Climatic analysis of effective jet streams frequency on extreme precipitations in west of Iran

In this study, the frequency of effective jet streams was analyzed in extreme and widespread precipitations in the west of Iran. For this purpose, the daily precipitation of 69 synoptic and climatic stations over 18,624 days (1961–2010) were selected. Then, 119 days of extreme and widespread precipitation in the study area were chosen based on generalized distribution for conducting related reviews and analyses. The frequency of jet streams in the geographical location from 0 to 120 E and 10 to 80 N were reviewed at four levels (250, 300, 400 and 500 hPa). Due to the large volume of information, only the highest and lowest levels (250 and 500 hPa) in relation to the surface were considered. According to the results, the highest frequency of jet stream was observed at 250 hPa. The second quarter of the jet stream core lay over the west of Iran (which is associated with increasing positive vorticity as well as upper-level divergence and lower-level convergence of the atmosphere). In general, the extension of jet stream up to 500 hPa indicated an unstable layer thickness, which can cause extreme and widespread precipitation in the west of Iran. The results of selected days based on cluster analysis and Lund correlation revealed that in rainy days, the wind speed was more than 50 m/s and the subtropical jet stream speed was over 40 m/s, leading to extreme precipitation in the west of Iran.


INTRODUCTION
The precipitation is mainly governed by various factors such as jet stream. As defined by the World Meteorological Organization (WMO), whenever the speed of these strips exceeds 30 m/s, the jet stream is created (Kaviani & Alijani ).
On the other hand, according to the definition in the dictionary of weather and climate, jet streams are known as very severe horizontal winds with speeds over 50 knots, or about 26 m/s which are blowing above the planetary winds (Geer ). Indeed, jet stream refers to velocity of cores which move in the bed of long and short waves and has convergence and divergence zones like them (Alijani ). The speed of jet stream cores declines from its center to the outside where this reduction toward poles is cyclonic or positive, while it is anticyclonic or negative toward the equator. The speed cores are located in westerly winds of equatorial margins, and the other is over the polar front of tropical regions which moves more than in other areas. Consequently, they create two relatively distinct contexts called polar front jet stream and subtropical jet stream. Polar front jet stream occurs in the mid and higher latitudes in the polar front and at a height of 9-12 km, in the parallels of 20-30 , at the height of 12-14 km, and forms discontinuous circles around the earth (Masoodian & Mohammadi ).
The place of jet stream formation matches the belt of maximum gradient of the tropopause. It means that it is contingent upon the place where the temperature gradient, as well as the transfer of energy from the equator to the poles, has reached its peak (Halabian & Hosseinali ).
The position of the jet stream, regulators and converters of atmospheric flows at all levels lies between the tropical and extratropical regions (Reiter ). This highlights the importance of studying this significant phenomenon. is that the aforementioned study has only analyzed the consecutive rainy days with a length of 1-5 days, while in the present study, extreme indices of heavy precipitation have been investigated.

MATERIALS AND METHODS
The type of this research is fundamentally experimental with an inductive approach. The geographic zone under study is western regions of Iran. The database of this research, which has an environmental to circulation approach, involves two groups of variables. To determine the threshold of extreme precipitation, we used the distribution of generalized extreme values. So, we selected the days with precipitation equal to or more than 22 mm and covered 30% of pixels as widespread and extreme precipitation. During the study period, 119 extreme precipitations were selected. Figure 1 shows the study area and its pixels, plus the data used in the present study.
MATLAB software was employed for calculations, whose results were presented in the form of maps. We (B) Calculating the transition coordinates for each pt point in the distribution from the following equation: (C) Calculating the rotation angle θ according to the following equation: Given the θ angle in the third stage, we can calculate deviation along y i and x i axes using the following formulas: In the following, extreme precipitation days were classi- value of 0.5 was selected as the correlation coefficient threshold.
Then, the representative of each class with a correlation coefficient of 0.5 or more and with the greatest similarity to others in that class is qualified as being a representative of the group.

RESULTS AND DISCUSSION
In         Thus, a high-speed wind tunnel has affected the northwest and west of Iran and created an unstable atmosphere engendering extreme precipitation on this day. Also, at the lower level 500 hPa, at 00 z, a tongue of subtropical jet stream core has extended to Iran's west with a speed of 30 m/s, which at 12 z has covered the whole north-west and west of Iran (Figure 10(c) and 10(d)).
December 1st, 1989, the third cluster representative As observed in Figure 11 level of 500 hPa, only a tongue from the polar front jet stream along the north-east south-west has extended to lower latitudes, where at 12 z, this tongue has been completely redirected to the north-west of Iran while keeping its extension with a speed of 30 m/s (Figure 11(c) and 11(d)).
February 1st, 1985, the fourth cluster representative Referring to Figure 12

CONCLUSION
The spatial analysis of jet stream frequency at the level of 250 hPa indicated that the highest frequency of jet streams Figure 11 | Distribution of the polar front and subtropical jet streams at 00 z and 12 z, at levels of (a, b) 250 hPa and of (c, d) 500 hPa, on December 1st, 1989.
belongs to the range from the south of the Red Sea to the south of the Mediterranean Sea. In other words, more than 70% of place of formation, establishment and part of jet streams affected extreme precipitation in the west of Iran. So, this leads to injection of moisture from the Red Sea to the precipitation systems from the west of Iran.
The spatial analysis of the jet streams frequency at the level of 250 hPa revealed that during the study period, the largest frequency of the jet streams covered the range from the northern part of the Red Sea to western and central parts of Iran. In other words, zones located in this range, in more than 50% of the cases, have been the place for formation and establishment of jet streams.
The spatial analysis of the jet streams frequency at the level of 500 hPa showed that during the study period, the largest frequency of the jet streams covered a range from the north-eastern part of the Red Sea to western regions of Iran.
The zones situated in this range in more than 50% of the cases have been the place for formation and establishment of jet streams. This range corresponds to the maximum frequency of jet stream occurrence; from this range to the east, the frequency of jet streams has significantly decreased.
The values of jet stream frequencies at the level of 500 hPa are unexpected mainly due to less occurrence of jet stream at this atmospheric level. Review of the maps of jet streams average speed has been in accordance with the highest frequency of jet streams occurrence, coinciding with occurrence of maximum jet stream speed in the study area.
The results indicated that the second quarter of the jet stream (associated with increasing positive vorticity as well as upper-level divergence and lower-level convergence of the atmosphere) is located in the west of Iran across all levels. So, it seems that this can provide the basis for instability at the time of extreme rainfall precipitation in the discussed geographical domain due to the expansion of the air mass at a high level or vertical upward movement. Figure 12 | Distribution of the polar front and subtropical jet streams at 00 z and 12 z, at levels of (a, b) 250 hPa and of (c, d) 500 hPa, on February 1st, 1985.
In general, the jet stream extension up to 500 hPa revealed an unstable layer thickness, which can cause extreme and widespread precipitation in the west of Iran.
Also, the results of the selected days obtained from cluster analysis and Lund correlation indicated that on precipitation days, the wind speed was more than 50 m/s and the speed of subtropical jet stream was over 40 m/s, making the atmosphere unstable and eventually causing extreme precipitation in the west of Iran.
One of the main problems with this research is the lack of access to New Year's data, which is generally available to researchers in Iran with delay. In future research, it is suggested to consider the possible effects of climate change on precipitation events using general climate models (GCMs).