Analysis of the Impacts of Small-Scale Orography on the Atmospheric Boundary Layer: Developing ICON-LES for the Perdigão Field Experiment

The response of the lower atmosphere to resolved versus parametrized orographic drag over moderately complex terrain is investigated. The larger terrain scales may trigger propagating gravity waves and generate flow blocking, while the smaller scales (smaller than 5 km) may modify the turbulent atmospheric boundary layer leading to turbulent orographic form drag (TOFD). We perform high-resolution numerical simulations to evaluate the ability of a TOFD parametrization to reproduce the impact of small-scale orographic features on the flow over complex terrain. The tool selected to perform the simulations is the Icosahedral Nonhydrostatic (ICON) numerical model, a unified modelling system for global numerical weather prediction (NWP) and climate studies. In the present study, the model is used in its limited-area mode. In the TOFD parametrization, the surface stress and its vertical distribution are formulated in terms of the orography spectrum, meaning that it only depends on the orography characteristics in the box (more specifically, only on the variance of the sub-grid scale orography).

As a first step, simulations using different grid spacings, from the km scale (NWP mode) to the O100 m scale (large-eddy simulations, LES), are carried out to reproduce the intensive observational period (IOP) of the Perdigão field experiment. The high-resolution LES is used to assess the performance of the TOFD parametrization (used in NWP mode) in simulating the surface stress, the near-surface atmosphere and to highlight possible issues resulting in a miss-representation of the flow over moderately complex terrain. The km-scale simulations are run continuously for the complete 49-day IOP using the ERA5 reanalysis dataset for initial and boundary conditions. LES at 130 m grid spacing, are run for the whole IOP nested into the NWP runs (offline nesting). Initial results of the NWP control simulation show good performance compared to the tower wind observations for the whole IOP. The model performance was especially good for the second part of the IOP when a high-pressure regime and weak synoptic forcing were observed.

Larger-scale thermally driven flows from the mountain ranges surrounding the Perdigão site impact the wind flow system at the double ridge, and when decomposed into along- and cross-valley, the performance of the model in the cross-valley direction tends to be very good. However, the model failed to reproduce the transition between anabatic- and katabatic-slope flow in the Serra da Estrela mountain range (NE of Perdigão). The latter negatively impacted the reproduction of the along-valley flow during the afternoon hours at the Perdigão site, when the most important deviations from observations were observed.