Synoptic Analysis and Path Simulation of Severe Dust Storms

According to the definition of W.M.O., dust storms are storms which reduce the view visibility to lower than 1 Kilometer. So in this survey the synoptic conditions associated with the occurrence of dust storms in southwest of Iran, are analyzed. To investigate the factors influencing the occurrence of dust storms, 500 cases of dust phenomena (code:06) were selected and analyzed using 4 synoptic stations data (Ahwaz, Khoramabad, Ilam & Lorestan) for the spring and summer seasons in a period of 13 years (2000-2012). For statistical analysis of these phenomena, in the first step, dusty days were categorized in 3 catergories according to their view visibility: less than 2 Km, 2-4 Km and more than 4 Km. Then distribution frequency analysis tests using SMADA software were run for cases with view visibility of less than or equal to 1 Km and finally cases with view visibility of 500 meters which were also comprehensive and recorded in all selected stations were extracted. In next step, 3 dust storms which had been selected were analyzed. After receiving the NCEP/NCAR reanalysis data, using GRADS software, maps of geopotential heights, wind speed and vorticity advection were produced and analyzed. In order to identify the source of dust generation, tracing and simulating the path of dust particles, HYSPLIT model Lagrangian approach of backward trajectory was used. According to the findings of the synoptic analysis, in all 3 severe dust storms, existence of low pressure cells in the central regions of Iran, led to the convergence and air intake at near-ground levels and divergence at higher levels and negative vorticity in southern regions of Iraq and relatively strong positive vorticity in the Iran's central areas, resulting in cyclonic movements (dust catcher movements) in the southwestern regions of Iran. These conditions resulted in the transfer of dust from the dry desert areas of Iraq and high speed winds in the lower levels of dust particles in the region accelerated the transfer rate. Based on the HYSPLIT model outputs, major sources of dust generation included arid zones of Southern Turkey, Iraq and Syria deserts which entered the under studied areas from west and northwest directions.