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 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 new revised parametric descriptions provide not only an improved prediction 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
Owing to ever growing computer resources, numerical resolution of weather and climate models steadily refines
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
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
Summary
Owing to ever growing computer resources, numerical resolution of weather and climate models steadily refines. 30 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. Long-term Single Column Model (SCM) runs are used to evaluate the turbulence scheme in terms of theoretical flux-gradient relationships, following the procedure of Baas et al (2017) Based on these results important modifications are made to the turbulence scheme. 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)) 55 are compared in detail with SCM runs of HARMONIE-AROME.
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