Abstract
Abstract. The parameterised description of subgrid-scale processes in the clear and cloudy boundary layer has a strong impact on the performance skill in any numerical weather prediction (NWP) or climate model and is still a prime source of uncertainty. Yet, improvement of this parameterised description is hard because operational models are highly optimised and contain numerous compensating errors. Therefore, improvement of a single parameterised aspect of the boundary layer often results in an overall deterioration of the model as a whole. In this paper, we will describe a comprehensive integral revision of three parameterisation schemes in the High Resolution Local Area Modelling – Aire Limitée Adaptation dynamique Développement InterNational (HIRLAM-ALADIN) Research on Mesoscale Operational NWP In Europe – Applications of Research to Operations at Mesoscale (HARMONIE-AROME) model that together parameterise the boundary layer processes: the cloud scheme, the turbulence scheme, and the shallow cumulus convection scheme. One of the major motivations for this revision is the poor representation of low clouds in the current model cycle. The newly revised parametric descriptions provide an improved prediction not only of low clouds but also of precipitation. Both improvements can be related to a stronger accumulation of moisture under the atmospheric inversion. The three improved parameterisation schemes are included in a recent update of the HARMONIE-AROME configuration, but its description and the insights in the underlying physical processes are of more general interest as the schemes are based on commonly applied frameworks. Moreover, this work offers an interesting look behind the scenes of how parameterisation development requires an integral approach and a delicate balance between physical realism and pragmatism.
Highlights
Due to ever-growing computer resources, numerical resolution of weather and climate models is steadily refined
The turbulence scheme describes the transport of heat, moisture, and momentum by the small-scale turbulent eddies in the boundary layer, whereas the convection scheme represents the transport by the larger-scale organised convective plumes
Optimisation of only one scheme will likely deteriorate the performance of another coupled scheme. This is why we describe in this paper the revision and optimisation of a tightly coupled triplet of parameterisation schemes for boundary layer turbulence, shallow cumulus convection, and clouds
Summary
Due to ever-growing computer resources, numerical resolution of weather and climate models is steadily refined. Prime atmospheric processes that remain to be parameterised at these scales are turbulent transport in the boundary layer, shallow cumulus convection, radiation, and cloud micro- and macrophysical processes of unresolved clouds. Optimisation of only one scheme will likely deteriorate the performance of another coupled scheme This is why we describe in this paper the revision and optimisation of a tightly coupled triplet of parameterisation schemes for boundary layer turbulence, shallow cumulus convection, and clouds. Several model intercomparison studies covering shallow cumulus, stratocumulus, and dry stable boundary layer conditions are used, most of which were based on observations collected during field campaigns For these intercomparison cases, results of the Dutch Large Eddy Simulation (DALES, Heus et al, 2010) are compared in detail with SCM runs of HARMONIE-AROME.
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