TransportMonthly-averaged wind fields of the year 1987 are interpolated to the MOGUNTIA grid. The vertical winds are derived from the divergence of the horizontal wind fields. The boundary conditions are applied at 1000 hPa and 100 hPa, where a no-flux boundary condition is requested. At the poles, two singular points are present.
The wind fields are made mass-conserving by small modifications in the horizontal winds, such that the boundary conditions are fulfilled (zero vertical winds at 1000 hPa and 100 hPa).
By the use of monthly-averaged winds, a lot of transport is not resolved: the average wind vector is usually smaller than the instantaneous vector. A measure for the non-resolved transport is the variance of the wind during a month. This variance (unit m2/s2) is converted into a diffusion coefficient (unit m2/s) by multiplying with a time-scale. In MOGUNTIA the Lagrangian time scale for mixing are used for this purpose (Murgatroyd, 1969; Zimmermann, 1983). The monthly averaged wind vectors and the diffusion coefficients are used to transport tracers over the globe. The second order upwind scheme QUICKEST is used to solve the advection-diffusion equations (Vested et al., 1992)
Due to the coarse (horizontal) resolution, a lot of meteorological transport processes are simply not resolved by the model. Examples are:
- Deep convection due to thunderstorms (scale about 10-100 km)
- Frontal activity due (small) low pressure systems (scale 100-1000 km)
An important effect of these processes is that tracers are vertically redistributed. Without taken these processes into account, the vertical distribution of many species would not correspond to measurements. This vertical mixing is a so-called sub-grid-scale (SGS) process, which has to be parameterised. This parameterisation (Feichter and Crutzen, 1990) consists of statistical data, which are used to redistribute tracers in a vertical column. The height of the convective column and the amount of air mass associated with the updraughts were derived from the amount of convective precipitation and the average large scale temperature and humidity distribution. The resulting vertical profiles of Rn222 compared much better to measurements than the profiles without convection.
In summary, transport of tracers in the MOGUNTIA model takes into account:
- transport by monthly averaged wind-fields
- diffusion based on the variance of the winds
- vertical redistribution by a deep convection parameterisation