Atmospheric moisture transport and river runoff in the mid-latitudes

Research output: Thesisinternal PhD, WU


Hydrometeorological extremes such as floods and droughts are the result of anomalous transport of moisture in the atmosphere, impacting the land. Recent examples of these impacts are the devastating floods in south Louisiana (United States) in 2016 and drought over western Europe in 2018. In this thesis we enhance our understanding of the water cycle and its extremes in present and future climate. More specifically, transport of moisture in the atmosphere is studied and linked to precipitation and river runoff. This is done from a global modelling perspective, making use of reanalysis data, simulations from a global climate model, a moisture tracking tool, and a global hydrological model (all described in Chapter 2). Both hydrometeorological events as well as yearly averages and seasonal cycles are studied for three regions in the mid-latitudes; the Mississippi river basin in North America, and the Rhine river basin and Norway in Europe.

In Chapter 3 we investigate atmospheric moisture transport and its relation to extreme precipitation over coastal Norway. We show a climatology (1979-2014) of cold season precipitation events based on the 99th percentile of daily precipitation from ERA-Interim reanalysis data, of which more than 85% is related to a so-called atmospheric river, an anomalous moisture flux. Characteristic patterns resulting in this anomalous moisture transport can be identified with significant time before the event (5 days), which is helpful to forecasting. However, the identification of large-scale patterns 5 days before the event is insufficient to predict the precise location of subsequent extreme precipitation.

In Chapter 4 we explore precipitation and moisture transport over the Mississippi River basin under present (2002-2006) and future climate conditions (2094-2098; RCP4.5) using simulations from the global climate model EC-Earth. More specifically, we determine the moisture sources of the Mississippi basin, by tracking precipitation falling over the basin backward in time using the Eulerian tracking model WAM-2layers. We find that the most important continental moisture sources of the Mississippi basin are evaporation from the basin itself, and the area southwest of it, while the most relevant oceanic sources are the Gulf of Mexico/Caribbean and the Pacific. Of course, those sources vary per season. We conclude that the moisture sources of the Mississippi River basin in the future 1) enhance over the oceans in winter, resulting in more future winter precipitation, and 2) show a relative decline over terrestrial areas in summer, indicating that land surface properties will have relatively less impact on precipitation over the Mississippi River basin in the future.

In Chapter 5 we move over the Atlantic back to Europe, where the extremely dry summers of 2003 and 2018 are studied with the ERA5 reanalysis dataset. Normally, evaporation over the Atlantic contributes to about half of the precipitation falling in the Rhine basin during summer. However, during the dry summers of 2003 and 2018 persistent blocking avoided moisture transport from the oceans towards the Rhine basin. Although this was both the case in 2003 and 2018, the normalized moisture sources in those years appear to be quite different, due to the slight different locations of the blocking systems. The large-scale circulation in 2018 was especially favorable for dry conditions over the Rhine, while in 2003 we find that local moisture recycling decreases because of the drying out of soils. The unique character of both extreme events indicate that hydrometeorological extremes should be investigated individually to enhance our understanding of these extremes, and there complex interaction from the larger-scale to the land-surface.

To study the global hydrological cycle and its response to climate change, we rely on global climate models and global hydrological models. In Chapter 6 we assess and compare the benefits of an increased resolution of a global climate model (GCM; EC-Earth) and global hydrological model (GHM; W3RA) for the Rhine and Mississippi River basins. Increasing the resolution of a GCM (1.125° to 0.25°) results in an improved precipitation budget over the Rhine basin, attributed to a more realistic large-scale circulation. These improvements with increased resolution are not found for the Mississippi basin, possibly because precipitation is strongly dependent on the representation of still unresolved convective processes. The (improved) monthly-averaged precipitation from the GCM is reflected in (improved) monthly-averaged actual evaporation and discharge from the GHM, although an increase in resolution in the GHM does not lead to significant changes in discharge. A straightforward resolution increase in the GHM is thus most likely not the best method to improve discharge predictions, which emphasizes the need for better representation of processes and improved parameterizations that go hand in hand with resolution increase in a GHM.

This work contributed to a better understanding of the water cycle and its extremes in the mid-latitudes, from a modelling perspective. To further enhance our knowledge on atmospheric moisture transport and the impact on land, combined efforts from climate science and the fields of meteorology and hydrology are needed.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • Hazeleger, W., Promotor
  • Weerts, Albrecht, Promotor
  • van Heerwaarden, Chiel, Co-promotor
Award date20 Nov 2020
Place of PublicationWageningen
Print ISBNs9789463955010
Publication statusPublished - 2020


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