Previous: Introduction Next: Model validation Up: Ext. Abst.

2. CTM2: Description of the model

The Oslo CTM2 isa global 3-dimensional chemical transport model (CTM) for the troposphere and the lower stratosphere, extending from the surface up to about 10 hPa where the uppermost layer is centered. The vertical grid comprises 19 layers defined in sigma-pressure hybrid coordinates, while the horizontal resolution can be varied between T21 (~5.6°x5.6°), T42 (~2.8°x2.8°), and T63 (~1.9°x1.9°).However, all simulations for this paper have been performed with T21 resolution. The model meteorology is determined by a self-consistent set based on ECMWF forecast data including horizontal winds, temperature, cloud liquid water content, cumulus convection, etc. for the year 1996. Model results can thus be compared readily with observations from the same time (Sundet, 1997).

Advective transport uses the concept of Second Order Moments (Prather, 1986), while convection is based on the Tiedtke mass flux scheme (Tiedtke, 1987), where the vertical transport of species is determined by the surplus/deficit of mass flux in a column. Transport in the boundary layer is treated according to the Holtslag K-profile scheme (Holtslag et al., 1990).

Emissions of source gases (CO, NOx, Methane, VOC compounds) for different source categories are taken from the GEIA and EDGAR data bases for natural emissions, and from Mueller (1992) for anthropogenic emissions. High-altitude emissions of NOx from lightning and aircraft are included based on Price et al. (1997a/b) and the ‘IPCC-2001’ aircraft inventory (IPCC, 2001), respectively. The calculation of dry deposition follows Wesely (1989). At the model top a constant mixing ratio boundary condition is applied using data from a multi-year simulation of the Oslo2D model.

For chemical integrations two separate modules are used covering tropospheric and stratospheric chemistry, respectively. The tropospheric chemistry scheme contains 51 species and has been thoroughly tested in the OSLO CTM-1 model (Berntsen and Isaksen, 1997). 86 thermal reactions, 17 photolytic reactions, and 2 heterogeneous reactions are integrated by the QSSA method (Hesstvedt et al., 1978) using a numerical time step of 5 minutes. The stratospheric chemistry solver is a well-tested extensive QSSA code developed by Stordal et al. (1985) and has been updated to include heterogeneous chemistry (Isaksen et al., 1990). It has been extensively used and validated in the OSLO 2D model (Isaksen et al., 1990) and in a stratospheric 3-D CTM (Rummukainen et al., 1999). 104 thermal, 47 photolytic, and 7 heterogeneous reactions involving a total of 57 species and 7 families are integrated in time steps of 10 minutes.

As a boundary between the tropospheric and the stratospheric chemistry regimes the 150ppbv ozone surface is chosen. Photodissociation rates are calculated on-line once every hour following the method described by Wild et al. (1999).

Previous: Introduction Next: Model validation Up: Ext. Abst.