Margarida Belo-Pereira

Understanding climate change projections for precipitation over western Europe with a weather typing approach

Precipitation over western Europe (WE) is projected to increase (decrease) roughly northward (equatorward) of 50°N during the 21st century. These changes are generally attributed to alterations in the regional large-scale circulation, e.g., jet stream, cyclone activity, and blocking frequencies. A novel weather typing within the sector (30°W–10°E, 25–70°N) is used for a more comprehensive dynamical interpretation of precipitation changes. A k-means clustering on daily mean sea level pressure was undertaken for ERA-Interim reanalysis (1979–2014). Eight weather types are identified: S1, S2, S3 (summertime types), W1, W2, W3 (wintertime types), B1, and B2 (blocking-like types). Their distinctive dynamical characteristics allow identifying the main large-scale precipitation-driving mechanisms. Simulations with 22 Coupled Model Intercomparison Project 5 models for recent climate conditions show biases in reproducing the observed seasonality of weather types. In particular, an overestimation of weather type frequencies associated with zonal airflow is identified. Considering projections following the (Representative Concentration Pathways) RCP8.5 scenario over 2071–2100, the frequencies of the three driest types (S1, B2, and W3) are projected to increase (mainly S1, +4%) in detriment of the rainiest types, particularly W1 (−3%). These changes explain most of the precipitation projections over WE. However, a weather type-independent background signal is identified (increase/decrease in precipitation over northern/southern WE), suggesting modifications in precipitation-generating processes and/or model inability to accurately simulate these processes. Despite these caveats in the precipitation scenarios for WE, which must be duly taken into account, our approach permits a better understanding of the projected trends for precipitation over WE.

Adjustment of the summertime marine atmospheric boundary layer to the western Iberia coastal morphology

During summer (June, July, and August), northerly winds driven by the Azores anticyclone are prevalent over western Iberia. The Quick Scatterometer Satellite 2000 to 2009 summertime estimates reveal a broad high wind speed (≥7 ms−1) area extending about 300 km from shore and along the entire Iberian west coast. Nested in this large high-speed region, preferred maximum regions anchored in the Iberian major capes, Finisterre, Roca, and S. Vicente, are found. Composite analyses of wind maxima were performed to diagnose the typical summertime synoptic-scale pressure distribution associated with these smaller size high-speed regions. The flow low-level structure was further studied with a mesoscale numerical prediction model for three northerly events characterized by typical summertime synoptic conditions. A low-level coastal jet, setting the background conditions to the marine atmospheric boundary layer (MABL) response to topography, was found in the three cases. The causes for wind maximum downwind capes were investigated, focusing on the hypothesis that western Iberia MABL responds to hydraulic forcing. For the three events supercritical and transcritical flow conditions were identified and expansion fan signatures were found downwind each cape. Aircraft measurements, performed during one of the events, gave additional evidence of the expansion fan leeward Cape Roca. The importance of other forcing mechanisms was also assessed by considering the hypothesis of downslope wind acceleration and found to be in direct conflict with soundings and surface observations.

Simulations of wave conditions on an open beach configuration: wind resolution, seaward forcing and whitecapping effects

In the present study, the SWAN model was employed to estimate the wave field on an open sandy beach at the west coast of Portugal. It is the purpose of this work to evaluate the sensitivity of nearshore wave propagation modelling to (i) the wind field resolution, (ii) the type of information used at the offshore boundary to drive the simulations, and (iii) the whitecapping formulation used by the wave model. Wave measurements from an acoustic Doppler current profiler (ADCP) sensor moored in 17 m depth (datum), and operating during a rather stormy period, were used to model comparisons. The comparison showed that no significant differences in the results were found between the two wind field resolutions tested and also that an improvement was observed when the model of van der Westhuysen et al. (2007) is used for the whitecapping dissipation, instead of the default formulation by Komen et al. (1984). Dans la présente étude, le modèle SWAN a été utilisé pour estimer le champ des vagues sur une côte sableuse de la côte Ouest du Portugal ouverte sur l’Océan Atlantique. Cette étude vise à évaluer la sensibilité de la modélisation de la propagation des vagues en zone côtière (i) à la résolution du champ de vent, (ii) au type d’information utilisé pour imposer les conditions sur la frontière maritime et (iii) à le formulation utilisé dans le modèle pour la dissipation due au moutonnement. Les mesures des vagues ont été faites avec un capteur ADCP mouillé par 17 m de profondeur, exploité pendant une période englobant deux épisodes de tempête. La comparaison a permis de montrer qu’il n’y a pas de différences significatives entre les deux résolutions du champ de vent testées, et également qu’une amélioration est observée lorsqu’ est utilisé le modèle van der Westhuysen et al. (2007) pour la dissipation par moutonnement à la place de la formulation par défaut de Komen et al. (1984).