Analysis of Wind Response to Resolved and Parametrized Orographic Drag Across Moderately Complex Terrain

We investigate the response of the lower atmosphere to resolved and parametrized orographic drag over moderately complex terrain. Typically, larger terrain scales may induce propagating gravity waves and create flow blocking, while smaller scales (less than 5 km) may alter the turbulent atmospheric boundary layer, leading to turbulent orographic form drag (TOFD). Through high-resolution numerical simulations using the ICON model we assess the capability of a sub-grid scale orography (SSO, only its blocking component) and a TOFD parametrizations to replicate the influence of small-scale orographic features on flow over moderately complex terrain.

On one hand, the SSO scheme explicitly addresses a phenomenon where a low-level flow is obstructed by sub-grid scale orography. This obstruction leads to flow separation on the mountain flanks, inducing form drag. Meanwhile, the upper portion of the low-level flow traverses over the orography, concurrently generating gravity waves (which is deactivated for this study). On the other hand, the TOFD parametrization in NWP diagnoses surface stress and the vertical distribution of the resulting momentum flux based on the orography spectrum, relying solely on statistical properties of the orography within the domain, specifically the variance of the sub-grid scale orography. 

For decades, a widely acknowledged length-scale threshold of 5 km has guided atmospheric modelling practices to determine the application of each parametrization (SSO and TOFD). However, as computational capabilities evolve and modelling grids become finer, the validity of this threshold necessitates revaluation. The application of the threshold depends on factors such as model grid resolution, terrain characteristics, and dynamical processes of interest. Therefore, ongoing evaluation and adaptation are essential to ensure its relevance and efficacy.

A series of numerical simulations examines the impact of both parametrizations over moderately complex terrain near the Perdigao and in the Serra daEstrela mountain range. First, simulations ranging from the kilometre to 100-meter scale, are compared with observational data from the intensive observational period (IOP) of the Perdigao field campaign to validate the model. Kilometre-scale ICON simulations in NWP mode, are run continuously for the 49-day IOP. High-resolution Large Eddy Simulations (LES) are run (O 100 m) using  low-resolution (O 1 km) orography to assess the impact of the small-scale orography on the near-surface wind field. The reference impact is compared to the contribution of each parameterization (SSO and TOFD) to the surface drag and the total vertical flux of momentum in the near-surface atmosphere.

The results indicate that even at high resolutions of around 1 km, the effects of the SSO parametrization remain significant, contributing more to total surface stress than the TOFD parametrization. This suggests that the commonly used 5 km threshold may not be applicable in simulations of this nature, underscoring the need for revaluation or refinement of the threshold.