Pollution transport by convective clouds in a mesoscale model

The effects of cumulus convection on the dispersion of passive tracers are considered in the framework of a mesoscale model, where the subgrid-scale clouds are parametrized. The parametrization uses a 'mass-flux' scheme, where guidance on vertical fluxes and cloud cover is obtained from a cloud-resolving large-eddy model run, and applied to the one-dimensional time-dependent vertical transport model. A set of three one-dimensional schemes is presented, the simplest of which contains approximations which are appropriate to, and commonly used in large-scale models. On the mesoscale, however, it is postulated that these approximations are no longer valid and a less idealised scheme is necessary. The schemes are compared using chosen initial tracer profiles, and their differences demonstrated in terms of the tracer evolution. It is found that the schemes diverge as the mass flux or grid-box area are decreased. There is some sensitivity to mixing between the clouds and their surrounding environment, where differences between the schemes are decreased if entrainment dominates and increased if detrainment dominates at in-cloud levels. The results imply a necessity for the scheme with fewest approximations when the host code is at mesoscale resolution, and this is implemented in the UK Meteorological Office non-hydrostatic model. A case is studied, 23 October 1992, in which convective activity is present in the cold air over the United Kingdom behind a frontal system which passed through on the previous day. The dispersion from hypothetical emissions of conserved tracers is modelled, and results compared both with and without the convective transport parametrization scheme. The convection is widespread over the Atlantic Ocean, the Irish Sea and the North Sea, and more intense during the afternoon. The intersection of a mid-level pollutant plume with the convection over the Irish Sea leads to a rapid vertical spread, both to the surface and cloud-top levels, which are mainly between 4 km and 5 km. It is then transported in this redistributed form over the UK mainland, and over the North Sea where it encounters more maritime convection. It is also found that pollutants initially below the convective-cloud-base level are, through the effects of boundary-layer turbulent diffusion, lifted to cloud-top levels.