This study aimed to investigate the temporal and spatial distribution of cold events in three northern provinces of the country using daily temperature records of at least 32 years (1996-2017) from 8 synoptic stations with a common statistical base.

To identify and diagnose cold fronts at the regional level, a threshold was extracted, which was conventionally the 95th percentile daily minimum temperature of each of the 8 stations. Based on the extracted threshold, any daily minimum temperature record that was lower than this threshold at each station was considered a station cold front record. The results of extracting the thresholds of minimum freezing temperatures at the level of the stations in the region, using the 95th percentile method of the minimum daily temperatures recorded at the stations based on daily statistics from 8 stations in Astara, Anzali, Rasht, Gorgan, Babolsar, Qarakhail, Ramsar, and Nowshahr during the statistical period of 1986-2017 (a 32-year statistical period), indicated that each station had its own freezing temperature threshold. At the Astara synoptic station, the minimum threshold for phrein was 1°C based on the 95th percentile method, and all days with a daily minimum temperature below this threshold were selected as days with a minimum phrein temperature.

This study aimed to investigate the temporal and spatial distribution of cold events in three northern provinces of the country using 32-year (1996-2017) daily minimum temperature records of 8 synoptic stations with a common statistical basis. To identify and diagnose cold spells at the regional level, a threshold was extracted, which was conventionally the 95th percentile daily minimum temperature of each of the 8 stations. Based on the extracted threshold, any daily minimum temperature record that was lower than this threshold at each station was considered a station cold spell record. The results of extracting the thresholds of minimum freezing temperatures at the level of the stations in the region using the 95th percentile method of the daily minimum temperatures recorded at the stations based on the daily statistics of 8 stations of Astara, Anzali, Rasht, Gorgan, Babolsar, Qarakhail, Ramsar and Nowshahr during the statistical period of 1986-2017 (32-year statistical period) indicated that each station had its own freezing temperature threshold. At the Astara synoptic station, the minimum freezing threshold based on the 95th percentile method was 1°C, and all days with a minimum daily temperature lower than this threshold were selected as days with a minimum freezing temperature. This threshold for the Anzali station was 1.6°C based on the statistical period of 1986-2017. At the two stations of Rasht and Qarakhail, the minimum temperature threshold was equal to zero degrees Celsius, which indicates that days were selected as days with minimum temperature thresholds that should have a temperature lower than the zero degree Celsius threshold. The minimum temperature threshold at the Gorgan station, which is the easternmost station in the study area, is -0.6 degrees Celsius, so the days that are selected as days with minimum temperature thresholds at this station have temperatures lower than -0.6 degrees Celsius. In general, the eastern parts of the region (Golestan province and also the parts far from the Caspian Sea water area) have a lower daily threshold than the western stations and the stations near the sea. In the eastern parts of the region, the threshold for detection and extraction of cold phrenic acid was less than -0.2 degrees Celsius, while in the western parts and parts near the coast, this threshold was higher and reached 1.5 to 2 degrees Celsius.

This study aimed to investigate the temporal and spatial distribution of cold events in three northern provinces of the country using 32-year (1996-2017) daily minimum temperature records of 8 synoptic stations with a common statistical basis. To identify and diagnose cold spells at the regional level, a threshold was extracted, which was conventionally the 95th percentile daily minimum temperature of each of the 8 stations. Based on the extracted threshold, any daily minimum temperature record that was lower than this threshold at each station was considered a station cold spell record. The results of extracting the thresholds of minimum freezing temperatures at the level of the stations in the region using the 95th percentile method of the daily minimum temperatures recorded at the stations based on the daily statistics of 8 stations of Astara, Anzali, Rasht, Gorgan, Babolsar, Qarakhail, Ramsar and Nowshahr during the statistical period of 1986-2017 (32-year statistical period) indicated that each station had its own freezing temperature threshold. At the Astara synoptic station, the minimum freezing threshold based on the 95th percentile method was 1°C, and all days with a minimum daily temperature lower than this threshold were selected as days with a minimum freezing temperature. This threshold for the Anzali station was 1.6°C based on the statistical period of 1986-2017. At the two stations of Rasht and Qarakhail, the minimum temperature threshold was equal to zero degrees Celsius, which indicates that days were selected as days with minimum temperature thresholds that should have a temperature lower than the zero degree Celsius threshold. The minimum temperature threshold at the Gorgan station, which is the easternmost station in the study area, is -0.6 degrees Celsius, so the days that are selected as days with minimum temperature thresholds at this station have temperatures lower than -0.6 degrees Celsius. In general, the eastern parts of the region (Golestan province and also the parts far from the Caspian Sea water area) have a lower daily threshold than the western stations and the stations near the sea. In the eastern parts of the region, the threshold for detection and extraction of cold phrenic acid was less than -0.2 degrees Celsius, while in the western parts and parts near the coast, this threshold was higher and reached 1.5 to 2 degrees Celsius