Identifying and tracking cut-off lows effective in Iranian heavy rains

Document Type : Original Article

Authors

1 Department of Physical Geography, Faculty of earth sciences, Shahid Beheshti University, Tehran, Iran

2 Department of R.S. and GIS Studies, Faculty of earth sciences, Shahid Beheshti University, Tehran, Iran

3 Payam Noor University, Parand Branch, Tehran, Iran

Abstract

Introduction
Cut-off  lows are cold -core cyclones in the upper troposphere that are identified as local minimums of geopotential elevation fields in the range of 500 and 200 hPa )Nito et al, 2008; Pinheiro et al, 2017; Muñoz et al, 2020(. Cut-off lows occur in most seasons, often in late winter and early spring. One of the synoptic patterns affecting temperature and precipitation as well as heavy rainfall in Iran is cut-off lows. This study has been compiled according to the importance and relationship between cut-off lows and heavy rainfall in the statistical period of 33 years (1986-2018). In this study, the climatology of cut-off lows systems at the level of 500 hPa of the northern hemisphere effective on heavy rainfall in Iran has been discussed. The ECMWF reanalysis data were used to identify and track cut-off lows that lasted for more than 48 hours in the statistical period of 33 years (1986-2018). For this purpose, geopotential, wind, and temperature data also were used.
 
Materials and Methods
In this study, from the average daily data: Geopotential height, wind orbit, and temperature level of 500 hPa of the re-analyzed ECMWF European site with a grid step of 0.125 * 0.125 in the statistical period of 33 years (1986-2018) Used in areas including 0 to 80 degrees north latitude and 0 to 80 degrees east longitude. The Tibaldi-Multney index was used to identify Cut-Off low. And the Cut-Off lows that were effective in Iran's heavy rainfall were studied.
 
Results and Discussion
Annual, monthly and seasonal distribution of cut-off lows during the statistical period
In the statistical period of 33 years (1986-2018), a total of 632 cut-off lows with a lifespan of 2 days and more were identified. The average was 18.87 days for each year. The trend line or regression during the statistical period shows that the amount of cut-off lows is increasing.
The share of cut-off lows in heavy rainfall in the four regions of Iran
In the northwestern region of Iran, the highest percentage of the effect of shear compressions on heavy rainfall is in Kashan, Takab, Mahabad, Maragheh, and Tabriz stations, respectively. The lowest percentage of the impact of this phenomenon on heavy rainfall was in Khorramabad, Karaj, Babolsar, and Parsabad stations, respectively. In the southeast region, the highest percentage in Chabahar, Bandar Abbas, and Sirjan stations and the lowest percentage were in Jask, Zabol, and Khash stations. In the southwestern region, the highest effect is in Bushehr and Kish stations and the least effect is in Omidieh and Masjed Soleiman stations. In the Northeast, the highest share was in Birjand, Ferdows, and Tabas stations and the lowest impact was in Gorgan, Mashhad, Quchan, and Bojnourd stations.
The amount of heavy rainfall and the effect of cut-off lows in it in different regions and Iran
The highest occurrence of heavy rainfall due to cut-off lows was in the northwest region with (36.62 occurrences) and the lowest in the southeast region with (8.5 occurrences). In research study stations (76 stations), the frequency of heavy rainfall during the study period was average (124.76 events) and the effect of shear cut-off lows on heavy rainfall in total (26.1 events) in the station has been.
 
Conclusion
A total of 632 cut-off lows with a lifespan of 2 days or more were identified in the research area. The trend line of the statistical period has a slight positive and increasing slope; the most occurrences of cut-off lows is in October, March, and January. The lowest incidence of cut-off lows is in July, August, and June. Seasonally, the highest number of cut-off low events was in spring, winter, and autumn. The highest occurrence of heavy rainfall under the influence cut-off lows was in the northwest region and the lowest in the southeast region during the study period. Therefore, it can be concluded that the entry of cut-off lows from the northwest and the proximity of the northwest region to the formation location of cut-off lows may be a factor contributing to the highest frequency of heavy precipitation events influenced by cut-off lows. Furthermore, the distance of the southeast region from cut-off lows indicates a lesser impact of this phenomenon on heavy precipitation in this area.

Keywords

Main Subjects

References

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Researches in Earth Sciences
Volume 15, Issue 3 - Serial Number 59
September 2024
Pages 137-155

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