Synoptic-Scale Control over Modern Rainfall and Flood Patterns in the Levant Drylands with Implications for Past Climates

Abstract

Rainfall in the Levant drylands is scarce but can potentially generate high-magnitude flash floods. Rainstorms are caused by distinct synoptic-scale circulation patterns: Mediterranean cyclone (MC), active Red Sea trough (ARST), and subtropical jet stream (STJ) disturbances, also termed tropical plumes (TPs). The unique spatiotemporal characteristics of rainstorms and floods for each circulation pattern were identified. Meteorological reanalyses, quantitative precipitation estimates from weather radars, hydrological data, and indicators of geomorphic changes from remote sensing imagery were used to characterize the chain of hydrometeorological processes leading to distinct flood patterns in the region. Significant differences in the hydrometeorology of these three flood-producing synoptic systems were identified: MC storms draw moisture from the Mediterranean and generate moderate rainfall in the northern part of the region. ARST and TP storms transfer large amounts of moisture from the south, which is converted to rainfall in the hyperarid southernmost parts of the Levant. ARST rainfall is local and intense, whereas TP rainfall is widespread and prolonged due to high precipitation efficiency and large-scale forcing. Thus, TP rainfall generates high-magnitude floods in the largest catchments; integration of numerous basins leads to sediment feeding from the south into the Dead Sea, exhibited in large sediment plumes. Anecdotal observations of the channel with the largest catchment in the region (Nahal HaArava) indicate that TP floods account for noticeable geomorphic changes in the channel. It provides insights into past intervals of increased flash flood frequency characterized by episodes of marked hydrogeomorphic work; such an increase is especially expected during intervals of southerly situated and southwesterly oriented STJs.

Characterising patterns of heavy precipitation events in the eastern Mediterranean using a weather radar and convection-permitting WRF simulations

“…While MCs move inland and towards the desert, a substantial amount of the moisture is lost, and rainfall occurrence and amounts are greatly reduced (Enzel et al, 2008). In this arid region HPEs are associated not only to MCs (Armon et al, 2018;Kahana et al, 2002), but also to the Active Red Sea Trough (ARST) (Ashbel, 1938;Krichak et al, 1997;De Vries et al, 2013) and, more rarely, to Tropical Plumes (Armon et al, 2018;Rubin et al, 2007;Tubi et al, 2017). Commonly, rainfall during ARSTs is of spotty nature, could reach far into the desert, and could have very high intensities (Armon et al, 2018;Sharon, 1972).…”

“…Conversely, during Tropical Plumes, rainfall is widespread and can simultaneously cover most of the region with moderate intensities. Desert HPEs frequently result in large and sometimes devastating flash floods (e.g., Armon et al, 2018;Dayan and Morin, 2006;Farhan and Anbar, 2014;Kahana et al, 2002;Saaroni et al, 2014;Seager et al, 2014).…”

“…The abovementioned TP synoptic pattern (Rubin et al, 2007;Tubi et al, 2017) is not a part of our classification because of the low frequency and because they do not appear in a near sea level pressure meteorological fields. Specifically, one HPE (#41; Table S1) was characterised, during its 5-day duration, first by the prevalence of TP (Armon et al, 2018) and then by MC; it was classified here as a MC. Despite the simplification, these two classes were recently shown to exhibit distinct characteristics of the rainfall intensity distribution (Marra et al, 2019).…”

“…Areal rainfall amounts are crucial drivers of the hydrologic response and are important for understanding rainfall structure and triggering mechanisms (e.g., Armon et al, 2018;Durrans et al, 2002;Kalma and Franks, 2003;Zepeda-Arce et al, 2000). To quantify and compare observed and simulated areal rainfall amounts, we used depth-area-duration (DAD) curves, which represent the areal extent in which given rainfall depths over given durations are exceeded (Zepeda-Arce et al, 2000).…”

Past, Present and Future Climate Trends Under Varied Representative Concentration Pathways for a Sub-Humid Region in Uganda

“…Dryland climates have generally received attention in terms of studies owing to the debilitating effects of extreme events especially droughts [16,17] and flash floods [18,19]. These studies to a large extent have focused on the historical to present climate patterns at a regional level.…”