A study of summer large-scale circulations and atmospheric boundary layer structure on days associated with dust in southeastern Iran (1987-2016)

This paper investigates the atmospheric condition of southeastern of Iran (Sistan-o-Balochistan) during the summer (June, July, and August) with emphasis on planetary boundary layer (PBL) characteristics and dust events. Studying the 30 years (1987-2016) averaged patterns by Era-Interim reanalyzed data from ECMWF showed that with a thermal low dominant in this region, in 850 hPa level, the mean temperature is over 33ºC in the center parts of province and relative humidity is less than 20% of the most parts of the province. It appears that corresponding to the decrease of temperature from north to south of Sistan-o-Balochistan, planetary boundary layer height reduces from 800m in the north to about 200m in the south of province in the coast of Oman sea. The frictional velocity in northern parts of the province (within the borders of Afghanistan) is more than 0.14 m / s, which can provide good conditions for dust emission from the earth surface. Observed meteorological parameters from 5 selected synoptic stations of Sistan-o-Balochistan include Zabol, Zahedan, Khash, Iranshahr and Chabaher are used to examine the summer trend in each station. In general, changes in time series at the selected stations do not follow the same procedure, and each station has special conditions. In Zabol synoptic station, the pressure decline corresponding to temperature enhancement was observed from 1978 to 2016. Moreover, maximum wind speed and relative humidity have deceased. Aerosol Optical Depth (AOD) at 550nm from MODIS-Terra has a downward trend from 2001 to 2016 at this station. The 30-year variation in dust storm index (DSI) for Zabol station is not statistically significant. But there is a significant downward trend from 2001 to 2016, corresponding to significant changes in other meteorological quantities, including significant downward trend in maximum wind speed and relative humidity with significant upward trend visibility at this station. Furthermore, calculating the correlation coefficients between maximum wind speed, visibility and DSI in the last 15-years, it shows that, as the wind speed increases, the DSI increases and, as expected, the horizontal visibility decreases. Using the reanalyzed data from NCEP/NCAR, we investigated the possible Indian summer monsoon effect as a large-scale phenomenon near this area on atmospheric conditions of southeast of Iran, especially dust events from 1987 to 2016. Calculating the correlation coefficients between dynamic Indian Monsoon Index (IMI) and meteorological parameters including temperature, geopotential height, horizontal and vertical components of wind, indicate that there is a significant decrease in geopotential height in northern and central parts of the province at lower levels of atmosphere, corresponding to increase in Indian monsoon intensity. There are no significant correlations between IMI and the other parameters in selected stations. Furthermore, there are no significant correlations between the convective index (CI) of the Indian monsoon and meteorological quantities in southeast of Iran during the summer. Therefore, it can be said that the intensity of convective activities in the Indian monsoon region does not have a significant effect on the air conditions (at low levels of atmosphere) in Sistan and Baluchestan province.