Analyzing the Role of Polar Vortex on Daily Extreme Precipitation in the Northwest of Iran

The polar vortex oscillation is one of the prominent manifestations of troposphere and stratosphere interaction, which plays an important role on the climate extremes. Therefore, in the present study, the concept of linking the daily extreme precipitation with the polar vortex and its control effect on synoptic systems was analyzed in the temporal-spatial analysis of the polar vortex at north-west stations of Iran. In this study, according to the statistic-synoptic approach, at first, the homogeneity of precipitation data and precipitation trend of all six stations were examined by linear regression test. 6 components explaining 90% of the data variance were identified in order to analyze the companionship and awareness of the trajectory and the effect of polar vortex on the studied stations, the location of the polar vortex investigated by T-type principal component analysis. Then, the position of vortex in each of these patterns was investigated by considering the polar vertex reagent contour in the geo-potential elevation maps of 500 hPa level. 6 general patterns were recognized by analyzing temporal and spatial position of the vertex during the given days in which spatial position, the extension and depth of the ridges were different. The highest positioning of the vertex was seen in the first pattern and the ridges obtained from vertex had the highest depth and expanse on the Black Sea. In all patterns, the daily extreme precipitation was caused by the placement of the receiving vessel from the polar vortex on the required area, which was due to the establishment of the massive Rex and Omega dams on Europe.
Extended Abstract
1-Introduction
Understanding the causes and nature of climatic extremes is one of the most important goals in monitoring climatic phenomena. In extreme precipitation, due to the continuity of phenomena, in addition to troposphere, stratosphere also has an active role in atmospheric interactions. The most prominent feature of the stratosphere is the polar vortex which is a large rotational cycle that occurs in winter and in both hemispheres with 50 to 90 degree distance above the tropopause (approximately 100 hPa) to the mesosphere (above 1 hPa) due to the lack of sun radiation; it is one of the dominant dynamic forms of winter rotation in troposphere and stratosphere. Thus, by the mechanism of radiation exposure, a polar vortex is formed and oscillates between the two states: strong and weak. The displacement of this system related to its normal state leads to some changes in the air of the middle point of north hemisphere, hurricane routes, extreme events such as heavy rainfall, etc. Recognition of the polar vortex, its displacement and movements at different atmospheric levels needs precaution and predictions that help to reduce heavy rainfall damage.
2-Materials and Methods
In this study, two environmental databases and an atmospheric database were used to identify the manner of atmospheric currents, then the homogeneity of the average annual rainfall data of the stations were examined by standard test (SNHT).  Then the significant level of its trend was examined by linear regression test. As 177 days of extreme precipitation were identified, the position of the polar vortex was analyzed by principal component analysis with T-array and rotation by Varimex method on the geopotential elevation data of the upper atmosphere to compare the synoptic and dynamic mechanisms in different positions of the polar vortex.
3- Results and Discussion
The standard normal test showed that the total annual intake during the study period is homogeneous in all stations. The use of linear regression test also showed that in Tabriz, Urmia, Khoy and Miyaneh stations, the process has significantly increasing trend and in Ardabil and Parsabad stations, it has decreasing trend. By applying the principal component analysis method with Varimax rotation and the initial matrix of 177 6 629, 6 primary factors were identified which represent the main arrangement at the level of 500 hPa. The first patterns had 34%, the second pattern had 22.5% and the third pattern had 21.1%, the fourth pattern had 4.4%, the fifth pattern had 3.6% and the sixth pattern had 3.5% data variance. The first days of heavy rainfall occurred with the formation of the first pattern (during the Black Sea) and the lowest heavy rainfall was seen in the sixth pattern (Eastern Mediterranean). In the first component, contour of polar vertex is from west Europe  to black sea and main rainfall is supplied from black sea. In the second component, the ridges with Caspian position and meridional curvature and great depth and several sources play a role in providing moisture. In the third component, ridges with a Mediterranean position and orbital curvature are seen with less depth. The fourth component of ridges has generally two domains with a chain connection from Central Asia to West Asia and appears as multiple sources of moisture supply. In the fifth component, in the east of the Caspian Sea, there are generally two slopes in a farther distance from the country. The sixth component with multi-range ridges is located in the eastern Mediterranean. Although the patterns are very different from each other and different sources play a role in providing moisture, one of the common features of all patterns in the middle level of the atmosphere, during high rainfall, is a strong thermal contrast due to the activity of two types of air masses with different temperature and origin, that have created the conditions for strong front currents and strengthened divergence and air ascent.
4- Conclusion
Checking the homogeneity or heterogeneity of the data and determining the jump points and changing the rainfall time series of the stations by the absolute standard normal test confirmed the homogeneity of the data in the mentioned stations. Then the significance of rainfall trend in all three stations was tested by linear regression method. The findings of this study showed that the general trend of rainfall in all stations except Parsabad and Ardabil was not significant. The study of heavy rainy days showed that Urmia station with 61 days had the most and the Mianeh with 20 days had the least heavy rainfall in 21 years.  Analysis of polar vortex position by principal component analysis and  with 177 6 629 matrix showed that 177 days of heavy rainfall occurs under 6 general patterns, polar vortex position analysis in 177 days of heavy rainfall showed that the common feature was in the middle level of the atmosphere in most different position patterns of polar vortex and the ridges on Europe flowed with cold polar vortex from the western edge of the ridge; the ridge on the lower latitudes with the arrival of hot and humid air through the eastern edge of the ridge have caused strong fronts and strengthened divergence and air rise. However, the patterns are so different from each other that they are clearly placed in 6 different patterns (positions).