سمانه نگاه

The continuous and expanding process of global warming, especially in the Asian region, has provided the conditions for increasing drought and the spread of desertification. Many deserts had ecologically balanced soil conservation conditions that until recently have become new sources of dust generation now. Numerous examples have occurred in Iran due to its special geographical location among some of the most important deserts in the world. Temperature anomaly (about 8º C) last winter in the Caspian Sea basin has created new dust sources for the southern coastal of the Caspian Sea. On 30-31 May 1400, dust emission was recorded in meteorological stations of Gilan province in terms of area and concentration. The implementation of HYSPLIT chemical backward models shows the emission of dust from the northwestern region of the Caspian Sea to the southern coastal of the Caspian Sea (Guilan province) for the first time with such intensity. The source and origin of this dust was identified in the Rhine desert in the northwest of the Caspian Sea. Continuous and unprecedented warming in the region and accompanied by strong north-south currents provided the conditions for the emission of this dust. Due to the origin of the emitted dust as well as the geographical and topographical conditions of the Caspian Sea basin, the level of this dust was assessed from the ground level to an altitude of less than 1500 meters. Analysis of synoptic conditions using NCEP / NCAR analysis data with 1 degree horizontal resolution indicates the establishment of high pressure air mass with a center of 1018 hPa on the northwestern parts of the Caspian Sea and the penetration of high pressure to the southern coastal areas of the Caspian Sea. Due to the appropriate pressure gradient and increasing wind speed, dust-producing springs are formed on the desert areas of the Rhine and with the dominance of the northern currents (south-south), the dust mass is sent to Gilan province.

The Manjil wind is one of the Iran atmospheric phenomena that is blowing in the southwestern of the Caspian Sea and northern region of the Iran plateau. Daily regime with the specified direction and high intensity of Manjil wind cause to the first windy farm is foremed in this region. In this study the reanalysis data of NCEP/NCAR were used to identify synoptic weather systems during the period 2010 -1980. At first, the pressure gradient between northern and southern Alborz were obtained and were compared with observation data of Manjil wind intensity then synoptic patterns of wind were obtained during different months. In synoptic scale, long -term mean of sea level pressure indicate Siberian high pressure system in the cold months and the AZORZ high pressure system from the West Europe to the southern coastal of the Caspian Sea in the warm months lead to the formation of the North flows along Sefid Rood valley. In the warm season, meriditional extension of Pakistan monsoon low -pressure and Saudi Arabian thermal low - pressure toward the Iran plateau cause to increasing the pressure gradient and wind speed. long -term pressure variation for hours of maximum wind speed (12UTC) and minimum wind speed (00UTC), indicate radiation and thermal conditions are very important factors on both sides of the Alborz mountains, especially during the hours of the on the wind formation mechanism. The narrowing of the Sefidroud valley as a secondary force is the main cause of channelization and increasing of the Manjil wind velocity

The Gap winds are low-level winds associated with geomorphological features such as mountain passes, canyon valleys, and gaps Gap winds are usually significant when there is a considerable pressure gradient on two side of the gap. The Gap winds, in addition to intrinsic values for study, also have practical aspects, including in the field of wind energy use, architecture and development plans and plans, maritime transport (if they are located in the vicinity of water areas), air transport, operations Military. In the present study, for the first time in Iran, the mechanism of formation of Gap Lut wind in the form of climate has been studied and the characteristics of this wind have been introduced including hourly, daily, monthly and seasonal behavior. Due to the wind in the eastern part of the desert plain, Lut desert and Jazmourian plain, the name of this wind, which is known as mountain wind in Shahdad region, was named Koloja wind. Ko, from the beginning of Kavir letter, Lu, from Lut and Ja, from Jazmourian.The data studied in this study include three main categories, which are:First, for the initial identification of the characteristics of meteorological quantities and their statistical analysis, the data of 75 meteorological stations have been used in the country at intervals of 3 hours and in the period from the beginning of establishment until 2018.reanalysis data (Era-Interim) of European Center for Medium-Scale Atmospheric (ECMWF) were used to study, the meteorological parameters and structure of patterns like sea level pressure, geopotential height and temperature for standard pressure levels, relative and Specific humidity in low-troposphere, wind field, relative vorticity, wind, vertical velocity, convergence and cross section of the relevant quantities were studied by temporal and spatial intervals 3-hour and 0.125 degree resolution (in terms of latitude and longitude during the period of 1987-2019The study of 10 m surface wind pattern with a scale of 10 km in the eastern regions of Iran shows a northerly wind with a sharp and specific pattern in these areas. This north wind is divided into two branches, western and eastern, on the north of Khorasan. This wind on the eastern borders of Iran on the Sistan plain is known as the 120-day wind. The western branch of this wind, which blows on areas east of the desert plain, Lut desert, Jazmourian, is less known in Iranian sources. The monthly pattern of both branches of this north wind is relatively the same and their peak activity is in August. The strengthening of surface heating conditions, both in the field of sea and land, has caused a spatial expansion in August; Increase the altitude and relative integrity of these two winds. This wind also has a specific daily pattern. The peak of north wind strengthening in both branches is 12 and 18 UTC. The presence of a high-pressure belt is in the areas between 50 and 70 degrees north with a European high-pressure core in the north of the Caspian Sea and west of the Urals. This high-pressure belt, in contrast to the low-pressure belt prevailing in the subtropical region over the Middle East, causes wind conditions to blow from the north to the eastern regions of Iran. Existence of high Pamir-Hindu Kush-Himalayan mountain system in the east of the region, especially its western massif, Pamir on the one hand, and the existence of lower and of course important mountain systems such as North Khorasan, South Khorasan as a secondary induction plays an important role in channeling the north wind. And have a bias towards the northeastern regions of Iran. In the southern part of low-pressure belt, the presence of low desert zones as well as the warm water zone of the Oman-Persian Gulf has provided the conditions for the formation of a large low-pressure center on this water zone. In addition, on the flat and desert areas of the region, the increase in solar radiation (snsible heat flux) caused by short-wave solar radiation, which is accompanied by a deep high-altitude system in the middle and upper troposphere, leads to the formation of local independent low-pressure cells such as Gang, Rigistan. , Baluchistan, Sistan, Lut, Jazmourian.The mountainous region not only lead to the formation of high pressures affecting the wind and strengthen its intensity, but also the topographic arrangement of the eastern region of Iran in the form of parallel meridian mountain ranges on both sides of Lut plain, is the main cause of strong winds in Koloja. The combination of regional and local synoptic conditions along with the mountain walls on both sides of the flat desert area of Lut has provided the conditions for the Bernoulli effect and the creation of a wind chat during the day in this area.The direction of the wind is slightly variable, depending on the surface topography and the rough arrangement of the stations. Shahdad station can be considered the best place to study the wind conditions. Warm season rosewind show a stronger role of this wind in the hot season than the cold season at all stations. This issue has the role and effect of different spatial radiation pattern conditions as well as the uneven role in pressure distribution compared to cold season synoptic patterns. The windrose show the 24-hour variations of wind behavior of Koloja. The behavior of this wind shows that this wind is highly dependent on daily radiation and heating conditions. At night, especially at 3 UTC, in all control stations, especially the southern stations located in the desert and lowlands, the prevailing wind is the wind that blows from the surrounding highlands and does not indicate the direction corresponding to the Koloja wind. The core of the speed of this wind is at an altitude of 1500 meters above sea level. Depending on the height of the study area, this accelerating current is formed at an altitude of 1000 meters above the surface (low level jet stream).

In this study, using reanalysis data  (Era-Interim) of European Center for Medium-Scale Atmospheric (ECMWF) ), the meteorological parameters and  structure of patterns  like sea level pressure, geopotential height and temperature  for  standard pressure levels, relative and Specific humidity in low-troposphere, wind field, relative vorticity, wind,  vertical velocity, convergence and cross section of the relevant quantities were studied by temporal and  spatial intervals  3-hour and 0.125 degree resolution (in terms of latitude and longitude during the period of 1987-2019. The results show the existence of independent thermal low-pressure systems in the form of single cells with local dimensions among the holes and plains of the Iran plateau corresponding to warm cores. Thus the theory that the low pressure conditions in the central regions of Iran, especially in the southeast and east, is the result of the spatial development of the Ganges system (Pakistan), was questioned. In other words, within the vast low-pressure belt of South Asia, different topographic, geographical, and surface cover conditions have led to differences in  radiation absorption  and the formation of local low-pressure centers. These independent low-pressure cells include the low-pressure systems of Dasht-e Kavir, Kavirlot, Rigistan Desert (Afghanistan), Balochistan Plateau (Pakistan) and Jazmourian. Rigestan low pressure and Lut desert low pressure have the highest frequency. The maximum height of these low pressures in Lut and Rigistan is up to about 500 hPa. The warming of the Iran plateau that increased their occurrence frequency in the warm season of the year,  more extension and high intensity, and negative pressure anomalies in the middle of the day, indicates the role of short-wave solar radiation of in amplifying their cyclonic rotation. The cross section of the relative vorticity and the vertical velocity indicate their limited extension to the depth of the troposphere and their thermal mechanism, and the positive and negative changes of these parameters confirm their local characters as separate cells. The formation of these low pressures in the vicinity of thermal mountain high-pressure systems has limited the spatial expansion and integration in the form of a large pressure system on the interior plains.

DissectionThe first wind pattern which was identified, northern wind that its intensity and direction and durability is considerable. This northern wind, blows due to the pressure difference between the Asian high pressure and the low pressure Oman-Persian Gulf. It has two main branches. Eastern branch and western branch. Its eastern branch is the famous and well-known 120-day wind of Sistan. The western branch of this seasonal wind blows in the eastern part of the desert plain and the Lut area. Due to blowing between the desert area of Kavir-Lut and Jazmourian plains, we named this wind, Koloja. The mentioned wind is one of the first new findings of this research. The synoptic pattern and mechanism of formation of the Koloja wind is similar to the 120-day wind of Sistan. In other words, they have the same origin, but their range of flux and emission is different as two separate parts according to the topographic conditions of eastern Iran. wind cross section, on 30 degrees latitude shows two high speed cores in the low troposphere in the range of 59◦ E to 64 ◦ E longitude, Sistan and Koloja separately Which confirms the result of synoptic patterns. When the pressure gradient decreases between the Central Asian and North Khorasan high pressures and the Oman-Persian Gulf low pressure belt, the role of Lut independent low pressure is strengthened. As the role of Lut low pressure is highlighted, the resulting convergence conditions can be displayed in the surface wind pattern. However, the pattern of winds in this region does not change significantly, especially in the Lut region, and does not necessarily reflect the convergence conditions due to the Lut thermal low pressure. The wind pattern on the Lut Desert shows the direction and intersection of winds from both the western and eastern regions of Lut, towards the center of the Lut Plain. The reason for this condition can be attributed to the constant presence of thermal high-pressure cells over the parallel high mountain range of Lut. Existence of high land areas above 4000 meters in the west of Lut plain, and the presence of Taftan Mountain along with the mountain of South Khorasan, leads to the creation of two permanent high-pressure cells, especially in the warm season in these areas. in the situation where see the formation of a strong local system on the Lut desert and the desert plain, which has a certain shape in terms of both spatial expansion and minimum pressure, the wind pattern on a large part of the central plains of eastern Iran becomes a broad convergence that The focus of this convergence is a large rotation due to the presence of this strong low-pressure cell, which is mostly located on Lut or east of the desert plain. The formation of this convergence is where thermal conditions cause the simultaneous formation of low-pressure cells on Lut and east of the desert plain.

Radar is a remote sensing instrument that sends electromagnetic waves with specific power to atmosphere and evaluates the amount of return power. It can then measure the difference between the send and retune powers and detect atmospheric phenomena as clouds. Using this tool, there is a wide, continuous and integrated monitoring of atmospheric phenomena. Like any remote sensing device that has, data of weather radars can also have errors. One of the most important measures to eliminate or minimize the radar data errors is calibration, and correction of radar index coefficients. The purpose of this paper is to extract an appropriate relationship for precipitation intensity related to radar reflectivity in Gilan. The Gilan ground based radar installed at the Kiashahr Port is a German-made GEMATRONIK MTEOR1600C type operating in the dual-polarization Doppler radar frequency band (c-band). In general, in order to calibrate the weather radars, “a” and “b” coefficients are required to modify in the Marshall Palmer initial formula for the target area. For this purpose, we tried to estimate the coefficients of this relationship (the relationship between precipitation intensity and radar reflection intensity) in a three years period (2012-2015) and to find new coefficients. In this study, the Doppler filter method (IIR Doppler Filter) was used to remove clutters. This filter was installed in the signal processor. In order to calibrate the Gilan radar, the rain gauge data of the Rasht airport synoptic station was compared with radar data. In this way, the precipitation statistics of the meteorological station were extracted using available meteorological and scdata software in the selected period and were separated based on two views of the season and precipitation severity.
Then the precipitation intensity was calculated based on radar data. Due to the large amount of raw data in Rainbow software, the data format was converted from binary to text. In the next step, the power regression is made between meteorological radar data ( ) and the automatic rain gauge (in mm / h), based on the existing default coefficients. Then, the new coefficient (a’ and b’) were determine by introducing the linear equation, a (a') and a new b (b'). In the third step, the precipitation intensity was re-calculated by applying new coefficients in radar measuremets. Now, there are 2 precipitation intensity values which are obtained by default and new coefficients. The intensity precipitation values were compared with observation of meteorological data using root mean square error in different seasons regardless of intensity. The same process was performed for the severity of observed precipitation and calculated precipitation by the radar regardless of seasons. The most important results of this study are relative improvement of radar estimation from precipitation intensity after correction of coefficients, which was 38% in March to May (spring) and 22% in December to February (winter). During the months of July to November (summer and autumn), there was almost no improvement. Also, based on precipitation intensity (regardless of seasons), average accuracy of precipitation was increased by 25% and severe precipitation by 47%. While in gentle precipitation, this method did not work and there was no improvement

In this research, effect of environmental forcing like topography, direction and sharpness of slope, shape and trend of topography on wind regime formation have been studied in mountainous stations of gilan. The regime of mountainous winds has major effects on the air conditions in mountain regions. This research had been done using hourly data of wind speed and wind direction and also analysis and fitness of topography conditions with theoretical fundamental of research. So it was observed that Masouleh station is one of the best mountains stations in the subject of mountains climatology. Suitable location of this station in the middle slope caused to affected region by upslope wind and down slope wind (katabatic and anabatic winds). Location of Deylaman station on southern slopes of Alborz Mountains had important effect on wind speed decreasing. Wind occurrence mechanism is Mountain wind type along vallay in Deylaman station. Secondary dominant wind was north-northwest thus daily and seasonal behaviors of wind and location of Caspian Sea and Alborz Mountain represent affecting the region by sea-land breeze system. Jirandeh station’s behavior is as an exclusive mountainous station that indicated deep combination but imperceptible influence of the mountain on atmospheric phenomena in Mountains range. Position of station on southern slope provided appropriate conditions to form slope wind occurrence in this region. However stretching of Totkabon- siahrood valley as a wide valley that is branching of sefidrood valley provided appropriate conditions to influx sea-breeze wind. This condition caused destruction of local mountain wind effects during day and night.

Without any doubt, the Gilan plain in the northern regions of Iran plateau has the special conditions faced with heavy snowfall hazards in terms of damage amount. In the recent decade, the increase in frequency of heavy snowfall events in contrast with earlier decades, has caused a huge attention to this atmospheric hazard in Gilan province and also Iran. In most studies that were done on heavy snowfall events, the target was analyzing the synoptic-dynamic structure of case study and long term events. Overall, the main question about heavy snowfall in Gilan plain was the cause of this event in the central plain and another was the movement of the maximum snow depth area in this region from east to center and west. So, what is the structure for super heavy snowfalls in Gilan central plain in recent decade? What were the causes for the formation of two different spatial patterns of these three snowfalls?
The snow depth of the west, central and east regions of Gilanplain in the super heavy snowfall events (Gilan Meteorological Office).
Total snow fall (cm) Western plain Central plain Eastern plain
Station Astara Talesh Bandar Anzali Agricultural Lahijan Roodsar
7th -12th Feb 2005 10 No data 3 170 120 No data
7th -14th Feb 2008 36 59 204 136 91 66
30th Jan-4th Feb
2014 34 3 95 40 45 75
Material and 1- GFS-FNL data with horizontal resolution of 0.5 degrees as an input to the WRF 3.5.1 version.
2- Hourly data of meteorological parameters in the 12 synoptic stations of Gilan province for statistical analysis of used atmosphere characteristics
3- Visible band images and 1-2-7 MODIS sensor for Terra and Aqua satellites at the time of the snow falling.
4- Kiashahr (Gilan) radar outputs at the time of the snowfall
In this investigation, the role of effective regional factors and geographical components on formation of the maximum snow depth patterns in three systems with super heavy snowfall in Gilan province of Iran is investigated using numerical model of WRF with horizontal resolution of 7 Km. The overall results are as follows: In large and mesoscales, the source of these atmospheric systems are the cold anticyclones of North Europe and the semi-permanent anticyclone of Siberia that extend to lower latitudes with strong depth and its cold flow affects the northern part of Iran and also the south coast of the Caspian Sea.
The precipitation output of WRF numerical model for 7 Km horizontal resolution, perfectly enhances the spatial pattern of snow depth in these three recent systems that includes Gilan central plain and near the south of Anzali wetland. The important point in 10-meter wind output in the region is that in all three systems with the southerly movement of cold high pressure center to lower latitudes, cooling over the Caucasus and the high mountains of this area is intensified. These situations are seen appropriately in the 2-meter temperature pattern. Formation of cold flow from mountain to plain f caused by the existence of cold cores over the big and small Caucasus Mountains, led to form secondary high pressure in local scale over the Kura plain.
Settlement of secondary high pressure over western coast of the Caspian Sea (Kura plain) and the weakening of pressure counter over the southern coast of the Caspian and also the temperature difference between water surface of the Caspian Sea and the land surface temperature in the west of the coasts (Kuraplain) causes the formation of easterly flows from the Kura to the Southern Caspian water surface.
These easterly flows acts as a forcing in local scale in contrast with the westerly wind that is from the high pressure mass in Northeast of the Caspian Sea and causes the convergence of wind flow in the conjunction part of them and towards the lower latitudes. The convergence of 10-meter wind flow in the form of convergence band along the western coast of Southern Caspian enters to the small area of Southwest of the Caspian in the Gilan central plain or near the Anzali wetland and causes the intensification of instability in the lower layers of Troposphere.
The maximum snow concentration area dependson the convergence part of the flows in Gilan central plain. The images of MODIS Terra and Aqua and also the Kiashahr (Gilan) radar output in the 2014 snowfallconfirm the formation of cloud band coincident to the wind convergence band over the western part of the Caspian coasts that enters to the southern parts.
In recent decade, Gilan central plain has been affected by three main precipitation systems with super heavy snowfall. In order to determine the effective weather factors in regional scale in organizing the spatial pattern of the maximum snow depth event, the structure of these systems is investigated using the WRF numerical model with horizontal resolution of 21 and 7 Km. The output of the model has an appropriate accuracy in simulating the amount of precipitation and enhancing the two cores of maximum height of snow depth. (One in the central plain of Gilan and another near the Anzali wetland). The source of these three systems is the cold and high pressure polar air mass coming from the northern part of Russia and the semi-permanent high pressure of Siberia and accompanying with the deep troughs of atmosphere medium levels. The anticyclonic circulation with strong center over the North and Northeast of the Caspian Sea causes advection of cold air in lower layers of Troposphere. The cooling forcing, caused by the development of cold air mass over Caucasus Mountains and spread of mountain toplain cold flows, caused governing of secondary anticyclone in local scales over Kura plain in the West of the Caspian Sea.
The temperature difference between Kura plain and the Caspian water surface and also thepressure gradient between Kura plain and the Caspian Southwest coasts, is accompanied with the eastern flow of the wind field from Kura secondary high pressure. That in conjunction with westerly flows caused by clock wise circulation of anticyclone over the Caspian Sea, causes convergence of surface wind direction to convergence band along with Western coast of Caspian water surface.
The Converging flows are bearers of humidity flux and enteredto the small part in Southwest of the Caspian coasts that exactly matched with the highest snow depth in Gilan central plain. In the convergence part of the cold flow in MODIS sensor images of Terra satellite and also the image of precipitation intensity of Gilan radar output, thecloudy band is seen that confirms the results from numerical simulation and synoptic analysis.

Heavy snow is one of the atmospheric hazards that sometimes occur in Gilan province. In this study, the maximum height of heavy snow location that occurred in February 2014 in Gilan province was recognized and the cause of the event was analyzed. For investigating this phenomenon, data from NCEP/NCAR (reanalysis), local synoptic stations, MODIS satellite image and WRF model were used. The results showed that the formation of a secondary high pressure center in the local scale, due to the cold advection forcing of Caucasus topography and the movement of cold air from Caucasus Mountains to the Kura plain caused to flow eastward wind cross with West-ward streams by polar high-pressure air mass which is located on the northeast of the Caspian Sea. The convergence of surface wind as the convergence band along the western shore of the Caspian Sea enters to a small range in the southern of Anzali Wetland. Cloud band is in accordance with the wind convergence zone that can be seen in the image of Terra satellite MODIS sensor and the Gilan radar precipitation intensity image. The maximum latent heat flux core on the Southern zone of the Caspian Sea as the thermodynamic forcing confirms the impact of regional factors on the spatial pattern of maximum snow depth in the East regions of Gilan.

Geographical factors directly create some weather phenomena or indirectly are effective on increasing or decreasing of atmospheric features intensity. One of these geographic factors consists of lakes or small to medium closed water bodies within the continent areas which have various effects on atmospheric conditions of their surrounding areas and have been historically known. Lake effect on climate covers micro to synoptic scale and varies with extent, depth and shape of the lake, speed and direction of winds, winter ice cover and global climate conditions facing the lake. One of the phenomena caused by the lake is the lake snow effect phenomenon. This is one of the reasons for the formation of snow, in addition to mountainous forcing, frontal and convergence activities. Lake snow distribution depends on various elements. The most important climatic factors are the difference between the temperature of lake surface and the temperature of air above it, wind speed and direction, stability, latent heat, and relative humidity. In addition to weather conditions, the geographical conditions of environment such as the extent of ice cover, passing path (fetch) etc. play a major role. Also several case studies have been done about Guilan heavy snows most of which related to synoptic patterns. In this study, we have tried to study the role of Caspian Sea on the heavy snowfall in Guilan plains using atmospheric, marine and land data and to answer the question that whether the mechanism of lake snow effect phenomenon is involved in heavy snowfall of Guilan central plains?
In this study, in order to test the occurrence conditions of lake snow effect in the southwest of the Caspian Sea as a well-known phenomenon which has a specific mechanism to heavy snowfall, factors affecting the formation and evolution of atmospheric systems caused heavy snowfall in southern coast of the Caspian Sea were studied for the first time using atmospheric, environmental and marine data. Thus, the research is organized into two main sections:In the first section, in order to study the meso-scale structure of atmospheric circulations and identify of synoptic-dynamic pattern in heavy snowfall hazard event of Guilan plains, the synoptic method of "environment to circulation" is used. To determine the days with heavy snowfall, data of snow height in Rasht meteorological station as the main station were used during (2012-1982). Based on the threshold index and local experiences, reviewing historical sources and long-term statistics and due to the extension of the crisis in society, snow depth of 40 cm in 24 hours was selected as the heavy snowfall. Guilan plain has experienced 7 years with heavy snowfall during the past 30 years. In the next step, in order to synoptic-dynamic study of these identified systems, the daily re-analysis data of sea level pressure and temperature fields, geopotential height, zonal wind vectors and vertical velocity of lower, middle and upper levels of atmosphere were collected from National Center for Environmental Prediction (NCEP/NCAR) on network including Iran, with horizontal resolution of 2.5 degrees. In the second section, the daily data of sea surface temperature (SST) of Caspian Sea were derived from AVHRR sensor of NOAA satellite in 0.25 degrees scale to prove the impacts of the Caspian Sea on the systems and investigation of physical and thermodynamic properties on the intensification of snowfall. Daily and monthly means of SST maps and maps of difference between SST and the 2-meters air temperature and also the sea surface temperature anomalies were produced and examined for these systems using GIS software.
The mentioned systems, were classified into two main categories of combined pattern of low pressure and high pressure and high pressure pattern based on the main sources of air mass at the surface which create them. In the next step, these two categories were separated into more distinct categories and studied based on the type of high pressure air masses with different origins such as polar, oceanic and continental, as follows: 1- Combined pattern of low pressure and high pressure (February 2005), 2- high pressure pattern which includes: 2.1- Siberian high pressure pattern (March 1985, January 2001), 2.2- European polar high pressure pattern (southeast of Scandinavia) (February 1993, January 2008), 2.3- migratory high pressures pattern of Europe West (January 1989, February 1982). In total, the high pressure air masses with different origins such as polar, oceanic and continental are exist in all 7 waves of heavy snowfall, which from the higher latitudes, alone or as pairs with lower latitudes low pressures (Mediterranean low pressures) led to heavy snowfall in the southern coast of the Caspian sea with dynamic and thermodynamic mechanisms that carry the heat flux and proper humidity. In addition, the mean daily of SST map show the warm area on the southern half of the Caspian Sea surface before snowfall. The high temperature contrast was on the southwest of Caspian Sea because of the transmission and the existence of cold air during the snowfall. In other words, cooling of the air in low troposphere creates temperature difference that cause the interaction between temperature and moisture characteristics of the two fluid and transmission of the surface fluxes to the air masses crossing over it. Despite these differences in the origins of air masses formation, there are some similarities in the mechanism and function of these systems; it seems that the vast water area of the Caspian sea has the significant impact on the transfer of moisture to the passing air and increasing its degree of instability.
Like the other mid-latitude lakes, location of the Caspian Sea on the poleward face of subtropical high areas has faced it to the cold polar and sub polar air masses. So the environmental conditions have provided a suitable conditions for the formation of lake snow effect phenomenon. The location of southwest coast of the Caspian Sea and the direction of coast line in the Guilan central plain face to the entry of atmospheric systems, flow direction and wind convergence have provided the conditions for the occurrence of this phenomenon and heavy snowfall in the Guilan plains. The shape of Caspian sea and its meridional extension lead to air advection in polar high pressure systems and, which pass fetch about 700 to 1000 km on the Caspian Sea. The fetch of flow over the Caspian Sea is more than all the water bodies in the northern hemisphere. The higher depths of the Caspian sea in the central and southern parts, are prevented from sea freezing in these parts. This issue removes one of the limiting factors in the occurrence of lake snows effect. In addition, the SSTs of southern parts of the Caspian sea are 10.5 and 10 degrees Celsius during the year in January and February, thus the Caspian sea is as a source of heat and moisture for transferring the surface fluxes and instability of air.
In all 7 identified systems, high pressure air masses with strong (1040 hPa) and with various origins such as oceanic or continental aspects play the main role in the mechanism of heavy snowfall. Their clockwise circulation in low troposphere on the Caspian Sea cause to enter of air flow to Guilan plains. Studying the 2- meter temperature and 10-meter wind show the transmission of cold air from higher latitudes to the southern coasts of the Caspian sea and air cooling in the low troposphere during the systems deployment. Daily and monthly mean of sea surface temperature (SST) of Caspian sea indicate the existence of warm water area, according to the maximum specific humidity over the South Caspian. The southward streams cause the advection of moisture to southwestern regions of the Caspian Sea. The difference of 2-meter air temperature and sea surface temperature (SST) on the Caspian, indicate the mean temperature contrast about 25 degrees Celsius. Surface fluxes patterns (latent heat flux and sensible heat flux) indicate maximum values of the two physical parameters on the south Caspian areas among the cold air masses from the high-latitudes and warm masses of water which represent the exchange of heat and moisture characteristics in the low troposphere.

In the recent decade, the southern coastal of the Caspian Sea is frequently facing dust hazards. This research attempts to identify the original source of dust generation that causes dust transmission to the southern coastal of the Caspian Sea in 19th and 20thof September 2014, and analyze the atmospheric circulation that leads to the emission of dust into this area. Therefore, Implementation of the HYSPLIT backward Lagrangian model is used, for determining the origin and direction of dust flow, the chemical atmospheric model (WRF-Chem) is applied to determine the concentration and distribution of dust, and to simulate the system’s pattern. The results indicate anew source of the dust in the eastern part of the Caspian Sea and across the deserts of Turkmenistan and Qara-Qum. The main cause of this dust event is the formation of thermal cyclone on the warm area in the eastern part of the Caspian Sea. Surface convergence due to cyclonic circulation of the low pressure and also dry desert coverage, caused dust transfer into the atmosphere column.Negative components of U-wind and V-wind at 10m height and increase in wind speed due to the intensified pressure gradient over the desert in the east of the Caspian Sea; lead to the formation of the Northeastern streams (Westward) and dust emission into the southern coast of the Caspian Sea. The Monitoring of Visible images from MODIS sensor and zoning of AOD confirms the results of numerical simulations in dust transformation to the northern slope of the Alborz Mountains.