Summary This thesis addresses the role of the Mediterranean Sea in the regional Mediterranean climate. It investigates the impact of fine-scale air-sea interactions and feedback mechanisms on sea surface heat fluxes, wind speed, and on potentially threatening Mediterranean cyclones (medicanes and Vb-cyclone). Additionally, it examinest the effect of horizontal atmospheric grid resolution on sea surface heat fluxes, wind speed, and medicanes. For this purpose, a new atmosphere-ocean regional climate model (AORCM) has been developed that includes a regional atmospheric climate model, COSMO-CLM (CCLM) and models for the Mediterranean Sea (MED) and/or the North- and Baltic Seas (NORDIC). Such a high-resolution coupled system allows for a better representation of the orography and detailed SST patterns. Several experiments were performed using CCLM in uncoupled and coupled configuration in the context of this study. Both the coupled and uncoupled models were able to reproduce the main characteristics of the Mediterranean climate. Addressing the research questions highlighted in Chapter 1, the new developed AORCM leads to the following conclusions. 1. Investigation of the sea surface heat fluxes and wind speed over the Mediterranean Sea: In investigating the impact of horizontal atmospheric grid resolution (0.08˚ ~ 9 km vs. 0.44˚ ~ 50 km) and ocean coupling on sea surface heat fluxes and wind speed at sub-daily, seasonal, and annual timescales over the Mediterranean Sea, the following conclusions were drawn: ▪ Fine-grid simulations gave a better representation of winds, particularly near the oastline and in the western Mediterranean Sea than coarse-grid simulations. Consequently, the fine atmospheric grid had a positive impact on turbulent fluxes. ▪ In addition, ocean coupling improved the turbulent fluxes compared to the uncoupled model. ▪ Radiative fluxes are slightly better represented in the coarse-grid simulations, due to the better representation of cloud cover. However, estimates of net heat flux (NH) are more realistic in the fine-grid coupled simulations. ▪ Furthermore, the coupled model simulates the sub-daily SST variations that are missing in ERA-Interim reanalysis. The results show that the sub-daily variations are of minor importance for the mean atmospheric state in the Mediterranean region. 2. Investigation of medicanes In investigating the impact of horizontal atmospheric grid resolution (0.44˚ ~ 50 km vs. 0.22˚ ~ 25 km, vs. 09 km) and ocean coupling on medicane tracks and intensity, the following conclusions were drawn: ▪ Model performance depends strongly on the horizontal atmospheric grid resolution. Although the large-scale disturbance is reasonably well simulated at all three resolutions, medicane signals were not detected at the 0.44˚ resolution. ▪ The mean sea level pressure and warm core structure were reasonably well captured in the 0.22˚ simulations, but wind speed was strongly underestimated. These features were more profound and refined in the 0.08 coupled and uncoupled simulations. In comparison to the uncoupled simulations, the coupled simulations did not show any significate improvement at the 0.44˚ and 0.22˚ resolution. ▪ At the 0.22˚ resolution, medicane tracks were almost identical in both the coupled and uncoupled simulations. However, at 0.08 resolution medicane track, warm core structure, and wind speed were largely improved in the coupled simulations compared to the uncoupled simulations. ▪ In the majority of the cases, medicane tracks in the coupled simulations with 0.08˚ resolution were longer and closer to the observed ones. These results suggest that a 0.08˚ grid resolution produced more detailed features of medicanes, particularly with the coupled model. ▪ The spectral nudging technique increased the accuracy in time and location of simulated medicanes. However, no improvement was observed in the mean sea level pressure and wind speed estimates. ▪ Similar to the simulations performed without spectral nudging, in the nudged simulations, medicane tracks and wind speeds in the coupled simulations were better represented compared to the atmosphere-only simulations. ▪ The intensities of the latent and sensible heat fluxes increased with ocean coupling and increasing atmospheric grid resolution. However, spectral nudging did not show any significant impact on the latent and sensible heat fluxes. Thus, the results suggest that the intensity of medicanes is strongly linked with surface heat fluxes and fine-scale features at the air-sea interface. 3. Investigation of Vb-cyclones In analyzing the impact of ocean coupling over the Mediterranean and North- and Baltic Seas on the trajectories and intensity of Vb-cyclones at a 0.22˚ (~25 km) atmospheric grid resolution and the performance of the AORCM which incorporates the coupling of two European marginal seas (MED+NORDIC), the following conclusions were drawn: ▪ The mean seasonal and annual SSTs simulated in MED+NORDIC were similar to MED for the Mediterranean Sea and to NORDIC for the North- and Baltic Sea. ▪ The annual mean values and trends of SST in the Mediterranean and the North- and Baltic Seas compared well with observations. ▪ Compared to uncoupled simulations or observations, SSTs simulated using the coupled configurations showed biases (1 ˚C) over the coupling regions, especially during winter and summer. ▪ The coupled model configurations were stable and robust, and the model’s SST uncertainties were promisingly small given the observational uncertainties in the marginal seas. ▪ In general, all model configurations were able to reproduce the trajectories of Vbcyclones and their main features, such as core geopotential heights. However, in most cases, the position, intensity, and life cycle of Vb-cyclones simulated in the coupled models were as close as or even closer than cyclone characteristics derived from reanalysis datasets than those from the uncoupled model. ▪ The positions of the precipitation maxima agreed well with the corresponding cyclone trajectories. ▪ The evaluation of simulated precipitation fields revealed that all models were able to capture intense precipitation patterns, but the patterns peaked and shifted differently. In general, the simulated precipitation underestimated EOBS precipitation data, possibly caused by the coarse model grid-spacing of 25 km, which does not sufficiently resolve orographic features and underestimates heavy convection in heavy Vb precipitation events. ▪ Additionally, coupling only the North- and Baltic Seas also showed a benefit in simulating Vb-cyclones, especially in the July 1997 case (also shown in Ho-Hagemann et al. 2015). This advantage is linked to the contribution of different moisture sources for each individual case (cf. Gimeno et al. 2010; Volosciuk et al. 2016; Messmer et al. 2017) Final remarks The models analyzed here are able to reproduce the main characteristics of the physical processes involved in the Mediterranean Sea surface heat fluxes, wind speed, medicanes, and Vb-cyclones. It was found that the coupled model together with fine atmospheric grid resolution (< 10 km) improved the representation of sea surface heat fluxes, wind speed, and medicanes. Regarding the simulations of Vb-cyclones, ocean coupling had an overall positive, although with a strong case-by-case variation, impact on the cyclone trajectories and intensity due to the variation in moisture source for each event. The ability of the AORCM to simulate its own highly resolved SST makes it more beneficial than the uncoupled atmospheric models. This study is a first step towards the use of new more complex modeling system. Further developments are required to overcome the current deficiencies in climate simulations.