Analysis of late spring frost, early fall frost, frost-free period and probability of their occurrence based on ENSO index in different climates of Iran

Thunderstorms are local, medium-scale weather systems of 20 to 50 km that depend on the height of boiling clouds. They are associated with thunderstorms and indicate the high level of development of convective conditions in the humid air. The occurrence of thunderstorms is a part of the nature of Iran's climate, which causes a lot of human and financial losses in different regions every year, including the damage caused by thunderstorms in Khuzestan province during the study period 2001-2006. West and northwest of Iran are among the areas where the occurrence of severe thunderstorms with heavy rainfall has a high frequency. This study tries to identify the dynamic and atmospheric factors affecting thunderstorms in the west and northwest to provide appropriate information about the situation of this phenomenon for forecasting and prognosis, damage reduction, crisis management, and increasing the level of productivity of provided water resources.

To achieve the effect of atmospheric circulation on the pervasive occurrence of thunderstorms in western and northwestern Iran, the current weather codes that indicated the occurrence of thunderstorms in the region were selected, which include codes 29, 95, 96, 97, and 99, Data related to these codes were obtained from the Meteorological Organization. Also, to determine the prevailing atmospheric circulation conditions during thunderstorm days in the study area, sea level pressure (SLP), geopotential height (hPa), and air temperature data were extracted for 850 and 500 hPa levels on a daily scale. These data were Prepared from the network databases of the National Center for Environmental Prediction / Atmospheric Science (NCEP-NCAR) with a spatial resolution of 2.5 * 2.5 degrees and the European Medium-Term Forecast Center (ECMWF). With 1 * 1 degree spatial separation. The hierarchical method was used to determine the prevailing atmospheric circulation patterns on days with thunderstorms, which were clustered based on geopotential elevation data at the level of 500 hPa. After identifying the atmospheric patterns that caused the storm, the average maps of the patterns at the desired levels were drawn using Surfer software. The composite temperature map was drawn at two levels of 850 and 500 hPa at the time of the phenomenon and all maps were compared with the anomaly maps because this shows the difference between the prevailing values at the time of the phenomenon and the average values. Then, the Nova HySplit model was used to identify the air mass affecting the occurrence of this phenomenon. The input of this model is meteorological data of sea level pressure, geopotential altitude, and air temperature, which were obtained from two databases of the National Center for Atmospheric Research and data of the European Center for Medium-Range Forecasting.

In the 29-year statistical period (1986-2014), a total of 27 days of widespread thunderstorms were identified. Examination of the monthly, seasonal and annual cycle of thunderstorms showed that the maximum occurrence of thunderstorms is in May and spring, and also the years 2005 and 2010 with 4 events had the highest frequency of thunderstorms. In terms of location, the provinces located in the west of the study area, including the provinces of West Azerbaijan and Kurdistan, experienced the most thunderstorms with a total of 49 cases, and Lorestan province, which is located in the south of this region, had the least thunderstorms with 8 cases. Maps of average sea level pressure during the days of thunderstorms showed that the occurrence of thunderstorms in the western and northwestern regions of Iran is associated with a low-pressure system throughout the region with values between 1007 and 1011 hPa. That is, the occurrence of thunderstorms in the region is possible under the negative anomalies of sea level pressure. In addition, average temperature maps at the 850 hp level during thunderstorm events show that the temperature has decreased to the northwest, and the map of temperature anomalies at the 850 hPa level shows that throughout the study area, the temperature decreases relative to Have experienced the average. Similar conditions were prevalent in the Middle Troposphere (500 hPa). The results of hierarchical clustering showed that the occurrence of thunderstorms is affected by three atmospheric patterns of blocking systems, cut-off-low, and shortwave, among which, cut-of-low with 14 cases, and the main pattern in the occurrence of thunderstorms in western and northwestern Iran. Back trajectory paths selected for thunderstorm days each pattern for 48-hour infiltration of air masses showed that the thunderstorm originated from different systems. The Mediterranean system is the main source of air mass affecting the western and northwestern regions during a thunderstorm, followed by the Sudanese system and the integration system in second and third place.

Thunderstorms are atmospheric phenomena that sometimes cause human and financial losses to humans. The results of this study showed that despite the local and regional nature of thunderstorms and their rarity, it is possible to study the atmospheric circulation conditions and the temperature of the lower and middle troposphere during the days of this phenomenon. The results of the statistical study showed that spring and April and May had the highest frequency of seasonal and monthly occurrence in the study area. Also, in the synoptic study, it was found that the occurrence of thunderstorms in the west and northwest of Iran, not only with sea surface pressure anomalies and 500 hPa but also with negative temperature anomalies in both levels. In the occurrence of thunderstorm atmospheric phenomena in the western and northwestern regions of Iran, atmospheric circulation patterns including blocking systems, cut-off-low, and shortwaves play a role in the middle level of the atmosphere. At the same time, Mediterranean, Sudanese and fusion pressures are forming on the Earth's surface. This adaptation of low surface pressure to high divergence enhances surface convergence and high divergence. This leads to increased vertical transmission and air ascent and instability, eventually leading to hurricanes.