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Tropical-extratropical interactions and extreme precipitation events in the Middle East


DeVries,  Andries J.
Atmospheric Chemistry, Max Planck Institute for Chemistry, Max Planck Society;

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DeVries, A. J. (2018). Tropical-extratropical interactions and extreme precipitation events in the Middle East. PhD Thesis, Universität, Mainz.

Cite as: https://hdl.handle.net/21.11116/0000-0003-1280-7
Extreme precipitation events in the arid Middle East can cause flooding with dramatic socioeconomic impacts. At the same time, these events can replenish fresh water resources that are of crucial importance for agriculture and ecosystems. Apart from Israel, such events in the region are largely unexplored in the literature. In this thesis, we present (1) a qualitative concept of an important weather phenomenon that is associated with the most severe flash floods in the Levant, (2) a multiple-perspective analysis and quantification of the larger-scale dynamics of three extreme precipitation cases in Saudi Arabia in different seasons, and (3) an identification method for extreme precipitation events in the Middle East based on synoptic-scale meteorological features. Our main finding is that extreme precipitation events in the Middle East, as in other dry subtropical regions, often result from tropical-extratropical interaction.

The Active Red Sea Trough (ARST) phenomenon is explained as a type of tropical-extratropical interaction whereby a transient midlatitude upper-level trough interacts with the semi permanent surface trough over the Red Sea. Enhanced moisture transport occurs predominantly over the Arabian and Red Seas through an intensified Arabian anticyclone, resulting in an intrusion of tropical moist air masses into the Levant. The devastating flooding of Jeddah, Saudi Arabia, in November 2009 showed the same dynamical characteristics as those of twelve ARST events that affected the Levant, implying that the phenomenon can affect a much larger part of the Middle East than previously assumed.

Three cases of extreme precipitation in Saudi Arabia in autumn (the Jeddah flooding of November 2009), winter, and spring, developed in a tropospheric environment controlled by the larger-scale circulation. Anticyclonic Rossby wave breaking resulted in an intrusion of a midlatitude upper-level trough into low latitudes that interacted with the tropical circulation. Eulerian and Lagrangian analyses revealed moisture transport from nearby and remote tropical regions, leading to above-normal tropospheric moisture content over Saudi Arabia. Upward motion was associated with orographic lifting of moist air masses (all cases), low-level moisture convergence and reduced static stability (autumn case), dynamical lifting and diabatic heating (winter case), and strong surface sensible heating (spring case), favoring the build-up and release of potential instability. The three cases exhibited unusual to extreme (>2 to >4 standard deviations) synoptic characteristics within the varying background large-scale circulation of the different seasons.

Building on these case studies, we developed an identification method for extreme precipitation events in the Middle East that combines stratospheric potential vorticity (PV) intrusions and structures of high poleward vertically integrated water vapor transport (IVT). These meteorological features represent tropical-extratropical interaction, and have previously been postulated as large-scale precursors of extreme precipitation, but had thus far never been combined in an algorithm for its detection. Combined PV and IVT features are intimately connected to the extreme precipitation intensity and seasonality of the region. The farther south a stratospheric PV intrusion reaches, the larger the IVT magnitude, and the longer their combined duration, the more extreme the precipitation. Importantly, IVT is a potentially more pertinent predictor of extreme precipitation than PV. Tropical-extratropical interactions contribute to a large fraction of the annual rainfall amounts (40-70%) and extreme precipitation days (50-90%) at many sites in the arid parts of the Levant and the Arabian Peninsula. This thesis contributes to a better understanding of the larger-scale atmospheric processes that drive extreme precipitation events in the Middle East and provides a tool for their identification that can support weather prediction and early warning systems as well as future studies on their mesoscale and climatological aspects.