Synoptic analysis of extreme and exteremly Wet Period in southern Iran

Document Type : Full length article


1 Department of Climatology, Science and Research Branch, Islamic Azad University, Tehran, Iran

2 Department of Physical Geography, School of Earth Science, The University of Shahid Beheshti (SBU), Tehran, Iran



Aridity and drought are inseparable features of every climate. But in arid and semi-arid climates, tarsal is an ideal opportunity to restore or compensate for the lack of water in the region. The southern part of Iran has a dry climate despite access to the huge moisture resources of the warm southern seas. To conduct the research, first, the daily rainfall data of all the synoptic stations of the southern provinces of the country, which had complete statistics in the 33-year statistical period (1986-2019), were extracted. Then, using ZCI, ZSI, and SPI indicators, droughts and droughts were identified. Finally, the years that were in extreme poverty in all three above indicators were selected as samples. In the next step, the data of specific humidity, orbital and meridional wind, geopotential height and omega for all atmospheric levels in the lower and middle layers of the Verdosphere were received from the NCEP/NCAR site for all rainy days. Examining the maps of subsurface levels of the Verdspehr (sea, 1000 and 925 hectopascals) showed that three main systems control the pattern of the subsurface layer of the Verdspehr. The Siberian, Tibetan and Mohajer high-pressure fronts spread over the warm waters of the Oman and Arabian seas from 3 to 7 days before the start of the system's rainfall and spread the necessary moisture into the Sudanese system. It plays a very important role in determining the entry path of the system, the expansion pattern of the Mediterranean trough and the duration of the activity of the precipitation system in the middle layer of the Arabian Wardspehr
Extended abstract
The southern part of Iran is one of the strategic regions in terms of industry, trade, port, population and agriculture. Any disturbance in any of the above sectors will result in irreparable economic, social, environmental and political damages. In the south of Iran, due to the thermodynamic nature of precipitation systems and the topographical conditions of the region, precipitation is generally very intense. As a result, most heavy rains are accompanied by heavy floods. This region has a arid and semi-arid climate due to its geographical location in relation to the general and regional circulation of the atmosphere. As a result, wet years are an opportunity to compensate for water shortages. This area is one of the cores of agricultural products production, especially in the field of grains, vegetables and fruits. All the country's need for vegetables and fruits in the cold period of the year, which cannot be cultivated in other places, is provided from this region. Also, only in the southern provinces, the cultivation period and the rainy season coincide, and the winter rains can be directly used for agriculture. Understanding the mechanism of rainfall in heavy wet years, in addition to improving the level of preparedness of managers to control and reduce the destructive effects of floods resulting from heavy rains, will help to manage excess water resources in heavy wet years.
Materials and methods
In this research, all the synoptic stations of the southern provinces of the country, which had complete statistics in the statistical period of 1986 to 2019 (corresponding to three solar cycles 22 to 24), have been selected. With this criterion, 17 stations were selected from the synoptic stations of southern Iran with appropriate distribution over the region. Then, the daily rainfall statistics of these stations during the region's predominant rainfall period (November to May) were extracted from Iranian meteorological data. These data, after sorting in the Excel environment, have been statistically reconstructed using conventional statistical methods. Then the data were averaged and using the annual rainfall of the stations and using the three indexes ZCI, SPI, and ZSI, the periods of wet and dray years were calculated. Finally, the years in which severe wet year occurred in more than half of the stations based on all three above indicators were selected as severe wet years. In the next step, the sea levels, 1000 and 925 hPa data were obtained from the NCEP/NCAR website. At this stage, using these data and the maps drawn for these levels, the dominant pattern in the lower layer of the troposphere was extracted. Then, based on the selected patterns and the entry path of precipitation systems, synoptic patterns were extracted. For these pattern, specific humidity, orbital wind and meridional wind, height and omega data for 1000, 850, 700 and 500 hPa levels were obtained from the NCEP/NCAR site. In the final step, with appropriate scripting, composite maps for selected synoptic patterns have been drawn and analyzed.
Results and Discussion.
The results of the assessment of wet and arid years showed that the frequency of droughts in the last three decades was much higher than droughts. This phenomenon has increased in the last decade. The results of this research also showed that, due to the diversity of topography and the extent of the region in terms of longitude, wet years are not widespread. This heterogeneity can be seen even in severe and ultra-extreme wet years. The results of the analysis of synoptic patterns in the lower levels of the troposphere showed that in all precipitation samples, regardless of the duration of the precipitation system and its entry path, from three to 7 days before the onset of the precipitation system activity, a tab of one of the high pressure systems of Migrant, Tibet or Siberia extends over the warm waters of the Oman and Arabian seas. With the expansion of this ridge over these warm waters, while intensifying the temperature gradient, with the advection of the moisture from the warm Arabian and Oman seas inside the Sudanese low pressure, it provides considerable energy and moisture for the formation of convection currents in the region. This high-pressure ridge is still active over the Arabian and Oman seas during the whole period of the activity of the rainfall system in the region.
In the middle levelsof troposphere, regardless of the path of Sudan's low pressure, from three to four days before the start of the rainfall activity of the system, a deep trough should spread over the African Sahara within the borders of the countries of Libya and Algeria. One to two days before the start of rain in the south of Iran, this trough was located over Egypt, and with the southward expansion and suitable vorticity advection over the sudan thermal low pressure, the necessary conditions are provided to strengthen the system. These troughs have had a significant southward expansion in all rainfall samples. So, the southern end of trough has extended to southern Sudan and northern Ethiopia. In all cases, the southern end of the trough extended below the 15° orbit at the 700 hPa level. The movement path of the sudan low pressure and its eastward movement trough is closely related to the location and eastward movement of the Arabian subtropical high-pressure. Precipitation systems with Sudanese origin enter southern Iran from three main routes. In long-term rainfall systems, this system gradually enters the region from the southwest and gradually moves to the east with the displacement of the Arabian subtropical high-pressure, and the entire southern region of Iran benefits from the rains of this system.
Despite the access to the huge sources of moisture of the warm waters of the southern seas, due to the prevailing circulation pattern in the region, especially the proximity to the strong dynamic Arabian subtropical high-pressure system on the one hand and the vast deserts of Arabia and Lut in the southeast of Iran, the south of Iran has a dry and semi-arid climate. As a result, water scarcity and poverty of surface and underground water resources are the characteristics of this region. Due to the strategic importance of this region, water scarcity can be a great risk for economic, social and industrial activities in the region. As a result, wet years are an opportunity to compensate for the lack of water and to store excess water resulting from heavy rains and floods, which are currently causing destruction and damage to infrastructure, soil and vegetation in the form of torrential flows. Due to the fact that the synoptic pattern leading to flooding rains is formed from three to 7 days before the start of rain, especially in the lower levels of the troposphere and can be estimated, by creating infrastructure and proper planning, most of these rains can be controlled and With surface storage and infiltration in the underground layers, it provided a suitable storage for droughts.
There is no funding support.
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
Conflict of Interest
Authors declared no conflict of interest.
We are grateful to all the scientific consultants of this paper.


Main Subjects

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