Stratospheric Chemistry plus GLOMAP-mode: Pinatubo test case

The GLOMAP-mode aerosol microphysics module in UKCA has recently been joined up with the UKCA Stratospheric Chemistry scheme (StratChem), with StratChem extended to additionally include a stratospheric sulphur chemistry scheme.

The GLOMAP-mode aerosol model has been modified to calculate the vapour pressure of sulphuric acid, which becomes significant in the stratosphere. The expression of Ayers et al (1980) is used, and this effectively specifies the sulphuric acid-water composition in the stratosphere.

The nucleation rate used in the model has also been updated to use the Vehkamakki et al (2002) scheme which is suitable for both stratospheric and tropospheric conditions.

As a test for this new configuration, the UKCA StratChem+GLOMAP model was integrated within N48L60 HadGEM3-A with 3 years spin-up prior to a simulated Pinatubo eruption in June 1991 whereby 20 Tg of SO2 is injected between the tropopause and 28 km at 15N, 120E.

The model was then integrated for a further 4 years post-Pinatubo to investigate the evolution of the stratospheric aerosol and its size distribution.

To evaluate the UKCA simulations, we have compared to the balloon-borne OPC profile measurements of size-resolved number concentration at Laramie, Wyoming. These measurements (Deshler et al, 2003) form an extremely good observational record against which to evaluate the model, with balloons launched at approximately monthly frequency throughout this period.

For example, UKCA_StratChem_GLOMAP_Pinatubo.pdf compares model profiles of the number concentration of particles larger than 10nm (black), 150nm (red) and 250nm (blue) against those from the observations 3 months prior to the eruption, and 9 months afterwards.