Browsing by Person "Milovac, Josipa"

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The added value of km-scale simulations to describe temperature over complex orography: the CORDEX FPS-Convection multi-model ensemble runs over the Alps
(2024) Soares, P. M. M.; Careto, J. A. M.; Cardoso, Rita M.; Goergen, Klaus; Katragkou, Eleni; Sobolowski, Stefan; Coppola, Erika; Ban, Nikolina; Belušić, Danijel; Berthou, Ségolène; Caillaud, Cécile; Dobler, Andreas; Hodnebrog, Øivind; Kartsios, Stergios; Lenderink, Geert; Lorenz, T.; Milovac, Josipa; Feldmann, Hendrik; Pichelli, Emanuela; Truhetz, Heimo; Demory, Marie Estelle; de Vries, Hylke; Warrach-Sagi, Kirsten; Keuler, Klaus; Raffa, Mario; Tölle, Merja; Sieck, Kevin; Bastin, Sophie
The increase in computational resources has enabled the emergence of multi-model ensembles of convection-permitting regional climate model (CPRCM) simulations at very high horizontal resolutions. An example is the CORDEX Flagship Pilot Study on “Convective phenomena at high resolution over Europe and the Mediterranean”, a set of kilometre-scale simulations over an extended Alpine domain. This first-of-its-kind multi-model ensemble, forced by the ERA-Interim reanalysis, can be considered a benchmark dataset. This study uses a recently proposed metric to determine the added value of all the available Flagship Pilot Study hindcast kilometre-scale simulations for maximum and minimum temperature. The analysis is performed using state-of-the-art gridded and station observations as ground truth. This approach directly assesses the added value between the high-resolution CPRCMs against their driving global simulations and coarser resolution RCM counterparts. Overall, models display some modest gains, but also considerable shortcomings are exhibited. In part, these deficiencies can be attributed to the assimilation of temperature observations into ERA-Interim. Although the gains for the use of kilometre-scale resolution for temperature are limited, the improvement of the spatial representation of local atmospheric circulations and land–atmosphere interactions can ultimately lead to gains, particularly in coastal areas.
The added value of simulated near-surface wind speed over the Alps from a km-scale multimodel ensemble
(2024) Molina, M. O.; Careto, J. M.; Gutiérrez, C.; Sánchez, E.; Goergen, K.; Sobolowski, S.; Coppola, E.; Pichelli, E.; Ban, N.; Belus̆ić, D.; Short, C.; Caillaud, C.; Dobler, A.; Hodnebrog, Ø.; Kartsios, S.; Lenderink, G.; de Vries, H.; Göktürk, O.; Milovac, Josipa; Feldmann, H.; Truhetz, H.; Demory, M. E.; Warrach-Sagi, Kirsten; Keuler, K.; Adinolfi, M.; Raffa, M.; Tölle, M.; Sieck, K.; Bastin, S.; Soares, P. M. M.
The advancement of computational resources has allowed researchers to run convection-permitting regional climate model (CPRCM) simulations. A pioneering effort promoting a multimodel ensemble of such simulations is the CORDEX Flagship Pilot Studies (FPS) on “Convective Phenomena over Europe and the Mediterranean” over an extended Alps region. In this study, the Distribution Added Value metric is used to determine the improvement of the representation of all available FPS hindcast simulations for the daily mean near-surface wind speed. The analysis is performed on normalized empirical probability distributions and considers station observation data as the reference. The use of a normalized metric allows for spatial comparison among the different regions (coast and inland), altitudes and seasons. This approach permits a direct assessment of the added value between the CPRCM simulations against their global driving reanalysis (ERA-Interim) and respective coarser resolution regional model counterparts. In general, the results show that CPRCMs add value to their global driving reanalysis or forcing regional model, due to better-resolved topography or through better representation of ocean-land contrasts. However, the nature and magnitude of the improvement in the wind speed representation vary depending on the model, the season, the altitude, or the region. Among seasons, the improvement is usually larger in summer than winter. CPRCMs generally display gains at low and medium-range altitudes. In addition, despite some shortcomings in comparison to ERA-Interim, which can be attributed to the assimilation of wind observations on the coast, the CPRCMs outperform the coarser regional climate models, both along the coast and inland.
Studies of soil-vegetation-atmosphere feedback processes with WRF on the convection permitting scale
(2017) Milovac, Josipa; Wulfmeyer, Volker
Land system models which can incorporate land-atmosphere and human-environment interactions are vital for reliable climate projections in heterogeneous agricultural landscapes. At resolutions fine enough to resolve detailed land use, models need a sophisticated representation of planetary boundary layer (PBL) and land surface processes in order to predict changes in key quantities like precipitation or temperatures. Assessment of turbulence schemes and land surface models (LSM) is fundamental therefore not only to advance model development, but also to understand important phenomena like feedbacks within the soil-vegetation-atmosphere (SVA) continuum. Up until now however, a lack of appropriate observations has impeded any comprehensive assessments. Here, through comparisons with so far unique profile measurements, the study investigates the impact of using different PBL schemes and LSMs, and explores how SVA feedbacks are simulated by the model. Using the Weather Research and Forecasting (WRF) model, a six member ensemble was run, at a convection permitting resolution, with varying combinations of LSMs (NOAH and NOAH-MP) and PBL schemes (two local and two non-local approaches). The analysis was performed for two case studies – a dry and a convective weather situation – in three different locations in Germany. During the dry case, key convective PBL (CBL) features were analysed, and the simulations were compared with high resolution water vapour differential absorption lidar measurements. For the convective case, the focus was on exploring the model representation of the pre-convective environment and the ensuing convection and precipitation. In both cases, the nature of the simulated SVA feedback processes was assessed through an innovative “mixing diagram” approach. Results show that the nonlocal PBL schemes produce a drier and higher CBL than the local schemes. These results are sensitive to parameters calculated in the surface layer schemes, which are themselves often paired with PBL schemes. Furthermore, the NOAH‑MP LSM produces drier atmospheric conditions than NOAH, with a difference in mixing ratio profiles ranging up to 1.4 gkg-1. These variations are more pronounced in the upper CBL than close to the ground. The mixing diagrams indicate that these deviations are mainly related to entrainment fluxes. In the dry case, NOAH-MP’s dry air entrainment is up to 6 times higher than with NOAH, while in the convective case the difference is not as pronounced (up to 1.5 higher with NOAH-MP). This suggests that the difference in the simulation of the CBL between the two LSMs is strongly linked to the surface energy partitioning – the higher the Bowen ratio, the greater the difference between the LSMs. Thus, WRF appears to be more sensitive to the choice of LSM at higher Bowen ratios. NOAH and NOAH-MP exhibit marked differences in representing atmospheric variables such as moisture. Those differences are not constrained to the lower atmosphere close to the land surface, but extended to the lower troposphere. The variations in free tropospheric moisture between the LSMs strongly affects the nature of the simulated convection, and associated precipitation. The degree of sensitivity of the spatial variability and amount of the precipitation with respect to the selection of LSM and PBL scheme shows a strong dependence on the analysed region. A distinct finding of this thesis is the greater sensitivity of WRF with respect to the PBL development to the selection of the LSM, than to the PBL scheme. Furthermore, the impact of this sensitivity is not constrained to the lower CBL, but extends up to the interfacial layer and the lower troposphere - for both dry and convective weather conditions. On the other hand, it is clear that the simulated coupling strength between the land surface and atmosphere is very sensitive to the surface Bowen ratio. The synergies between high resolution measurements and model simulations, with an advanced representation of the land surface processes, will facilitate not only further development of parameterization schemes, but also an improvement in our understanding of land-atmosphere interactions.