Development of the Mesoscale Model GRAMM-SCI: Evaluation of Simulated Highly-Resolved Flow Fields in an Alpine and Pre-Alpine Region

Abstract

In this study, new developments implemented in the mesoscale model GRAMM-SCI are presented. GRAMM-SCI has been specifically developed for providing flow fields in the sub-kilometer range. A comprehensive model evaluation using wind, temperature, radiation, as well as soil moisture and soil temperature observations in an alpine valley and in a hilly pre-alpine region in Styria (Austria) is presented. Three one-way nested model domains were used, whereby the coarse model run (5000 m horizontal resolution) was initialized and forced using ERA5 reanalysis data. The grid sizes for the two inner domains were set to 1000 m and 200 m, respectively. Comparisons were carried out for a five-day period in October 2017 which was dominated by clear-sky conditions. Though, the observations reveal quite complex flow structures governed by interactions between synoptic flow and thermally-driven local flows, GRAMM-SCI was able to reproduce the main features satisfactorily. In addition, the new version of GRAMM-SCI shows significant improvement with regard to simulated air temperature compared with the previous one. Finally, microscale flow-field simulations were carried out for some monitoring sites that are apparently influenced by nearby buildings or vegetation.

The performance of GRAMM-SCI and WRF in simulating the surface-energy budget and thermally driven winds in an Alpine valley

“…Overall, most model evaluation studies are restricted to readily available quantities such as specific humidity, radiation, wind, and temperature (Tomasi et al 2017;Jiménez-Esteve et al 2018). A number of studies have evaluated the performance of mesoscale models in simulating flow fields in the Alps (Giovannini et al 2014;Gsella et al 2014;Cantelli et al 2017;Schmidli et al 2018;Schlager et al 2019;Oettl 2021a), but comparisons with observations of all surface-energy fluxes are still scarce in the scientific literature (one exception being Sun et al 2017), especially in mountainous terrain and with respect to small-scale spatial variations. The above studies have highlighted some challenges in simulating the near-surface atmosphere over mountainous terrain.…”

“…The performance of GRAMM-SCI and WRF for an Alpine valley is crucial to resolve small-scale wind structures (Jiménez-Esteve et al 2018;Schmidli et al 2018;Schlager et al 2019;Oettl 2021a), processes impacting the surface-energy budget components, such as topographic shading (Schlager et al 2019), surface albedo (Tomasi et al 2017), and land-use class (Jiménez-Esteve et al 2018), have equally been identified as strongly impacting the modeled near-surface temperature and wind fields. Understanding model deficiencies in reproducing observed surface radiation, sensible, and latent heat fluxes over mountainous terrain is thus paramount to further improve numerical simulations.…”

“…Finally, a discussion and conclusions are given in Section 5. (Oettl 2021a). Since 2019, GRAMM has been further developed by the Air Quality Control unit of Styria, Austria, to use ERA5 reanalysis data 1 for initialization and for prescribing transient boundary conditions.…”

“…The model description and previous evaluation studies for Alpine regions have been published by Oettl (2020Oettl ( , 2021a. In the following, model details and parameterizations relevant for this study will be briefly outlined.…”

The Performance of GRAMM-SCI and WRF in Simulating the Surface-Energy Budget and Thermally Driven Winds in an Alpine Valley

“… 2018 ; Schlager et al. 2019 ; Oettl 2021 ), but comparisons with observations of all surface-energy fluxes are still scarce in the scientific literature (one exception being Sun et al. 2017 ), especially in mountainous terrain and with respect to small-scale spatial variations.…”