فهرست مطالب اعظم صانعی

The near-surface temperature in the Arctic has increased about four times greater than the global average temperature in recent decades, the feature called rapid Arctic warming or Arctic amplification. Different dynamical pathways have been hypothesized through which Arctic amplification may influence the midlatitude weather. There is also some evidence that extreme weather in midlatitudes of the Northern Hemisphere is linked to rapid Arctic warming. We analyzed the ERA5 data for the period 1965–2021 to investigate the impact of rapid Arctic warming on weather patterns in West Asia. We defined pre-Arctic and post-Arctic amplification periods as 1965–1991 and 1995–2021, respectively. We analyzed wind speeds between 400 and 100 hPa to investigate the response of the upper-tropospheric jet stream in West Asia to rapid Arctic warming. We defined the meridional extent of an isopleth of the geopotential height at 500 hPa (Z500) as the difference between the maximum and minimum latitudes reached by a single Z500 isopleth each day. By averaging these daily values in different seasons during the pre-Arctic and post-Arctic amplification periods, we obtained seasonal averages of the meridional wave extent in these two time periods. In winter, the altitude of the maximum geopotential height in the middle troposphere has increased in the post-Arctic amplification period. Moreover, the latitudinal extension of the maximum amplitude of Rossby waves has decreased. These two changes imply that the speed of the eastward propagation of Rossby waves has intensified in the post-Arctic amplification period.In summer, the subtropical jet stream has wakened and shifted toward the equator in the post-Arctic amplification period. Due to the weakening of the meridional temperature gradient, both the subtropical and subpolar jet streams become weaker and shift poleward in summer. The poleward shift of the subtropical jet stream has also occurred in the post-Arctic amplification period, while the jet has weakened compared to the pre-Arctic amplification period. The analysis of the meridional circulation index (MCI) shows significant positive and negative values in most parts of West Asia in the post-Arctic amplification period, implying that Rossby waves have become wavier and their speed has decreased, associated with which is a higher possibility for the occurrence of extreme weather events in summer. In autumn, in the post-Arctic amplification period, the jet stream has intensified in the eastern parts of West Asia compared to the climatology and the pre-Arctic amplification period. Between 50 and 75 ºE, the subtropical jet stream in West Asia has shifted toward the equator in the post-Arctic amplification period. As the subtropical jet stream separates the warmer tropical air from the colder air of higher latitudes, its equatorward shift in the post-Arctic amplification period implies the development of negative temperature anomalies in most parts of West Asia.

Hail related to summertime thunderstorms is a small-scale phenomenon, and it often has short time duration. Hail will be produced by a thunderstorm for typically a fraction of an hour, and the width of the track of hail can be as narrow as a few hundred meters. In this research, radar data were analyzed for severe thunderstorms that produced hail across Tehran, Ahvaz and East Khazar radar stations of the Iran during the 2008–2010. A dataset containing 6 reports of hail that sounding, radar and synoptic stations covering them simultaneously. There are several methods for the detection of hail using single polarizationRadar. The methods with high accuracy of detection hail occurrence and warning threshold have been selected.this method is based on criterion of the Waldvogel hail algorithm that uses the maximum altitude at which a reflectivity of 45dBZ is found in relation to the height of the freezing level. The results show that for height difference greater than 4.5 km, the probability of hail detection is 100%. This value depending on warm and cold climate can be varied up to 1 km.