Investigation of Winter Severity Index in Northwestern Iran

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In different countries affected by snowfall, sub-zero temperatures and freezing temperatures, special maintenance and control is maintained for roads and transportation networks each year with large budgets. The winter severity index is an index that combines different air effects into a single value to help the organization compare and normalize costs geographically and temporally. Researchers such as Strong and Shvetov (2006) have invented models of winter traffic, for example, Qui (2008) on winter road traffic and maintenance, Boselly and Edward (1993) on road maintenance, and Cerruti and Decker (2011) on human activities. Given that much of northwestern Iran is mountainous, the occurrence of freezing and snowfall is not the same in intensity and duration, and the necessary budget is not evenly distributed. Therefore, the necessity of knowledge based on the climatic facts of the region can be an important step in the proper functioning of the relevant organizations in the process of budget allocation for the maintenance and control of the road network in the North West region.
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Study area
Northwest of Iran has some kind of topographical unity (Taleghani, 2013). In spite of the mountainous area, the flatlands and troughs are located in it. The most famous ones include the plain of Moghan, the plains of Tabriz and Urmia, and other plains around Lake Urmia and Lake Urmia itself. In general, the Northwest region has cold and mountainous climate due to its geographical location and topographic dispersion, especially in the cold season. And most of the rainfall in this season is in the form of snow falling and freezing, which is one of the highlights of this season' climate.
Data and numerical analysis

In this study, the mean daily temperature of 18 synoptic stations in Northwestern Iran (see Figure 1) was used from October 1 to the end of March of the following year in the 1986-2015 statistical period. Accordingly, temperatures below zero were assigned a positive algebraic signal and temperatures above zero were assigned a negative algebraic signal. Then, the cumulative amount of frost day degree was calculated during the above months. In this function, if the cumulative amount of frost day degree is negative for a given period, the cumulative value is zero and the new cumulative sum begins the following day. In this regard, it was done based on Asell's (1980) classification to obtain an adequate understanding of the degree of winter severity. Accordingly, the intensity of winter was determined in the study stations and was classified into five groups. Based on this classification, the cumulative frequency distribution of frozen degree-days more than 95% took place as the most severe and less than 5% as the mildest classes. Also, 15% high distances (80 to 95%) as more severe than normal, and 15% lower distances (5-20%) were classified as milder than normal, and 20% to 80% classified as normal.
Synoptic analysis

To measure the changes in the general atmospheric large-scale circulation and to study the impact of atmospheric circulation on extreme climatic conditions, the Mean Circulation Composites data for the period of 1961-1985 as a representative of the past climate and the 1986-2016 as a period with the occurrence of climate change (WMO, 1989) were obtained based on NCEP / NCAR data. The difference maps were then plotted by subtracting the new statistics period from the old statistical period to show the changes in the general atmospheric circulation in these two periods using Grads software.
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Results of numerical analysis
The survey of the mean frost degree day map of the region shows the difference between the regions. Sarab Station with 392 degrees in east and Maku station with 322 degrees northwest and Takab with 324 degrees in the south have the most significant winter intensity, whereas Pars Abad with 13-degree day shows the lowest winter intensity during the statistical period. Around Lake Urmia, especially in the south, relatively low values were observed. In addition, the survey of the map of minimum frost degree day's values of the region indicates the existence of a focal point in the east of the region at the Sarab station. In other words, even in the years when the region has mild winters, the intensity of winter cold at the Sarab station is reduced; however, it significantly differs from other stations. The investigation of the map of maximum frost degree days in the region also shows the intensification of winter cold in east (Ardebil with 773 degree days and Sarab with 709 degree days) and southeast (Takab with 830 degree days) and northwest (Maku with 623 degree days). The examination of the start and end dates of the cold season shows that in the region's severe winter foci, the cold season begins in the third decade of November and lasts until the third decade of March, while it begins in the first decade of December and ends in the first half of February in parts such as Parsabad.
Results of synoptic analysis
The mean geopotential height anomaly map of 500 hPa, for the months of October to March (cold period) from 1988 and 1989 in the study region (northwest of the country) shows that the negative anomaly of geopotential height is even as low as -10 m, which indicates a decrease in the thickness of the atmosphere during this time period. The 1988 surface air temperature anomaly map shows a negative temperature anomaly in the north and northwest of the country as compared to the long-term average, reaching more than -2 degrees in West Azerbaijan Province. This indicates colder autumn and winter than other years, which fully confirms the results of the winter severity index. The cold period anomaly map of 1998 also shows an increase in temperature this year as compared to the long-term average across the country, reaching as high as 3 degrees Celsius for the northern parts of West Azerbaijan province according to results from Calculate winter severity index.
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The surveys of northwestern Iran based on the use of total daily negative temperatures indicate that the Sarab, Maku, Takab stations have been the strong winter hotspots in the region. The most severe winters have also occurred in Ardebil and Sarab stations in the east, while Takab and Maku in the west of the region and the Takab station also have the highest record during the statistical period with 830 freezing degree-days. However, it was found that in severe winter foci, the cold season begins earlier and ends later. Based on the synoptic analysis, it was found that in 1998 the decrease in the thickness of the atmosphere caused more instability; thus, cold airflow at high latitudes and lower air temperatures, and ultimately increased winter cold intensity. The increase in atmospheric thickness in the winter of 1998 resulted in greater stability as well as the intensification of downstream currents due to delayed subtropical high-pressure retreat (STHP), leading to an increase in air temperature and a warm winter experience this year. Accordingly, it seems that the relevant road transport agencies of the country should take the necessary measures to establish fully equipped winter toll in Sarab, and Ardebil in the east and Takab and Maku in the west of region; in fact, with proper management of resources and costs, they work best while saving money.