Statistical-Synoptic Analysis of Heat Waves above 40○C in the West of Iran

Temperature effects on human activities and natural processes are undeniable. Temperature increase، in particular، in the form of heat waves affects all facets of life including architecture، welfare، transport، agriculture and industry. When the intensity and frequency of the waves are high، they can produce major problems. In recent years، the number and intensity of these heat waves has been increased in the world، so that it is one of the major causes of atmosphere related mortality in many countries. The aim of this study is to analyze the temperatures above 40°C and specify their synoptic patterns in the west of Iran using statistical-synoptic method. Study area: Study area in this paper consists of five western provinces of Iran، including Kermanshah، Kurdistan، Hamedan، Lorestan and Ilam. From the climatic point of view، southern and western parts of the region including Khorramabad and Ilam stations are placed in the outer foothills climatic zone، and the other parts of this region are placed in the mountainous zone. Western low altitude strip has a warmer climate; But the other parts because of having different altitudes and latitudes، have colder climate. To achieve the statistical-synoptic analysis of heat waves، the daily maximum temperature data of six synoptic stations of the region within a 32-year period (1976-2007)، and the relevant synoptic maps were used in this study. Different criteria have been proposed for selecting a numerical threshold for defining the superhot temperatures and the occurrence of heat waves. In the present study، reaching a maximum daily temperature of 40°C and higher has been the basis for defining the heat wave based on the region''s temperature characteristics. Accordingly in each of these stations the dates in which the maximum temperature had reached 40°C and above during all those years were extracted. Then، based on the January 1st، each calendar date was converted to Julian date، and after that، the different characteristics of these superhot days like start date، duration and frequency were analyzed. Next، heat waves were derived based on two criteria: incidence of 40°C temperature and above at least in three stations، and three consecutive day occurrence of the above mentioned conditions. The beginning day of each wave was defined as the day when the temperature has reached 40°C or higher at the desired stations for the first time in compliance with the above conditions، and، the end day was defined as the day when the temperature reached the mentioned threshold for the last time. To calculate the mean temperature of a specific wave، maximum temperatures equal to 40°C and above at the all stations on all days of the occurrence of heat wave were used. To study the synoptic occurrence of heat waves، various data including geopotential height، sea level pressure، surface temperature، 1000-500 hPa thickness and meridional wind component at 200-hPa level were taken from NOAA''s Website. In the next step، different maps comprising mean sea level pressure and anomaly، 500-hPa geopotential height، warm air advection and 1000-500 hPa thickness were drawn using GrADS software. The assessment of maximum temperatures in the region resulted in the extraction of 27 heat waves. The usual date for the start of heat waves and rising the temperatures to 40°C or higher in the west of country is July 23. The highest frequency of heat waves in 1977 with 4 waves، the longest wave in 1995، continuing for 28 days، and the most severe wave in 1998 with an average temperature of 42. 4°C has occurred. The average length of waves in the region was 10 days، with the range of 3 to 28 days. In the mentioned period، Khorramabad and Kermanshah stations with respectively an annual average of 22. 5 and 13. 4 days had the highest amounts of 40°C temperatures and above، and in terms of duration and severity were among the hottest points of the region. Kermanshah station had the maximum number of superhot days after Khorramabad due to the high altitude and thin atmosphere. In contrast، duration and intensity of heat waves in Hamedan station was the least in the region. The results obtained from the synoptic analysis of heat waves in the west of Iran showed that during the heat wave occurrences، warm and dry air from deserts of Saudi Arabia and northern Iraq has been drawn to the western half of Iran through low pressure air masses located over these two countries، and consequently، this influence is more sensible in the western and southern stations like Khorramabad and Kermanshah than other stations، such as Hamedan، Sanandaj and Saghez. In most of the studied hot waves، Azores subtropical high pressure extends as a belt from North Africa to Iran. 1000-500 hPa thickness maps showed that a warm air core is extended from Iraq to Egypt with southwest-northeast direction، and covers western half of the country. All in all، two synoptic patterns، one related to the strongest waves and the other near-normal waves were extracted. In the first pattern (near-normal waves) Azores subtropical high pressure creates a secondary high pressure over Saudi Arabia، and the deep trough which has been created in the West Urals، weakens the Saudi Arabian High. Thus subtropical systems move to more southern latitudes and consequently have less impact on the western half of the Iran. In the second pattern (strongest waves)، Azores subtropical high pressure advances and expands over northern latitudes، up to about 35°N. In this pattern Azores high pressure belt covers the area from North Africa to Iran، and this way the stronger heat waves affect western half of Iran. Warm air advection maps too verify this status. So that the warm air advection amounts at 200-hPa over the Middle East are positive، which indicates the greater thickness of atmosphere compared to the usual state. As expected، from synoptic point of view، the occurrence of heat waves in the region has been simultaneous with thermal low pressure formation at the ground level and subtropical high pressure at the upper levels. The number of superhot days in the stations has increased on average at the rate of 2. 06 days in the study period، which can be indicative of warming trend in the region. This warming trend is consistent with results of studies in Greece (Maheras et al، 2006:161)، United States of America (Bumbaco et al، 2013:1618)، Europe (Twardosz and Kossowska، 2013:627)، and Iran (Esmailnejad، 2014:39; Darand، 2014:167). Comparing the results of this study with other studies in the future will reveal the similarities or differences of statistical characteristics and synoptic patterns of heat waves in different parts of the country.