Two adaptive schemes for the radiative transfer parameterisation in the Lokal-Modell

We introduce the term “adaptive parameterisation scheme”, for a scheme, which uses spatial and temporal correlations to make parameterisations computationally more efficient and thus to be able to include more physics in the parameterisations. As an illustration of this general idea, we will present two adaptive radiative transfer (RT) parameterisation schemes for the Lokal-Modell (LM) of the German meteorological service.

In an adaptive parameterisation scheme, the computation is split into a more complex, physical calculation and a simple, adaptive algorithm. As physical calculation for our two adaptive schemes, we use the 2-stream RT parameterisation used operationally in the LM. Most other NWP models use less-physical, simpler schemes.

We have achieved an error reduction of a factor 3 compared to the operational practice of time constant fluxes for one hour. Users of other models now have a RT scheme that is more physical and computationally light enough for frequent calls.