Stavros Solomos

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.

Using NWP Simulations in Satellite Rainfall Estimation of Heavy Precipitation Events over Mountainous Areas

AbstractIn this study, the authors investigate the use of high-resolution simulations from the Weather Research and Forecasting Model (WRF) for evaluating satellite rainfall biases of flood-inducing storms in mountainous areas. A probability matching approach is applied to evaluate a power-law relationship between satellite-retrieved and WRF-simulated rain rates over the storm domain. Satellite rainfall in this study is from the NOAA Climate Prediction Center morphing technique (CMORPH). Results are presented based on analyses of five heavy precipitation events that induced flash floods in northern Italy and southern France complex terrain basins. The WRF-based adjusted CMORPH rain rates exhibited improved error statistics against independent radar rainfall estimates. The authors show that the adjustment procedure reduces the underestimation of high rain rates, thus moderating the magnitude dependence of CMORPH rainfall bias. The Heidke skill score for the WRF-based adjusted CMORPH was consistently higher...

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.

An overview from hygroscopic aerosols to cloud droplets: The HygrA-CD campaign in the Athens basin

The international experimental campaign Hygroscopic Aerosols to Cloud Droplets (HygrA-CD), organized in the Greater Athens Area (GAA), Greece from 15 May to 22 June 2014, aimed to study the physico-chemical properties of aerosols and their impact on the formation of clouds in the convective Planetary Boundary Layer (PBL). We found that under continental (W-NW-N) and Etesian (NE) synoptic wind flow and with a deep moist PBL (~2-2.5km height), mixed hygroscopic (anthropogenic, biomass burning and marine) particles arrive over the GAA, and contribute to the formation of convective non-precipitating PBL clouds (of ~16-20μm mean diameter) with vertical extent up to 500m. Under these conditions, high updraft velocities (1-2ms-1) and cloud condensation nuclei (CCN) concentrations (~2000cm-3 at 1% supersaturation), generated clouds with an estimated cloud droplet number of ~600cm-3. Under Saharan wind flow conditions (S-SW) a shallow PBL (<1-1.2km height) develops, leading to much higher CCN concentrations (~3500-5000cm-3 at 1% supersaturation) near the ground; updraft velocities, however, were significantly lower, with an estimated maximum cloud droplet number of ~200cm-3 and without observed significant PBL cloud formation. The largest contribution to cloud droplet number variance is attributed to the updraft velocity variability, followed by variances in aerosol number concentration.

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.

Moving toward Subkilometer Modeling Grid Spacings: Impacts on Atmospheric and Hydrological Simulations of Extreme Flash Flood–Inducing Storms

AbstractFlash floods develop over small spatiotemporal scales, an attribute that makes their predictability a particularly challenging task. The serious threat they pose for human lives, along with damage estimates that can exceed one billion U.S. dollars in some cases, urge toward more accurate forecasting. Recent advances in computational science combined with state-of-the-art atmospheric models allow atmospheric simulations at very fine (i.e., subkilometer) grid scales, an element that is deemed important for capturing the initiation and evolution of flash flood–triggering storms. This work provides some evidence on the relative gain that can be expected from the adoption of such subkilometer model grids. A necessary insight into the complex processes of these severe incidents is provided through the simulation of three flood-inducing heavy precipitation events in the Alps for a range of model grid scales (0.25, 1, and 4 km) with the Regional Atmospheric Modeling System–Integrated Community Limited Are...

Effects of Airborne Particles on Cloud Formation and Precipitation: A Modeling Study

The amount, size distribution and chemical properties of cloud condensation nuclei (CCN), depend on the type of the prevailing air mass and also on local production and transportation of natural and anthropogenic particles. The aerosol properties vary in both space and time and impose significant amount of uncertainty on atmospheric research and also on future climatic projections. Cloud microphysics are very complex in nature and most of the times it is difficult to separate and identify the links and feedbacks between air quality and atmospheric processes under real atmospheric conditions. This presentation focuses on the interactions between air quality, clouds and precipitation for the area of greater Mediterranean. Mixtures of Saharan desert dust with sea-salt or with particles of anthropogenic origin, mainly sulphates and nitrates, may lead to the formation of new aerosols with different physiochemical properties. The effectiveness of airborne particles to act as CCN is examined towards their chemical composition and size distribution. Several modeling experiments, in-situ and airborne observations are analyzed and first results on the role of natural and anthropogenic particles on cloud formation and precipitation are discussed.

From Hygroscopic Aerosols to Cloud Droplets: The HygrA-CD Campaign in the Athens Basin—An Overview

The international experimental campaign Hygroscopic Aerosols to Cloud Droplets (HygrA-CD), organized in the Greater Athens Area (GAA), Greece from 15 May to 22 June 2014 provided an unprecedented record of data on aerosols, clouds and meteorology. HygrA-CD brought together different active/passive remote sensing and in situ instrumentation/teams for the purpose of fostering our understanding on the physico-chemical properties of aerosols and their impact on the cloud formation in the lower troposphere, with emphasis on the top of the Planetary Boundary Layer (PBL). The use of well-established numerical weather prediction and atmospheric modeling, such as the Flexible particle (FLEXPART) dispersion model and the Weather Research and Forecasting (WRF) model enabled us to simulate air mass back-trajectories and atmospheric variables in the PBL, such as PBL height (PBLH), relative humidity, wind fields etc. This paper presents the major findings and highlights of the HygrA-CD campaign.

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.