Simulation of Radiative Forcingof Middle Eastern Mineral Dust in Western Iran

IntroductionDust storms frequently occur throughout the desert regions of the world, injecting large amounts of mineral dust aerosols into the atmosphere. Dust aerosols have a wide range of potential consequences for ambient air quality, global climate, atmospheric chemistry, and biogeochemical processes. Higher levels of particulate matter during dust storms can lead to serious health problems.
Mineral dust aerosols from arid and semi-arid regions are important constituents of the atmosphere and climate system that influence the radiative budget of the earth directly and indirectly by acting as condensation nuclei for the formation of both, cloud droplets and atmospheric ice particles and thereby impacting microphysical and optical properties of clouds in both regional and global scales. Considering only the indirect effects, the ice phase has a particularly strong influence on cloud properties by affecting cloud lifetime and precipitation processes. Depending on their physical and optical properties as well as their chemical composition, aerosols exert a cooling or warming influence on the atmosphere and underlying surface.
Every year, about 3000 trillion grams of dust are entrained into the atmosphere from arid and semiarid regions and the Asian deserts and Middle East are two of the largest dust-producing regions in the world. Regarding to the geographical location of Iran, it is frequently exposed to local and regional dust systems. Over the last decade, dust storm in the Middle East especially in the Arabian region (including Saudi Arabia, Iraq, Syria and Jordan) has significantly been increased and such condition injected annually millions grams of dust into the atmosphere and affects radiation budget and climate of this region. Thus, being in vicinity of vast deserts, the west and southwest of Iran are characterized by high-levels of dust events, which have adverse consequences on human health, ecosystems and environment. The aim of this study is to explore the radiative forcing and direct effects of dust storms events in the Middle East and west of Iran.
Study AreaThe study area located in western Iran and including three Kermanshah, Ilam and Khuzestan provinces. The region bounded between 29˚ 58ʹ N to 35˚ 17ʹ N and 45˚ 20ʹ E to 50˚ 39ʹ E. The study area is geographically bounded by the high western Iran’s high Mountains on the West, and coasts of Persian Gulf on the south. Depending on the topographical features of the landscape, the climate of the region varies from north to south, such that there is a drier and warmer climate on the southern area compared to the middle and northern parts of the study area.
Material and MethodsThe WRF-Chem model version 3.7 was applied to simulate the global distribution of mineral dust and its radiative forcing. The Weather Research and Forecasting with Chemistry (WRF-CHEM) model is the most advanced online coupled model in terms of the number of available schemes, complexity of the aerosol, and chemical schemes, as well as the coupling between gases, aerosols, and meteorology. The GOCART simple aerosol scheme used in this study only focuses on the simulation of dust particles and their impact on radiation.
Simulation of mineral dust inthe Middle East from April 12 to 15 2011 and its impacton radiation fluxes over the domain with 30-km grid spacing is estimated using two simulations: a standard model that did not include dust aerosols feedback; and aninteractive experiment that included dust aerosols and theirfeedback to the atmosphere.Finally for validation ofWRF-CHEM simulation of mineral dust, the data fromaerosol products of AERONET (Aerosol Robotic Network) and satellite measurements, along with environmental observation close to dust sources had been used. For this, the hourly ground-based measurements (e.g., dust concentration, PM10 and PM2.5 in Iran’s environmental stations and AOD in AERONET measurements) and also satellite observations (e.g., MODIS and MISR) were used. Via these, the regional model deficiencies in the representation of the spatio-temporal distribution as well as optical properties (e.g., aerosol optical depth; AOD) of mineral dust had been identified.
Results and DiscussionWe found that for simulation of mineral dust in the West Asia domain by the GOCART dust module, it is needed to make some changes in erosion maps and source codes. By these changes, the model will give the simulation closer to reality. By these changes, the diurnal variation of the simulated hourly PM10 mass concentration inAhwaz is qualitatively close to the hourly observations made by the Khuzestan Department of Environment. The model captures diurnal cycle of the observed PM10 concentration during most of the simulation period, although the model predictions overestimated PM10 concentration in time of maximum PM10 concentration. Totally coupling WRF model with GOCART aerosol scheme has a good performance in simulation of hourly PM10 mass concentration.
Furthermore, thecomparison of the simulated aerosol optical depth (AOD) by GOCARTaerosol scheme and the observations in Solar Village site of the global AERONET Network show thatthe model has a good performance in simulation of (AOD). In the other hand, the model has a good performance in spatial distribution in case of dust storm in the Middle East.
The spatial distributions fromApril 12 to 15, 2011 average of SW, LW,and net radiation (SW) dust direct effect at surface showthe SW DRE (direct radiative forcing) is mostly negative at the surface, reaching -50Wm-2.The average of surface SW DRFin western Iran is -36Wm-2 and tendsto cool the surface. The daily averaged surface LW DRE ismostly positive.
The maximum average LW DREat the surface is 12Wm-2and LW average in areas that are affected by dustis 5Wm-2.At TOA, the maximum average valueof outgoing LW radiation(OLR) is -25Wm-2that the average in areas that are affected by dustis -15Wm-2which demonstrates warming of the TOA earthatmosphere system. At the surface, the net radiation DRE is negative in most of the region and such condition leads to surface cooling.
The surface air temperatureover the majority of the study domain cools down in response tothe net dust radiative effect. The temperature changes by upto -1° K in Middle East. The instantaneous surface sensible heat flux decreases over the area that is affected by dust with an average value of -21Wm-2 in the presence of dust aerosolsover land. The value for surface latent heat flux is -2Wm-2in average of dusty region.
ConclusionThe results of this study show that the model has a good performance in simulation of (AOD) and diurnal cycle of the observed PM10 concentration; however, the model predictions overestimated PM10 concentration in time of maximum PM10 concentration.
The results of radiative forcing of dust storm in the Middle East show that mineral dust cause decrease of shortwave radiation and net radiation and increase of longwave radiation at the surface. Such condition leads to decrease on surface temperature and tendsto cool down the surface up to -0.6° K in western Iran. Additionally, the presence of mineral dust cause decrease of surface latent and sensible heat flux and temperature. Hence, dust directly influences the earth’s radiative budget and causes surface cooling. At top of atmosphere, presence of dust leads to decrease of OLR which causeswarming of the TOA earthatmosphere system. Some documents showed that dust storm in the Arabian Peninsula leads to decrease of surface shortwave radiation, increase of surface longwave radiation, and totally decrease of net radiation in the surface. In this study, we approved such results for the west of Iran.