Linking extreme precipitation events and the associated moisture transport
- Autores: Danica Ciric
- Directores de la Tesis: Raquel Nieto Muñiz (dir. tes.), Luis Gimeno Presa (dir. tes.)
- Lectura: En la Universidade de Vigo ( España ) en 2019
- Idioma: inglés
- Tribunal Calificador de la Tesis: Srdjan Glisovic (presid.), Marta Vázquez Domínguez (secret.), Juan José Taboada Hidalgo (voc.)
- Programa de doctorado: Programa de Doctorado en Física Aplicada por la Universidad de A Coruña y la Universidad de Vigo
- Materias:
- Enlaces
- Tesis en acceso abierto en: Investigo
- Resumen
The research presented in this PhD dissertation aims to characterise the transport of moisture from the detected main moisture source regions using a Lagrangian perspective at the regional and global scales, and to link this transport with extreme precipitation events that have recently occurred around the world. The role of anomalous moisture transport for extreme rainfall events that occur due to the impact of climate change is important for current and future research related to the occurrence of intense precipitation, flood events, and accompanying risks in several regions.
This work consists of four published articles in scientific journals included in the SCI list, listed in “Chapter 4: Collection of Publications” of this PhD thesis, and the independent Chapter 5, which presents “A Global Atlas of Precipitation and Contribution of the Main Moisture Sources in the Peak Precipitation Month”. The published papers describe moisture transport at the regional scale in two related regions in Europe, the Danube River Basin, and the Mediterranean Basin.
The first studied region is the Danube River Basin (DRB), an important European catchment and the second largest in Europe by size. Three research activities were conducted in the DRB: i) detection of the main moisture sources for precipitation; ii) ranking the extreme precipitation events; and iii) investigation of the moisture contribution from the Mediterranean Sea to the extreme precipitation events detected.
The Mediterranean Basin was analysed next. It is one of the main oceanic moisture sources producing continental precipitation (Gimeno et al., 2010, 2012). Its effect on the mean and extreme precipitation on the surrounding continental areas, and the sink for its moisture, were examined.
Finally, in a worldwide study, the role of the major global moisture source regions in the occurrence of extreme precipitation over the continents was investigated. This work forms the basis of the Global Atlas of Precipitation and Contribution of the Main Moisture Sources in the Peak Precipitation Month.
Throughout this work, the Lagrangian model FLEXPART, v9.0, was used to analyse moisture transport for precipitation. The model requires a division of the atmosphere into approximately 2 million virtual air particles recorded every 6 hours, with constant mass and motion along the trajectories allowed by the three-dimensional wind fields inferred by the input model data, the ERA-Interim reanalysis. Following the particles’ trajectories in a backward mode, it is possible to detect regions where particles gain moisture along their pathway, thus identifying regions that are the sources of moisture. Tracking particles’ forward trajectories allows identification of regions where particles lose moisture, indicating the main moisture sink regions. To identify regions that are moisture sources and sinks, changes in specific humidity along the particles’ trajectories were tracked, and the fresh water balance was calculated as evaporation rate minus precipitation rate (E-P). Regions where evaporation exceeds precipitation (E-P > 0) are defined as moisture sources, whereas in moisture sink regions, precipitation exceeds evaporation (E-P < 0).
The main source regions were detected using this methodology by following the backward trajectories of air particles that ultimately reach the DRB. The results show that seven different moisture source regions are responsible for precipitation over the Danube basin: the Danube River Basin itself, the Mediterranean Sea, Caspian Sea, Black Sea, North Atlantic Ocean and two additional sources located over land areas, including part of North Africa, and continental parts of Central and Eastern Europe.
Once the climatological moisture sources for the DRB were identified, the next important step in the study was characterizing the extreme precipitation events (wet spells) over the region, and linking these with the moisture sources. Wet spell events of different durations (1, 3, 5, 7, and 10 days) were detected by applying a ranking methodology developed by Ramos et al. (2014; 2017). Daily precipitation data from the Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) during the period 1981 to 2015 was used. The ensuing ranking was used for two different analyses. The first one was based on the top-ranked 1-day extreme precipitation event, which occurred on 23 September 1996. The roles of the moisture sources for this event were investigated, and a complete synoptic analysis was done to better understand this extraordinary event. The larger second analysis took into consideration the top 100 extreme precipitation events across all time scales (1, 3, 5, 7, and 10 days). In this second analysis, moisture contribution from the Mediterranean Sea only, one of the most evaporative regions in the world, was considered. The analysis was done at the annual and seasonal scales.
The Mediterranean Sea was also investigated in terms of its contribution to climatological and extreme monthly precipitation over the surrounding continental areas. The monthly Multi-Source Weighted-Ensemble Precipitation (MSWEP) database was used to determine climatological and extreme precipitation over the region each month from 1980 to 2015, while the Mediterranean sea’s moisture contribution was computed via the FLEXPART Lagrangian model. In this case, the air particles were followed forward from the Mediterranean Sea to compute the moisture that generates precipitation over the continental areas. This study highlighted that the Mediterranean Sea, as a moisture source for precipitation, has a similar spatial pattern for monthly precipitation during both extreme and regular climatological conditions. Moreover, significant differences were recorded over the European continent; in some areas the Mediterranean Sea plays a significant role in extreme precipitation events, but is not an important source of moisture for climatological precipitation.
As mentioned above, in this thesis the characterization of extreme precipitation events started from a regional perspective (Danube River Basin and Mediterranean Sea) and then expanded to a global perspective. For this purpose, a Global Atlas of Precipitation and Contribution of the Main Moisture Sources in the Peak Precipitation Month was prepared. The Atlas connects the monthly provision of moisture by the main global moisture sources with precipitation during the peak precipitation month (PPM). The study was performed on a grid of 0.25° resolution in longitude and latitude. The forward mode was applied to the Lagrangian trajectories to compute precipitation over continental areas from each moisture source. This data was combined with the MSWEP monthly precipitation dataset to construct the Atlas, which consists of 26 types of maps. Fourteen of these maps are presented in Chapter 5; 12 more are located in Appendix A: Supplementary Material. The first map in the Atlas provides new, much-needed information about the contribution of moisture sources to precipitation on a monthly scale; previously, this information existed only at an annual scale. The following maps detail the PPM grid-by-grid, using MSWEP monthly precipitation data around the globe. The remaining maps aim to better characterize the moisture supply from the main sources during the PPM, both from a climatological point of view and during the occurrence of extreme precipitation events over each grid square.
