N. Hatzianastassiou

Climatology and trends of aerosol optical depth over the Mediterranean basin during the last 12 years (2002–2014) based on Collection 006 MODIS-Aqua data

The Mediterranean basin is a region of particular interest for studying atmospheric aerosols due to the large variety of air masses it receives, and its sensitivity to climate change. In this study we use the newest collection (C006) of aerosol optical depth from MODIS-Aqua, from which we also derived the fine-mode fraction and Ångström exponent over the last 12years (i.e., from 2002 to 2014), providing the longest analyzed dataset for this region. The long-term regional optical depth average is 0.20±0.05, with the indicated uncertainty reflecting the inter-annual variability. Overall, the aerosol optical depth exhibits a south-to-north decreasing gradient and an average decreasing trend of 0.0030 per year (19% total decrease over the study period). The correlation between the reported AOD observations with measurements from the ground AERONET stations is high (R=0.76-0.80 depending on the wavelength), with the MODIS-Aqua data being slightly overestimated. Both fine-fraction and Ångström exponent data highlight the dominance of anthropogenic aerosols over the northern, and of desert aerosols over the southern part of the region. Clear intrusions of desert dust over the Eastern Mediterranean are observed principally in spring, and in some cases in winter. Dust intrusions dominate the Western Mediterranean in the summer (and sometimes in autumn), whereas anthropogenic aerosols dominate the sub-region of the Black Sea in all seasons but especially during summer. Fine-mode optical depth is found to decrease over almost all areas of the study region during the 12-year period, marking the decreasing contribution of anthropogenic particulate matter emissions over the study area. Coarse-mode aerosol load also exhibits an overall decreasing trend. However, its decrease is smaller than that of fine aerosols and not as uniformly distributed, underlining that the overall decrease in the region arises mainly from reduced anthropogenic emissions.

Aerosol shortwave direct radiative effect and forcing based on MODIS Level 2 data in the Eastern Mediterranean (Crete)

A spectral radiative transfer model was used to quantify the aerosol direct radiative effect and forcing over the island of Crete in the Eastern Mediterranean. Computations were performed for the 11-year period from 2000 to 2010. MODIS Level 2 data (daily, 10 km × 10 km spatial resolution) were used as input to the model. Output includes the radiative fluxes and the aerosol direct radiative ffect at the top of the atmosphere, within the atmosphere and at the surface. The corresponding forcing components were evaluated based on MODIS fine mode aerosol data. Results show a decreasing trend of the aerosol radiative effect. The analysis of the contribution of anthropogenic and natural aerosols shows major peaks of natural aerosol effects occurring mainly in spring and autumn, while a summer maximum is attributed to anthropogenic aerosol.

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.

Direct Effect of Aerosols on Solar Radiation Over the Eastern Mediterranean Basin on a Daily 1° by 1° Resolution

The climatically sensitive Mediterranean basin, which is characterized by high solar radiation amounts and aerosol loadings, is the focus of this study that aims to determine the direct effect of aerosols on solar radiation (DRE) over the eastern basin. The objective is to reveal detailed patterns of aerosol DRE that are smoothed when obtained at coarse resolution. Therefore, computations of DRE are performed at a concurrent spatial and temporal resolution that has not been achieved to date. The DREs are computed for 1×1° latitude-longitude grids with the FORTH detailed spectral radiative transfer model (RTM) using daily input data for various atmospheric and surface parameters, such as clouds, water vapor, ozone and surface albedo, taken from the NASA-Langley Global Earth Observing System database. Key aerosol optical properties, namely aerosol optical depth, single scattering albedo and asymmetry parameter, necessary for the RTM runs, are taken from combined climatologies, like the Global Aerosol Data Set, and the satellite-derived datasets of Total Ozone Mapping Spectrometer and Advanced Very High resolution Radiometer that satisfy daily availability at the RTM required spectral and 1×1° resolution. The aerosol DREs are computed at the surface, the top-of-atmosphere and within the atmosphere, over the period 1985–1995.

Study of the Total and Ultraviolet Solar Radiation in the Alpine Region of Ioannina (Northwestern Greece)

The FORTH deterministic spectral radiative transfer model is used to estimate the incoming surface solar radiation (ISSR) at the alpine city of Ioannina in northwest Greece. The model is initialized with high spatial (10 km × 10 km and 5 km × 5 km) and temporal (daily) resolution satellite Level-2 Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) data for aerosol, cloud and other atmospheric and surface parameters. The daily total and ultraviolet (UV) solar radiation is computed with the model for the period 2008–2012. The model ISSR is successfully validated against corresponding ground-based measurements of total and UV radiation from a station located at Ioannina for the study period. The instantaneous total, UVA and UVB ISSR values are as high as 1070, 45 and 2.25 W/m2 in summer, exhibiting a significant day to day and seasonal variability, primarily driven by the top of atmosphere solar radiation and clouds. The primary role of clouds for ISSR, versus aerosols and ozone, is also verified by model sensitivity tests, which reveal however an increasingly stronger role of these two parameters for UV radiation.

A Satellite Based Algorithm for Identification of Biomass Burning Aerosols Over the Southern Atlantic Ocean

In the present study, an algorithm is developed for identifying the presence of biomass-urban (BU) aerosols all over the globe, with emphasis to the southern Atlantic Ocean, which is neighbour to the greatest global biomass burning source areas, namely the Amazonian basin and sub-Sahel, central and southern Africa. The algorithm uses NASA’s satellite daily aerosol optical properties, at 1° × 1° latitude-longitude resolution, for the period 2005–2015. The identification of BU aerosols is achieved by using a variety of aerosol properties, namely spectral Aerosol Optical Depth, Angstrom Exponent and Fine Fraction (FF) from Collection 006 MODIS-Aqua and Aerosol Index (AI) from OMI-Aura. Maximum frequencies of BU aerosols, reaching 57 days/year and 22 days/month) are found over global areas with the greatest biomass burning activities, namely in South Africa and South America. The algorithm outputs are in line with the known seasonality of biomass burning, with two distinct periods (local dry seasons) in Africa and a single one in Amazonia. The good performance of the algorithm is supported by the agreement with MODIS satellite active fire counts data, which enable the distinction between BU and BB aerosols.

High Resolution Aerosol Optical Depth in the Broader Greek Area Using MODIS Satellite Data

We study the spatial (50 km × 50 km) and temporal distribution of aerosol optical depth (AOD) over the broader Greek area on a mean annual, seasonal and monthly basis. Daily visible Level-2 AOD data (AOD 550 nm ) from the MODIS database are used for the period March 2000–February 2008. The mean annual AOD values for the study region range from 0.13 to 0.37, with a mean annual regional AOD value equal to 0.19. Maximum AOD values are found in spring and late summer (August), and minimum in winter. The highest values occur in Eastern Greece (particularly during spring and summer) and near the Aegean coasts of Turkey (Smyrni) and Istanbul (during autumn and winter). The smallest AOD values are found in Western Greece, at some areas of the neighboring Balkan countries, and in the interior of Turkey (during all seasons) as well as in marine areas south and east of Crete (during summer). A decreasing tendency of regional AOD is found from 2000 to 2008, equal to Δ(AOD) = −0.05, implying decreasing aerosol amounts over the study region.

Spatial Variability of Daily Extreme Surface Air Temperatures Over Europe During 1950–2009

The spatial variability of extreme surface air temperature has been studied across Europe over the 60-year period 1950–2009. Data of daily maximum and minimum temperature, Tmax and Tmin, respectively, taken from the European Climate Assessment & Dataset (ECA&D), were used to investigate their spatial variability, through parameters, DTi = [max(Ti) − min(Ti)] and STDV(Ti), standard deviations, where i = max,min, expressing the absolute and overall spatial variability, respectively. The data are measurements from 181 stations uniformly distributed over Europe with adequate availability. Our results indicate that there is a significant spatial variability of daily Tmax and Tmin values over Europe. The computed values of DTmax,min range from about 17°C to 58°C, whereas those of STDV(Tmax,min) range from about 3.0–11.5°C, depending on the year and season. It is found that the spatial variability of maximum temperatures over Europe has decreased from 1950 to 2009, as indicated by the computed values D(DTmax) = −0.008°C and D[STDV(Tmax)] = −0.003°C. Similarly, the overall spatial variability of minimum temperatures over Europe has decreased as well, as indicated by our computed values D(DTmin) = −0.016°C and D[STDV(Tmin)] = −0.006°C. The seasonal analysis, however, reveals contrasting tendencies, with decreasing D(DTi) and D[STDV(Ti)] in winter and spring and increasing in summer.

Cloud Radiative Effects on Solar Radiation Over the Eastern Mediterranean Basin from 1984 to 2007

The cloud radiative effect (CRE) on solar shortwave (SW) radiation is investigated over the broader Mediterranean basin, for the 24-year period from 1984 to 2007, using a deterministic spectral radiation transfer model (RTM) and quality monthly cloud data taken from the ISCCP-D2 satellite database. The model simulates the interaction of solar radiation with all relevant physical parameters, namely ozone, carbon dioxide, methane, water vapour, aerosols and atmospheric molecules as well as surface reflection. The ISCCP cloud model input data include cloud cover and optical depths for low-, mid-, and high-level clouds, separately for ice and liquid water clouds. The model computes CREs at the top of atmosphere (CRETOA), within the atmosphere (CREatmab), and at the surface (effects on downwelling and absorbed solar radiation, CREsurf and CREsurfnet). The determined perturbation of regional SW radiation budget by clouds is important, undergoing significant intra-annual and inter-annual changes.

Day to Day Variability of Air Temperature over Greece for the Period 1957–2002

In the ongoing discussion of global warming and current climatic changes, an open issue is the increased variability of climatic parameters and the changing frequency and intensity of their extreme values. In this framework, the present work deals with the day-to-day variability of maximum (ΔTmax) and minimum (ΔTmin) air temperature and diurnal temperature range (Δ(DTR)) over the Greek area for the period 1957–2002. The data used are daily values from ten stations of the National Hellenic Meteorological Service (N.H.M.S.) which are uniformly distributed over the study area. The absolute values of the considered parameters, for each station, are analyzed in terms of intra-annual and inter-annual variation, whereas the existence of statistically significant trends on a seasonal and annual basis is also examined. The analysis reveals stations with common behaviour regarding temperature variability mainly determined by their geographical location and other geophysical characteristics. In many cases, especially for day-to-day variability of ΔTmin and Δ(DTR) there appear statistically significant trends, mainly increasing, either on a seasonal basis or annual.