The CONVECTion submodel calculates the process of convection.
Via namelist one of the implemented parametrisations can be chosen and the necessary calculations are
performed automatically. At the momentary state there are three similar schemes available. Those all are based on the Tiedtke convection scheme developed at ECWMF, but have different closures: the default value is the Nordeng closure, a second option the original Tiedtke closure and a third option, a hybrid closure.
All of the Tiedtke schemes calculate after initialisation and determination of the cloud base a
first ascent with entrainment and detrainment in absence of downdrafts. In a second step now the downdrafts and changes of temperature, humidity and tracer concentrations due to these effects are estimated. As next step the final ascent, now including the changes of the downdrafts, is performed. Finally the fluxes are adjusted and the values of the larger scaled fields get changed due to the convection. Because for chemistry it is very important that the tracer concentrations are positive definite an update to the code by S.Brinkop from the DLR is implemented that guarantees this condition, but at the momentary state this only works for the Nordeng closure.
The other closures as well as other schemes that do not explicitly calculate tracer transport should
therefor better be used with the related CVTRANS module. As the tracer fluxes can be changed by wet deposition in the convective cloud and/or precipitation, there is a link to the SCAVenging module.
All the necessary parameters can be easily stored and afterwards used by other processes.
In addition to this there is a small routine that estimates a convective cloud cover. The original Tiedtke scheme uses a fixed, grid size independent fraction of 5%. This might be a good estimate, but is not only dependent on the grid size, it also does not give any information of the strength of the convective event. Therefor a new convective cloud cover is estimated from the strength of the updraft mass flux.
In the extended version of the module additional convection schemes are implemented. These are:
- The operational ECMWF convection scheme (IFS-cycle 29r1),
- The convection scheme of Zhang / Hack / McFarlane as it is implemented in the CTM MATCH with two different
treatments of cloud water,
- The convection scheme of Bechtold, that is/was used at ECMWF and in the French MESO-NH,
- The Emanuel convection scheme,
- an experimental version of the Donner scheme
The Zhang / Hack / McFarlane Scheme starts after initialisation with calculation of the buoyancy strength by determining the convective available potential energy (CAPE). In a second step the microphysical part is calculated followed by the closure. At the end the changes of temperature and humidity due to the convection are determined as well as the rain parameters. This scheme does not change the winds in our version and yields quite different temperatures and humidity fields compared to the Tiedtke scheme, but the effect on climate calculations has not been determined yet. The scheme is not implemented with its own tracer transport mechanism, so the CVTRANS module should be used for this task.
The Bechtold Scheme is also a scheme following the mass flux approach. First it is determined by a trigger function whether convection will take place in that column or not. Afterwards the LCL for the column will be calculated and from this level the updraft, downdraft and condensation. It is closed by a CAPE closure. The scheme has the capability of calculating ensembles for the deep convection process, i.e. it can calculate the process of deep convection with 3 additional perturbed starting conditions.