A new orographic drag parameterization package for the GLOBO model: implementation and evaluation

Orography plays a fundamental role in shaping the atmospheric circulation and affects key atmospheric processes. Therefore, weather and climate models must adequately represent its effects to obtain accurate predictions. Since all orographic scales are found to influence the atmospheric flow, the parameterization of unresolved orographic drag has been recognized as crucial to simulate a realistic mid-latitude circulation. Moreover, in the last few years, it has become clear that orographic gravity wave drag (OGWD) and turbulent orographic form drag (TOFD) parameterization schemes play a crucial role in reducing some of the long-standing circulation biases affecting climate models. However, they are still considered a potential source of errors, due to the uncertainties which affect some poorly constrained physical parameters. Furthermore, these schemes need boundary conditions suitable to characterize the physical features of sub-grid orography. The strategies for the generation of such boundary conditions can vary a lot between different modelling centres, and it has been shown to be an important source of uncertainty. 

GLOBO is a global atmospheric general circulation model developed at the Institute for Atmospheric Science and Climate of the Italian National Research Council (ISAC-CNR). It is currently in use within many operational frameworks, including a global monthly probabilistic forecast system that contributes to the Subseasonal-to-seasonal (S2S) project database. In an effort to improve and modernize the model, we implemented a novel orographic drag parameterization package, based on state-of-the-art OGWD and TOFD schemes. Simultaneously with the development of the orographic drag parameterizations, we developed a novel software package, OROGLOBO (OROGraphic ancillary files generator for GLOBal atmospheric mOdels) designed for the generation of the orographic boundary conditions. This unique open-source tool is designed to exploit a state-of-the-art, high resolution global Digital Elevation Model to generate boundary conditions for OGWD and TOFD schemes, gathering the main algorithms and techniques available in the literature in a single software. 

Here, we present the results of this model update. A new set of retrospective forecasts was performed, consisting of an 8-members ensemble, initialized every 5 days and integrated for 35 days, during the period 2001-2020, including the developments in orographic physical parameterization and boundary conditions. This set of simulations is compared to the corresponding hindcasts set performed with the standard model configuration and used to calibrate the operational ensemble of global sub-seasonal probabilistic forecasts. We evaluate the impact of the improved representation of unresolved orographic drag on the simulation and prediction of the Northern Hemisphere mid-latitudes circulation. We assess the change in prediction skill for atmospheric blocking events and associated extreme temperature and wind conditions.