عنوان مقاله [English]
The occurrence of precipitation in an area requires several conditions within the Earth's atmosphere. Moisture availability, deep instability, and cooling availability are the three basic conditions for precipitation to occur. Precipitation, no matter what happens, requires a source to supply the moisture pluvial systems. This moisture can be supplied from the site itself or nearby or more remote areas. The processes and factors involved in the phenomenon of precipitation have long been considered by researchers and have been studied and researched from various aspects. Due to the location of Iran in the transition from a tropical climate to mid-latitude climate and severe spatial and temporal variability of precipitation, issues related to precipitation and water resources have long been one of the most important issues in this land. There are no significant sources of moisture inside Iran. Inland lakes or rivers are not large enough to provide moisture precipitation to adjacent areas. They are mostly local and change the Absolute and Relative humidity of the air. As a result, Iran's moisture precipitations are supplied from nearby water sources such as the Caspian Sea and southern waters or more distant sources such as the Mediterranean Sea, the Indian Ocean, etc. According to the rich research literature on the dynamic, physical and synoptic properties of moisture flux convergence on local, regional and global scales, there are still some questions, especially about the spatial and temporal characteristic of moisture transfer in the form of moisture flux convergence functions during pervasive wet and dry periods in the Middle East, especially in Iran and this study is trying to answer some of them. Therefore, the most important questions in this study that are to be answered are as follows:
• What are the vertical variations of moisture flux convergence in the vertical dimension during pervasive wet and dry periods?
• What are the vertical variations of moisture flux convergence in the horizontal dimension during pervasive wet and dry periods?
This research investigates the characteristics of moisture flux convergence at the time of occurrence of pervasive wet and dry periods in Iran. To achieve this goal, two different databases were used. The first database was monthly precipitation data of 63 synoptic stations in Iran for a period of 30 years (1986-2016), which was obtained from the Iran Meteorological Organization. The second database included gridded data of atmospheric variables such as the geopotential height, sea level pressure, zonal component of wind, meridional component of wind, temperature, and specific humidity which were obtained from the website of the European Center for Medium-Range Weather Forecasting (ECMWF) as monthly observations. The Standardized Precipitation Index (SPI) was used to analyze Iran's droughts on three-time scales: monthly, seasonal and annual. Then, based on a spatial index, wet and dry periods, which affected about 75% or more of the stations studied, were defined as pervasive wet and dry periods. Finally, with the identification of pervasive wet and dry periods at different time scales, moisture flux convergence variations during pervasive wet and dry periods were determined in both vertical and horizontal dimensions. The results of analysis of moisture flux convergence profiles on the studied time scales showed that the height and thickness of the moisture transmission layers inside Iran play a very important role in the occurrence of pervasive wet and dry periods. So that in pervasive wet periods, moisture is transported into Iran from lower levels and with greater thickness, and in pervasive dry periods, this transition takes place from higher levels and with less thickness. The spatial arrangement of moisture flux convergence in the horizontal dimension at different levels also showed that the 850 hPa level can reveal the cause of pervasive wet and dry periods more than other levels studied. At the 850 hPa level, pervasive wet periods are observed as a relatively continuous strip of moisture flux convergence with a west-east direction from the Mediterranean Sea between two latitudes of 35-40 degrees to the northwest of Iran. At the 850 hPa level of pervasive wet periods, we observe a relatively continuous strip of moisture flux convergence with a west-east direction is extended from on the Mediterranean Sea between two latitudes of 35-35 degrees to the northwest of Iran. From the south, a relatively wide and strong strip of moisture flux convergence with the source of moisture the Arabian Sea extends in a south-north direction to the western half of Iran. These two strips of moisture flux convergence are connected in northwestern Iran. But in pervasive dry periods, we see almost different spatial patterns than moisture flux convergence at the 850 hPa level on the study area. The continuous west-east strip of moisture flux convergence, which was from the Mediterranean Sea during the pervasive wet periods, moves to higher latitudes in dry pervasive Octobers, between 40 and 50 degrees latitude. The same displacements range from 40-35 to 50-40 degrees, diverting the entry of precipitation systems of the Mediterranean Sea into Iran.