the study of snowfall hazard mechanisms in gilan state (1963-2012)

1. introduction heavy snow hazards, although have less frequency in comparison with extreme weather events such as freezing, but are major hazards in midlatitudes (smith, 2009, 196). heavy snow damages manmade structures, crops, casualties, even between urban and rural ways of blocking.(briant. 69, 2005. ohara et al, 2009, 1611). various definitions have been offered for heavy snow, those are classified in terms of 'intensity', 'height', 'visibility'. note that in heavy snow conditions, geographical features and the climate of each region should be considered. 2. study area central part of gilan plain has located in the southwestern of caspian sea where alborz mountain belt are surrounded the area from south of it. according to the available snow data, this plain sometimes has affected by heavy snowfall. in the recent 50 years, it has experienced 12 year with heavy snowfall. 3. material and methods in order to achieve the days with heavy snowfall, we used a combination of data that obtained from reputable newspapers (70 years) and interviews with knowledgeable people because of lack of long-term data. these snowy days were compared with daily synoptic stations data (1963-2012) to ensure that the event was heavy snow. snow storms were studied through three major methods 1- snow volume classification, 2- synoptic atmospheric conditions, and 3- case studies of major events separately. the second and third approaches are considered in this study. to study of synoptic and dynmical mechanism of heavy snowfall, zonal and meridional component of wind, geopotential height, omega, temperature, and sea level pressure with netcdf format were used from ncep/ncar on iran ,then other parameters like stream, vertical motion, convergence, relative vorticity and cross section of these parameters were plotted. 4. results and discussion during 50 years (1963-2012), 80 days with heavy snowfall were identified in 30 waves that classified in two patterns. in the first pattern, high-pressure air mass with different origins, such as oceanic and continental (southeast scandinavian arctic high pressure, siberian high-pressure and western europe high-pressure) was attended. in the second pattern, combination of low pressure and high pressure were seen, that 10 cases of 30 cases of heavy snowfalls were in second pattern. in this pattern, the role of caspian sea, mediterranean sea, black sea, red sea and persian gulf that the air masses originated there or has been strengthened with the establishment, is undeniable. deep-tropospheric dynamical cyclones from the west, northwest and southwest are imported and carrying heat and moisture fluxes to the southern coast of the caspian sea. influence and establishment of dynamical or semi permanent anticyclones from higher latitudes accompany by anticyclonic circulation and cold air advection (negative vorticity ) at lower level of the troposphere in south coast of caspian sea. this situation considerably increases the pressure gradient in the northwestern part of the iran and the southwest of caspian sea. eastward moving of trough with strong positive vorticity and considerable circulation counterclockwise in the middle and upper levels of the troposphere cause to strengthening of ascending motion in the middle and lower levels of the troposphere and convergence. temperature fields in the middle and lower levels of the troposphere show a deep temperature trough and cold advection to the southwest coasts of the caspian sea. with the formation of cold front and significant reduction in temperature, all factors appropriate for conversion liquid phases of precipitation to snow. high pressure pattern is the main pattern of heavy snowfall that 20 cases of 30 cases of heavy snowfall in the region are included. however the caspian sea is the source of heat and humidity and enhances upward motions, the role of the alborz mountains as dynamical factors in high pressure vertical motion cannot be ignored. these dynamical and semi-permanent anticyclones that are imported from the north and northwest, cause to strengthening anticyclonic circulation associated with negative vorticity at lower levels of all regions of the atmosphere in the northern part of the caspian sea. cold air advection from higher latitudes and available humidity conditions, cause a sharp reduction in temperature and snow is formed. in addition to the lower levels of the troposphere, barotropic conditions cause a positive vorticity and convergence which increase vertical movement at mid-troposphere. surface cyclones in the south of the alborz mountain and the mid-tropospheric trough cause omega be positive that represents the upward movement. despite the establishment of a strong anticyclone in the lower troposphere in guilan, omega-negative values (upward movement) is observed due to a mechanical climb mountains reveals. because of the trough in the mid-tropospheric level, omega be positive and matches with the convergence field (positive). the establishment of a surface anticyclone in the lower levels of the troposphere from guilan plain until north caspian, a negative relative vorticity and anticyclonic circulation occurs. with height increasing (from the level of 700 hpa), because of the unstable waves and their associated cyclonic circulation, the vorticity is positive and it is maximum in the middle troposphere. because of the surface cyclone in southern alborz mountains, cyclonic circulation is observed and positive vorticity increases with altitude. 5. conclusion surface anticyclones or in pairs with cyclones are the main factor for cooling and northern flux. the intensity of performance due to the location of formation, establishment and spread of air mass, strengthening and attenuation of cyclone and anticyclone center, changes of dynamical parameters (vorticity, vertical motion, convergence) due to mountain forcing, thermodynamic factors due to pass of air mass over water (surface flux forcing) during the system activity, change the speed of westerly current in middle troposphere. in some cases, exist of omega blocking in middle level of troposphere from eastern europe to eastern caspian sea, cause to stay deep trough on north of iran. cross section of wind shows a convergence zone in low level of troposphere in sefidrod delta between shahrod valley until north of manjil valley and there is an instability between 850 to 700 hpa level too. however negative vorticity, because of surface anticyclone on caspian sea, cause the descending motion, but existence of ascending motion indicative dominance of role of alborz topographic forcing in mechanical rising of air mass.