Chemical transport model
Chemical transport models and general circulation models
While related general circulation models (GCMs) focus on simulating overall atmospheric dynamics (e.g. fluid and heat flows), a CTM instead focuses on the stocks and flows of one or more chemical species. Similarly, a CTM must solve only the continuity equation for its species of interest, a GCM must solve all the primitive equations for the atmosphere; but a CTM will be expected to accurately represent the entire cycle for the species of interest, including fluxes (e.g. advection), chemical production/loss, and deposition. That being said, the tendency, especially as the cost of computing declines over time, is for GCMs to incorporate CTMs for species of special interest to climate dynamics, especially shorter-lived species such as nitrogen oxides and volatile organic compounds; this allows feedbacks from the CTM to the GCM's radiation calculations, and also allows the meteorological fields forcing the CTM to be updated at higher time resolution than may be practical in studies with offline CTMs.
Types of chemical transport models
CTMs may be classified according to their methodology and their species of interest, as well as more generic characteristics (e.g. dimensionality, degree of resolution).
- (Eulerian) "boxes" through which fluxes, and in which chemical production/loss and deposition occur over time
- (Lagrangian) the production and motion of parcels of air ("puffs") over time
- discussion of gridding in CLaMS
- Lagrangian and Eulerian coordinates
- discussion of the continuity equation in Jacob's Introduction to Atmospheric Chemistry online
Examples of Eulerian CTMs
- CMAQ, CMAQ Website
- GEM-MACH (semi-lagrangian)
Examples of Lagrangian CTMs
Examples of Semi-Lagrangian CTMs
Species of interest
CTMs typically focus on one species, but in order to realistically model its dynamics, the CTM may be forced to account for many related species, such as precursors or tracers. E.g. the MOZART model focuses on ozone, but additionally models over 100 related species (including aerosols) and several hundred reactions.