Impact of Soil Moisture-Atmosphere Interactions on the Development of Extreme Weather in the Western Mediterranean

The Western Mediterranean (WMed) region is frequently affected by weather extremes such as heat waves and droughts in summer and heavy precipitation events (HPEs) and flash floods mostly in autumn. Both dry and wet extremes lead to damages which cost billions of euros and frequently cause human fatalities. The projected climate change indicates a considerable increase in the frequency and intensity of such extremes in this region. Therefore, a better prediction of these phenomena is essential in order to develop strategies for the prevention and the adaptation of the Mediterranean society and infrastructure to such hazards in a changing climate. The WMed is a transitional region between dry subtropical and wet mid-latitude climates, where soil moisture (SM) is a crucial component controlling the partitioning of surface heat fluxes over land. This region is prone to extreme weather events due to its complex physiographic terrain which is characterised by strong ocean-land-atmosphere interactions. Due to its long-term memory SM variability affects the near-surface conditions that can favour extreme events on different time scales. However, the contribution of SM-atmosphere interactions to the initiation and evolution of extremes in the complex terrain of the WMed is still not fully understood due to the lack of adequate SM observations and the inaccurate modelling representation of SM feedbacks with the atmosphere. Thus, the aim of this thesis is to assess the relevance of SM-atmosphere interactions for the development of extremes in the WMed and to gain better process understanding about SM feedbacks. A multi-scale modelling approach using the model of the Consortium for Small-scale Modeling (COSMO) was carried out at spatial scales of convection-permitting (∼3 km) and convection-parametrised (∼7 km) resolutions and at temporal scales ranging from hours up to several months. This approach encompasses the most relevant scales of the involved processes. The analysis is divided into three main parts. In the first part, the autumn period 2012 was selected to analyse wet extremes. For this period the Special Observation Period 1 (SOP1) campaign of the HYdrological cycle in the Mediterranean EXperiment (HyMeX) provides unprecedented observational data sets with a focus on wet extremes. In the second part, state-of-the-art 1 km satellite-derived surface soil moisture (SSM) observations from the Soil Moisture and Ocean Salinity (SMOS) mission were used for the first time to initialise the COSMO model in order to improve the initial SM status. Extreme heat waves in the summer of 2003 and 2015 were investigated in the third part of the analysis. All seasonal simulations were performed with a convection-permitting resolution over large domains containing the entire WMed and for the study of dry periods extending to continental Europe. Various experiments were conducted with physically-based extreme values of initial SM to investigate the sensitivity of the atmospheric conditions on extreme SM conditions. In autumn 2012, the representation of precipitation timing and of intensity in the WMed corresponds well between the seasonal convection-permitting simulation and the observations. In this respect, the representation of the higher resolution is more accurate than in the coarser convection-parametrised simulation. In this period, wet and dry SM initial conditions in the COSMO model induce a positive SM-atmosphere coupling with seasonal mean changes in near-surface heat fluxes, atmospheric moisture and instability. In the wet initial SM scenario the modified atmospheric conditions cause a positive SMprecipitation feedback with an increase in mean moist convection and low-cloud cover which leads to an increase in seasonal mean precipitation of about +25 % in semi-arid regions. Depending on the regional climate conditions the effects of the initial SM conditions on extreme precipitation varies. The wet initial SM scenario leads to an increase in the frequency and intensity of daily extreme precipitation events in semi-arid regions while the dry initial SM scenario causes higher hourly precipitation rates and an increase in extreme precipitation in moderately humid regions. Four HPEs were selected during autumn 2012 to test the realistic initialisation with 1 km SMOS-L4 3.0 SSM data. For this purpose, a SMOS-L4 SM profile product was created by applying a cumulative density function (CDF)-matching bias correction and an exponential filter. The implementation of this product for SM initialisation in COSMO improves the representation of all selected HPEs with an optimal time of realistic SMOS initialisation of 3 days in advance. The dry summer seasons in 2003 and 2015 reveal a strong response of seasonal mean atmospheric conditions to the modified initial SM in spring with feedback mechanisms on temperature, precipitation and the large-scale conditions. The reduction of initial SM in the whole investigation domain results in a seasonal mean increase in maximum temperature of about +2.5 °C in the Iberian Peninsula (IP) subdomain and about +4 °C in the Central Europe (ME) subdomain. The temperature increase is double the existing observed temperature anomaly in both years. At the same time, the seasonal mean decrease in precipitation is about 40 % in both regions. The extent of the temperature and precipitation differences originating from remote sources reaches 30 % and 50 %, respectively. Analyses of dry extreme indices demonstrate that the SM-atmosphere interactions induced by a dry SM initialisation in spring lead to an increase in the number of hot days and the heat wave duration by a factor of 3 in the ME subdomain and by a factor of 1.5 in the IP subdomain. Contrariwise, wet SM spring conditions reduce the number of hot days and the duration of heat waves by about 50 %. This thesis demonstrates the potential of combining state-of-the-art observations and high-resolution modelling to improve the understanding of SM-atmosphere interactions and feedbacks in the WMed. The importance of local and remote SM-atmosphere interactions for the development of heat waves and HPEs in the WMed and continental Europe is highlighted in seasonal convection-permitting simulations. The presented high-resolution realistic SM initialisation and modelling are expected to have real applicability for future weather to climate modelling in order to improve the prediction uncertainties of extremes in the WMed and Europe.