Hossein Asakereh; Skineh Khani Temeliyeh
Abstract
Extended Abstract
Introduction
As an influential element of climate, precipitation affects human activities and societies. It is thus considered to be the essence of any study conducted as a part of environmental and economic planning. Precipitation in Iran, especially in its west and southwest is ...
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Extended Abstract
Introduction
As an influential element of climate, precipitation affects human activities and societies. It is thus considered to be the essence of any study conducted as a part of environmental and economic planning. Precipitation in Iran, especially in its west and southwest is affected by thermal, dynamic, and thermodynamic low-pressure centers such as the Red Sea trough. The trough is an extension of Sudanese low-pressure with a central pressure of about 1006 hPa. The Red Sea is stretched in a southeast to northwest direction and thus connects tropical and subtropical regions. Considering the importance of the Red Sea low-pressure system for precipitation events in west and southwest Iran, any change in this system will affect precipitation patterns in the region. Analyzing the activity of this system and resulting precipitation in west and southwest Iran will thus provide more accurate understanding of the climate of this region.
Materials and methods
Environmental and precipitation data retrieved from Asfezari national database and atmospheric data (geopotential height) extracted from the European Center for Medium-Range Weather Forecasts (ECMWF) were utilized in the present study. A numerical algorithm was also used to identify the cyclones. The algorithm identified 459 cyclones in the statistical period.
Results and discussion
Time distribution of days in which the Red Sea trough is active showed increased activity in summer (198 days) especially August (99 days) and spring (178 days) especially April. However, the Red Sea trough showed decreased activity in autumn and winter. Activities of the Red Sea trough have shown a slightly decreasing but significant annual trend during the statistical period. A sharply and significantly decreasing slope can be observed in summer which results in a decreasing annual trend. Average daily precipitation of the study area in the statistical period ranged from 0 to 2.5 mm. The minimum average precipitation (less than 1 mm) was observed in 29.58% of the study area while maximum average precipitation (more than 2 mm) was observed in 3.64% of the study area. The largest part of the study area (66.87%) experienced an average daily precipitation of 1 to 2 mm. Moreover, 24.28% of the region with minimum precipitation (less than 1 mm) was located in the south and southwest of the study area. This indicates a relatively less severe impact of the Red Sea trough in this area. Around 70.88% of the study area has experienced a precipitation between 1 and 2 mm. Subtracting average daily precipitation recorded throughout the statistical period from the average daily precipitation occurring simultaneously with the activities of the Red Sea trough showed a positive anomaly (more than 0.4 mm) in the north and northeast of the study area. Therefore, it can be inferred that most of the precipitation in this area is originated over the Red Sea. It seems that the presence of the Zagros Mountains has also had a significant effect on precipitation in the study area. Areas with a negative anomaly (less than -0.4 mm) in which precipitation is not affected by the Red Sea trough include spatially scattered regions in Khuzestan, and Kohkiluyeh and Boyer-Ahmad provinces (0.74% of the study area). In other words, precipitation associated with the activity of the Red Sea trough was less than the total precipitation, and thus, most of the precipitation in these regions has other sources.
Conclusion
Results indicated that during the statistical period, minimum average daily precipitation has occurred in south, southwest, and northeast of the study area. Moreover, south and southwest of the study area experienced precipitation simultaneously with the activity of the Red Sea trough. The maximum precipitation in either cases (during the statistical period and also during the activity of the Red Sea trough) has been concentrated in parts of the northwest, west, and east of the study area (along the Zagros mountain range). Significant latitude difference between the north and south of the study area, and existence of the Zagros Mountains and consequently the heterogeneous topography have created two different zones in the study area experiencing minimum and maximum precipitation. In the presence of the Red Sea trough, a higher percentage of the study area experienced maximum precipitation. The frequency of days with more than one millimeter precipitation and their spatial distribution showed that under general conditions, the maximum precipitation has occurred in the north, northwest, west, and east covering 61.11% of the study area. Kurdistan province has recorded a maximum precipitation in more than 3500 days under the influence of different air masses. More than 73% of the factors associated with precipitation in Iran, especially in its northwest, west, and southwest are various synoptic systems (cyclones and short waves) entering the country from the Mediterranean with westerly winds. The minimum number of rainy days during the whole statistical period and also during the low-pressure activity of the Red Sea were also recorded in the southern and southwestern parts of the study area.
Farshad Pazhooh; Farzaneh Jafari
Abstract
Extended Abstract
Introduction
Due to its specific geographical situation,Iranhas an especial precipitation pattern. In other words,despitehaving a precipitation equal to one-third of global average,Iran experiences a strong fluctuation in its rainfall regime. According to global classifications, floods ...
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Extended Abstract
Introduction
Due to its specific geographical situation,Iranhas an especial precipitation pattern. In other words,despitehaving a precipitation equal to one-third of global average,Iran experiences a strong fluctuation in its rainfall regime. According to global classifications, floods are considered to be among the most important natural disasters. In recent decades, humaninterferencesin the environment and improper management of land usehave resulted in increasing severity and higher frequency of these natural disasters (Abbas ZadehTehrani et al., 2010: 78). Extreme floodingcaused by climate changeshave resulted in severe damages in different parts of the world during recent decades and the effects of these changes are more significant in dry environments (Negaresh et. al., 2013: 15). Increasing urbanization and constructions has naturally reduced permeable areasin different basins. The resulting impenetrable surfacesare incapable of absorbing the rainfall, and consequently, the total volume of runoff in the city has increased (TaheriBehbahani and Big Zadeh, 1996).
Materials and methods
Two typesofground level data and data collected from higher levels of the atmosphere were used in the present study:
A) Precipitation data collected during the first ten daysof April 2019 by stations in Western and South Western Iran obtained from the Iranian meteorological organization.
B) Data collected from higher levels of the atmosphere including revised geopotential heights, sea level pressure, meridian and orbital winds, omega and especial humidityobtainedfrom the National centre for environmental surveys at Colorado, USA.
For synoptic analysis, environment to circulation approach was used to detect heavy rainfall peak periods and then their synoptic dimensions were reanalysed in the spatial range of 10 to 70 degrees north latitude and 10 to 80 degrees east longitude. Based on the analysis ofprecipitation data, April5th and11th,2019 were selected as having the highest rainfall resulting in the highest level of flooding and damage in the western and southwest regions of Iran.
Results and Discussion
On April 5th,2019 most regions of Iran have receiveda rainfall of more than 20 mm. The maximum levels of rainfall wererecorded in Koohrangstation(187 mm), Izehstationin Khuzestan (155 mm) and Yasoujstation(151 mm). OnlySistan and Baluchestan, Kerman and South Khorasan Province have experienced a stable situation without any precipitation on this day. However, on April 11th,2019, the highest level of rainfall has occurred inwestern stations of the country. The maximumlevels of rainfallon this day were recorded inNahavand and Tuyserkan stations (Hamedan Province) and Noorabad(LorestanProvince) with 126 and 122 mm, respectively. Central and northwesternregions of the country have experienced the next highest level of rainfallfollowing western regions. Figures 1 to 3 show a part of precipitation values in the western and southwestern regions of Iran during rainfall peak periods. Precipitations in more than 16 provinces in the western, southwestern, and central regions of the country have damagedagricultural, economic and social sectors. More than 45 people were killed in thesedays.The highest number of deaths and injurieshas occurred in Shiraz. In the western parts of the country, Poldokhtar and Mamoualn were most severely damaged. Moreover, heavy rainfall and floodinghave damaged 700 thousand hectares of agricultural land and resulted in 4600 billion USDlosses. In the construction sector, the country has suffered from 1,600 billion USD losses (Hamshahri Newspaper, 1398).
Conclusion
The present study have focused on synoptic and thermodynamic analysis of systems causing pervasive, heavy and hazardous precipitation onApril 5th and 11th in the western and south western regions of the country. The synoptic and thermodynamic analysis of maps indicated that the contrast between the influence of southern and western low pressure fronts such as Saudi Arabia, Sudan and the Mediterranean on the southwestern areas of the country and the cold high pressure frontover the Caspian Sea have caused a strong pressure gradientand formed a strong front condition over the country and the region under study at the sea level. In the middle and upper atmosphere, deep multiple amplitudetroughsformed over the North Pole passed through Russia as bipolar and low pressureblocks, cyclonic centressettled over the eastern Mediterranean regions and the eastern half of the trough formed as a result of blocking settledover the western and southwesternregions of the country. These have resulted in severe, and widespread negative omega and divergence of warm and humid southern weather over the country and the region.