عنوان مقاله [English]
The dust has affected most parts of Khorasan Razavi in July 1, 2014. The purpose of this study is to simulate this event by RegCM model and validate its results with observation and remote sensing data. For this purpose, horizontal visibility were prepared from the Iran Meteorological Organization and AOD of Aqua and Merra-2 satellites from Giovanni web.
The result shows that the development of low heat pressure on Pakistan along with north and northeast winds of 20 m/s in the east of the Razavi province leads to the transfer of dust to the region. The HYSPLIT model and the output of the RegCM model show the dust source in Turkmenistan and western Afghanistan.
The optical depth of aerosol obtained from the RegCM model is in good agreement with the visibility in the east and south stations of the Razavi province, So that the correlation coefficient in Gonabad was -0.98, Torbat Jam -0.66 and Sarakhs -0.61. In addition, the correlation coefficient of the RegCM model and the Merra-2 optical thickness of aerosol obtained in Sarakhs and Torbat-e Jam located in in the east of the province, including, were -0.45 and -0.78, respectively.
The result of comparing the time series showed the model output in three-hour intervals is in good consistency with the visibility, the Aqua optical depth aerosol, and the Merra-2 optical thickness aerosol in the east and south stations of the Razavi province. However, the RegCM model does not estimate dust and has a time delay of 6 to 9 hours in the central and western stations.
RegCM model is a numerical climate model that has been developed very fast in recent years. The RegCM model is a climate model, but in order to evaluate the ability of the model in long-term dust simulations, firstly it is better to simulate a dust case in a regional area and analyze its results. Khorasan Razavi has a complex mountain structure. Roughness in northeastern Iran affects temperature, direction and speed of wind, so by removing the mountain structure in the RegCM4 model, the structure of temperature and local winds change. Therefore, the operation of numerical models is very important in these areas. The purpose of this study is to investigate the accuracy and ability of the RegCM4.1 model in simulating dust storm on July 1, 2014 in Khorasan Razavi province.
Material and methods
In this study, visibility data was investigated in some synoptic weather stations in Khorasan province to show visibility reduction during the dust storm. Also, ERA5 reanalysis data was used to synoptic study in this case. HYSPLIT model implemented backwardly to investigate the dust particles sources too. MERRA2 model AOD outputs were compared with RegCM4.1 model output for evaluation RegCM4.1 model AOD amounts. Also, MODIS AOD data was used in this paper.
Results and discussion
Horizontal visibility reduction has been reported in Sarakhs, Gonabad, Torbat-e Jam, Mashhad, Sabzevar, Torbat-e Heydarieh, Khaf, Dargaz and Quchan in the case study. In Gonabad and Sabzevar, visibility reached less than 1000 meters. The HYSPLIT backward trajectory shows that the origin of the dust storm was in Turkmenistan and dust has been transferred to Khorasan Razavi province in a north-south direction. The maximum MERRA2 AOT (0.8) was in the southeast of the province and the western and eastern regions have a lower value. But the MODIS/Aqua and MERRA2 model AOD was less than 0.4 in northern part of the province. The RegCM model did not simulate the southwestern dust transfer well. The RegCM4.1 model shows the maximum AOD on some areas in Turkmenistan which is considered as the location of dust source in the dust storm.
On July 1, 2014, a severe dust storm affected most parts of Khorasan Razavi and in Gonabad wether station, the horizontal visibility decreased to less than 500 meters. The results of the synoptic analysis show that the reason for the formation of dust is the development of low thermal pressure over Pakistan and the high pressure in northern Afghanistan. The interaction of the two leads to the formation of north and northeast winds with 20 meters per second speed. In the study of the output of the HYSPLIT model showed the source of the dust particles was in the deserts of Turkmenistan and western Afghanistan.
The comparison of the results from the RegCM4 model with Aqua AOD showed that they are in good agreement with MERRA2 AOT in the eastern parts of the province. But in the central and western stations of the province, the RegCM4 model did not simulate the dust well.
The result of time series comparison showed that the model output in three-hour time intervals is well coordinated with horizontal changes in the east and south stations of the province. So the correlation coefficient between them was -0.98 in Gonabad, -0.66 in Torbat Jam and -0.61 in Sarakhs. In general it can be concluded that, the RegCM model is a climatic model and it has not a reasonable performance in dust cases where the dust period is short and less than 12 hours.