Tropical-extratropical interactions and extreme precipitation of Iran in Connection with Tropical Plums (Case study March 25 and 26, 2019)

Document Type : Full length article


1 PhD student in Climatology, Zanjan University, Zanjan, Iran

2 Assistant Professor Department of Climatology, Zanjan University, Zanjan, Iran

3 Professor Department of Climatology, Kharazmi University, Tehran, Iran

4 Professor, Department of Climatology, Zanjan University, Zanjan, Iran

5 Assistant Professor, Department of Climatology, Zanjan University, Zanjan, Iran


Extended Abstract
The interactions of circulation patterns are the interference or synergies of different circulation patterns from different latitudes and at different atmospheric levels. When interacting with circulation patterns, they will have different effects on the environmental phenomena of the Earth's surface than when acting individually. The mechanics of moving the telescopic link between the tropical zone and the subtropics is a major issue in geographical research. Considerable evidence suggests a dynamic relationship between tropical regions and mid latitudes. Tropical-extratropical interactions occur in a wide range of processes and at different scales.Tropical cloud plumes, reflects tropical- extratropical interaction in relation to the transfer of moisture from tropical to extratropical. TP was first defined by McGregor et al. (1984); a continuous strip of upper and middle clouds that are at least 2000 km long . clouds with minimum latitude of 20 degrees and maximum longitude of 5 degrees are in the tropical region. considers tropical cyclones as long bands of mid- and upper-level clouds moving from the tropical region to the polar-eastern direction to the subtropical region, especially continued by a tropical jet and a trough in its eastern part. TPs are relatively narrow at low latitudes (20–15 degrees) and widen at about 30 degrees north latitude. Studies of TP have expanded since the advent of satellite imagery in the 1960s. Prior to satellite imagery, McGurick et al. Described the development of TP in adaptation to a deep trough in subtropical jets and the turbulence of tropical winds, which ceases whenever this alignment is lost. The location and time of occurrence of plum clouds have been reported differently in different studies. In this study, we seek to investigate the Plum tropical clouds and the dynamic climbing factors that lead to their transfer to the midlatitude as a source of tropical moisture that leads to rainfall in Iran.
In order to investigate the role of TP event as an important factor in tropical-extraterrestrial interactions and as a source of moisture in the days with inclement rainfall, the 25th, 26th days of 2019 that Iran has experienced extreme precipitation were selected. The data used in this study are satellite imagery and atmospheric data. Since TPs are clouds that can only be seen in satellite imagery, IR Meteosat satellite imagery was used. Required daily atmospheric data, geopotential height (in meters), wind speed (in meters per second) and wind direction were used on an hourly time scale. The origin, path and direction of the clouds were identified using satellite images. In order to identify synoptic patterns at the time of TP occurrence, synoptic maps of geopotential height and jet stream for atmospheric indices (200 and 300 hPa) were drawn in GRADS environment using ERA5 data. Also, combined images of clouds with geopotential height and tornadoes were used for better investigation.
Results and discussion
On March 25, at 0:00 a.m., the TP oceanic strip reaches 17 degrees along the North Atlantic cut-off low divergence zone, and along the orbital currents of westerly winds entering the tropics below the equatorial equator the subtropical jet stream is flowing. The bifurcation of TP corresponds to the bifurcation of polar front and subtropical jet streams. TPs correspond exactly to the kernels of jet stream. Jet stream core has not reached the east, southeast and parts of the northwest, so TP is not observed in these areas. The widening of the clouds occurred in the divergence zone of western trough over Iran and in the cut-off divergence zone of the Atlantic Ocean over western North Africa. At 06.00, with most of the jet stream core entering the northeastern and northern regions of Iran, TP has entered these areas. At 0:12, the wind speed of the core of jet stream has several degrees of southward displacement, which has led to the entry of TP into the southern and southwestern regions of Iran. At 0:18 East and TP continents originate from the Central African equator and enter Iran in the direction of the divergence zone of west trough and jet stream. Jet stream entered Iran in a more southerly direction, which prevented TP from entering northwestern Iran. On March 26, the subtropical jet stream is not orbital, unlike the day before. This factor can prevent the transmission of the TP orbit of the oceanic band to the east. The TP oceanic band reaches the Mediterranean with a south-north direction in the direction of the cut-off divergence of the Atlantic Ocean. Due to the location of jet stream core on the southwest-northeast diameter of Iran, the highest volume and extent of TP is observed in these areas of Iran. The entry point of TP corresponds to the entry point of jet stream core. At 18:00, the presence of the highest TP on the northeast of Iran indicates the presence of a speed core on these areas. On March 26, we see the southern transfer of TP in the direction of southwest-northeast diameter, which is in line with the southward displacement of the jet stream core to more southern widths.
On March 25 and 26 ,2019 The tropical intrusion of the extratropical dynamic factors has occurred. These factors are the deep extratropical western troughs and polar front jet streams. Their tropical intrusion has led to the transfer of tropical moisture to the extratropical region. The place of penetration of the western troughs into the tropics has determined the origin of the formation of clouds in the tropics. Also, the path and direction of the clouds from the tropics to the subtropics have been determined by the western trough divergence zone and the polar front jet stream at the level of 300 hPa and the subtropical jet stream at the level of 200 hPa. TP has entered Iran with both oceanic and continental origins and through the Red Sea and the Arabian Peninsula along the western trough divergence zone and speed core of the subtropical jet stream and polar fronts in a southwest-northeast direction. Two cloud bands merge on the Red Sea, Which lead to an increase in the Cloud moisture capacity on Saudi Arabia and Iran. With Transfers of clouds over Iran due to insufficient instability, clouds have led to precipitation. Tropical Plume clouds can be a source of moisture for offshore precipitation Tropical plume clouds can be a source of moisture for precipitation in extratropical. The presence of such a moisture source along with lower level moisture sources can increase the capacity of moisture, which in the presence of instability and lifting factors, cause more precipitation.


Main Subjects

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