The effect of atmospheric rivers on cold-season heavy precipitation events in Iran

Atmospheric rivers (ARs) as massive and concentrated water vapour paths can have a critical impact on extreme events in arid and semi-arid areas. This study investigated the effect of ARs on heavy precipitation events during the cold, rainy months (November–April) in Iran for 11 years. The results showed that 107 ARs had an influence on heavy precipitation, which providing partial moisture for Iran’s precipitation. On average, 11 heavy precipitation days were linked to the presence of ARs in the six cold months of each year. During the study period, ARs accounted for almost 20–50% of the country’s total heavy precipitation monthly. Although most ARs entered the country from the south through coastal areas, the western part of Iran, especially elevated stations along the western slope of the Zagros Mountains, received the highest heavy precipitation. Accordingly, about 66% of ARs directly originated from the Red Sea and the Gulf of Aden. Moreover, December experienced the highest frequency of ARs linked to heavy precipitation during the statistical period. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wcc.2020.259 om http://iwaponline.com/jwcc/article-pdf/12/2/596/865818/jwc0120596.pdf er 2021 Neda Esfandiari Hassan Lashkari (corresponding author) Department of Physical Geography, School of Earth Science, The University of Shahid Beheshti (SBU), Tehran, Iran E-mail: h-lashkari@sbu.ac.ir


INTRODUCTION
Iran is an arid and semi-arid country with limited water resources. Despite this limitation, water demand is growing every day due to the large waste of water for drinking and agricultural uses. Therefore, managing the use of such scarce water resources makes the issue even more important. It should be noted that heavy precipitation occurs annually in different parts of the country, influencing many sectors including water resources, environment, agriculture, energy, health and financial security. Moreover, increasing climate variability and change has augmented the risk of such natural disasters (Van Aalst ), so that in recent years the number of these incidents has multiplied in Iran. Forecasting Iran's climatic condition also shows a rising occurrence in flood events across the country, as To meet these challenges, the government needs to be equipped with excellent operating infrastructure and warning systems. Flood alert for risk management is thus one of the most effective non-structural methods of flood management (Rezaee & Shakur ). Although it is possible to predict floods after the beginning of precipitation according to hydrological practices, there is not usually enough time for preparation operations and flood damage is in any case unavoidable. However, their occurrence can be predicted by means of flood patterns if effective mechanisms and circulation patterns of this event are identified (Taghvaei & Soleimani ).
The main purpose of this study was to identify the importance of the atmospheric phenomenon called 'atmospheric river' (AR) in the high-risk area of Iran. Numerous studies in this regard have revealed that the given phenomenon was potentially associated with heavy precipitation, ARs are relatively narrow and concentrated structures of water vapour in the lower troposphere moving from lower to higher latitudes in each hemisphere. They also play an important role in the hydrological cycle and account for 90% of water vapour transport over mid-latitudes (Zhu & Newell ). A bulk of water vapour transported by ARs can cause heavy rainfall or flooding in affected areas if accompanied with appropriate synoptic and thermodynamic conditions. Therefore, ARs play a crucial role in the global water cycle and weather extremes. Several researchers have studied the role of ARs in extreme weather events. Waliser & Guan () showed that 'ARs contributed to the distribution of half of the extreme winds and rainfall events, especially in mid-latitudes'. They attributed 40-75% of extreme events mentioned above in more than 40% of the world's coastal regions to ARs. Lavers & Villarini (a) studied the relationship between ARs and extreme precipitation in Europe and indicated that ARs were responsible for many extreme precipitation days. Gimeno et al. () investigated the mechanisms of moisture transport and their role in extreme precipitation. Neiman et al. () surveyed landfalling ARs along the west coast of North America for eight years. Their results showed that the north coast received more ARs than the south coast, and most of them occurred in north (south) coasts during warm (cool) seasons, respectively. global floods and water availability. They found that ARs accounted for approximately 22% of the total runoff in more than 50% of the world. They also reported that ARs would increase the probability of flooding occurrence up to 80% in susceptible areas. Moreover, they showed that nearly 300 million people worldwide were affected by floods and droughts due to ARs. Skelly () investigated the relationship between a flash flood and landfalling ARs over ten years in California and indicated that ARs were present during most cold-season days and caused floods. Papineau & Holloway () studied ARs during the warm months in Alaska and concluded that not all ARs produced much rainfall despite having a large amount of moisture.
The importance of research on ARs is also due to their relation with overall precipitation. Between 20 and 45% precipitation in central and northern California was linked to ARs (Dettinger et al. ; Dettinger ). Moreover, ARs are responsible for about 20-30% of the total precipitation in western Europe (Lavers & Villarini ).
Only two studies were carried out on ARs in Iran. Shademani () investigated the impact of ARs on two heavy rainfall events, causing floods in the west and south of the country. She identified the characteristics of ARs for the selected events. In another study, Salimi & Saligheh () identified ARs at different atmosphere levels for three years (2011)(2012)(2013). They showed that south and southwest ARs had the highest amount of moisture, which, in some cases, led to floods and runoff in southern cities. However, they did not consider the total column water vapour flux in their study.
ARs may account for a proportion of heavy precipitation due to their geographical location. Identifying ARs leading to heavy rainfall events can help to create the necessary flood warnings and thus minimize the resulting damage.
This study examined whether ARs were associated with heavy precipitation in Iran during the cold months from November to April as the highest precipitation in more than three-quarters of the country occurs within the cold season (Masoudian & Ataei ). In the warm season, due to the dominance of the subtropical Saudi Arabia anticyclone in Iran, almost all its territory being affected by the tongues of this strong weather phenomenon (Alijani ).
As a result, rainfall is interrupted, except for a few areas in the north and the south-east of Iran.
This study aimed at fulfilling four basic goals: (1) identi- the flux data are more applicable. Therefore, this study has used the second approach. The 6-hourly data from the European Center for Medium-Range Weather Forecasts (ECMWF) ERA-Interim (ERAINT) Reanalysis were used to identify ARs. 'One of the advantages of using reanalysis products, rather than satellite product, is that they use data assimilation of observations to produce the best available three-dimensional dynamical representation of the atmosphere. ERAINT is provided at high resolution and is an obvious choice given the fine filament structure of Ars' (Blamey et al. ). Hence, eastward and northward vertically integrated water vapour transport parameters with 0.5 × 0.5 resolution were retrieved to calculate the IVT magnitude. Moreover, daily precipitation data of a total of 340 synoptic stations with their complete statistics were obtained from the Iran Meteorological Organization (IRIMO) and used to determine heavy precipitation.

Detection of ARs
In general, geometric characteristics and IWV/IVT are investigated in order to identify ARs. Moisture structures with a length of at least 2,000 km and a maximum width of 1,000 km while reaching above a certain threshold (IWV exceeding 20 mm) have been typically defined as ARs (Ralph IVT value was too high to detect ARs in arid and semi-arid areas. Therefore, the 85th percentile of IVT was used to detect ARs. First, the IVT magnitude was calculated in the geographical domain of 33 to 69 E and 13-43 N (Figure 1(a)) using the eastward and northward IVT. The mentioned range was selected to locate the ARs in the threshold calculation domain as much as possible and to consider all potential humidity supply sources. The IVT is calculated from 1,000 to 300 hPa levels using a Eulerian framework as follows: qv dp where q is the specific humidity (kg kg À1 ), u and v denote the zonal and meridional components in (m s À1 ), g is the gravity acceleration and dp is the pressure difference between the two adjacent levels ( Waliser ) or centres of AR gravity (for one time step as the representative), then, a line was drawn following these points. The axis of each river was also measured on the map with the km measuring instrument as AR length. For calculating the AR width, the length line was divided into four equal segments, and a line was drawn from these points perpendicular to flow direction. After measuring the distance of all segment lines, the mean width was obtained.
Using this approach is simpler as it considers the ARs with a highly curved or unusual shape and works with any map projection (Guan & Waliser ). In this study, the axes were used to investigate the paths of rivers. To accurately measure the length of ARs and to determine the first location of their axes, the plotted coordinate was thus set in an extensive range between 0 and 50 N as well as 15 and 85 E.
In order to identify the paths of ARs, they were also classified according to the origin, location and time of entry into Iran. Thus, the axes of ARs were classified based on where they had started, as well as where they had entered into Iran, and when they had formed (monthly).
All possible entry locations of ARs into Iran were also surveyed for classifying the entry paths; then, they were classified into five regions based on provincial divisions in different geographical directions (Figure 2(a)). This approach could help to have a better insight from the spatial distribution and the frequency of ARs entering into Iran.

Heavy precipitation determination
There are different absolute thresholds to determine heavy precipitation that vary regionally. They have not been

Identifying ARs associated with heavy precipitation
Among the 364 identified ARs, 107 were associated with heavy precipitation. Characteristics of ARs leading to heavy precipitation are depicted in Table 1. Investigating    Figure 3.

The role of ARs in heavy precipitation of Iran
The annual occurrence rate of ARs obtained from November 2007 to April 2018 with the sum of heavy precipitation associated with them is displayed in Figure 4(a of ARs occurring each year was in full conformity with the sum of their heavy precipitation, as shown in Figure 4(a).
On average, 11 days of AR events were linked to heavy precipitation, which accounted for about 3,000-11,000 mm of precipitation in the cold months.
The estimates of heavy precipitation linked to ARs compared to the total (November-April) heavy precipitation during 2007-2018 along with the monthly frequency of AR events is shown in Figure 4(b). On average, AR events accounted for about 30% of the total heavy precipitation during the statistical period. This contribution was variable from 17% in November to 51% in December. Moreover, the highest and lowest frequency of AR events occurred in December and April, respectively.

The paths and location formation of ARs associated with heavy precipitation
The paths and initial location of AR axes associated with heavy precipitation ( Figure 5) indicated that the greatest  Table 2. Seventy-one ARs were formed from the Gulf of Aden and the Red Sea, which mostly entered Iran from the south (32) and southwest (23). The Arabian Peninsula and Sudan were the next areas where ARs originated. As can be observed, the Mediterranean Sea did not play a significant role in the formation of ARs which led to heavy precipitation; however, its contribution to strengthening ARs should be studied. A total of 48 ARs from the south, 32 ARs from the southwest, 19 ARs from the west and 8 ARs from the northwest entered Iran, while no AR axis entered the country from the southeast.

Spatial impact of ARs on heavy precipitation
The frequency of heavy precipitation associated with ARs for the cold months is displayed in Figure 6. In November, heavy precipitation was highly concentrated in the southwest (including the two provinces of Chaharmahal and Bakhtiari and Kohgiluyeh and Boyer-Ahmad) west and northwest of Iran. Similarly, most of the AR axes were concentrated in these regions during this month. The main concentration of heavy precipitation on high-altitude provinces showed the key role of topography in intensifying rainfall in these geographical regions. In December, the affected zones were widened and covered the entire highlands of the Zagros Mountains, from the southwest to the northwest, and even to some extent to the western range of the Alborz Mountains. Also, ARs were extended to Lorestan, a high-altitude province. Further, the number of ARs increased so that ARs associated with heavy precipitation influenced these provinces for up to ten days. Previous studies also showed that December was one of the rainiest months in the south and southwest of Iran, that their landfalling systems were being formed from the Sudanese lowpressure system and the tropical convergence zone (Mohammadi & Lashkari ). In January, the impact zone of ARs was slightly more limited in the west and southwest, but increased in the south, and even in some parts of southeast Iran. According to Mohammadi & Lashkari  Figure 6(h)). Therefore, the potential for flooding in these provinces is higher than in other parts of Iran.

CONCLUSION
The arid and semi-arid nature of the Middle East and West Asia means that even light precipitation leads to devastating floods. This area has been highly prone to flooding due to improper use of soil and vegetation over many years, (floodlist.com n.d.). Therefore, studying the causes of heavy precipitation can help local planners to manage floods.
The purpose of this study was to identify the importance of an atmospheric phenomenon called the atmospheric river, whose relationship with heavy precipitation, potentially, in many parts of the world has been thus far confirmed. Considering that precipitation occurs in more than three-quarters of Iran during six cold months (November to April), this rainfall period was selected to study the At this time, precipitation that originated from the southern systems will be interrupted, but during the rainy season with the eastward displacement of this anticyclone, the precipitation systems increase in these areas. The results of the present study also showed that the number of ARs leading to heavy precipitation had amplified from the south and southwest during the colder months, especially from December. In addition, it was found that the rivers' paths had lengthened in colder months and extended to Central Asia.
On approaching the end of the rainy season in April, the entered ARs had shortened and shifted westward. As a result, heavy precipitation was focused on the west and the northwest of the country. In terms of temporal frequency, December and April had the highest and the lowest number of ARs associated with heavy precipitation, respectively.
In terms of spatial distribution, altogether, the mountai- Another important result of the study was the similarity of 40 AR event dates over different years, implying the sequence of repetitive systems in AR events occurring.
These findings raise the possibility of predicting the occurrence of ARs and can be used to take preventive measures against flood hazards.
This study highlighted the potential of ARs for causing heavy precipitation in Iran, which had not been studied previously, and provided a better understanding of the paths and origins of these rivers. In addition, investigating the spatial and temporal distribution of heavy precipitation associated with ARs revealed the areas at risk of flooding as this phenomenon occurs.
With regard to ARs affecting many regions of Iran and making a contribution to water supply of these areas, analysing the effect of climate change on ARs using climate models to project their frequency and intensity in future conditions is suggested (e.g., Filipiuk & Hu ; Moazenzadeh et al. Homsi et al. ). This work helped to partially improve the historical data on ARs associated with heavy precipitation events.

;
The results are also based on reanalysed data with records of synoptic stations. However, it has been found that selection of the type of data and the method can sometimes influence the results. Regarding the identification of ARs due to the high accuracy of the data used in this study and the magnitude of the occurrence scale of this phenomenon, it is unlikely that the present results are affected (Lavers et al. ). However, employing different threshold methods and criteria to identify ARs in future studies is recommended for better comparisons. In addition, the results revealed that not all identified ARs (364 cases) had caused heavy precipitation in Iran. Since the synoptic conditions of the river path could play a decisive role in converting this abundant moisture into heavy precipitation, it is suggested to reflect on effective synoptic patterns in heavy precipitation.
One of the main limitations of the study was no accessibility to station data from the Arabian Peninsula and other countries to investigate precipitation occurring along the AR paths. Therefore, it is suggested to examine the effects of ARs on heavy precipitation in the Arabian Peninsula in future studies to reveal the potential of ARs on this event.
Further investigations into the persistence and sources of moisture enhancing ARs can also be undertaken.

ACKNOWLEDGEMENT
We acknowledge the Shahid Beheshti University (SBU) for partial support of this work.