Atmos. Chem. Phys., 18, 18169–18186, 2018 https://doi.org/10.5194/acp-18-18169-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Characteristics and evolution of diurnal foehn events in the Dead Sea valley Jutta Vüllers1, Georg J. Mayr2, Ulrich Corsmeier1, and Christoph Kottmeier1 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany 2Department of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria Correspondence: Jutta Vüllers ([email protected]) Received: 18 May 2018 – Discussion started: 9 August 2018 Revised: 7 November 2018 – Accepted: 5 December 2018 – Published: 21 December 2018 Abstract. This paper investigates frequently occurring foehn 1 Introduction in the Dead Sea valley. For the first time, sophisticated, high- resolution measurements were performed to investigate the In mountainous terrain the atmospheric boundary layer, and horizontal and vertical flow field. In up to 72 % of the days thus the living conditions in these regions, are governed by in summer, foehn was observed at the eastern slope of the processes of different scales. Under fair weather conditions, Judean Mountains around sunset. Furthermore, the results the atmospheric boundary layer (ABL) in a valley is often also revealed that in approximately 10 % of the cases the decoupled from the large-scale flow by a strong tempera- foehn detached from the slope and only affected elevated ture inversion (Whiteman, 2000). In this case mainly local layers of the valley atmosphere. Lidar measurements showed convection and thermally driven wind systems, which are that there are two main types of foehn. Type I has a duration caused by differential heating of adjacent air masses, such of approximately 2–3 h and a mean maximum velocity of as slope and valley winds, determine the valley ABL (e.g. 5.5 m s−1 and does not propagate far into the valley, whereas Atkinson, 1981; Zardi and Whiteman, 2013). They influence type II affects the whole valley, as it propagates across the the diurnal temperature and humidity cycle (e.g. Alpert et al., valley to the eastern side. Type II reaches mean maximum 1997; Bischoff-Gauß et al., 2008) and also determine aerosol wind velocities of 11 m s−1 and has a duration of about 4–5 h. dispersion and, thus, air quality (e.g. Kalthoff et al., 2000; A case study of a type II foehn shows that foehn is initiated Corsmeier et al., 2005; Fast et al., 2006). Mesoscale pro- by the horizontal temperature gradient across the mountain cesses, such as topographic and advective venting, accom- range. In the investigated case this was caused by an ampli- pany the thermally driven flows under fair weather conditions fied heating and delayed cooling of the valley boundary layer (e.g. Fast and Zhong, 1998; Adler and Kalthoff, 2014). When in the afternoon, compared to the upstream boundary layer the large-scale flow is not negligible it also impacts the valley over the mountain ridge. The foehn was further intensified ABL. The interaction takes place via turbulent transport or by the advection of cool maritime air masses upstream over dynamically driven flow phenomena, which occur when the the coastal plains, leading to a transition of subcritical to su- large-scale flow is affected by the orography. Gravity waves, percritical flow conditions downstream and the formation of wave breaking, downslope windstorms, hydraulic jumps and a hydraulic jump and rotor beneath. These foehn events are rotors can occur on the lee side of the mountains. The inten- of particular importance for the local climatic conditions, as sity and extent of the developing phenomena depend on the they modify the temperature and humidity fields in the valley shape of the mountains, the stratification of the atmosphere, and, furthermore, they are important because they enhance the strength of the valley ABL inversion, the wind speed, evaporation from the Dead Sea and influence the aerosol dis- and the direction of the large-scale flow (Whiteman, 2000). tribution in the valley. Stratified flow theory as well as hydraulic flow theory were both used successfully to explain the aforementioned phe- nomena (e.g. Schär and Smith, 1993; Corsmeier et al., 2005; Published by Copernicus Publications on behalf of the European Geosciences Union. 18170 J. Vüllers et al.: Characteristics and evolution of foehn at the Dead Sea Durran, 2003; Jackson et al., 2013). Field campaigns were The foehn events influence the lake evaporation, as evapora- performed to gather observations of these phenomena along tion is driven by wind velocity and vapour pressure deficit, large mountain barriers (Alps, Pyrenees) during field cam- as shown by Metzger et al.(2018). The diurnal maximum of paigns such as ALPEX (Davies and Pichler, 1990), PYREX evaporation is reached, untypically, shortly after sunset when (Bougeault et al., 1990), and MAP (Bougeault et al., 2001) the foehn sets in (Metzger et al., 2018). Finally, the foehn and also for individual valleys, e.g. T-REX (Grubišic et al., events can also cause an air mass exchange and remove the 2008), MATERHORN (Fernando et al., 2015), or craters like aerosol particles and the often-occurring haze layer from the in METCRAX (Whiteman et al., 2008; Lehner et al., 2016). valley, improving air quality (Levin et al., 2005; Holla et al., This paper contributes to this research by investigating a fre- 2015). quently occurring mesoscale flow phenomena which influ- Even though these foehn events apparently have such a ences weather and climate in the Dead Sea (DS) valley. large influence on the atmospheric conditions at the DS, a de- The Dead Sea with a water level of currently −430 m tailed analysis of their three-dimensional structure and their above mean sea level (a.m.s.l.) forms the lowest part of the characteristics, such as height, duration, and intensity, and Jordan Rift Valley, which is an over 700 km long north–south further insights regarding their evolution are missing. Hence, oriented depression zone extending from the northern Israeli the following questions are addressed in this study. (i) Is a border to the Gulf of Aqaba. The complex and steep orogra- differentiation between radiation and density-driven downs- phy, together with the land surface heterogeneity in the val- lope flows possible? (ii) What are the typical characteristics ley, introduced by the lake, results in a strong local forcing of the foehn events? (iii) What are the key mechanisms dur- and triggers pronounced thermally driven wind systems, such ing the foehn evolution? as a lake breeze, slope, and valley winds (Kottmeier et al., For the analysis the first sophisticated high-resolution li- 2016). Additionally, regional forcing influences the atmo- dar measurements of foehn events in the DS valley, along spheric processes in the valley, resulting in a distinct diurnal with long-term near-surface observations, were used. The wind pattern. In particular, strong westerly downslope winds measurements were performed in the framework of the in- are observed frequently in the evening. Ashbel and Brooks terdisciplinary virtual institute DEad SEa Research VEnue (1939) first described the westerly winds in the northern part (DESERVE) (Kottmeier et al., 2016). In the following sec- of the DS as the Mediterranean Sea breeze (MSB) entering tion, Sect.2, a short geographical overview, information on the valley, but not with the typical characteristics of a sea the measurement sites and the instrumentation as well as the breeze at the coast, i.e. a steady, cool, and moist air flow; they applied methods is presented. Section 3.1 presents results of rather described it as a very dry, hot, and gusty wind. Follow- an objective occurrence frequency analysis, characteristics of ing Ashbel and Brooks(1939) various observational near- the foehn events are shown in Sect. 3.2, and a detailed case surface studies (e.g. Bitan, 1974, 1976; Lensky and Dayan, study of a strong foehn event and the processes leading to 2012; Naor et al., 2017) and also numerical studies (e.g. it are presented in Sect. 3.3. Section4 provides a summary, Doron and Neumann, 1978; Alpert et al., 1982; Segal et al., including a conceptual model of the processes, and conclu- 1983, 1985) have been carried out to study the penetration of sions. the MSB into the Jordan Rift Valley. Studies showed that the downward penetration of the MSB results from the tempera- ture difference between the cooler maritime air mass and the 2 Methodology warmer valley air mass. A density-driven flow, which accel- 2.1 Study area erates and warms while descending into the valley (Alpert et al., 1990), i.e. a foehn wind, when following the defi- The DS is the lowest reachable place on earth with a current nition of the WMO (WMO, 1992). At the DS they occur water level of −430 m a.m.s.l. The valley is north–south ori- most frequently in summer and enter the Jordan Rift Valley ented, with the Judean Mountains to the west, with a mean first in the north around Lake Kinneret and at approximately ridge height of about 895 m a.m.s.l., and the Moab Mountains D C 18:00 LT (LT UTC 2) at the DS (Bitan, 1974, 1976; Se- to the east, which reach up to 1200 m a.m.s.l. (Fig.1a). The gal et al., 1985; Naor et al., 2017). These foehn events have a cross section in Fig.1b illustrates the steep orography at both large impact on the atmospheric conditions in the DS valley. sides of the DS, extending over 1600 m in the vertical. The −1 Mean hourly wind velocities of 5 m s in the south and up DS is about 100 km long and 15 to 17 km wide.