C. Spyrou

Impact of natural aerosols on atmospheric radiation and consequent feedbacks with the meteorological and photochemical state of the atmosphere

This paper addresses the aerosol effects on radiation and the feedback on meteorology and photochemical activity, applying the online model RAMS/ICLAMS. The model treats meteorology and chemical pollutants on an interactive way. Cloud condensation nuclei (CCN), giant cloud condensation nuclei, and ice nuclei are treated as predictive quantities. The calculation of the aerosol optical properties accounts for size-resolved mineral dust and size- and humidity-dependent optical properties of sea salt. The simulations with and without aerosol impacts reveal the complex direct and indirect mechanisms through which the alteration of radiation fluxes influences meteorology and photochemical processes. For the specific dust event, the reduction in the surface shortwave radiation over cloudless regions affected by dust averages at ~ −75 W m−2 at 12:00 UTC per unit dust loading (1 g m−2). The increase on downwelling longwave radiation over the same areas and time averages at ~ 40 W m−2 per unit dust loading (1 g m−2). Surface upwelling longwave radiation over Mediterranean exhibits a complex daytime behavior. During midnight, the inclusion of dust leads to larger upwelling longwave radiation fluxes over the African continent. The net downward longwave radiation over cloudless areas exhibits an increase both during noon and midnight with the inclusion of dust. The results show that the vertical structure of the dust layer governs the magnitude of the feedback on radiation. The activation of natural particles as CCN causes small changes in radiation fluxes and temperature. Precipitation is influenced more by the indirect rather than the direct and semidirect effects.

Forecast errors in dust vertical distributions over Rome (Italy): Multiple particle size representation and cloud contributions

[1] In this study, forecast errors in dust vertical distributions were analyzed. This was carried out by using quantitative comparisons between dust vertical profiles retrieved from lidar measurements over Rome, Italy, performed from 2001 to 2003, and those predicted by models. Three models were used: the four-particle-size Dust Regional Atmospheric Model (DREAM), the older one-particle-size version of the SKIRON model from the University of Athens (UOA), and the pre-2006 one-particle-size Tel Aviv University (TAU) model. SKIRON and DREAM are initialized on a daily basis using the dust concentration from the previous forecast cycle, while the TAU model initialization is based on the Total Ozone Mapping Spectrometer aerosol index (TOMS AI). The quantitative comparison shows that (1) the use of four-particle-size bins in the dust modeling instead of only one-particle-size bins improves dust forecasts; (2) cloud presence could contribute to noticeable dust forecast errors in SKIRON and DREAM; and (3) as far as the TAU model is concerned, its forecast errors were mainly caused by technical problems with TOMS measurements from the Earth Probe satellite. As a result, dust forecast errors in the TAU model could be significant even under cloudless conditions. The DREAM versus lidar quantitative comparisons at different altitudes show that the model predictions are more accurate in the middle part of dust layers than in the top and bottom parts of dust layers.

Natural and anthropogenic aerosols in the Eastern Mediterranean and Middle East: Possible impacts

The physical and chemical properties of airborne particles have significant implications on the microphysical cloud processes. Maritime clouds have different properties than polluted ones and the final amounts and types of precipitation are different. Mixed phase aerosols that contain soluble matter are efficient cloud condensation nuclei (CCN) and enhance the liquid condensate spectrum in warm and mixed phase clouds. Insoluble particles such as mineral dust and black carbon are also important because of their ability to act as efficient ice nuclei (IN) through heterogeneous ice nucleation mechanisms. The relative contribution of aerosol concentrations, size distributions and chemical compositions on cloud structure and precipitation is discussed in the framework of RAMS/ICLAMS model. Analysis of model results and comparison with measurements reveals the complexity of the above links. Taking into account anthropogenic emissions and all available aerosol-cloud interactions the model precipitation bias was reduced by 50% for a storm simulation over eastern Mediterranean.

Ten-year operational dust forecasting – Recent model development and future plans

The Sahara desert is one of the major sources of mineral dust on Earth, producing up to 2x108 t yr-1. A combined effort has been devoted during the last ten years at the University of Athens (UOA) from the Atmospheric Modeling and Weather Forecasting Group (AM&WFG) to the development of an analysis and forecasting tool that will provide early warning of Saharan dust outbreaks. The developed tool is the SKIRON limited-area forecasting system, based on the Eta limited area modeling system with embedded algorithms describing the dust cycle. A new version of the model is currently available, with extra features like eight-size particle bins, radiative transfer corrections, new dust source identification and utilization of rocky soil characterization and incorporation of more accurate deposition schemes. The new version of SKIRON modeling system is coupled with the photochemical model CAMx in order to study processes like the shading effect of dust particles on photochemical processes and the production of second and third generation of aerosols. Moreover, another new development in the AM&WFG is based on the RAMS model, with the incorporation of processes like dust and sea-salt production, gas and aqueous phase chemistry and particle formation. In this study, the major characteristics of the developed (and under development) modeling systems are presented, as well as the spatiotemporal distribution of the transported dust amounts, the interaction with anthropogenically-produced particles and the potential implications on radiative transfer.

Mechanisms of Climate Variability, Air Quality and Impacts of Atmospheric Constituents in the Mediterranean Region

This chapter describes the physicochemical mechanisms that formulate the air quality over the Mediterranean region and the resulted impacts on the regional climate. At first, a detailed description of the teleconnections and regional flow patterns that dominate in the region is provided. The dominant flow patterns during the different seasons of the year determine the transport paths of air pollutants and aerosols towards and across the study area. The analysis on the characteristics of the air pollution transport is separated for the different parts of the Mediterranean region (eastern, western and entire), since the sources of pollutants that reach at different points in the region vary, while specific pollutant transport paths may influence the wider Mediterranean area. Similarities and differences in patterns are discussed. The air quality over the region, as recorded from black/organic carbon, ozone, aerosol observations, is extensively discussed, along with seasonal variabilities and annual trends. There is particular discussion on the suspension of naturally-produced aerosols and especially desert dust particles in the region and their spatial influence on the aerosol levels. At the last part of the chapter, the major impacts of the transport and transformation processes (natural and anthropogenic pollutants) on the regional climate are discussed. The impacts of aerosols are distinguished in direct (the impacts on radiation budget), health (the amounts of inhaled particles and impacts on health) and indirect effects (impacts on clouds and precipitation), are discussed on qualitative and quantitative way.

Eta model simulations using two radiation schemes in clear-sky conditions

This work evaluates the performance of two radiation parameterization schemes in 30-day clear-sky runs of the Eta model over a region in Southeast Brazil. Two versions of the Eta model are compared: a version using the radiation scheme developed by the Geophysical Fluid Dynamics Laboratory (GFDL) and a recently developed version using the Rapid Radiative Transfer Model for GCM (RRTMG). These simulations are compared against CMSAF satellite data and surface station data. The simulation using RRTMG produced downward surface shortwave radiation fluxes closer to observations and reduced the systematic positive bias of the Eta simulation using the GFDL scheme. The 2-m temperature negative bias found in the Eta-GFDL simulations is reduced in the Eta-RRTMG simulations, which results from a larger net total radiation in the Eta-RRTMG simulations. The new version has better accuracy than the Eta using the GFDL scheme for most of the evaluated variables, particularly for clear-sky conditions.

Direct radiative impacts of desert dust on atmospheric water content

ABSTRACT The direct and indirect radiative impact of naturally produced dust particles influences climate from regional to global scale, introducing one of the largest uncertainties in future climate projections. By absorbing and scattering solar radiation, aerosols reduce the amount of energy reaching the earths surface, while at the same time they enhance the greenhouse effect by absorbing and emitting longwave radiation (direct dust effect). In this study an attempt is made to quantify the feedback of this energy redistribution in the atmospheric water content in the Arabian Peninsula (one of the main sources of atmospheric mineral dust globally). To this end the SKIRON/dust modeling system was used for 2 years (2014–2015) and two sets of simulations were performed: in the first one the dust particles exert no feedback on the radiative transfer due to dust particles (control run), while in the second set dust interacts with radiation (direct radiative effect). Both simulations have been evaluated in their ability to describe the impacts on surface humidity, with the simulations including the dust feedback showing improved results. These direct feedbacks lead to an increase in the mass of water in the atmospheric column that can reach a maximum daily average of 0.5 g per kg of dry air. Water vapor is the most important greenhouse gas and through this process dust enhances its own greenhouse effect, further increasing the surface temperature and humidity, making life difficult for people living in an already harsh desert climate.

The Implementation of a Mineral Dust Wet Deposition Scheme in the GOCART-AFWA Module of the WRF Model

The principal objective of this study is to present and evaluate an advanced dust wet deposition scheme in the Weather and Research Forecasting model coupled with Chemistry (WRF-Chem). As far as the chemistry component is concerned, the Georgia Tech Goddard Global Ozone Chemistry Aerosol Radiation and Transport of the Air Force Weather Agency (GOCART-AFWA) module is applied, as it supports a binary scheme for dust emissions and transport. However, the GOCART-AFWA aerosol module does not incorporate a wet scavenging scheme, nor does it interact with cloud processes. The integration of a dust wet deposition scheme following Seinfeld and Pandis into the WRF-Chem model is assessed through a case study of large-scale Saharan dust transport over the Eastern Mediterranean that is characterized by severe wet deposition over Greece. An acceptable agreement was found between the calculated and measured near surface PM10 concentrations, as well as when model estimated atmospheric optical depth (AOD) was validated against the AERONET measurements, indicating the validity of our dust wet deposition scheme.

Highly Hygroscopic Particulate in Cloud Environment

Highly hygroscopic aerosols, such as sodium chloride or sulphates are often present in the atmosphere. They can be produced through several natural or anthropogenic processes (ocean spray, fires, volcanoes, anthropogenic emissions). Their hygroscopicity depends on their chemical properties and thus some of them can serve as cloud condensation nuclei (CCN) easier than others having different impacts on the cloud formation. While the interactions of hygroscopic aerosols with water in the atmosphere is more clearly understood, the impact of aerosols in the resulting precipitation remains inconclusive (Rosenfeld et al. 2008). The thermodynamic state, the background aerosol composition of the atmosphere and the topographical variation of the region can modify these impacts. In this study we use a fully coupled modeling system (atmospheric and chemical—RAMS/ICLAMS) in order to study the impact of highly hygroscopic particles in a cloud system, representing the average thermodynamic conditions of winter convective clouds in the eastern Mediterranean. Of particular interest is the analysis of the level of background pollution in such sensitivity studies. For this reason we applied the material dispersion processes to two characteristic air masses with different pollution levels: clean air masses and highly polluted. This study focuses on the contribution of the material dispersion on the size and number of cloud droplets as well as the liquid and ice mass of the respective cloud system. The dispersion of NaCl (Material 1) resulted in decrease of the amount of ground precipitation, while the background pollution affected the distribution of liquid and ice masses as well as the size of the hydrometeors. In respect to the time of cloud development the effect of the material dispersion was more evident in the mature phase of the cloud system.

The Effects of Naturally Produced Dust Particles on Radiative Transfer

Mineral dust has a profound effect on the radiative budget and energy distribution of the atmosphere. By absorbing and scattering the solar radiation aerosols reduce the amount of energy reaching the surface. In addition aerosols enhance the greenhouse effect by absorbing and emitting longwave radiation. Desert dust forcing exhibits large regional and temporal variability due to its short lifetime and diverse optical properties further complicate the quantification of the Direct Radiative Effect (DRE). The complexity of the above processes, indicate the need of an integrated approach in order to examine these impacts. To this end the radiative transfer module RRTMG has been incorporated into the framework of the SKIRON model. The updated system was used to perform a 6-year long simulation over the Mediterranean region. As it was found, the most profound effect dust clouds have in areas away from the sources is the surface cooling through the “shading” effect. The long wave radiation forcing below and above the dust cloud is considerable and drives changes in the tropospheric temperature. In general dust particles cause warming near the ground and at mid-tropospheric layers and at the same time cooling of the lower troposphere.