formation and evolution of a heavy dust storm over middle east: a numerical case study

introduction atmospheric processes and their interactions with ground surface responsible for formation and evolution of a heavy dust storm are important for identifying dust transport pathways. spatial and temporal distribution of middle eastern dust for a severe dust event during 4-8 july 2009 was analyzed by weather research and forecasting with chemistry (wrf/chem) model simulations, in-situ and remote sensing observations. analyses of simulated weather conditions, during maximum dust concentrations in low level, were occurred with high pressure gradient and high advection in present of cyclonic system. this condition was caused strong surface wind shear over known dust emission sources over iraq. this frontal cyclone was forced soil to eroded and dust to dispersed and transported for a significant long distance. the fine dust particles (< 1µm) with significant concentration were crossed zagros mountain range and affected tehran mega-city on 6 july 2009 and after two days left iran from north east. the simulation results were found to well reproduce spatial and temporal distribution of mineral dust concentrations according to visible images based on the high-spatial resolution nasa modis, and reported hourly visibility in synoptical stations network. the performance of wrf/chem was acceptable for simulation of spatial and temporal distributions of dust storm along affected area during simulated period. study area in this study, one heavy dust storm that was formed over iraq and then was crossed iran to turkmenistan and afghanistan north in early july 2009 is analyzed. materials and methods for this study, the wrf/chem regional model was applied to simulate the formation and evolution of a heavy dust storm during the days 26 june to 8 july 2009. the ncep final analysis data were used as meteorological initial and boundary conditions. the simulation was configured with 3 nested domains (with 81, 27 and 9 km horizontal resolution and with 41 vertical levels to 50 hpa) that the inner domain covers the area under influence of selected dust storm. the initial and boundary dust concentrations was considered to be zero during the simulation. the dust physics of gocart model, such as dust emissions, transport, dry deposition, and gravitational settling in wrf/chem was applied for dust storm simulation. in this study, the mineral dust aerosols was partitioned into 5 size bins with mean radius of 0.6, 1.2, 2.4, 4.5, and 8.0 μm, respectively. the dust emission flux in each bin p (fp) is size-resolved, which is calculated by taking into account the soil particle fraction, erosion factor, surface wind velocity, and the threshold velocity of wind erosion. result and discussion the summer shamal winds regime is prevailing phenomenon in middle east central region (from late may to early july∼40 days) that lifting dust from iraq, iran and adjacent regions are usually associated with low pressure anchored over southern iran that forms a strong baroclincal gradient with a semi-permanent anticyclone over northern saudi arabia. the convergence zone between the two pressure systems induces high-velocity, turbulent winds for regional dust transport during a time of intense convection over the tigris–euphrates floodplain due to very high surface temperatures. from mean see level pressure patterns, on july 6th, summer shamal formation mechanism is established over tigris and euphrates basin and a frontal cyclone over mediterranean region formed with significant upper level forcing. after that along the intensification of this frontal cyclone and its entrance over iraq, the dust formation and its ascending is accelerated over this source. the final erosion by this system was formed a heavy pre-frontal dust, that along its track, heavy dust was crossed zagros mountain range and was formed a long-rang dust transportation. in the beginning storm (on 4th july), iraq was suffered by high dust concentration background especially over the north due to frontal cyclone forcing and strong dust emission in this area. after approaching of cold front and surface wind shear to the iraq south, it is occurred a huge dust emission over tigris and euphrates basin. on 5th july, due to passage and forcing of frontal cyclone over zagros mountain range, great dust emitted and ascended has allowed to cross the mountains. on 6th july, due to presence of frontal cyclone over central of iran, mass of dust suffered deeply this area such as tehran and on other side; iraq was experienced lower dust concentration in this time. the presented dust distributions in last section are fully observed in spatial distribution of simulated dust concentration. the shamal wind pressure pattern is still active on 2th july and it was caused significant emission and on 4th july, shamal wind pressure pattern is destroyed by frontal cyclone forcing and it is coasted huge dust emission over iraq north and on 6th july, the frontal cyclone was positioned over central of iran with significant dust transportation and on 8th july, the shamal wind pressure pattern was again activated and dust plum was left tehran nearly. the most dust emission flux and near surface concentrations have occurred over third bin with effective radius equal to 2.4 µm and on other side, first bin with effective radius equal to 0.5 µm have played most mobility and with significant concentration over long distance from its sources and showed biggest fraction of concentration over tehran. also sand distribution in fifth bin with effective radius equal to 8.0 µm showed significant concentration only over sources regions especially during dust storm formation on 2th july. conclusion it can be concluded from this study that in late of summer with presence of shamal wind regime over iraq and after forcing of a dry deep frontal cyclone is able to produce a long rang dust storm alone the middle east. in this study, it is become clear that wrf/chem with gocart dust physical parameterization is nearly sufficient technique for dust storm simulation. it means that this configuration is able to simulate correctly spatial and temporal dust distributions during intense dusty conditions. it is clarified that dust source in iraq such as tigris–euphrates floodplain have high potential for huge emission during summer shamal and intense frontal cyclone activity during hot and dry conditions. by frontal cyclone forcing, clay and silt particles, smaller than 2 µm are able to transport several thousand kilometers from their sources from middle east central region, especially during summer. it suggests testing other physical parameterizations in order to study of sensitivity of simulations of dust storm to them and also suggests testing of meteorological and chemical data assimilation techniques by forcing in-situ and satellite observations to the simulations.