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.

With the aim of revealing the at revealing the synchronic role of mid-latitude atmospheric cut-off lows and low-level jets in the continuation of daily rainfall of the west and northwest in Iran, maps of geopotential height, thickness, vorticity, vertical motion (omega), specific humidity, zonal and meridional winds of ECMWF and NCEP has been used. Daily rainfall data for the period 1995-2014 have been received from the Iran Meteorological Organization. Nieto and Reboita methods and Bonner criterion have been used to identify cut-off low and low-level Jet, respectively.  The results showed that out of the total rainfall periods, 60 cases are directly related to atmospheric cut-off lows. These cut-off lows were located in three regions East of the Mediterranean Sea, the Middle East and East of the Black Sea. The highest frequency of this phenomenon was related to winter and was identified with a maximum in January. In the study of the life cycle of these systems, it was found that cut-off lows with a duration of two days have the highest durability and in the days of the occurrence of the cut-off low, low-level Jets accompanied them in 58 occurrences. Low level Jets, by intensifying the convergence in the low levels of the atmosphere, on the one hand, provide the conditions for intensifying the upper-level divergence caused by the cut-off low phenomenon and on the other hand, increasing the moisture transport, so that the intensification of low level Jets, intensifies the instability caused by the cut-off lows.The amount of atmospheric moisture transport reveals that the main sources of moisture in Continuous rainfall periods caused by cut-off Low are Indian Ocean, Atlantic Ocean, Mediterranean Sea, Red Sea, Arab Sea and Black Sea respectively.

One of the world's major mineral dust source regions lies along the border between Iran and Afghanistan. In this study it is hypothesized that a low-level jet may play in role in generating the intensity of this source region. The presence of a low-level jet east of the Seistan mountains is documented here for the first time. The jet exists mainly from May toSeptember and has a core at 850 mb. Maximum speeds are at 18 UTC and 0 UTC and occur in July-August. In the mean the core speed attains 20 ms-1 in those months. The jet attains a maximum at night, a minimum during the day. However, the vertical motions associated with it do not fluctuate greatly over the course of the day. The development of the jet during a dust outbreak in 2002 is also described. It arises a day or so before the outbreak and disappears as the outbreak ends.

Low-level jets play a key role in the formation of dust storms in arid regions. The ‎purpose of this study was to investigate the ability of the WRF_Chem model to simulate low-level jet ‎and determine its criteria in local dust formation in Khuzestan province.For this purpose, dust was ‎selected during March 13 and 14, 2012. The synoptic structure and wind field were ‎analyzed using ERA-Interim data. The simulation ‎was also performed with dust shceme GOCART and AFWA and planetary boundary layer ‎scheme ACM, Bo, MYN, MYJ, UW and YSU. ‎ ‎ Wind profiles with LLJ characteristics were illustrated of those stations that ‎located to the right of the maximum wind 925 hPa . The LLJ broke in the mid-day results in a increasing surface wind speed up to 13 〖ms〗^(-1)and local dust emissions. The model simulated low-level jet metrics including wind speeds 20 to 23, positive wind shear ‎between 8 and 18 and negative shear 4 to 13 〖ms〗^(-1)‎‏.‏The WRF_Chem model simulated stronger ‎LLJ than the ERA_I data. The correlation coefficients of 0.83 and 0.7 for surface wind speed and of 0.68 ‎and 0.72 for PM10 obtained by MYN scheme with GOCART and AFWA models respectively. The most matching daily variation of PM10 were obtained ‎with the MYN boundary layer and the AFWA dust scheme. The MYN boundary layer scheme was ‎chosen as the best scheme for local dust simulation in this dust strom. However, all ‎of the schemes ‎showed a surface PM10 concentration of more than PM10 observations.‎

As a core of wind speed, Low Level Jet (LLJ) of the Persian Gulf is made on the Persian Gulf and its surrounding in the low levels of the atmosphere during the hot period of the year.  Known as north wind, this jet appears in the body of a more extensive current of wind with the northern, northwestern, southern, southeastern direction. North wind often blows from the mountainous regions of Turkey and Iran to the southern regions. Except for topographic reasons, the formation of this wind is influenced by hollow topography of low regions of Mesopotamia and Khouzestan appearing as a corridor. Reaching the Persian Gulf, this phenomenon is intensified as the water area of the Persian Gulf is besieged as a low hollow by Zagros Mountains and Arabic Peninsula aggravating the wind.

Nocturnal low-level jets (NLLJs) occur frequently in many parts of the world. These low level jets are important in heat, dust, moisture and even insects transport for long distances; hence their characteristics have been the subject of many studies. There are many regions in mountainous areas of Iran that experience NLLJs for which the NLLJ over the Dashte Kavir (DK) is an important one that occur in summer months. The occurrence and other detailed characteristics of NLLJs over the DK in the north desert region of Iran and south of the Alborz chain are not well known. There are not much observational wind profiles available to study the jet in the region. So, we have used the ERA-Interim Reanalysis data that provide long enough historical grid data that can be used for this kind of studies. This paper climatologically presents the occurrence of NLLJs and its characteristics over the DK by analyzing multi-year ERA-Interim reanalysis. We have compared the reanalysis data with surface observations from 11 synoptic stations in the DK region in the long period (1979-2017) and also a 40m tall observation platform to find correspondence between the two. It is found that the ERA-Interim data set can capture the real atmospheric parameters and thus can be used to study NLLJs’ features in the region. The NLLJs occur in most of the summer nights, which are primarily easterly to northeasterly. The jet core typically appears at 850 hPa at the top of the surface inversion layer with monthly average speed of 9 to 14m/s in its core. Based on the Inertial oscillation theory or Blakadar (1957) effect, most NLLJs are located above the nocturnal inversion during the warm season nights, while during the cold season, the wind regime changes and weak westerly winds dominate in lower levels in this area. Based on the 6 hours’ resolution of the available reanalysis data, NLLJs above the inversion have strong daily oscillations and the maximum wind speed occurs at 00 and the minimum at 12 UTC and an annual cycle with a mean monthly maximum speed in June. We quantified mean monthly NLLJ parameters using some definitions and construct magnitudes of the NLLJ in every grid point in the DK region. The magnitude of the momentum in the lower atmosphere from the top of the surface layer to the top of the mixed layer is large for NLLJs of the warm season and using the bulk Richardson number, in a few case studies, the downward momentum transfer in weak stability conditions leads to dust rising in the region. The winds below the NLLJ core to the desert surface gain strength in summer, and these summer winds are coincident with an enhancement of rising dust that reduces visibility in the cities in the desert margins. The nocturnal jet seems to flow along the southern mountain range of Alborz that extents far west even to the Tehran greater plain. Such strong flow may have implication for air pollution ventilation of the Tehran area in summer. This phenomena can be interesting subject for future study in this area (Tehran) that suffers from acute air pollution episodes.

Since the wind pattern on various activities in islands as well as its effect on other meteorological parameters is important long – term temporal and spatial variations of the wind field are studied. Here, the warmest month (July) and the coldest month (January) 2015, are selected in order to test the sensitivity of low-level wind simulations of the Weather Research and Forecasting (WRF) model to the parameterizations of the boundary layer (PBL), the surface layer (SL) and the land surface (LSM) over Qeshm Island. As this work was focused on the simulation of near-surface and vertical wind profiles, the physical options related to the parameterizations of boundary layer processes (SL, PBL and LSM) that have significance influence for this purpose are validated. Although more physical options are available in the model (for cumulus convection, short and long wave radiation, microphysics and etc.), it is not feasible or necessary to include all the model configuration options in the sensitivity analysis to obtain an efficient model configuration optimization. The model grid comprised of four nested domains at horizontal resolutions of 45, 15, 5 and 1 km respectively. The innermost domain (D4) with 1 km spatial resolution covered the chosen area to simulate PBL wind field over Qeshm island region. The results of the simulations under five different configurations are validated with the observational wind speed data of Qeshm Airport and Marine Qeshm Stations. The results demonstrate that in both episodes, the ACM2 boundary layer scheme has presented the best performance in combination with the Pleim - Xio surface layer and the Noah land surface schemes because it considers vertical mixing both local and non-local in simulation of planetary boundary layer wind structure. The simulations of WRF are sensitive to warm and cold seasons as well as selected parameterizations. After selecting the appropriate configuration, the simulation of the wind field for one year was carried out to investigate the low level wind field, the vertical structure of the boundary layer wind and the impact of the land mask distribution on and around the Qeshm Island. These simulations indicate higher wind speed in spring and summer and the roughness of the island causes a low level wind convergence, then turn to the left on the Strait of Hormuz with decreasing wind speed. Monthly average of the wind direction during the daytime of reference month of each season are generally simulated to be southwesterly (January, April, July, October) and during the nights of January and July it is southerly to southeast and in April and October it is simulated southwesterly. The direction of the wind has significant variations at sunrise and sunset due to changes in regional scale forcing and baroclinicity behavior between the sea and the coast. Surface roughness in coastal areas, strait narrowing and sea breeze, enhance the low-level jet during summer and spring middays at altitudes of about 180 to 200 meters. In other words, we can say these low-level jet (Shamal winds) during summer and spring occurs as a result of the interaction of two pressure systems; the heat low pressure cell (low level cyclone) over Iran and a semi-permanent high over northwestern Saudi Arabia and it acquires some convergence because of the these factors.

In this paper, using reanalysis ERA5 data and observational data, the effect of Low-Level Jet (LLJ) on dust in the west and south-west of Iran during the period 2007-2017 is studied. The vertical wind shear transfers the momentum from the jet level to the surface and then transfers dust from the sources, so to identify the LLJ besides the maximum wind speed, the vertical wind shear is also considered. Moreover, the horizontal and vertical range of LLJ, average of long term frequency of LLJ and monthly and seasonal frequency of dust are calculated in this study. The result of the comparison of observational data with ERA5 data shows that 10 m wind speed of ERA5 is larger than of observational data. Strong north to south pressure gradient due to the high pressure system over Iraq and Zagros and the low pressure system over the south-west of Iran, is the most important synoptic forcing that causes strong north and north-westerly and LLJ winds over the study area. Furthermore, region topography, land-sea breeze, weak surface tension over the Persian Gulf, temperature differences and special heat capacities between land and sea are other factors that form and affect the intensity of LLJ. The monthly wind speed distribution shows the maximum average wind speed 10 m/s in July and its minimum 2 m/s in January. The average height of the boundary layer varies from 2400 meters by days to 150 meters at nights, which is one of the main causes of LLJ. The highest frequency of dust in Ahvaz was in July 2009 with 30 days of dust

In this work, the role of the Persian trough and low-level jet (LLJ) in the north wind (Shamal) intensification were studied as the main cause of extensive dust storm (EDS) formation in the warm period of western Iran. In this regard, visibility reduction criteria of less than 1000m and the code of 06 (as a dust event) in more than 50 percent of the stations were refined in 21 stations in the west of Iran (Khuzestan, Ilam, and Kermanshah provinces) during 2000-2009. From a total of 346 dusty days, 28 specks of dust with duration of 1 to 12 days were detected, of which 20 are related to the warm period of the year (especially in the months of June and July). Further, the pressure patterns, potential vorticity and temperature, divergent field, wind vector and vertical profiles using NCEP/NCAR data and ERA-Interim data with 2.5 and 0.125 degrees resolution, respectively, in the range of 10N Up to 60N and 20E to 75E were extracted at all atmospheric levels. The results showed that the warm period dust storms in western Iran were related to the low-level pressure gradient in the Persian trough, which although it is a source of low-pressure monsoon, the strengthening of this low-pressure is due to the local and topographic factors of the western windward of Zagros. The deepening of this system by strengthening the northern wind is the cause of the formation of dust storms in western Iran. In addition, EDS rarely extend beyond 1000m due to the limitation of strong winds to low levels. The speed of these winds often exceeds 50 km/h. The severe nocturnal inversion, which generally extends at 400-450m height, causes an intense wind speed gradient, and developed a Jetstream, often overnight, at 250-350m height, that affected by the Blackheads heating system.

Tropical storm is a one of the major hazards that threat the southern coastal zones of Iran. Understanding of such hazards and knowledge of the time of their occurrence could be useful in the management of accidents caused by them. The aim of this study is analysis of the dynamism and hazards of recent tropical storm formed over the Arabian Sea is known as the Hurricane Nilofar. The data used include re-analyzed data of SLP (Sea Level Pressure), Geopotential Height, wind (U and V components), Omega, specific humidity, CAPE (Convective Available Potential Energy), Vorticity advection and SST (Sea Surface Temperature) for Nilofar storm activity in end days of October 2014 obtained and plotted than were analyzed. The results showed that the depth of the trough level of 500 mb with the axis southwest - northeast, creates a cut of low on 25 and 26 October on the Arabian Sea that following the practice causing a divergence in the eastern side of the cut of low on level of 500 mb and creating a strong convergence zone in the lower levels of the atmosphere and on the surface of the sea. Eastward movement of trough on third day of the formation of hurricanes and out of the activity storm, also, its change the mechanism of action following the availability of energy from the ocean surface (conversion of thermal energy into mechanical) to strengthen the updraft and downdraft currents on the wall of the eye and eye of storm has helped, as of this day and the next day the storm activity, increase speed to low level jet stream than the upper levels of atmosphere, causing the energy source the storm is chanced from upper levels to the lower levels of the atmosphere, also the interaction of tongues and anticyclonic centers located on the Arabian Sea, direction and movement of the storm has created to overturn it on 31 October.