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In order to improve the quality of the Mediterranean high-impact weather (HIW) numerical predictions, this study proposes to modify the potential vorticity (PV) field of the model initial state, taking advantage of information provided by the water vapour (WV) channel of the METEOSAT-7 satellite. The implemented PV field modifications aim to reduce the mismatch between the upper-level PV features and the WV brightness temperatures guided by the known relation between these two fields (PV-WV technique). The PV-WV technique effectiveness is evaluated on two HIW events, and is also compared with two additional PV modification techniques from an earlier study. The chosen episodes occurred on 9-10 June 2000 and 9-10 October 2002 and produced heavy precipitation over both Spain and France. The main difference between these two episodes is found in the driving mechanism, a mesoscale cyclone for the June 2000 event and a larger low-pressure centre for the October 2002 case. The two additional PV modification techniques introduce perturbations along the zones highlighted by the MM5 adjoint model calculated sensitivity zones (PV-adjoint) and along the three-dimensional PV structure presenting the locally most intense values and gradients of the field (PV-gradient). A close examination of both case studies of the forecast rainfall fields and several objective verification indices show that the PV-WV technique performance exceeds the control (or non-perturbed) forecast skill while remaining inside the distribution obtained by both PV-gradient and PV-adjoint techniques. This PV-WV technique could be used to increase the ensemble spread introducing higher amplitude modifications. Thus, a more skilled ensemble prediction system could be built by taking advantage of the subjectivity inherent to this method (manual perturbations) and also of the uncertainty present in the initial state. [ABSTRACT FROM AUTHOR]

Flash floods are common in small Mediterranean watersheds and the alerts provided by real-time monitoring systems provide too short anticipation times to warn the population. In this context, there is a strong need to develop flood forecasting systems in particular for developing countries such as Morocco where floods have severe socio-economic impacts. In this study, the AROME (Application of Research to Operations at Mesoscale), ALADIN (Aire Limited Dynamic Adaptation International Development) and WRF (Weather Research and Forecasting) meteorological models are evaluated to forecast flood events in the Rheraya and Ourika basin located in the High-Atlas Mountains of Morocco. The model evaluation is performed by comparing for a set of flood events the observed and simulated probabilities of exceedances for different precipitation thresholds. In addition, two different flood forecasting approaches are compared: the first one relies on the coupling of meteorological forecasts with a hydrological model and the second one is a based on a linear relationship between event rainfall, antecedent soil moisture and runoff. Three different soil moisture products (in-situ measurements, European Space Agency's Climate Change Initiative ESA-CCI remote sensing data and ERA5 reanalysis) are compared to estimate the initial soil moisture conditions before flood events for both methods. Results showed that the WRF and AROME models better simulate precipitation amounts compared to ALADIN, indicating the added value of convection-permitting models. The regression-based flood forecasting method outperforms the hydrological model-based approach, and the maximum discharge is better reproduced when using the WRF forecasts in combination with ERA5. These results provide insights to implement robust flood forecasting approaches in the context of data scarcity that could be valuable for developing countries such as Morocco and other North African countries. [ABSTRACT FROM AUTHOR]

The Maritime Continent is a major active convective area and one of the most challenging regions in terms of atmospheric modelling. Rainfall in the Maritime Continent is driven by both large-scale phenomena such as the Madden-Julian Oscillation and ENSO, and fine-scale processes such as land-sea breeze and tropical convection. In combination, they produce very particular precipitation regimes that models consistently fail to reproduce across spatial resolutions, regardless of their dynamic formulation. Climate simulations at convection-permitting scales are now becoming possible. Rather than simply increasing spatial resolution, these experiments constitute a conceptual advance since they no longer rely on many of the assumptions of convective schemes. Here, we use the Weather Research and Forecasting model operating at multiple resolutions (32, 16, 8, 4 and 2km) to determine the role of the spatial resolution and the representation of convection on the realism of precipitation in the Maritime Continent. We show that the key aspect to better simulate precipitation in the region is the explicit representation of deep convection, while the benefit of increasing spatial resolution is limited. Both explicit convection and parameterized shallow convection produce precipitation at the right time of the day because they do not transform CAPE as readily as the deep convection scheme. Thus, not only precipitation but also other processes linked to convective circulation such as land-sea breeze are better represented when deep convection is explicitly resolved. In spite of generating a diurnal cycle that better matches the observations, there are features that remain a challenge even at the kilometre scale, such as the land/ocean distribution of total precipitation amounts. Explicitly resolving deep convection has implications for the lower atmosphere mixing, specially over the ocean, where the model does not generate enough energy to trigger convection and produce rainfall. As such, the microphysics and the planetary boundary layer take on particular importance in the challenge of simulating tropical convective precipitation in very high-resolution experiments. [ABSTRACT FROM AUTHOR]