فهرست مطالب امید علیزاده چوبری

The El Niño-Southern Oscillation (ENSO) cycle of alternating warm El Niño and cold La Niña events occurs when the tropical Pacific Ocean and its overlying atmosphere change from its natural state for at least several consecutive months (Neelin et al., 1998). The neutral phase of the El Niño-Southern Oscillation is derived by the strong zonally asymmetric state of the equatorial Pacific and is characterized by surface easterly trade winds along the equatorial Pacific, rising motion, deep convection and heavy rainfall over the western equatorial Pacific, westerly winds at upper levels and sinking motion over the eastern equatorial Pacific (Bjerknes, 1969). El Niño is characterized by weak and La Niña by strong zonal SST gradients, accompanied respectively by weakening and strengthening of the trade winds across the equatorial Pacific (McPhaden et al. 2006). As a result, compared to the neutral phase of the El Niño-Southern Oscillation, convective systems intensify in the western tropical Pacific and slightly shift to the west during La Niña events, but shift to the central and eastern tropical Pacific during El Niño events. Since this early recognition of the coupling between the atmosphere and the Pacific ocean by Bjerknes (1966) and Bjerknes (1969), major advances have beenmade toward a comprehensive understanding of the physics of the El Niño-Southern Oscillation. This is particularly achived through development of complex climatemodels for realistic simulation of the El Niño-Southern Oscillation cycle (Bellenger et al., 2014), and great observational advances that have been made during the international Tropical Ocean-Global Atmosphere (TOGA) program conducted between 1985 and 1994 (McPhaden et al., 1998). El Niño or the warm phase of the El Niño-Southern Oscillation is a quasi-periodic natural phenomenon that occurs in the tropical Pacific Ocean. The El Niño-Southern Oscillation not only influences the climate of nearby regions, but it is the most important natural climate factor that contributes to the interannual climate variability over many regions across the globe, including North America (e.g. Yu et al., 2015; Guo et al., 2017), the Middle East (e.g. Alizadeh-choobari, 2017; Alizaeh-Choobari et al., 2018a, Alizaeh-Choobari et al., 2018b), East Asia (e.g. Feng and Li, 2011), Southeast Asia (e.g. Lee et al., 2017) and the Indian subcontinent (e.g. Kumar et al., 2006). Depending on the location of the maximum sea surface temperature in the eastern or central equatorial Pacific, the eastern Pacific El Niño or the central Pacific El Niño are identified, while a mixed event of the eastern and central Pacific El Niño events has been also diagnosed.

In this study, using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis Interim (ERA-Interim) monthly dataset with a horizontal resolution of 0.75º  0.75º, and the Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5) dataset, the phase of ENSO and the type of El Niño events during the period 1979-2016 are determined. In addition, characteristics of the 1997-98 eastern Pacific El Niño, the 2009-10 central Pacific El Niño and the 2015-16 El Niño, which is a mixed event of the eastern and central Pacific El Niño, are investigated.

Analysis indicateed that during the period 1979-2016, 1979-80, 1982-83, 1986-87, 1987-88, 1991-92, 1994-95, 1997-98, 2002-03, 2004-05, 2006-07, 2009-10, 2014-15 and 2015-16 have been the years for which three-month running means of the Oceanic Niño Index (ONI) for 5 consecutive periods became greater or equal to 0.5 degree Celcius, indicating the occurrence of El Niño in these years. Using the empirical orthogonal function (EOF) and by examining spatial correlation between sea surface temperature anomalies in the equatorial Pacific Ocean and results of the empirical orthogonal function, the eastern and central Pacific El Niños during the period 1979-2016 are determined. The 1997-98 and 2015-16 El Niño events are both categorized as extreme El Niño events. The 2009-10 El Niño is weaker than the other two events, but over the last century, it has been the strongest central Pacific El Niño event. Results indicated that the onset of all these three events was in June, while some differences are found between termination of the 1997-98 and 2015-16 El Niños, including different time of dissipation for these events. All these three events have shown characteristics of classic El Niño events, such that anomalous positive and negative sea surface temperature are seen in the eastern and western equatorial Pacific, respectively. Nevertheless, maximum positive sea surface temperature is formed in the eastern equatorial Pacific during the 1997-98 El Niño, which is different from the 2009-10 El Niño event with the maximum sea surface temperature in the central (near the dateline) equatorial Pacific. In fact, maximum positive sea surface temperature anomalies are located in the eastern and central equatorial Pacific during the 1997-98 and 2009-10 El Niño events, respectively, while it extends from central to eastern equatorial Pacific during the 2015-16 El Niño. Intensities of the maximum sea surface temperature anomalies and mean sea level pressure have been greater during the 1997-98 El Niño compared to those during the 2009-10 event, indicating that central Pacific El Niños are generally less intense than eastern Pacific El Niño events. It is shown that positive sea surface temperature anomalies in the 2015-16 El Niño event cover a larger area, extending from the central to the eastern equatorial Pacific. It is found that both sea surface temperature and mean sea level pressure anomalies in the equatorial Pacific were larger during the 1997-98 eastern Pacific El Niño than that of the 2009-10 central Pacific El Niño. This suggests that the central Pacific El Niño events are generally weaker than the eastern Pacific El Niño events. In all of the three El Niño events, positive (negative) geopotential height anomalies at 300 hPa pressure level in the equatorial Pacific are collocated with positive (negative) sea surface temperature anomalies. Geopotential height anomalies in the upper levels over the tropical Pacific influence weather patterns of other regions.It is discussed and shown that different geopotential height anomalies at upper levels of the equatorial Pacific during the three El Niño events have led to different teleconnections across the globe. For example, temperature anomalies in the Antarctic during the 2009-10 El Niño were opposite to those during the 1997-98 and 2015-16 El Niño events.

Analysis of the ERA-Interim dataset with the horizontal resolution of 0.75º  0.75º for the period 1979-2016 indicated that the eastern, central and mixed El Niño events have generally different characteristics in the equatorial Pacific. As a result, teleconnection patterns of these events across the globe are also found to be different.

Dust aerosols make a considerable contribution to the climate system through their radiative effects due to their abundance in the atmosphere. Recent observations suggest that over the past decade, dust events have become more frequent in many parts of Iran, especially in the west and southwest. Through their radiative forcing, dust aerosols have significant effects on the regional radiation budget of the atmosphere, while their adverse effects on human health have also raised serious concerns. The primary aim of the present study is to examine the radiation effects associated with a severe dust storm that occurred in west and southwest Iran on 16 to 21 June 2012. To this end, the Weather Research and Forecasting with Chemistry (WRF-Chem) model was used. Two simulations were conducted: a model setup that did not include dust aerosols, and the one that included dust aerosols and their feedback to the atmosphere. A two-way interactive nested domain (nesting ratio:1:3) simulations were performed using 98 Í 90 and 151 Í 139 horizontal grid points, respectively. In the vertical, 27 σ-levels were used. The grid spacing for the two domains were 45 and 15 km, respectively. Simulations ran from 16 to 22 June 2012, and the first 24 hours was considered as the spin-up time. Meteorological initial conditions were obtained from the Global Forecast System (GFS) data at 0.5˚Í 0.5˚ resolution. The performance of the model was evaluated using the available observed data, including PM10 observations in Ahwaz located in southwest Iran, available AErosol RObotic NETwork (AERONET) data in nearby areas, and aerosol products of the Moderate Resolution Imaging Spectroradiometer (MODIS), the Ozone Monitoring Instrument (OMI) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) carried on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) spacecraft. Results indicate that PM10 concentration in Ahwaz is overestimated by the model, while simulated aerosol optical depth (AOD) is underestimated compared to the observed AERONET data. Relatively, good agreement is found between the model results and satellite products, and temporal evolution of the dust events is also well-simulated. Thus, generally, the performance of the model is acceptable for accurate simulation of the dust event. Our analysis indicated that radiative effects of dust particles cause cooling at the surface and top of the atmosphere, but warming in the middle of the troposphere. On average, perturbation of shortwave radiation by dust aerosols in the west, and southwest Iran is estimated to be -7.27, 1.79 and -5.47 W m-2 at the surface, in the middle and at the top of the atmosphere, respectively. Average perturbation of the longwave radiation by dust aerosols over the same region was estimated to be 2.2, -1.61 and 0.59 W m-2 at the surface, in the middle and at the top of the atmosphere, respectively. Thus, the net (shortwave longwave) radiative effect of dust aerosols averaged in west and southwest Iran is found to be -5.07, 0.19 and -4.88 W m-2 at the surface, in the middle and at the top of the atmosphere, respectively.

Observations unequivocally show that climate change is happening in most regions of the globe. Warming which has been observed in most regions of the globe, particularly in recent decades, is the best manifestation of the climate change. In contrast to the warming of the most regions of the world, many places across the globe have experienced different changes in the amount and intensity of precipitation, such that under the global warming both increases and decreases of precipitation have been reported. Using meteorological records of fifteen ground stations across Iran for a 63-yr period from 1951 to 2013, trends of the minimum, maximum and daily mean near-surface air temperatures and annual accumulated precipitation are examined. Results indicated that the annual minimum, maximum and daily mean near-surface air temperatures in most regions of Iran have experienced increasing trends. Thus, Iran, like most regions of the world, has been rapidly warming over the past few decades. The observed increasing trend in air temperature is mostly attributed to the increase of the greenhouse gases due to human activities. In most regions of Iran, the increasing trends of the minimum temperature have been greater than those of the maximum temperature, the feature which has been mostly attributed to the urbanization development. Indeed, through blocking the outgoing longwave radiation, the urbanization development has effectively contributed to the more significant increase of the minimum temperature than the maximum. In addition, the urban air pollution decreases the incoming shortwave radiation reaching to the Earth surface; thereby partly contributes to the less increase of the maximum temperature compared to the minimum. As a result, a decreasing trend in the diurnal temperature range (the difference between the daytime maximum and nighttime minimum temperatures) is identified. Temperatures in most regions of Iran have experienced a changing point either in 1980s or 1990s, such that the mean temperature of the all regions during the period after the changing point was approximately 1.2 ˚C greater than the mean temperature during the period before the changing point. Under the warming, most regions of Iran have experienced decreasing trends in the annual accumulated precipitation, although most of the trends have not been statistically significant. The decrease of precipitation, and the increase of air temperature imply that Iran has become drier and more vulnerable to drought over the past few decades. The observed decreasing trend in precipitation over Iran is in contrast to the trend of global mean precipitation, for which the increase of precipitation under the global warming has been noted. Indeed, previous studies have indicated that 1K rise in temperature is associated with 2 percent increase in the global mean precipitation. However, the results of the present study are consistent with previous studies conducted over the subtropical regions. In a warmer climate, saturation of the atmosphere takes alonger time, which delays the onset of precipitation. Thus, in the arid and semi-arid regions of Iran with the dominant subtropical climate, more water vapour can be transported to higher latitudes by the general circulation of the atmosphere before precipitation can form. In contrast, previous studies have indicated that precipitation increases in both subpolar and tropical regions. We, therefore, argue that depending on the geographical location, the intensity and frequency of precipitation vary in response to the warming of the climate.

Although cloud properties and precipitation formation are primarily affected by atmospheric dynamics, cloud microphysical features also play key roles. The aerosol number concentration strongly influences cloud microphysics and precipitation formation, mainly through affecting the formation of cloud droplets and ice crystals.
In the current research, using the Thompson aerosol-aware microphysics scheme implemented on the Weather Research and Forecasting (WRF) model, the effects of aerosol number concentration was investigated on the precipitation formation of a heavy rainfall in Tehran. The aerosol number concentrations were obtained from the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model, while the National Center for Environmental Prediction Final Analysis (NCEP/FNL) dataset was used for the initial and lateral boundary conditions. Two numerical simulations were conducted, referred to as the clean and polluted experiments. The initial hygroscopic aerosol number concentrations, compared to the values obtained from the GOCART model, were reduced to one-fifth and increased by a factor of 5 in the clean and polluted experiments, respectively. The model simulations were run with three nested domains, with horizontal resolutions of 21, 7 and 2.3333 km, and 45 levels in the vertical position, reaching up to the 50 hPa level. Simulations were conducted for 30 hours, starting from 18:00 UTC April 13, 2012, from which, the first 6 hours were considered as the model spin-up. The Rapid Radiative Transfer Model (RRTM; Mlawer et al., 1997) was used for the shortwave and longwave radiation, respectively. The land surface scheme and surface layer scheme were based on the five-layer thermal diffusion and the revised MM5 similarity theory, respectively (Zhang and Anthes, 1982). The non-local Yonsei University (YSU) scheme was employed for the parameterizations of the boundary layer processes (Hong et al., 2006). The Kain-Fritsch scheme (Kain, 2004) was used to parameterize moist convection in the mother and first nested domains, while it was explicitly modelled in the innermost domain.
Results indicated that changes in the aerosol number concentration are associated with changes in the spatial distribution of precipitation. Stronger updraft cores were found in the polluted experiment, entailing higher precipitation, longer growth times, and larger sizes of hydrometeor; accordingly, more raindrops survived from the evaporation after falling from the cloud base, increasing the surface precipitation. On the other hand, surface precipitation decreased in the downstream, primarily due to the decrease in the effective radii of ice crystals, reducing the riming processes and the amounts of graupels. Results further indicated that the increase in the aerosol number concentration is associated with the increase in the rate of precipitation under high relative humidities, while the reverse is true when the available water vapour is relatively low.

Through modifying the number concentration and size of cloud droplets, aerosols have complex impacts on radiative properties of clouds, which consequently change the radiation balance of the Earth, and modify the atmospheric air temperature. By conducting numerical experiments for a mid-latitude cloud system in April, the indirect effects of aerosols on shortwave and longwave radiation, and subsequent impacts on the near-surface air temperature are investigated over Tehran. To this end, three numerical experiments (control, clean and polluted) with initial identical dynamical and thermodynamic conditions, but different cloud-nucleating aerosol concentrations were conducted using the Weather Research and Forecasting (WRF) model. Simulations were conducted over three nested domains with two-way interactions (nesting ratios: 1:3:3; horizontal resolutions: 21, 7 and 2.333 km). A two-moment aerosol-aware bulk microphysical scheme, recently developed, discussed and tested by Thompson and Eidhammer (2014), was used. In the control experiment that represents conditions of the current era in terms of the aerosol number concentrations, concentrations of atmospheric aerosols were derived from 7-yr global simulations of the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model which include mass mixing ratios of sulfate, dust, black carbon (BC), organic carbon (OC), and sea salt. Hygroscopic aerosol number concentrations were reduced to one-fifth in the clean experiment, and increased by a factor of 5 in the polluted experiment. The meteorological initial and lateral boundary conditions in the three experiments were derived from the National Center for Environmental Prediction final analysis (NCEP/FNL) data at 1˚ horizontal resolution and 6 h temporal intervals. Results indicate that increasing (decreasing) cloud-nucleating aerosol concentrations in the polluted (clean) experiment is associated with more (less) numerous cloud droplets of overall smaller (larger) size. Indeed, mean cloud droplet number concentrations (effective radius of cloud droplets) in cloudy grid points averaged over the innermost domain and during the simulation period were found to be approximately 46, 158 and 417 cm-3 (8.5, 6.1 and 5.2 μm) in the clean, control and polluted experiments, respectively. Thus, the total droplet cross-sectional area of cloud droplets increases in the polluted experiment, leading to an enhancement in the shortwave cloud radiative forcing (or cloud albedo), such that less shortwave radiation reaches to the Earth surface. In contrast, the total droplet cross-sectional area of cloud droplets decreases in the clean experiment, leading to a reduction in shortwave cloud radiative forcing (or cloud albedo). In contrast to the significant changes in the shortwave cloud radiative forcing by aerosols, results indicate that changing the number and size of cloud condensation nuclei in the polluted and clean experiments has little impact on longwave cloud radiative forcing. Values of shortwave and longwave cloud radiative forcing indicate that as the positive longwave cloud radiative forcing in all experiments are nearly half of the negative shortwave cloud radiative forcing, clouds have an overall cooling effect on the climate system, counteracting the warming caused by increases in concentrations of the atmospheric greenhouse gases. Comparing the near-surface air temperature of the three experiments reveals that the enhancement of cloud albedo in the polluted experiment leads to a reduction in the near-surface air temperature, while reduction of cloud albedo in the clean experiment leads to the enhancement of the near-surface air temperature.

Mineral dust is produced from both natural and anthropogenic sources. Dust aerosols can be transported over long distances in the atmosphere. They reduce the incident shortwave radiation to the surface by absorbing and scattering the solar radiation; thereby leading to a cooling effect at the surface and lower tropospheric temperature. On the other hand, by absorption and re-emission of longwave radiation, they increase the net surface longwave radiation at the surface. This direct interaction of dust aerosols with shortwave and longwave radiation, known as the direct radiative impact, plays a key role in the radiation budget of the atmosphere. Although mineral dust is one of the most significant aerosols in the atmosphere, according to the Intergovernmental Panel on Climate Change (IPCC, 2007), uncertainty in its spatial distribution and radiative forcing, remains as a great challenge in climate studies. In the present study, the Weather Research and Forecasting with Chemistry (WRF-Chem) regional model is used to simulate distribution of mineral dust and its impacts on radiation fluxes on the global scale. The model was executed using 335 × 168 horizontal grid points with a horizontal spacing of 120 km, and 28 vertical levels for January and July 2011. The National Centers for Environmental Prediction (NCEP) Final Analysis (FNL) re-analysis data were used as meteorological initial conditions. The GOCART (Goddard Global Ozone Chemistry Aerosol Radiation and Transport) simple aerosol scheme was used for the simulation of dust emission and airborne dust distribution. Two experiments were conducted: the control simulation with no dust; and the interactive simulation for which dust aerosols feedback to the atmosphere. Differences between these two simulations indicate the perturbation of radiation by dust. Results indicate that the concentration of dust particles is generally much higher in the Northern Hemisphere than the Southern Hemisphere. The main sources of dust are located over the Sahara and Sahel, the Middle East, and East Asia, especially the Gobi Desert of China and Mongolia. The Eyre Basin in central Australia was identified as the most important source of dust in the Southern Hemisphere. Over the Sahara, dust emission was most intense in January, but substantially decreased in July. In contrast, in response to drier soils and higher wind speeds, sources of dust in the Middle East were more active in July than January. The Gobi Desert was also found to have much more dust activity in January than July, primarily due to stronger wind speeds during this month. On the global scale, monthly-averaged dust optical depth (DOD) was estimated to be 0.046 and 0.069 in January and July, respectively. Globally, perturbation of shortwave and longwave radiation by dust at the top of the atmosphere (TOA) was estimated to be -1.84 and 1.34 W m-2 in January, and -2.38 and 0.68 W m-2 in July, respectively. At the surface, it was estimated that perturbation of shortwave and longwave radiation to be -2.07 and 0.82 W m-2 in January, and -4.14 and 1.02 W m-2 in July, respectively. It was also found that perturbation of radiation is larger closer to the sources of dust. For instance, the perturbation of shortwave radiation exceeds -20 W m-2 over the Sahara. Globally, we identified that dust has a negative effect on the shortwave, but a positive effect on the longwave radiation at the surface. However, in snow covered regions (such as over the Tibetan Plateau, northern parts of the Scandinavia and the United States in January) deposition of dust on the surface increases the net shortwave radiation reaching the surface (due to reduction of surface albedo) and decreases net longwave radiation by increasing outgoing longwave radiation from the surface.