Synoptic analysis of the most severe and widespread extreme cold wave hazard in Iran

Despite global warming, cold waves (CWs) remain one of the most severe, frequent, and damaging climatological hazards in most regions of the world. In this research, the most intense and widespread CW in Iran during the statistical period of 1836–2015 has been identified and analyzed. To identify and analyze the synoptic patterns of CWs, the following data were used: Minimum Daily Temperature (MDT) at the 2-meter land surface, Sea Level Pressure (SLP), Geopotential Height (GH) between the 1000 and 500 hPa levels, atmospheric thickness maps (500 to 1000 hPa), and temperature data from the lower and middle levels of the atmosphere. These data were obtained from the daily reanalysis dataset of the gridded NOAA-CIRES-DOE 20th Century Reanalysis V3. The criteria for selecting severe CWs include a temperature threshold of -20 °C or below, a wide spatial extent (regional or covering at least half of Iran), and a duration of at least two days. Based on the determined intensity and extent index, the most severe and widespread extreme CW in Iran, which occurred in January 1925, was selected for synoptic analysis. The synoptic analysis of the selected wave revealed that the extreme CW was caused by the combined influence of the Siberian high-pressure system and a migrating western high-pressure system with a pressure of 1030 hPa. The main cold core featured a high-pressure system of 1035 hPa associated with the migrating western system at the surface level. At upper atmospheric levels (1000 to 500 hPa), a blocking pattern over the North Atlantic Ocean played a significant role. The arrangement of these systems directed very cold northerly air currents toward Iran. At the 1000 to 850 hPa levels, wind flow divergence and temperature gradients caused the cold air to settle. Meanwhile, at the 700 and 500 hPa levels, blocking and the descent of cold air from the polar region and North Scandinavia, combined with a deep trough over Iran, were the primary factors contributing to the cooling and the occurrence of the extreme CW in January 1925.