Submitted as: model description paper 11 Nov 2020

Submitted as: model description paper | 11 Nov 2020

Review status: this preprint is currently under review for the journal GMD.

Mesoscale nesting interface of the PALM model system 6.0

Eckhard Kadasch1, Matthias Sühring2, Tobias Gronemeier2, and Siegfried Raasch2 Eckhard Kadasch et al.
  • 1Deutscher Wetterdienst, Offenbach, Germany
  • 2Institute of Meteorology and Climatology, Leibniz University Hannover, Hannover, Germany

Abstract. In this paper, we present a newly developed mesoscale nesting interface for the PALM model system 6.0, which enables PALM to simulate the atmospheric boundary layer under spatially heterogeneous and non-stationary synoptic conditions. The implemented nesting interface, which is currently tailored to the mesoscale model COSMO, consists of two major parts: (i) the preprocessor INIFOR, which provides initial and time-dependent boundary conditions from mesoscale model output and (ii) PALM's internal routines for reading the provided forcing data and superimposing synthetic turbulence to accelerate the transition to a fully developed turbulent atmospheric boundary layer.

We describe in detail the conversion between the sets of prognostic variables, transformations between model coordinate systems, as well as data interpolation onto PALM's grid, which are carried out by INIFOR. Furthermore, we describe PALM's internal usage of the provided forcing data, which besides the temporal interpolation of boundary conditions and removal of any residual divergence includes the generation of stability-dependent synthetic turbulence at the inflow boundaries in order to accelerate the transition from the turbulence-free mesoscale solution to a resolved turbulent flow. We demonstrate and evaluate the nesting interface by means of a semi-idealized benchmark case. We carried out a large-eddy simulation (LES) of an evolving convective boundary layer on a clear-sky spring day. Besides verifying that changes in the inflow conditions enter into and successively propagate through the PALM domain, we focus our analysis on the effectiveness of the synthetic turbulence generation. By analysing various turbulence statistics, we show that the inflow in the present case is fully adjusted after having propagated for about 1.5 eddy turn-over times downstream, which corresponds well to other state-of-the-art methods for turbulence generation. Furthermore, we observe that numerical artefacts in the form of under-resolved convective structures in the mesoscale model enter the PALM domain, biasing the location of the turbulent up- and downdrafts in the LES.

With these findings presented, we aim to verify the mesoscale nesting approach implemented in PALM, point out specific shortcomings, and build a baseline for future improvements and developments.

Eckhard Kadasch et al.

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Eckhard Kadasch et al.

Data sets

Supplementary material to 'Mesoscale nesting interface of the PALM model system 6.0' Eckhard Kadasch and Matthias Sühring

Model code and software

Supplementary material to 'Mesoscale nesting interface of the PALM model system 6.0' Eckhard Kadasch and Matthias Sühring

Eckhard Kadasch et al.


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