dust, following recent new data evaluations of the state-of-the-art CALIOP measurements. A cold temperature bias in the polar lower stratosphere results in an overestimated PSC areal coverage in SOCOLv3.1 by up to 100 %. Furthermore, the occurrence of mountain-wave induced ice, as observed mainly over the Antarctic Peninsula, is continuously underestimated in the model due to the coarse model resolution and the fixed ice number density. However, overall we find a good temporal and spatial agreement between modeled and observed PSC occurrence and composition, as well as reasonable modeled denitrification and ozone loss. Based on constraining three important parameters by means of the CALIOP measurements, this work demonstrates that also a simplified PSC scheme, which describes STS, NAT, ice and mixtures thereof with equilibrium assumptions and avoids nucleation and growth calculations in sophisticated, but time-consuming microphysical process models, may achieve good approximations of fundamental properties of PSCs needed in CCMs.