Investigation of Rainfall Variation of Sudan Low during the Historical Process in Southwestern Iran

Document Type : Full length article


1 PhD Candidate in Climatology, Shahid Beheshti University, Tehran, Iran

2 Associate Professor of Climatology, Shahid Beheshti University, Tehran, Iran


Rain is one of the most important elements of the atmosphere. In addition to supplying water of the natural ecosystems, the rainfall plays an irrefutable role in the atmosphere and thermodynamics. The purpose of this research is to study the behavior of the southwestern region of Iran as one of the important agricultural and industrial poles in the country. Therefore, understanding changes in rainfall from the past to now, and the readiness for the changes, should be one of the most important goals of the administration.
Material and methods 
At first, daily precipitation data of 22 stations were obtained from the Meteorological Organization of Iran. The rainfall data were used from 1957 to 2017 for a period of 8 months (October to May). In order to determine the prevalence of rainfall, the rainfall criterion is considered to be above 5 mm. In the next step, by determining 3 priorities, the frequency of the system was obtained with a continuity pattern per day. The first priority is to see the daily rainfall occurrence over 5 mm in common at all synoptic stations. The second priority is that the rain above 5 mm has occurred at least in 50% of the selected stations. If the two top priorities are not observed, in the third priority, and the precipitation is above 5 mm for at least one third of the stations (7 stations), it is acceptable as a result of atmospheric pressure on that day. The purpose of this research is to investigate the historical trend of Sudan's low pressure system in terms of durability and intensity. Thus, for selecting rainwater systems due to Sudan's low pressure, we have used surface-level maps (slp) and pressure levels of 1000 hP from database of the National Center for Atmospheric Research (NCEP / NCAR), with spatial resolution 2.5 * 2.5 degrees, for all continuity patterns. Using the optical analysis method and using the results of the Lashkari (2013) and Alfandi (1950) based on the determination of the spatial displacement of the Sudan low pressure system during the cold period; we investigated logging of the systems into the southwest region of Iran. 
Result and discussion  
There are a total of 227 days or a one-day billing system. The highest number of overnight days is in January with 53 records. The lowest number of day offs was in October and May, with 2 records. Precipitation frequency with continuity of 2 days with 306 repetitions over the course of eight months from the past to today has been superior to the prevalence of precipitation occurrence with one to several days. In other words, from October to May, the share of precipitation with duration of 2 days during the historical process is rising relative to the share of other precipitation with different lengths. Meanwhile, the maximum amount of monthly rainfall with a 2 day continuation in January is 68 with a minimum share in October and May. The precipitation with the duration of one day and the most frequent repetition occurred in January and the lowest in October. However, with the increase in the duration of the precipitation, the share of January will be lower in these rainfalls. With the exception of May, the prevalence of rainfall distribution is approximately the same for all three months. These conditions can be counted with less frequency for 4 days persistence, with the difference that they have a very small contribution to the 4-day rainfall in April and May. Rainfall with duration of five days in March and February was the most frequent with 4 and 3 occurrences, respectively. Precipitation is not formed for six days only in November and April. The precipitation of seven days is just one case in January. Here is the question that how the flow pattern of the Southwest region in the historic process has been dominated, which is increasing with two days' persistence over other rainfall. To understand the reasons for these changes, other studies are required to investigate changes in air masses and circulation patterns in the southwestern region of Iran.  
Compared with other months of the cold season, January has had the most rainfall in most stations (about 17 stations). This month can be important for agriculture and cultivation in the southwestern region of Iran. At all stations, from October to January, the slope of the high rainfall variations shows a rising pattern. Since February, except for the Yasuj station, in all other stations the frequency of precipitation is reduced at the same station. In fact, a general overview of the monthly rainfall variations in Khuzestan province shows that this area is much weaker than in other provinces from February to the end of the cold season. Certainly, this disrupted changes in the availability of atmospheric precipitation in the production, industry, agriculture and even supply of drinking water in the area. Precipitation survey in the pattern of continuity in the day showed that during the historical process from the past to the present day, the frequency and severity of precipitation with duration of 2 days was more than the precipitation occurred with duration of one or more. As the frequency of precipitation systems varied from one to five days or more, there is an increasing trend in Sudanese systems entrance into the South West region. Therefore, it can be concluded that the contribution of Sudanese low pressure precipitation is increasing in the region.


Main Subjects

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