Improvements to stratospheric chemistry scheme in the UM-UKCA (v10.7) model: solar cycle and heterogeneous reactions

Abstract

Abstract. Improvements are made to two areas of the United Kingdom Chemistry and Aerosol (UKCA) module, which forms part of the Met Office Unified Model (UM) used for weather and climate applications. Firstly, a solar cycle is added to the photolysis scheme. The effect on total column ozone of this addition was found to be around 1–2 % in mid-latitude and equatorial regions in phase with the solar cycle. Secondly, reactions occurring on the surfaces of polar stratospheric clouds and sulfate aerosol are updated and extended by modification of the uptake coefficients of five existing reactions and the addition of a further eight reactions involving bromine species. These modifications are shown to reduce the overabundance of modeled total-column ozone in the Arctic during October to February, southern mid-latitudes during August, and the Antarctic during September. Antarctic springtime ozone depletion is shown to be enhanced by 25 DU on average, which now causes the ozone hole to be somewhat too deep compared to observations. We show that this is in part due to a cold bias of the Antarctic polar vortex in the model.

Stratospheric Ozone Changes From Explosive Tropical Volcanoes: Modeling and Ice Core Constraints

“…The changes to the stratospheric chemistry are tested by running the model for 30 years in a year 2000 time slice experiment using CMIP6‐prescribed trace gases and sea surface temperature forcings. The model is mostly able to reproduce the observed total column ozone and the results are similar to those found by Dennison et al (2019) in which a more thorough discussion of the changes can be found. The improved match with observed TCO resulting from our model updates is shown in Figure 1a.…”

“…This could be due to the vortex breakup being delayed and is a known issue in a number of comprehensive chemistry‐ climate model (Butchart et al, 2011; Eyring et al, 2010; McLandress et al, 2012). Overall, our changes to the chemistry scheme lead to an ozone distribution that is very similar to Dennison et al (2019).…”

“…We add eight new heterogeneous reactions to the model involving chlorine and bromine species in a similar way to Dennison et al (2019), following the previous work by Yang et al (2014), with the main difference being the explicit treatment of the reactions of four additional chemical species: Cl 2 , Br 2 , ClNO 2 , and BrNO 2 . These species are also photolyzed to produce Cl and Br radicals.…”

“…Reaction rates depend on the probability of a gas molecule colliding irreversibly with the surface of the aerosol and this is given by an uptake coefficient. We update the calculation of the uptake coefficients using the same scheme as Dennison et al (2019) with the differences listed in Table A1 in the Appendix. The model data used in this paper is archived on the Centre for Environmental Data Analysis (Ming, 2020).…”

Implementation of U.K. Earth System Models for CMIP6

“…As noted in section , UKESM1 includes photolytic reactions in its interactive simulation of ozone, but the photolysis rates use fixed solar inputs and therefore have no dependence on the solar variability. Dennison et al () show that including solar variability in photolysis calculations can have a noticeable effect on ozone production, resulting in variations of order 1% in extratropical total column ozone. This mode of ozone variability will therefore be absent from the UKESM1 CMIP6 simulations but will be a priority for inclusion in future versions of UKESM.…”