Study and evaluation of dust emission scheme in WRF-Chem model of duststorm in the east and southeast of the Iran (Case study 11 to 13 August 2018)

Since dust phenomenon has become one of the major problems in many parts of the world, a large number of models have been developed in order to predict the concentration of dust particles in the atmosphere. In the east and southeast of Iran, the occurrence of dust phenomenon causes severe financial, human and environmental losses every year. Therefore, accurate forecasting in order to provide the necessary warnings can reduce the damage as much as possible. From 11 to 13 August 2018 the report of the current weather code 07, 30, 31 and 32 in the synoptic stations of southeastern Iran, including Zabol, Zahedan, Nehbandan and Iranshahr, indicates the occurrence of storms and the horizontal visibility of the region during the storm occurred less than 1000 meters. In this study, ERA5 (ECMWF) reanalysis data set was used to obtain the required fields including sea level pressure, geopotential height, temperature and relative vorticity at 500 hPa, and geopotential height and temperature at the 850 hPa level. The vertical cross section of the wind speed from the ground level to the level of 400 hPa along with the vertical velocity are also plotted and analyzed for latitude 31 degrees north and longitude 53 to 63° E. Moreover, weather research and forecasting model coupled with chemistry (WRF-Chem) with three emission schemes, GOCART, AFWA and SHAO1,SHAO2,SHAO3, were used to simulate the dust load and amount of particulate matter (PM10) and its transportation over the studied region. The initial and lateral boundary conditions of the model simulations are provided by Global Forecast System (GFS) data with the horizontal resolution of 0.5°. Atmospheric pattern analysis shows that during the storm (120-day winds in warm season), with high pressure around the Caspian Sea and Turkmenistan and on the other hand intensified seasonal thermal low pressure on the southeast of Iran, India and Pakistan and cool air advection from the north and then strong gradients of temperature and pressure lead to strong winds. Storm simulations showed that the implementation of the model clearly showed the beginning of dust from the southeast and its emission to the east. In general, the output of the model in terms of scale and temporal variations gave a reasonable estimate of dust particles in the study area. The AFWA and GOCART emission schemes of the WRF-Chem model simulate very similar the emission source and dust storm concentration changes of 11 to 13 August 2018. However, the simulation results of SHAO schemas, especially SHAO1 and SHAO2, are completely different and have a significant error. Quantitative comparison of the model output for different emission schemas with the observational data of Zabol station shows that the AFWA scheme simulated the dust concentration more than the observations, but the trend of its changes is acceptable. The dust concentration predicted by the GOCART scheme is lower in quantity than observed. Simulation of hurricane dust particles in eastern Iran, Zabol region, Zahedan and southern regions of Kerman and Sistan and Baluchestan province (Kahnooj, Regan, Nikshahr and Qasrgand), Zahedan, Zabol (consecutive report weather code 07 of these stations) and east part of Hormozgan porivence (Jask and Minab) as the main source dust of the region. In terms of compliance at the time of the occurrence of severe dust, it has an acceptable compliance with the PM10 data of Zabol station. The simulated concentrations showed good validity of the temporal and spatial distribution of dust concentrations with respect to satellite RGB images and observation data. Synoptic analysis in the warm season indicates the presence and intensification of seasonal thermal low pressure in Pakistan, Afghanistan and east and southeast of Iran and on the other hand high pressure in Turkmenistan in the north of the Caspian Sea and in the cold season of Siberian high pressure and dynamic low pressure in eastern and southeastern part of Iran. The activity of low-level jets (severe northern currents) near Zabol region and alternating vertical motion are other effective factors in the instability of the region and the occurrence of dust storms and have provided favorable conditions for dust formation. The dry bed of Lake Hamoon, the low rainfall of recent decades, is also a very important factor in the occurrence of dust in east and southeast of Iran.