جستجوی مقالات مرتبط با کلیدواژه "low level jet" در نشریات گروه "جغرافیا"

Mesoscale Convective Systems (MCSs) are the convective precipitation structure that is most frequently associated with floods at mid-latitudes, mainly due to the high degree of organisation, which allows the structure to be maintained for a longer period of time and to become more extensive. Moreover, MCSs are an important link between atmospheric convection and larger-scale atmospheric circulation. Based on the results of previous studies, it can be claimed that Sudanese low pressure systems in many cases are the cause of the formation of MCSs, especially in southwestern Iran. Although many studies have been done in Iran on these systems and how they are formed, but the role of some environmental components of their formation and intensification, such as vertical wind shear, High and Low Level Jets (HLJ and LLJ) has received less attention. Therefore, the purpose of this study is to investigate the role of these factors in addition of the known factors that cause the formation of these systems. For this purpose, the flood of 24 and 25 march 2019 in the south and southwest of Iran has been selected as a case study. To track and investigate the spatial characteristics of MCSs in this study, IR channel of the second-generation Meteosat imagery (MSG) on March 24 and 25, 2019, with a spatial resolution of 3 km and a temporal resolution of 15 minutes from Eumetsat site was extracted. After calibration and georeference of the images, the brightness temperature was calculated. The exact choice of temperature threshold for the identification of convective systems is optional and depends on the spatial resolution and wavelength of imagery. The size distribution obtained from the 207 or 218 k thresholds are not very different, especially for larger convection systems. Therefore, in this study, a threshold of 218 degrees Kelvin was used. Also, there is no agreement among researchers on the criterion of minimum length or area in the definition of MCSs, and this criterion is mostly determined by the characteristics of the region and the selected temperature threshold. In this study we select a threshold of 10 thousand square kilometers. In other words, the system was identified as MCSs, which at some point in life had an area of more than 10,000 square kilometers. The daily precipitation data of GPCC database were used to investigate the scattering of precipitation produced by these systems. Also, to understand the synoptic and environmental conditions of occurrence of MCSs on studied days, first geopotential height data, zonal and meridional wind components, potential temperature, relative humidity, vertical velocity and CAPE from ECMWF database were extracted and then the required maps and diagrams were prepared to synoptic and environmental analyses. In general, the results of this study showed that three MCSs on March 24 and 25, 2019 affected different parts of Iran. The maximum area of ​​the cold core of the first system is about 73,000 square kilometers and has traveled from west to north of Iran. The second system, which affected Iran from the west to the northeast, had a maximum area of ​​about 660 thousand square kilometers. The cold core of the third quasi-stable system with a linear extension (northeast-southwest) and a maximum area of ​​about 440 thousand square kilometers, has moved slightly to the southeast. The synoptic conditions of the formation of these systems have been the same as the common pattern of the formation of Sudanese low pressure systems and MCSs. In this pattern, Azores high pressure can bring the cold air of the high latitudes to the middle latitudes and hot and humid air is injected by the high pressure over the Oman Sea and the Arabian Sea, which activates the Red Sea convergence zone along with the Mediterranean system. These conditions have led to the formation of the minimum potential temperature zone in the eastern Mediterranean with significant temperature and pressure differences compared to its environment, resulting in the formation of LLJ. This LLJ has been very effective in transferring hot and humid air to western Iran. So that in the peak hours of convective activity in the center of Iran, a potential temperature difference of about 30 degrees Kelvin with the environment has created that has played an effective role in the formation of convective storms. The transfer of hot and humid air by the LLJ has led to the formation and continuation of convection and the release of latent heat to enhance the convergence and longer life of convection systems. On the other hand, the coupling of LLJ and HLJ, by strengthening the MCSs in the western part of Iran and strengthening the divergent flow at higher levels, has strengthened the HLJ, which in turn has led to strengthening the convective system. Vertical wind shear probably also led to the formation of new convective cells in areas far from the origin of the primary convective cells. During the peak hours, unstable convective activity was observed over a large part of Iran, especially the southern and western parts, and its maximum was observed from the southern half of the Red Sea along the convergence zone to the west of Iran. Therefore, various components of the Sudanese low pressure system play an important role in the formation, continuity and development of mseoscale convective systems. It seems that low-level jet, vertical wind shear and its interaction with the Red Sea convergence zone and the outflow of primary convective cells have a very effective role in the occurrence of this phenomenon. Thus, more detailed studies of this issue using mesoscale numerical models will probably identify unknown aspects of Iranchr('39')s climate.

The most notable of convective systems are Mesoscale Convective Systems (MCSs) that develop when clouds occurring in response to convective instability amalgamate and organize upscale into a single cloud system with a very large cirriform cloud structure and rainfall covering large contiguous areas (Houz, 2004). Detection and monitoring of MCSs is very important in southwest Iran because they produce hazardous weather, such as lightning, heavy rainfall, hail and strong winds. Several factors influence the development of MCSs such as the flow generated by a weak midlevel trough and the occurrence of low level jets (LLJs). LLJs transport moisture at the jet level, increase the low-level convergence and are responsible for sustaining convection especially at night.
The aim of this study was to assess the influence of low level jets on MCSs formation across the southwest of Iran over the period from 2001 to 2005. The months of January, Mars, April and December was selected because of more MCSs occurrence. Event days were selected using synoptic station data (a set of storm reports such as thunderstorm, lightning, and shower and precipitation) across the study area. IR brightness temperature data from Meteosat 5 were utilized for detecting MCSs. It has a resolution of 4 km with temporal resolution of 30 min. Detection of MCSs was performed on the basis of brightness temperature and areal extent thresholds. In this approach ‘convective cells’ are connected zones of pixels below the temperature threshold that exceed the areal extent threshold (Woodley et al., 1980). The best threshold for detection of area characterized by deep moist convection was determined 228 K. Based on Morel and Senesi (2002), area threshold selected 1000 km2. Those systems have considered as a MCS which reached at least an area of 10000 km2 during its mature stage and lasted at least 3 h.
To determine the influence of low level jet on MCSs development, the occurrence percent, maximum extension and duration of MCSs was analyzed in both LLJ and NOLLJ condition. The detection of low level jet events is based on Bonner (Bonner, 1968). According to this classical definition, a low level jet event is detected when the wind speed is equal to or higher than 12 m/s. In addition, the wind speed should decrease by at least 6 m/s to the next higher minimum. Furthermore the moisture fluxes at 850 hPa are analyzed to identify low level jets role in moist air advection. Moisture flux (MF850) is calculated by multiplying the specific humidity and wind speed (Remedio, 2013). The regions with intense moisture transport are identified during the mean monthly conditions as well as during the composite of low level jet events.
The result of this study showed that the most of MCSs has triggered and developed during low level jet event in all months. So that, 85% of MCSs in January, 96% of MCSs in Mars, 84% of MCSs in April and 88% of MCSs in December has formed during Low Level Jet event. Thus MCSs triggering without low level jets assist was rare. Analyzing of the 850-mb isotachs showed that there was the Low Level Jet many hours before the organized convective systems is established in most of cases. The center of Low Level Jets was in Persian Gulf vicinity mostly. Its speed was equal to 14 - 18 m/s approximately and its axis was in north to south direction. The high wind speeds generally advect the warm and moist air from the Arab and red sea towards the southwest of Iran. These conditions induced to the release of latent heat and increase the low-level convergence which was favorable for the development of convection and MCSs formation.
Westerly wind whit low speed prevailed during the mean monthly conditions at 850 hPa. But it was southwesterly during the composite of low level jet events which transmitted heat and moisture to the study area. The result of this research revealed the biggest and most lasting formed MCSs in days whit low level jet event was bigger and more lasting in contrast whit the biggest and most lasting formed MCSs in days whit no low level jet event. But the mean extension and duration of MCSs in two different condition showed no significant difference.