Atmospheric conditions of extreme precipitation events in western Turkey for the period 2006 – 2015

This paper investigates the precipitation types and background physical mechanisms of extreme precipitation events (EPEs) over western Turkey during the period 2006– 2015. The EPEs are described as the precipitation values above the 90th percentile obtained from the hourly precipitation dataset, which has high spatial resolution. Precipitation types associated with EPEs are identified by using radar outputs and the Lamb weather type (LWT) approach. It is found that EPEs occurred more frequently in the Marmara and Aegean regions during autumn and winter months. In Marmara, mainly 21 %, 17 %, and 15 % of total autumn EPEs show convective, cyclonic, and sea-effect precipitation characteristics, respectively. While convective EPEs are seen more commonly in the southern portions, cyclonic and sea-effect-originated EPEs mainly affect the southwest and northeastern parts of Marmara. Among these three precipitation types, convective mechanisms generally produce more intense daily precipitation (66.1 mm on average) in the Marmara Region under the proper synoptic conditions (highpressure center over the Balkan Peninsula and low-pressure center over the eastern Mediterranean). Based on the hourly observations, convective types of extreme precipitation (EP) show two peak values during afternoon and evening times of the day and are linked to diurnal heating. In terms of the Aegean Region, cyclone-originated EP, which includes 65 % of the total winter EPEs, is more common in the whole territory and reaches its peak value during the early hours of the day.


Introduction
The occurrence of extreme precipitation events (EPEs) and background physical mechanisms triggering these episodes become a fundamental issue in the last decade due to its great impacts on agriculture, health, energy, and tourism. From this perspective, many researchers firstly identified the EPEs by applying fixed (e.g. Brooks and Stensrud 2000;Ralph and Dettinger 2012;Hitchens et al. 2012Hitchens et al. , 2013 or percentile 10 based precipitation thresholds (e.g. Piccarreta et al., 2013;Krichak et al., 2014) to the daily precipitation. In the later studies, the main atmospheric systems that cause extreme precipitations (EPs) are investigated in detail by focusing the role of large-scale (e.g., Madden-Julian Oscillation (MJO), ENSO, PDO) (Jones, 2000;Higgins et al. 2000;Deflorio et al. 2013). or the synoptic scale circulations for the selected regions in US 15 (Schumacher and Johnson 2006;Warner et al. 2012;Moore et al. 2015). Afterwards, the characteristics of the EPEs are defined by using radar (Moore et al. 2015), outgoing longwave radiation (Carvalho et al., 2002) or horizontal temperature advection data (Milrad et al., 2010).
Owing to the spatial complexity, rugged topography, and land-sea interactions of 20 the Mediterranean Basin, many devastating flash floods occurred in the various part of the region in the last decade. Therefore, researchers have analyzed the atmospheric conditions that cause these extraordinary events by focusing on the selected flood days (e.g., Ferretti et al. 2000;Nuissier et al. 2008;Pastor et al. 2010). Only a few researchers analyzed the climatological and general synoptic behaviors of the EPEs for this large territory (Ricard et al. 2011;Reale and Lionello 2013).
Turkey is located at the east Mediterranean and EPEs there, in general, cause sudden flash floods resulting with deaths and economic losses in infrastructure and agriculture. As a result of the EPEs in the last decade, numerous flash floods and 5 landslides occurred in some particular regions of Turkey. During September 2009, Ayamama creek in Istanbul (NW of Turkey, most populated city in Europe) was overflowed as the consequence of the dense daily precipitation episodes, which produced more than 250 mm rainfall over 3-day, and 32 people died together with millions of dollars of economic losses (Kömüşçü and Çelik 2013). During 9 October 10 2011, 238 mm rainfall total was measured during an 6 hour time-period at the province of Antalya (south of Turkey) and damaged the infrastructure of the tourism center of the country (Demirtaş, 2016). During August 2015, torrential rainfalls ended up with a devestating landslide in Hopa district (NE Turkey, sloppy domain of the country) and 11 people died during this natural hazard (Baltaci, 2017). Turkey and its sub-basins are 15 mainly influenced by these EPEs in all seasons in the variety of the atmospheric conditions such as baroclinic waves and cyclones, mesoscale convective systems, landsea interactions and orographic forcing.
In literature, numerous studies investigated the influence of large-scale circulation patterns or synoptic weather types on precipitation mechanism over Turkey 20 and its sub-regions (Karabörk and Kahya 2003;Karabörk et al. 2005;Unal et al. 2012;Baltacı et al. 2015Baltacı et al. , 2017. Only a limited number of these studies explored the atmospheric conditions that caused extreme precipitation over Turkey for a set of selected episodes (Kömüşçü et al. 1998;Kömüşçü and Çelik 2013;Demirtaş 2016).
Although a number of prior studies have focused on the synoptic characteristics of the EPEs ending up with life or economic losses over Turkey, environmental characteristics of these EPEs and underlying causes were not studied in detail. To overcome this deficit, we identified the types of the EPs, which are taken from ten-year (2006-2015) high-resolution precipitation datasets, by using the Lamb Weather Type (LWT) 5 approach (Fig. 1a) and radar outputs in the western Turkey. Therefore, the goal of this study is to document the spatio-temporal and environmental characteristics of the EPEs, and investigate the synoptic-scale patterns associated with EPEs.
In Section 2, description of the precipitation characteristics of EPEs, along with the data and methods used, are described. Results of the EPEs and related discussion are 10 presented in Section 3. The last part, Section 4, is devoted to the summary and conclusions.

Precipitation dataset
Values of meteorological parameters in Turkey had been recorded manually 15 from the late 1920s to the beginning of the 21 st century. After the year 2003, starting from the western regions, existing meteorological stations were replaced by automatic ones (Automatic Weather Observing Systems, AWOS) and also virgin land was covered with AWOS stations by the support obtained from large projects. These projects can be explained in four parts as follows: 20 1) AWOS 206: Excessive rainfalls on May 21- 25,1998, which also triggered landslides, resulted in many flash floods over the western Black Sea region of Turkey. In order to eliminate damages originated from floods, TEFER project (Turkey Emergency Flood and Earthquake Recovery) was introduced that was financially supported by International Public Works and Development Bank 3) AWOS 246 and 350: To expand the spatial density of the meteorological stations, 246 and 350 AWOS stations have been started to be used in the following years. By the 246 stations, new districts were aimed to be 15 meteorologically covered, which did not have any active meteorological stations before (blue stars in Fig. 1b). Later, due to high topographical differences of the country, 350 new automated meteorology stations were mainly located at the higher elevation points and started to operate since 2016 (black squares in Fig.   1b). 20 In our study, for the first time, we aimed to obtain the atmospheric conditions of EPEs in Turkey with high resolution and coverage. Therefore, we have chosen long-term hourly precipitation dataset of AWOS stations. For this reason, hourly precipitation records of 206 AWOS stations were selected for the investigation for the environmental characteristics of EPEs. Firstly, daily total precipitation amounts (00-24 UTC) were calculated from hourly precipitation records. Quality controls of data were done by RCLIMDEX method which was explained by Zhang and Yang (2004) and Baltacı et al. (2018). The years having more than 10 % missing data days and stations that are subjected to relocation were eliminated from the study. As a consequence of the quality 5 control and assurance of precipitation data in the period 2006-2015, we selected 97 stations densely located in the west Turkey (Fig. 1c). From 97, 51 stations are found to be located in the Marmara (NW Turkey, pink points), and 46 to be located in the Aegean (W Turkey, light brown points) regions of Turkey.

10
The Marmara Region is located in the northwest of Turkey, between latitudes 29°N and 32°N and longitudes 38°E and 42°E and covers an area of 67000 km 2 .
Marmara has different climatic characteristics in itself. While inland areas have temperate continental climate, milder climate of places on the Black Sea coast resembles more of an oceanic climate, typical to other areas of Turkish Black Sea coast. 15 The coasts in Marmara and Aegean parts have Mediterranean (Med) climate and this region is second smallest Turkish region in size after Southeastern Anatolia. Only south and east parts of the region are more mountainous.
In terms of Aegean, this region has a Med climate with mean annual precipitation changing from 450 to 1200 mm yr -1 (Asikoglu and Benzeden 2014). 20 Although climatic behavior of the Aegean is similar to the Med climate, it is shown obvious differences in landscape. Unlike the more parallel mountains found along the Med, Aegean mountains often cut directly into the sea.

Radar data over Turkey
First meteorology radar over Turkey was installed in Ankara for nowcasting purposes during the year of 2000 (Fig. 2). Afterwards, Istanbul, Zonguldak, and Balikesir radars were installed during 2003 by the TEFER project. Later, to detect EPEs that can be effective over Mediterranean and Black Sea, another six C band radars were setup to Izmir, Mugla, Antalya, Hatay, Samsun, and Trabzon cities during 2007. Due to 5 the forecast difficulties of convective precipitation by the numerical weather prediction models, another four C band radars (Bursa, Afyon, Karaman, and Gaziantep) were

Lamb Weather Type (LWT) methodology
The subjective version of the Lamb's work (Lamb 1972) was firstly developed as an objective version by Jenkinson and Collison (1977) and refined by Jones et al. (1993) to indicate the circulation types (CTs) influencing British Isles. According to the objective methodology, vorticity and directions of the geostrophic flows are calculated 20 using sea level pressure (SLP) fields over a predetermined central point. As a consequence of the six parameters and certain thresholds for the defined region, totally 27 different CTs were defined (16 hybrid, 8 directional, cyclonic, anticyclonic, and unclassified types). In this study, we used daily mean SLP values on 16 grid points (between 5°W-55°E and 30°N-60°N, Figure 1a), centred over Marmara Region and separated by 5° from each other. The six parameters, namely the westerly flow (WF), southerly flow (SF), resultant flow (FF), westerly shear vorticity (WSV), southerly shear vorticity (SSV), are computed as follows: where p i is the daily mean SLP at grid point i ( Figure 1a). Finally, classification of CTs is done according to the following criteria: • Directional types (N, NE, E, SE, S, SW, W, NW) is found by tan -1 (WF/SF), adding 180° to the final value if WF is positive. 45° is allocated for each sector.
• If |Z|<FF, CT is one of the eight pure directional types listed above 15 • If |Z|>2FF, the CT is either Cyclonic or Anticyclonic • If FF<|Z|<2FF, the CT is one of the 16 hybrid types: a combination of directional and vorticity types.
• If |Z| or FF<6, then the CT is 'unclassified' For the Aegean region, the equations based on six circulation parameters for Marmara were also recalculated by using different 16 grid points (centred over Aegean) and coefficients (due to latitudinal difference).

Identification of EPEs and precipitation characteristics
An extreme precipitation event is generally defined as a daily amount exceeding 5 a certain threshold (e.g. Brooks and Stensrud 2000;Ralph and Dettinger 2012;Hitchens et al. 2012Hitchens et al. , 2013. For example, Karl et al. (1996)  Marmara and E and SE types for Aegean were chosen. In terms of cyclonic (C) EPEs, low-pressure center over Marmara and Aegean was selected as cyclonic CT in accordance to LWT methodology. 5 The physical mechanisms behind the EPs were investigated by using NCEP/NCAR Reanalysis products (Kalnay et al. 1996)

Spatial variation of EPEs in the west Turkey
For the first time, different daily precipitation threshold limits of 97 stations 15 were constructed from a 10-year dataset (Fig. 3a). According to the results, highest daily precipitation rates exceeding 100 mm are observed on the southern Aegean region where it can be classified as 'rich' in terms of extreme amounts of precipitation. This suggests that if the daily precipitation amount of a station located in the south Aegean exceed this limit, that day is recorded as an EPE for that station. Daily precipitation 20 threshold ranging from 60 to 100 mm is shown to be mainly located on the coastal regions of the west Turkey. When one move towards interior continental areas, daily EP threshold decrease from 60 to 20 mm level. The lowest limits are observed in the semiarid continental areas of the Aegean and Marmara region as having threshold value lower than 40 mm, as illustrated with blue color in Fig. 3a and can be classified as 'poor' in terms of extreme amounts of precipitation.
The annual contribution of EPs for each station (cumulative totals of EPs for each station divided by 10) is shown in Fig. 3b. We observe that the largest normalized annual amounts of EPEs is located mainly on the southwest of Aegean, middle-south 5 and northeast of the Marmara region with values larger than 60 mm. It is interesting to see that the interior continental areas of the Aegean and Marmara region that was characterized as poor in terms of extreme amounts of precipitation (Fig. 3a), now exhibit a better picture in their normalized value as generally having a better value between 40-60 mm. The reason of this can be the convective precipitation, generating 10 intensified rain that can accumulate higher amounts of precipitation during a single rainstorm. On the other hand, western regions of Marmara that exhibited considerably larger threshold value with precipitation totals larger than 60 mm (Fig. 3a), show a worse image with the normalized values as having precipitation values between 40 to 60 mm. 15 As an example on October 28, 2010 intense daily rainfalls and associated many flash floods occurred on the western parts of Turkey. As a result of this extraordinary event, daily precipitation amounts exceeded 70 mm in the Bandirma province (south seaside station of Marmara Sea, in Fig. 4). During this day, daily precipitation totals exceeding 50 mm is shown in yellow color on Fig. 4 that is extending from coastal 20 Aegean region towards Marmara as an enlarging region and reaching up to Black Sea passing over Gulf of Izmit and Silivri. This squall line affected majority of the Marmara region and to a lesser extends the Aegean region. However, many stations located outside of this critical yellow region also had rainfall totals above their extreme daily precipitation limits.
Seasonal distribution of the EPE frequencies can provide important information to understand the physical mechanisms forcing these extreme events. For this reason, we analyzed total counts of EPEs for four seasons and the results are depicted in Fig. 5. 5 It can be stated from Fig. 5 that winter (DJF) and autumn (SON) are more significant than the other seasons (Figs. 5a, d). During winter, two cores over Aegean result in more than 6 extreme precipitation days (Fig. 5a). Spring is mainly characterized as having EPEs between 2 to 4 days on the eastern portions of the Aegean region (Fig. 5b).
During summer, highest count of the EPEs with 3 days is shown to be located over the 10 Black Sea effected areas of the Marmara region (Fig. 5c). Seasonally, second highest frequency of EPEs can be found in the autumn. In this season, an area extending from northeast to south of Marmara receive a frequency considerably higher than 6 days (Fig.   5d). From this point of view, a detailed analysis of the atmospheric systems generating EPEs and effecting Aegean region mainly during winter and Marmara region during 15 autumn becomes important. Next section is focusing on this aim.

Regional features of the seasonal EPEs
In this section, we carried out frequency analysis of the seasonal EP events for Marmara region and documented the results in

Precipitation characteristics of EPEs over Marmara with its
background synoptic-scale atmospheric conditions 10 As discussed in the previous section, we mainly focused on the months of autumn to analyze the spatial distribution of daily mean precipitation, to determine the counts of station based EPEs in the Marmara region and investigate the synoptic-scale atmospheric conditions responsible from the development of these extreme precipitation events. In this respect, 2006-2015 period autumn mean precipitation values, counts of 15 EPEs and their associated average weather maps are illustrated in Figure 6.
During cyclonic CTs, highest daily mean precipitation amounts exceeding 8 mm are shown to exist on the southwestern parts of the region. Similarly, the count of EPEs is higher on this portion of the Marmara region (Fig. 6a). When the synoptic composite maps are analyzed, one can see the low-pressure center that probably came from west 20 (Karaca et al. 2000) and located over Aegean Sea and west of Marmara. Sea surface temperature varies between 19 and 20 °C and temperature in the low level of the atmosphere (approx.. 1.5 km high from the ground) is shown to be between 7.5 and 10 °C (Fig. 6b).
During NE types, north and northeast part of Marmara gets higher daily mean precipitation amounts (between 6 and 8 mm in Fig. 6c). Totally 28 extreme precipitation cases in the northeastern stations exceed their threshold levels at this part of the region.
It is shown from the previous studies that primary factor for the formation and intensity of sea-effect precipitation is known to be the temperature difference between sea 5 surface and the air at 850 hPa level (Holroyd, 1971;Niziol, 1987;Steenburgh et al., 2000). If the SST-T 850 difference becomes higher, the chance of precipitation increases due to higher convective instability. Millan et al. (1995) argued that enhanced evaporation resulting from temperature differences between European continental air and the relative warm Mediterranean Sea in fall can become a key factor in determining 10 the onset of precipitation. Pastor et al. (2015) have shown that regions of high heat/moisture air-sea exchange over the Mediterranean Basin are prone to enhancing convection, leading to torrential rain. At the later study, Baltacı et al. (2015) mainly emphasized that 13 °C temperature difference between the sea surface and the 850-hPa level can cause above-normal precipitation records at the northeast of Marmara. Our 15 results indicate that as a consequence of the combination of a high-pressure center (HPC) located over eastern Europe and a low-pressure center (LPC) over southern Turkey, strong northeasterly flows can be generated owing to high pressure gradient force bringing significant amounts of moisture from the relatively warm Black Sea (21 °C) to the northeast of Marmara. The temperature difference between SST and 850-hPa 20 level exceeds 13 °C threshold level and this increase the strength of instability conditions (Fig. 6d).
As explained by Ricard et al. (2012), the orographical properties of the area induce mesoscale convergence and lift of the low-level conditionally unstable flow.
Most active regions for deep convection in Mediterranean Basin (MB) are the Alps, the western Croatian coast, the south of France and the wider area of Tunisia (Dayan et al., 2015). Alhammoud et al. (2014) found a maximum frequency of deep convection over MB in September-October and a minimum one in June and July. Similar to the previous studies for MB (e.g. Funatsu et al., 2009;Melani et al., 2013;Alhammoud et al., 2014), 5 convective EP events over Marmara are mostly shown during autumn season. Although daily mean precipitation amounts are lower (between 0 and 2 mm) in the convective (E) type, extreme cases are seen more common in the southern part of the region (Fig. 6e).
Mountainous area (Mt. Uludag over 2500 m high)) of Marmara is located in this part and due to the interaction between HPC over eastern Europe and LPC over western 10 Turkey, strong easterly flows coming from flat land areas meet with highland barriers producing higher amounts of orographic enforced convective EPs, if the atmospheric condition such as temperature exchange (will be explained in Section 3.6) is suitable (Fig. 6f).
As an example, convective activity in southern Marmara started afternoon times 15 on 28 September 2015 (Fig. 7). Due to the movements of the single cells to the easterly directions their spatial area expanded. When the cells met with orographic barrier over Bursa (Mt. Uludag, red star in Fig. 7) quasi-stationary conditions developed the convective instability. As a result, extreme precipitation amounts were recorded in the western part of the mountainside in a very short time.

Precipitation characteristics of EPEs over Aegean with its background synoptic-scale atmospheric conditions
As mentioned above, winter months are more important for extreme precipitation events over Aegean. As previously explained by Ulbrich and Christoph (1999), high-pressure conditions in southeastern Europe tend to divert the Mediterranean storm track southwards, resulting in increased precipitation in the eastern Mediterranean. As a consequence the positioning on cyclones over Aegean in this season, more daily precipitation amounts occur in the southern corner of the region (above 14 mm) and we observe higher EP cases close to the coastal stations (Fig. 8a). In 5 addition, during appropriate synoptic conditions, cyclonic activity can result in intense rainstorms at the majority of the stations of Aegean region, especially at those located in the south. When compared with cyclonic CT over Marmara, more deepened LPC is located over Aegean Sea. In this case, cold air aloft coming from north can meet with the relatively warm Aegean Sea and the convergence of warm air above the cold air can 10 generate cyclogenesis which can result in heavy precipitation (Fig. 8b).

Interannual and hourly variation of EPEs
Annual distribution of the total counts of EP days together with their precipitation characteristics was also analyzed for Marmara and Aegean regions during autumn and winter months and depicted in Fig. 9. In terms of Marmara, cyclonic CTs belonging to the autumn and winter months (Fig. 10). In Marmara, highest daily mean extreme precipitation is shown to occur under the convective types, and followed by Black-Sea effected and cyclonic CTs. During convective activity, we showed a peak during afternoon hours of the day. Main reason of this event can be the diurnal heating 10 and this is further investigated in the next section. For the Black Sea effected EPs, we observe an hourly peak of the precipitation close to noontime and this suggests that when maximum solar radiation reaches the sea surface, significant amount of moisture and heat are transferred by northerly flows to Marmara, generating a considerable amount of precipitation (Baltacı et al. 2015(Baltacı et al. , 2017. During the cyclonic CT, the region 15 takes dense hourly precipitation at the mid-afternoon of the day. In regard to Aegean in winter, cyclones generally release dense precipitation potentials from night to noon times.

Relationship between extreme daily precipitation and surface
temperature 20 From the previous studies, it can be said that the link between precipitation intensity and temperature was explained by Clausius-Clapeyron (C-C) relation. C-C relation presents the moisture-holding capacity of the atmosphere to temperature, hinting a roughly 7% increase in atmospheric moisture storage per degree Celcius. Pall et al. (2007) found a high agreement between the C-C relation and the changes in the rainfall extremes at midlatitudes. Lenderink and van Meijgaard (2008) found for Netherlands that changes in hourly and daily precipitation intensity generally increased at the 7% °C -1 rate anticipated by the C-C at temperatures below 10 °C, but that hourly precipitation exhibited a "super C-C" relation (increase greater than 7% °C -1 ). Later, 5 Lenderink and van Meijgaard (2009) considered that stronger updrafts due to greater latent heat release are the main physical mechanism in the formation of the super C-C relationships. Haerter and Berg (2009) suggested that super C-C scaling may be prevalent in regions that have a relatively balanced co-existence of both convective and large-scale rainfall events. 10 In this part, to examine C-C relation for the convective EPs, we extracted 10 daily mean temperature records and extreme hourly precipitation records of the extreme precipitation days for the selected south (Bursa) and east (Kocaeli) stations of Marmara in autumn (Table 3). It was shown that hourly extreme precipitation is more linked to daily mean temperature (r=0.69, statistically significant at 95% confidence level) in the 15 south of Marmara under the proper synoptic conditions and daily mean temperature changes from 12.2 to 18.4 °C.
The characteristics of the EPEs in each region were analyzed objectively using Lamb Weather Type (LWT) methodology and radar products. Physical mechanisms behind • At Aegean region, 61% of the total EPs occur from the cyclonic activity during winter and torrential rainfall is found to be experienced at the majority of the stations, especially those located in the south. This condition can be explained by cold air transfer from north that meet with the relatively warm Aegean Sea and thus, convergence of warm air above the cold air generates cyclogenesis which results in heavy precipitation.
We conclude by noting that the methods and the results of the current study can serve as a basis for future research related to EPEs in the western Turkey and elsewhere. The 5 methods applied to identify EPEs can be adopted for use in other geographical regions in Turkey.