UMSLIMCAT chemistry-climate model

UMSLIMCAT is a coupled chemistry-climate model (CCM) which combines the Met Office Unified Model (UM) v4.5 with the well-established TOMCAT / SLIMCAT stratospheric chemistry scheme. The model is designed to study the long-term changes in stratospheric ozone due to changes in chemical composition and greenhouse gas loading.

The Met Office UM is a hydrostatic, primitive-equation, grid-point atmospheric general-circulation model (GCM) with a hybrid vertical coordinate. Different versions of the UM exist but UMSLIMCAT is based on the ‘climate’ version v4.5 L64. The model has a horizontal resolution of 2.5◦ latitude × 3.75◦ longitude and 64 vertical levels from the surface to 0.01 hPa (approximately 80 km). The vertical resolution in the stratosphere is ∼1.3 km. This version of the UM uses the orographic gravity-wave drag scheme from the surface to 20 hPa. The scheme accounts for the anisotropy of sub-grid orography in the calculation of the surface stress. Above 20 hPa, non-orographic gravity-wave drag is represented by a Rayleigh friction term. UMSLIMCAT uses sea-surface temperatures (SSTs) from observations or ocean model output. UMSLIMCAT uses the radiation scheme of Edwards and Slingo with some modifications in the middle atmosphere. This scheme treats absorption by the gases CO2, H2O, O3, N2O, CH4, CFC-11 (CFCl3), and CFC-12(CF 2 Cl2). In the standard UM, these gases are specified by climatological values, though certain UMSLIMCAT simulations can generally use interactively calculated profiles. In UMSLIMCAT the v4.5 UM’s default tracer advection scheme has been replaced by the QUINTIC-MONO scheme. This scheme performs much better in maintaining realistic tracer distributions in the presence of strong gradients.

The UMSLIMCAT coupled chemistry version of the UM takes the full stratospheric chemistry modules from the TOMCAT/SLIMCAT off-line 3D chemical transport model (CTM). This scheme is both comprehensive and computationally efficient. The scheme contains all of the Ox, NOy, Cly, Bry and HOx species believed to be important to describe stratospheric chemistry. The scheme also includes a detailed CH4 oxidation scheme and a range of long-lived source gases. During the model simulations, the abundance of the three halogen-containing source gases (CFCl3, CF2Cl2 and CH3Br) are scaled to give the required chlorine/bromine loading (to account for the contributions from source gases not considered). The scheme uses 28 advected tracers in the UM (including a ‘passive’ Ox tracer to diagnose polar ozone loss).

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