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Abstract
Over recent years, the extreme temperature events (cold and heat waves) have been a burning issue driving the scientific society to carry out extensive researches to investigate and discover the trend, frequency, intensity, and scope of their occurrence. The goal of this study is also the recognition of the trend and spatial analysis of the frequency and maximum of extreme cold days in northwest Iran. For this purpose, the data of minimum daily temperature from 21 synoptic stations of the country for the months of January, February, March, April, November, and December from 1986 to 2017 were used. By using the standard index coefficient, the cold days with an intensity of less than -1/2 were extracted as the extreme cold days for each station monthly. The Mann-Kendall test and spatial analysis with the Anselin Local Moran's I and  hotspot index were used to identify the behavior of extreme cold days. The results demonstrated that the presence of a decreasing trend of extreme cold days in most months (except for a few stations) had no statistical significance, and a significant decreasing trend occurred only in March in almost 50% of the stations. The spatial analysis of the trend of March indicated that the significant decreasing trend had created a cluster and a cold spot. Moreover, the results obtained from spatial analysis indexes in the northwest of the country demonstrated that the frequency and the maximum number of events of extreme cold days in the area in the studied months had formed a high cluster pattern (positive autocorrelation) and a low cluster pattern (negative autocorrelation), while more than 50% of the area of the studied zone had no spatial pattern. The cluster patterns in different months of the year formed in various places and the spatial arrangement of autocorrelation patterns and hotspots were not similar in the studied months. Furthermore, the obtained results based on the  index confirm the formation location of clusters in the Anselin Local Moran's I.

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Drought is a climatic phenomenon that occurs in almost all climatic zones and more than any other hazard affects human life and the environment. In order to statistically analyze the synoptic occurrence of dry falls in Northwestern Iran, the monthly precipitation data records of 15 synoptic stations were received from the Iranian Meteorological Organization during the statistical period of 1987-2016. Standard rainfall index and DIC software were applied to identify and differentiate dry fall seasons within the study area in the recorded data period. The results of SPI method showed that the frequency of occurrence of normal situation and then drought in mild and moderate intensities occurred more in the stations of the region during the anticipated time cycle. Then, daily data of 500 hPa and ground level with coordinates of 10 to 75°eastern longitude and 20 to 70° northern latitude in dry periods were received from the total reconstructed data of NCEP/NCAR site and by applying the PCA analysis of synoptic patterns for effective factors in the occurrence of dry falls, the results were interpreted and analyzed by the use of 500 hPa level maps and sea level pressure (SLP). The results of this study showed that in dry falls, severe positive surface anomalies are observed due to high pressure infiltration and positive anomalies are seen at the level of 500 hPa due to the high-level governance in the region and the establishment of high-altitude system on Iraq and Syria which is the most important drought factor. Also, by examining Hoff Müller diagrams, the 700 hPa level in the dry summers of the Northwest indicates a significant decrease in the humidity of this level, especially in the southern latitudes (15 to 30°north) in 4 of the 5 study samples, which indicates the importance of low pressure systems. The Sudanese and Sudanese-Mediterranean combination of pressure play a major role in the injection of moisture into the study area.